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Strong Westerly Wind Events in the Strait of

CLIFFORD F. MASS,MICHAEL D. WARNER, AND RICK STEED Department of Atmospheric Sciences, University of , , Washington

(Manuscript received 19 February 2013, in final form 27 August 2013)

ABSTRACT

Strong westerly wind events occur regularly within and downstream of the Strait of Juan de Fuca of 2 Washington State. During such strong gap wind periods, flow can accelerate to 30–50 kt (1 kt 5 0.51 m s 1), with gusts sometimes reaching 80 kt or more. Strong winds in the strait and downstream of its eastern exit have caused substantial damage and loss of life. Strait of Juan de Fuca westerly wind events are often associated with sharp short-wave troughs coming out of the northwest, strong lower-tropospheric northwesterly geostrophic flow roughly paralleling the axis of the strait, and a large along-strait pressure gradient near the surface. Strong westerly flow in the strait also accompanies the passage of intense low pressure centers across northwest Washington State and southern . The climatology of westerly wind events in the strait is presented, as well as their composite synoptic evolution. Using both mesoscale ob- servations and high-resolution numerical simulations, it is shown that modern modeling systems can re- alistically simulate the mesoscale evolution of strait wind events. It is shown that strait gap wind events are associated with downward mixing of strong gap-parallel geostrophic winds in the lower troposphere and acceleration down a low-level pressure gradient produced by the passage of a synoptic trough and terrain- induced pressure perturbations.

1. Introduction 2000; Sharp and Mass 2004; Gabersek and Durran 2004). For situations in which gap flow is relatively The Strait of Juan de Fuca, a major gap in the terrain deep and well mixed so that hydraulic effects are sec- of the west coast of North America, occasionally expe- ondary, and when the lower-tropospheric geopotential riences strong westerly wind events, with sustained height lines are aligned along the gap, both geostrophic (2-min average) winds of 30–50 kt and gusts reaching 2 60–80 kt (1 kt 5 0.51 m s 1). Such powerful westerly and ageostrophic effects can be significant and syner- winds occur within and downstream of the strait, with gistic. This paper examines such a situation for the the highest wind speeds often occurring when a sharp Strait of Juan de Fuca. upper trough moves through the region from the west The Strait of Juan de Fuca is a sea level gap, roughly or northwest. These events have resulted in substantial 130 km in length and 20–25 km in width, between the damage and economic loss. The region downstream of high terrain of the of Washington the strait is heavily populated and includes the U.S. Navy State (general crest level of roughly 1800 m) and the homeport at Everett, which accommodates an aircraft mountains of , with crests of approxi- carrier and support fleet. mately 1300 m. A terrain map for the strait and its en- There is a substantial literature on gap flow in moun- vironment is shown in Fig. 1. tainous terrain, with much of it describing the contribu- The earliest literature on winds in the Strait of Juan de tions of downgradient acceleration, vertical momentum Fuca dealt with strong easterly flow that can accelerate mixing, or hydraulic effects (e.g., Overland and Walter westward down the gap, reaching maximum speeds over 1981; Lackmann and Overland 1989; Colle and Mass its western terminus near Tatoosh Island (e.g., Reed 1931). These early studies suggested that winds in the strait are highly ageostrophic, with air accelerating Corresponding author address: Prof. Clifford F. Mass, Dept. of downgradient from high to low pressure, particularly as Atmospheric Sciences, University of Washington, Box 351640, Seattle, WA 98195. low pressure centers approach the coast from the west. E-mail: [email protected] While Reed (1931) correctly identified the along-gap

DOI: 10.1175/WAF-D-13-00026.1

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FIG. 1. The Strait of Juan de Fuca and the surrounding region. The red X and white line show the locations of the model sounding and model cross section, respectively. pressure gradient as being important for flow accel- Shelikof Strait1 during a gap flow event confirmed eration, he incorrectly postulated that the Venturi Overland’s analytic findings (Lackmann and Overland mechanism was a primary cause of the large down-gap 1989), with a nearly geostrophic balance in the cross- acceleration. Based on simple mass conservation prin- strait direction, and a three-way balance between ac- ciples, the Venturi effect dictates that the strongest wind celeration, drag, and pressure gradient forces in the and lowest static pressure should be found at the nar- along-gap direction. The observed ageostrophic accel- rowest part of a gap, with flow decelerating in the exit eration was approximately 55% of that predicted by region. However, aircraft-based observations of gap the Bernoulli equation using the observed along-strait flow in the Strait of Juan de Fuca (Colle and Mass, 2000, pressure gradient. The remaining pressure gradient Overland and Walter 1981) found that the strongest force was balanced by drag due to surface friction winds and lowest pressures are generally located over and entrainment at the top of the flow. Mass et al. (1995) the exit regions of this gap. Specifically, Colle and Mass examined the force balances in the nearby (2000) documented the horizontal and vertical struc- valley mesoscale gap, demonstrating that adding a drag tures of easterly flow in the strait using dual-Doppler term to the Bernoulli relationship resulted in a gap-exit radar data acquired from a National Oceanic and At- wind speed that closely approximated reality. mospheric Administration (NOAA) WP-3 aircraft, show- Although strong easterly flow in the Strait of Juan de ing that the strongest easterly winds were just downstream Fuca has received considerable attention in the litera- of the western gap exit. Overland and Walter (1981) ture, few papers (e.g., Colle et al. 1999) have examined completed a scale analysis of the along-strait momentum westerly flow in this gap, even though some westerly equation, finding that the pressure gradient and inertia strait wind events have produced gusts reaching 60–80 kt (advection) terms were of primary importance. Thus, and caused considerable property damage. For example, the down-strait momentum equation reduced to a form a particularly strong westerly wind event in the strait of the Bernoulli equation, which produced realistic (al- occurred on 17 December 1990. As a well-defined upper though overestimated) winds in the gap. trough approached Washington State from the northwest Overland (1984) showed that a large along-gap Rossby number implies ageostrophic flow parallel to a gap axis, with the along-gap pressure gradient bal- 1 Shelikof Strait is a high-latitude sea level gap between the anced by along-gap acceleration and drag. Approximate Alaskan Peninsula and Kodiak Island. It is about 200 km long and geostrophic force balance was diagnosed perpendicular 50 km wide, making it relatively wide compared to most other gaps to the axes of mesoscale gaps. Aircraft data gathered in mentioned in this review.

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FIG. 2. Radar radial velocity data at a 0.58 elevation angle from the Camano Island National Weather Service WSR-88D radar at 0954 UTC 15 Dec 2006 for the region covering northwest Washington State. Wind speeds are in kt with cold colors (green and blue) indicating winds approaching the radar. and an associated front moved through the region, terminal. An important aspect of the 28 October event strong winds pushed into the central and eastern strait. was the excellent 1-day forecast by the fifth-generation Over a period of a few hours, sustained winds of 40– Pennsylvania State University–National Center for At- 60 kt, with gusts to 60–70 kt, extended across Whidbey mospheric Research Mesoscale Model (MM5), run daily Island into Snohomish County, causing considerable at the University of Washington at 12-and 4-km grid tree falls and loss of power (see Fig. 1 for locations). By spacing, suggesting the potential predictability of such the end of the storm, nearly 141 000 Snohomish and Is- events. land County customers had lost power and many roads Strong westerly winds in the strait also occur when were impassable (NOAA Storm Data, 1990). According deep Pacific low pressure centers move across north- to the Snohomish County Public Utility District, the west Washington from the west or southwest (Mass and 17 December event was the ‘‘worst in history from the Dotson 2010). As the low center reaches the northern standpoint of disruption, hardship, and dollar damage,’’ interior of western Washington or southern British eclipsing the destruction of even the great 1962 Co- Columbia, an intense pressure gradient is established lumbus Day storm. Winds were particularly severe in along the strait, resulting in strong westerly winds, often Everett Harbor, where a 62-kt gust was observed in- reaching 40–70 kt. This situation occurred on 15 December shore and sustained winds of 55–80 kt occurred imme- 2006 during the Chanukah Eve storm, when a westerly diately offshore, with the strongest winds between 0300 wind surge in the strait occurred after the passage of the and 0400 UTC 18 December 1990. The Washington low center, producing westerly gusts to 65 kt over Smith State Elwha, docked for repairs in the Everett Island, at the eastern exit of the strait. Doppler radial waterfront, was thrown against its pier, crushing the winds from the nearby Camano Island Weather Sur- vessel’s car deck and heavily damaging the pier, result- veillance Radar-1988 Doppler (WSR-88D) at 0954 UTC ing in millions of dollars of damage. 15 December 2006, near the time of peak low-level On 28 October 2003, another major westerly wind westerly winds, reached at least 60 kt between approxi- event resulted in extensive tree damage, power outages mately 600 and 900 m MSL over the central and eastern for nearly 100 000 customers, one death, and the destruc- portions of the strait (Fig. 2). tion of a popular restaurant on (NOAA Strong westerly winds in the strait are also important Storm Data, 2003). This storm had much in common due to their impact on waves. Wind-driven waves over with the 17 December 1990 event, with a sharp, upper- the interior waters of western Washington are generally level trough embedded in northwesterly flow. As an modest due to a lack of fetch for the usual directions of associated Pacific front moved over the strait, winds strong winds (southerly, southeasterly, and southwest- increased to over 50 kt across , nearby erly). Strong northwesterly flow through the Strait of Smith Island, and Snohomish County, while large wind- Juan de Fuca is the exception, with a continuous fetch of driven waves struck the eastern shore of Puget Sound. greater than 100 km along the strait to its opening on the Such waves destroyed Ivar’s Mukilteo Landing restau- Pacific Ocean. Furthermore, the strait is the only inland rant, located on the water near a Washington State ferry waterway that is open to swell originating over the Pacific.

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associated with flow down the Strait of Juan de Fuca. A lesser peak is apparent for northerly winds that originate from the or out of the Fraser River valley. Strong southeasterly flow tends to be of longer duration than strong westerly flow at this location; in- tense westerly flow is generally associated with the passage of a transient upper-level trough or low pressure center, while southeasterly flow is driven by the inter- actions of sustained southerly flow with the Olympic Mountains (Mass and Ferber 1990). Table 1 shows the statistics of strong sustained (2 min) winds that are roughly parallel to the strait (2508–3108) for both the cool (15 October–1 March) and warm (1 May–30 September) seasons for Smith Island and Race Rocks. Specifically, Table 1 shows the numbers of

FIG. 3. Scatter diagram of 1-h peak winds (kt) vs direction at days in which various wind speed thresholds are met Smith Island for January 1994–December 2003. The peak wind is during 1 h of the day. In general, Race Rocks, the more the highest 5-s wind during the past hour. western location at the immediate exit of the strait, has higher wind speeds than Smith Island, with greater fre- As a result, the greatest significant wave heights for the quencies at all higher-wind thresholds for both seasons. inland waters of Washington State occur at the eastern During the summer, neither location has winds ex- end of the strait (Finlayson 2006). ceeding 50 kt, while during the winter, Race Rocks had This paper begins by describing the climatology of three such events. The greater frequency of moderately major westerly wind events in the Strait of Juan de Fuca, strong winds (30–39 kt) during the summer reflects the and particularly at and downwind of the eastern exit strong inflow into the strait on warm summer days and where westerly flow is generally strongest. Synoptic com- during onshore–marine push events (Mass et al. 1986). posites at several levels are provided for major cases, and As described below, the greater frequency of the more three westerly wind events are described using both severe wind events (40 kt and more) during the winter observations and model simulations. Finally, the dy- reflects the impact of the passage of major low centers or namics and structural characteristics of these events sharp upper-level troughs, with the previously men- are discussed. tioned 1990, 2003, and 2006 cases being prime examples.

2. Climatology of westerly wind events 3. A composite westerly wind event Since westerly wind events in the strait are generally To illustrate the typical synoptic characteristics asso- strongest on the eastern side of the gap, the relative ciated with strong westerly Strait of Juan de Fuca wind frequencies of such winds were examined at two loca- events, we have composited the first hour of the 13 tions: Race Rocks, British Columbia (located near Vic- events for which the sustained winds at Smith Island, toria, British Columbia) and Smith Island, Washington, at the eastern end of the strait, exceeded 39 kt be- roughly 50 km to the east (see Fig. 1 for map). A peri- tween 1994 and 2003. This compositing made use of the od with complete hourly data at both locations was 3-hourly National Centers for Environmental Prediction selected, specifically January 1994–December 2003.2 (NCEP) North American Regional Reanalysis (NARR) Figure 3 presents a scatter diagram of 1-h peak winds versus dataset, which is available at 32-km grid spacing. The direction at Smith Island. Two major peaks of strong most temporally proximate 3-hourly grids were used for winds are evident: one associated with strong south- the composites. easterly flow (1108–1608) moving northward from Puget Figure 4 shows the composites of geopotential heights Sound, and the other for westerly directions (2508–2908) at 500 and 850 hPa, as well as sea level pressure, near the time of strongest winds for these events. At 500 hPa, a trough extends from western British Columbia into western Washington, with height contours, and thus 2 The Race Rocks data were acquired from Environment Can- ada’s National Climate Data and Information Archive (www. geostrophic winds, oriented roughly parallel to the strait. climate.weatheroffice.gc.ca), and Smith Island observations were At 850 hPa, the trough is farther inland, with a large height available from NOAA’s National Data Buoy Center (NDBC). gradient, associated with northwesterly geostrophic flow,

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TABLE 1. Number of days in which 1 h during the day reaches a sustained (2 min) wind speed (kt) at various thresholds for Race Rocks and Smith Island, based on 10 yr of winter and summer data from 1994 through 2003. Only directions between 2508 and 3108 are considered.

Winter Summer 15 Oct–1 Mar 1 May–30 Sep Sustained wind Smith Race Smith Race speed (kt) Island Rocks Island Rocks 30–34 28 119 21 280 35–39 21 38 4 90 40–44 9 29 1 15 45–49 2 9 0 2 501 0300 Total 60 198 26 387 over the region. The sea level pressure composite in- dicates a low center (995 hPa) over southern British Columbia, high pressure offshore, and a large sea level pressure gradient over western Washington and south- western British Columbia, including a substantial pres- sure gradient down the strait. Thus, this composite suggests that the strong winds within the strait are as- sociated with strong northwesterly geostrophic flow aloft (e.g., at 850 hPa and above) and a significant sea level pressure difference down the gap. To explore the synoptic evolution associated with these events, Fig. 5 shows the 500-hPa and sea level pressure composite evolutions during the 24-h period leading to strong westerly winds (40 kt or more) at Smith Island. During that period, an upper-level trough moves rapidly southeastward from the Gulf of Alaska to the western interior of the Pacific Northwest. At sea level, 24 h before the strongest winds, there is a low center over the northern Gulf of Alaska, with a trough extending southward over the eastern Pacific. Twelve hours before the strong winds, the trough moves to just off the coast of British Columbia and the northwest , with an offshore pres- sure gradient over the strait. During the next 12 h, this trough rapidly deepens into a deep, closed, low pressure area centered over southern British Columbia and a re- gion of high pressure builds offshore, producing a sub- stantial onshore pressure gradient over the strait.

4. Case studies Although damaging westerly wind events in the Strait $ of Juan de Fuca occur on an annual basis, the two FIG. 4. NARR composites for times with winds 40 kt at Smith Island for (a) 500-hPa geopotential height (m), strongest events of the past 25 yr, based on damage and (b) 850-hPa geopotential height (m), and (c) sea level pressure economic impact (NOAA Storm Data), occurred on 18 (hPa). December 1990 and 28 October 2003. Both of these events were associated with a sharp upper trough mov- ing southeastward across the region and are described

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FIG. 5. Composites of (left) 500-hPa geopotential height (m) and (right) sea level pressure (hPa) for (top to bottom) 24, 12, and 0 h before westerly high wind events ($40 kt) at Smith Island using the NARR dataset. below. The other type of event, albeit less frequent of the western entrance of this gap (see Fig. 1 for loca- (every few years), is connected with the passage of an in- tion). Before the event (1100 UTC 17 December), winds tense midlatitude cyclone across northwest Washington. A were southwesterly and light near the surface, veering to prime example of this type of event, occurring on 14–15 west-northwesterly at 30 kt by 850 hPa (Fig. 7). Below December 2006, is also reviewed. 800 hPa the air was near saturation and close to moist adiabatic, with a drier, stable layer aloft. Twelve hours a. December 1990 later (0000 UTC 18 December), immediately before the As noted in the introduction, winds during the 17–18 period of strongest surface winds, the flow had veered to December 1990 event gusted to 60–70 kt over and westerly at the surface and west-northwesterly imme- downwind of the eastern exit of the Strait of Juan de Fuca. diately above, with a strengthening of the surface winds The synoptic configuration at 0000 UTC 18 December to 20 kt and 850-hPa winds to 50 kt. There was little di- 1990, a few hours prior to the strongest low-level winds, is rectional shear above the surface and the saturated, shown in Fig. 6. All the canonical elements of a westerly well-mixed layer extended above 850 hPa. wind event in the strait are evident, including the 500-hPa Figure 8 displays surface observations and sea level trough moving southeastward out of British Columbia, pressure analyses prior to and during the December ridging both aloft and at the surface over the eastern 1990 westerly wind event. At 1800 UTC 17 December, Pacific, strong 850-hPa height gradients over the strait sustained westerly winds of 20–30 kt were found over the consistent with northwesterly geostrophic flow, and a western side of the strait, while on the eastern side, the substantial sea level pressure gradient down the strait. winds were lighter and southerly. There was coastal The nearest upper-air station to the strait is at pressure ridging upstream and inland lee troughing down- Quillayute, Washington, roughly 50 km to the southwest stream of the Olympic Mountains and the mountains of

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strait (reaching sustained values of 40 kt), with winds turning westerly downwind of the strait over Smith and Whidbey Islands. The along-strait pressure gradient increased to 1.7 hPa at this time. By 0000 UTC 18 December, winds on the coast strengthened and veered, with strong northwesterly flow along the west coast of Vancouver Island. The pressure difference down the strait (TTI 2 NUW) jumped to 5 hPa, and a well-defined lee trough was evident to the east of the Olympic Mountains. Such lee troughing is often observed when strong westerly or northwesterly flow approaches the Olympics (Mass and Ferber 1990). Strong westerly winds, some sustained at 40 kt, pushed eastward to Whidbey Island at this time. By 0300 UTC 18 December, dam- aging westerly winds exiting the strait extended east- ward to northern Puget Sound and Snohomish County. The pressure difference down the strait remained at 5 hPa and lee troughing east of the Olympics was still apparent. Figure 9 shows the winds at Tatoosh and Smith Islands, at the entrance and exit of the strait for westerly flow, as well as the pressure difference between these two locations. The winds at Smith Island are closely synchronized with the along-strait pressure gradient, thus suggesting the importance of the pressure gradient acceleration. In contrast, the winds at the upstream lo- cation (Tatoosh Island) have less temporal modulation and increase modestly a few hours prior to the acceler- ation at Smith Island and the rapid increase of the on- shore pressure gradient. The coarse (90-km grid spacing) operational model available at this time, the National Weather Service Nested Grid Model (NGM), was unable to resolve ei- ther the strait or the regional mountain barriers and predicted a strengthening of the winds to only 20–30 kt over the eastern Strait of Juan de Fuca. To evaluate the potential of modern high-resolution mesoscale models for predicting such events and to use model simulations to better understand their evolution, the Weather Re- search and Forecasting (WRF) model was employed to simulate the December 1990 case at 36-, 12-, and 4-km grid spacings. These simulations were initialized at FIG. 6. Geopotential heights (m) at (a) 500 and (b) 850 hPa, and 0000 UTC 17 December 1990 and forced on the lateral (c) sea level pressure (hPa) at 0000 UTC 18 Dec 1990 from the boundaries by the NARR grids, which were available NARR dataset. every 3 h. Figure 10 shows the 500-hPa, 850-hPa, and sea level Vancouver Island. The pressure difference between pressure forecasts from the 12-km WRF domain at Tatoosh Island (TTI), on the northwest corner of the 1200 UTC 17 December and 0000 UTC 18 December, at the strait entrance, and Whidbey the latter only a few hours before the time of strongest Island Naval Air Station (NUW), downstream of the winds. At 500 hPa, a sharp short-wave trough over eastern exit, was only about 1 hPa (higher at Tatoosh southeastern Alaska moved southeastward to British Island). Three hours later (2100 UTC), westerly winds Columbia during this 12-h period, resulting in a strength- strengthened at the western entrance and within the ening of geostrophic northwesterlies over northwest

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FIG. 7. (top) Temperature (solid, 8C), dewpoint (dashed, 8C), and winds (kt) from vertical soundings at the Quillayute upper-air station for (left) 1200 UTC 17 Dec 1990 and (right) 0000 UTC 18 Dec 1990. Dry and saturated adiabats are shown by solid and dashed gray lines, respectively. (bottom) Model soundings at the same locations and times from the WRF model run at 4-km grid spacing.

Washington State. Similarly, the 850-hPa height gradi- pressure gradient down the strait strengthened. This ent tightened during the 12-h period, with northwesterly synoptic evolution is consistent with the observed syn- 850-hPa flow nearly aligned with the strait at 0000 UTC optic structures shown in Figs. 6 and 8. 18 December. At the surface, coastal winds were west- As a further check on the realism of the model, sim- erly at 1200 UTC 17 December, but during the next 12 h ulated soundings at Quillayute, upstream of the Olym- the isobars and low-level winds veered into a north- pics and near the entrance to the strait, are shown in westerly direction. As the winds turned and intensified, the Fig. 7 along with the observed soundings at 1200 UTC

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FIG. 8. Surface observations and sea level pressure analyses for (a) 1800 and (b) 2100 UTC 17 Dec 1990, and (c) 0000 and (d) 0300 UTC 18 Dec 1990. The contour interval is 1 hPa and sea level pressure is shown at some locations (format 10XX.X hPa). Winds (kt) are also shown.

17 December and 0000 UTC 18 December 1990. At Figure 11 shows the simulated 10-m winds from a 1200 UTC, both the observed and model soundings subset of the WRF model’s 4-km domain. At 1200 UTC showed southwesterly flow near the surface, veering to 17 December, winds were generally modest, with 10– northwesterly by 900 hPa. The thermodynamic sound- 20-kt southwesterlies over the Pacific coast and weaker ings are quite similar, with the exception of the air being southerly flow over the inland waters of western Wash- drier above 800 hPa in the observed. Twelve hours later, ington. Six hours later (1800 UTC) the winds greatly the model and observed winds are quite similar, with strengthened over the coast and in the strait, and by west-northwesterly winds near the surface veering to 2100 UTC the simulated winds increased above 40 kt northwesterly aloft as they strengthened. Both model east of the strait’s exit, with strong flow reaching Whidbey and observed soundings show a saturated layer from just Island. The winds had turned west–northwesterly along above the surface to roughly 800 hPa that appears well the coast and, thus, were directed along the strait axis. By mixed (approximately saturated adiabatic). Considering 0300 UTC, winds downstream of the gap had accelerated the fidelity of the model soundings of the incoming flow above 40 kt and extended across Whidbey Island and and the realism of the synoptic–mesoscale mass and wind toward the mainland. Although the simulation did not fields, the analysis of the model simulation should provide extend the strongest winds as far east as observed, it useful insights into the nature of these events. produced very strong winds in and downstream of the

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FIG. 9. Wind speed (kt) at Smith Island and TTI and the along-strait pressure difference (TTI 2 Smith Island, hPa) during the wind event of 17–18 Dec 1990. strait that clearly delineated a threat to marine traffic. 900 hPa. A stable layer is found immediately beneath the All of the simulated features compare well to the obser- strong westerly winds aloft, while near the surface the vations shown in Fig. 8. Comparisons to coarser-resolution southerly flow is more stable than in the postfrontal simulations (not shown) revealed the importance of the westerlies. Six hours later (1800 UTC) low-level westerly higher-resolution domain for securing a realistic simu- winds pushed through the strait, with substantial strength- lation of winds in and downwind of the strait. eningaloftto60ktormore.By0000UTC18December, Model soundings from the WRF 4-km domain down- low-level westerly winds of 25–35 kt extended through stream of the eastern exit of the strait (see Fig. 1)are the entire domain, as the winds above 800 hPa continued shown in Fig. 12. Initially (0300 and 1200 UTC 17 De- to intensify. Three hours later (0300 UTC), near the time cember), low-level winds east of the gap are southeasterly, of the strongest winds in and downstream of the strait, the with increasing westerly–northwesterly winds aloft that most intense winds below 800 hPa were downstream of strengthen and descend toward the surface over time. A the eastern strait exit. The flow was generally well mixed stable layer is found above a well-mixed layer encom- in the lowest 100 hPa, as suggested by the lack of a po- passing the southeasterlies. By 1800 UTC, westerly tential temperature gradient and strong winds. winds pushed into the eastern strait at low levels, and the b. 28 October 2003 stable layer was greatly attenuated. During the next 9 h the low-level winds strengthened and veered into the The 28 October 2003 event produced gusts that west-northwest, with temperatures indicating a well- reached 50–60 kt in the strait and 30–50 kt over northern mixed layer to approximately 700 hPa. Substantial Puget Sound. The synoptic structure near the time of midtropospheric drying followed the upper-level trough. strongest winds (Fig. 14) includes a sharp 500-hPa trough Model vertical cross sections along the strait of po- embedded in northwesterly flow; a 850-hPa low center tential temperature, wind speed, and wind direction are over southeast British Columbia, southwest , shown in Fig. 13. At 0600 UTC 17 December 1990, low- and western Montana; and a similarly placed low in sea level southerly winds were found over the east and west level pressure. As with the 1990 case, this configuration ends of the strait, with light winds at its center. Aloft, is associated with northwesterly geostrophic flow in the winds turned westerly and then northwesterly, strength- lower troposphere and a substantial onshore pressure ening with height. By 1200 UTC, westerly winds pushed gradient at and near sea level. into the central strait with the surface front, while west- The sounding at Quillayute, south of the western en- erly winds aloft strengthened and descended to roughly trance to the strait, at 1200 UTC 28 October showed

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FIG. 10. The geopotential heights, temperatures, and winds at (top) 500 and (middle) 850 hPa, and (bottom) sea level pressure, 10-m winds, and 925-hPa temperatures at (left) 1200 UTC 17 Dec and (right) 0000 UTC 18 Dec 1990 from the 12-km WRF simulations initialized 0000 UTC 17 Dec 1990. Geopotential heights are in m, temperatures in 8C (color shading), and winds are in kt.

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FIG. 11. Simulated sustained 10-m winds (kt) and sea level pressure (contours, hPa) at (a)–(d) 1200, 1800, and 2100 UTC 17 Dec, and 0300 UTC 18 Dec 1990. Wind speeds (kt) shown by shading and barbs indicate the wind speed and direction at a subset of model grid 2 points [full barb is 10 kt (5 m s 1)]. The black line in the Strait of Juan de Fuca is the international boundary. The forecasts are from the WRF 4-km domain, initialized at 0000 UTC 17 Dec 1990. modest southeasterly flow near the surface, with veering close relationship, with the acceleration of the Smith toward the southwest and northwest aloft and consid- Island winds nearly simultaneous with the increase of erable strengthening (to 50 kt) by 700 hPa (Fig. 15). The the pressure difference along the strait. Both peaked at atmosphere was stable (nearly isothermal) below 850 hPa virtually the same time, followed by a simultaneous de- and nearly saturated. Twelve hours later, the winds be- cline. In contrast, the winds at the entrance to the strait at came northwesterly throughout the lower troposphere, Tatoosh Island accelerated far earlier in the day with the and far drier at about 750 hPa. Much of the lower layer (to passage of a strong front. The winds at Tatoosh decreased roughly 800 hPa) was well mixed and less stable, with the substantially as the Smith Island winds increased, followed atmosphere close to saturated adiabatic. by an acceleration to a second peak at 0300 UTC 29 A comparison of the winds at Smith Island, down- October 2003 as the upper trough moved over the region. stream of the strait’s eastern exit, and the pressure gra- The October 2003 event was simulated using the WRF dient along the strait (Fig. 16), shows an extraordinarily model driven by the NARR grids and again a realistic

FIG. 12. Model vertical soundings over the eastern exit of the strait (see Fig. 1 for sounding location, indicated by an X), showing temperature (red, 8C), dewpoint (blue, 8C), and winds (kt) for (left to right) 0300, 1200, and 1800 UTC 17 Dec and 0000, and 0300 UTC 18 Dec 1990.

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FIG. 13. Vertical cross sections along the Strait of Juan de Fuca at (a)–(e) 0600, 1200, and 1800 UTC 17 Dec 1990, and 0000 and 0300 UTC 18 Dec 1990. The cross-section location is shown in Fig. 1. Wind barbs and shading (kt) and potential temperature (contours, 8K) are shown. simulation was produced. A comparison of the observed directions were well modeled at 0000 UTC 29 October, and modeled soundings at Quillayute, on the northern with the model accurately showing well-mixed flow be- Washington coast, reveals that at 1200 UTC 28 October low roughly 750 hPa. The model was too moist, partic- the observed temperature and dewpoint structures were ularly near 750 hPa. closely duplicated, as were the veering winds with height Figure 17 shows the modeled 10-m winds and sea level in the lower troposphere. Only at the surface was there pressures during the event. At 0600 UTC 28 October a minor discrepancy in wind direction (southwesterly 2003, southerly flow dominated offshore, with weak instead of southeasterly flow). The wind speed and winds within the strait. Six hours later, a front moved

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while the winds in the strait were greatly enhanced. The strongest sustained winds were downstream of the strait, reaching 40–45 kt, and extended across Whidbey Island to the mainland. The 24-h forecast realistically simu- lated both the distribution and wind speeds associated with this event, illustrating the general predictability of this mesoscale evolution. The model soundings downstream of the eastern strait (location indicated in Fig. 1), shown in Fig. 18, illustrate substantial changes in low-level wind shear and stability during this event. At 1200 UTC 28 October, there was weak southerly flow near the surface, with increasing westerly–northwesterly flow aloft. The shear was main- tained by the large stability in the lowest 100 hPa of the sounding. At 0000 UTC 29 October, northwesterly flow extended to the surface, the lower troposphere became much drier, and the lapse rate was dry adiabatic to 925 hPa and far less stable above. Little directional shear was evident in the lower troposphere. Vertical cross sections of potential temperature and wind along the strait during the 28 October event doc- ument the passage of the front, low-level destabilization, and the strengthening of the winds near the surface (Fig. 19). At the initial time (0600 UTC), weak winds were found at low levels over the eastern and western portions of the strait, with winds strengthening and veering to northwesterly aloft. By 1200 UTC, low-level west-southwesterly flow pushed into the central strait, while westerly winds strengthened considerably aloft. A stable layer over the eastern exit of the strait separated moderate southerly flow at low levels from westerly winds aloft (approximately 925 hPa and above). By 1800 UTC the low-level winds were westerly or northwesterly over the entire domain and low-level stability declined, with little directional shear with height. Finally, at 0000 UTC, near the time of the strongest surface winds, the most intense low-level westerly winds were over the eastern portion of the domain, with 50-kt winds immediately above the surface. The temperatures cooled consider- ably at low levels, and the air was well mixed to ap- proximately 900 hPa over the west half of the strait, as the postfrontal air flooded the region.

FIG. 14. As in Fig. 6, but at 0000 UTC 29 Oct 2003. c. 15 December 2006 The previous two cases illustrated the most typical eastward down the strait, while winds veered to westerly synoptic evolution associated with strong westerly wind over the nearshore waters. By 1800 UTC the coastal events in the Strait of Juan de Fuca: the passage of a winds had rotated to the northwest, troughing developed sharp trough embedded in northwesterly flow. But as along the eastern slopes of the Olympics, and the winds noted earlier, very strong westerly winds in this gap have accelerated down the strait, reaching a sustained 30 kt occurred after the passage of deep low pressure centers within and downstream of the eastern exit of the gap. moving southwest to northeast across northern Wash- Finally, at 0000 UTC 29 October, the offshore surface ington State and southern British Columbia. Such an winds veered to the northwest and increased to 20–30 kt, episode occurred on 15 December 2006 during an event

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FIG. 15. As in Fig. 7, but at 1200 UTC 28 Oct and 0000 UTC 29 Oct 2003. known as the Chanukah Eve storm, when sustained Figure 21 shows sea level pressure, 10-m winds, and westerly winds of 40–50 kt and gusts to 50–70 kt were 925-hPa temperature from a 12-km WRF model simu- observed over and downstream of the eastern exit of the lation for 0000–1200 UTC 15 December 2006. This sim- strait. For example, Smith Island (see Fig. 1 for location) ulation, which closely followed the observed evolution had sustained westerly winds reaching 49 kt and peak (Mass and Dotson 2010), shows the movement of a deep winds to 66 kt (Fig. 20). The strongest winds occurred low center (973 hPa) across central Vancouver Island and as the low center passed northeast of the strait (1000– into southern British Columbia. The pressure pattern 1200 UTC). over the strait changed profoundly during this period:

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FIG. 16. As in Fig. 7, but during the wind event of 28–29 Oct 2003. from strong offshore pressure gradients during the first The heights, winds, and temperatures at 850 hPa from 6 h, to a regime with an intense pressure gradient roughly a higher-resolution (4-km grid spacing) simulation are normal to the axis of the strait. Low-level coastal winds shown in Fig. 22 for a time of peak winds in the eastern transitioned from moderate southwesterly to strong strait (1000 UTC). West-northwesterly flow, approxi- north-northwesterly. mately parallel to the axis of the strait, pushed over the

FIG. 17. Simulated sustained 10-m winds (kt) at (a)–(e) 0600, 1200, and 1800 UTC 28 Oct, and 0000 UTC 29 Oct 2003. Wind speeds are shown by color shading and wind barbs represent wind speed and direction at a subset of model grid points. Sea level pressure (interval of 2 hPa) indicated by brown contours.

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The push of strong surface (10 m) winds down the strait, with localized acceleration on its eastern side, is shown in Fig. 23, which also presents sea level pressure. At 0600 UTC 15 December 2006 there were moderate southeasterlies and southwesterlies east and west of the strait, respectively. Two hours later (0800 UTC), strong northwest winds (sustained at 45 kt or more) developed over the coastal waters and pushed into the strait. During the next 2 h the strong winds extended eastward, with the highest wind speeds (sustained winds exceeding 52 kt) east of the strait exit. At this time there was a FIG. 18. Model vertical soundings over the eastern exit of the substantial (5 hPa) sea level pressure gradient down the strait (see Fig. 1 for location), showing temperature (red, 8C), dewpoint (blue, 8C), and winds (kt) for (left) 1200 UTC 28 Oct and strait. During this time and the previous 4 h, there was (right) 0000 UTC 29 Oct 2003. notable pressure troughing on the lee side of the Olym- pics Mountains, contributing to the along-strait pressure region. Such northwesterly flow led to weak windward gradient. ridging upstream and stronger lee troughing downstream In summary, the December 2006 event, although of the Olympic Mountains, resulting in the development embedded in an event with far stronger regional winds, of a modest onshore 850-hPa height gradient along the had many characteristics of the trough-initiated westerly axis of the strait. wind events noted above: strong lower-tropospheric

FIG. 19. Vertical cross sections along the Strait of Juan de Fuca at (a)–(d) 0600, 1200, and 1800 UTC 28 Oct 2003, and 0000 UTC 29 Oct 2003. Cross-section location shown in Fig. 1. Wind barbs and shading (kt) and potential temperature (contours, K) are shown.

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situation, is when an intense low pressure center passes east or northeast of the strait. The passage of a sharp short-wave trough in north- westerly flow results in the development of strong northwesterly geostrophic flow that is aligned with the strait. Initially, strong (40 kt or more) westerly or north- westerly flow in the region is only found aloft (above roughly 900–850 hPa), while low-level flow east of the strait exit is from the south or southwest at modest speeds. As the upper trough and its associated surface- based front moves across the region, strong westerly flow descends toward the surface. Prior to trough/front passage, the lower troposphere is relatively stable, with substantial deflection by the Olympics enhancing southwesterly flow along the coast of the Olympic Peninsula. After the frontal passage, cold-air advection aloft produces a less stable and more mixed lower at- mosphere. The result is that the low-level shear is reduced, strong westerly momentum is mixed to the surface, and the deflection by the terrain is lessened. Strong near-surface winds, particularly over and down- wind of the eastern strait, are enhanced by the develop- ment of large eastward-directed surface pressure gradients along the axis of the strait. This increasing onshore

FIG. 20. (a) Sustained and peak wind speeds (kt) and (b) 10-m wind gradient is produced by the inland movement of the direction on 15 Dec 2006 at Smith Island. Times are in UTC. synoptic trough as well as lee troughing to the east of the Olympics/Vancouver Island mountains as northwesterly flow strengthens in the lower troposphere. The con- flow parallel to the strait and a down-strait pressure tribution of low-level ageostrophic acceleration down gradient associated with both synoptic-scale pressure the along-strait pressure gradient is suggested by the differences and mesoscale troughing to the lee of the existence of the strongest winds at and downstream of Olympic Mountains. As with strong upper-trough pas- the eastern exit of the gap. Furthermore, it was shown sages in northwesterly flow, the strongest winds were above (Figs. 9 and 16) that there is a very close relationship observed/simulated over and downstream of the eastern between the evolution of low-level winds over the eastern exit of the strait. strait (e.g., Smith Island) and the pressure gradient down the strait. The importance of the along-strait pressure gradient 5. Discussion and conclusions in modulating strait winds is highlighted in Fig. 24, Strong westerly winds within and downstream of which shows a scattergram of the winds at Smith Island Washington State’s Strait of Juan de Fuca have resulted (downstream of the strait) and the along-strait pressure in major power outages, millions of dollars in property gradient for both the 1990 and 2003 events. The corre- damage, and loss of life. Sustained winds in and down- lation coefficient between the winds and the pressure stream of the strait exceeding 39 kt typically occur five or gradient is high (0.87). The slope deviates from linear, six times per year, with sustained winds of 50 kt or more which would be expected from the inviscid Bernoulli occurring roughly every 3 yr. Maximum surface westerly form of the momentum equation in which the square of gusts in the most extreme cases can reach 80 kt or more. the wind speed is related to the pressure gradient down The most intense westerly strait winds typically occur the gap (Mass et al. 1995). In addition, drag, which is during the winter season, although modest westerly flow wind speed dependent, would contribute to a preferen- can occur any time of the year. tial lessening of the high wind speeds. Two major synoptic evolutions are associated with Gap flow in the strait during strong westerly events is such strong westerly gap wind events. The most frequent consistent with the theoretical and observational study is the passage of a sharp, short-wave trough in north- of Lackmann and Overland (1989) for the gap winds westerly upper-level flow, and the second, less-frequent in the Shelikoff Strait of Alaska. They showed that gap

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FIG. 22. Forecast 850-hPa heights (contours, m), winds (barbs, kt), and temperature (shading and blue lines, 8C) for 1000 UTC 15 Dec 2006.

wind acceleration is controlled by the down-strait pres- sure gradient, with wind speeds being reduced by sur- face drag and drag at the top of the gap flow associated with turbulent momentum fluxes produced by large vertical wind shear. In contrast, for the Strait of Juan de Fuca, the lack of directional shear in the lower tropo- sphere, the increasing winds with height, and lessened postfrontal stability act to strengthen low-level winds in the strait. For both the Shelikoff Strait and the Strait of Juan de Fuca, the overwater path in the gap results in only weak drag at the surface. In summary, a strong along-strait pressure gradient in an environment of small drag at the surface and aloft allows the development of powerful gap winds in the strait when strong lower- tropospheric flow is aligned along the strait axis. Both observations and the simulations for the cases described above show that although the strongest surface winds occurred within and downwind of the strait, strong winds were found along the entire length of the strait due to the downward mixing of northwesterly geostrophic momentum. Downgradient acceleration due to the along- strait pressure gradient allows for additional acceleration as the low-level air moves eastward. The down-strait alignment of the flow at all levels reduces the three- dimensionality of the low-level flow (Colle et al. 1999), in contrast to the highly complex airstreams and substantial directional change with height associated with easterly gap flow through the strait (Colle and Mass 2000). There is little suggestion that hydraulic effects are significant in strait gap flow events, consistent with a lack of a lower, dense-air layer upstream of the strait and the well-mixed lower troposphere during these events.

FIG. 21. WRF model forecasts of sea level pressure (hPa), As noted above, the three events presented in this 925-hPa temperature (shading, 8C), and 10-m winds (kt) for (a)–(c) paper were simulated realistically by the WRF model 0000, 0600, and 1200 UTC 15 Dec 2006. The simulation was ini- tialized at 1200 UTC 14 Dec 2006.

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FIG. 24. Scattergram of sustained winds speeds vs the along-strait pressure difference (TTI 2 Smith Island, hPa) for the December 1990 (red circles) and October 2003 (blue triangles) Strait of Juan de Fuca westerly wind events.

et al. 2003). Such good predictability reflects two major improvements: 1) better initialization and subsequent forecasts for synoptic models such as the GFS due to greater use of satellite data and better data assimilation and 2) the ability of high-resolution numerical forecast models to simulate and predict mesoscale phenomena produced by the interaction of synoptic-scale flow and regional terrain features.

Acknowledgments. This research has been supported by the National Science Foundation under Award AGS- 1041879. Mark Albright contributed the surface pres- sure analyses for several case studies and Neal Johnson helped to access data resources required for completion of this research. Three anonymous reviewers provided FIG. 23. Model forecasts at 4-km grid spacing for (a)–(c) 0600, 0800, and 0900 UTC 15 Dec 2006. The fields shown are 10-m sus- helpful comments and suggestions. Beth Tully prepared tained wind speed (barbs and shading, kt) and sea level pressure several of the figures. (contours, hPa).

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