Eastern Region Technical Attachment No. 2015-05 October 2015 Using the Froude Number to Improve Orographic Snow Forecasts in the Green Mountains of Vermont Michael Muccilli* NOAA / National Weather Service Burlington, Vermont ABSTRACT Orographic snow is a very challenging aspect of forecasting in the northeastern United States. This paper provides a method for improving these forecasts using the Froude number. Past research has shown that variations in the non-dimensional Froude number during upslope snow events can be correlated to the nature of the orographic snow bands, and the distribution of snowfall accumulations. Twenty-five observed northwest flow orographic snowfall cases from 2007 to 2012 in the Green Mountains of Vermont are examined with a focus on the Froude number during the events. Composite synoptic analyses show 500-hPa northwest flow and cyclonic north to northwest flow at 850 hPa. In these cases, the Froude number has a critical level of 1.0, at which events lower than this critical level, the flow becomes blocked and precipitation backs up into the lower elevations of the western slopes of the Green Mountains. With Froude numbers less than 0.5, precipitation affects the Champlain Valley and the Burlington metropolitan area. At Froude numbers above 1.0, air flows more freely over the Green Mountain spine, and the majority of the precipitation falls along the mountains and downwind. The forecast Froude number, in addition to other fields from numerical weather prediction models, can assist forecasters in determining the potential for and possible distribution of orographic snow. _____________________________________________________________________________ _____________________________________ *Corresponding Author address: Michael Muccilli, NOAA/National Weather Service, 1200 Airport Drive, South Burlington, VT 05403 E-mail: [email protected] 1. Introduction (Fig. 1). The key terrain feature of Vermont is the Green Mountain chain which runs Orographic snow is one of the more primarily north to south through the central challenging aspects of forecasting in areas of portion of the state, with peaks as high as complex terrain, and more specifically in the 1340 m (4395 ft) (Mount Mansfield). case of this paper, Vermont. Orographic snow events are highly localized, but can The goal of this paper is to make it evolve into high impact events that affect easier to identify the different regimes of portions of Vermont. Depending on the orographic snow events and the distribution mesoscale progression of events, impacts of heavy snow using the Froude number, as can range from only a few inches along the well as profiles of low to mid-level peaks of the Green Mountains to a highway- humidity, wind, and stability. In addition, a closing 1 to 3 foot snowfall along the set of “most-favorable” conditions for western slopes of the Green Mountains and orographic snow in three main areas (the even into the Champlain Valley. One of the Champlain Valley, the Western Slopes, and main ingredients for these events is the the Eastern Slopes as indicated in Fig. 1) orographic lift over the terrain of Vermont will be identified. Figure 1. Locations of the Champlain Valley, the Western Slopes, the Eastern Slopes, and of the eleven sites used in Table 1. Topography is shaded blue for Lake Champlain, dark green for 2 lower elevation valleys, light brown for mid-elevation hills, and dark brown for higher elevation mountain. a) Synoptic Pattern Equivalent potential temperature decreasing with height in the low Over the northeastern United States, levels northwesterly lower-tropospheric flow Event duration of at least 12 hours regimes occasionally produce periods of heavy precipitation over the higher terrain of The daily composite synoptic northern New York and northern New pattern of all 25 upslope cases between 2007 England. This is primarily due to the and 2012 (for multi-day events, the day with interaction of the wind regime and the low the majority of the snowfall was chosen) level moisture with the complex terrain of shows a northwest flow at 500 hPa over the the region, producing orographic eastern United States and Vermont with a precipitation. negatively tilted trough extending southeastward from eastern Canada (Fig. The state of the atmosphere leading 2b). At 850 hPa, the composite mean shows up to an orographic (upslope) snow event north to northwest moist cyclonic flow over for the Green Mountains of Vermont has Vermont with the center of the cyclone in been examined in previous studies, most the Bay of Fundy between the Canadian notably St. Jean et al. (2004). It can be provinces of New Brunswick and Nova characterized by an upper level trough or Scotia (Fig. 3a). This configuration, closed upper low progressing through the combined with lingering low and mid-level region, typically with a trailing vorticity moisture, as shown by 850-hPa relative maximum, and a surface and low level humidity plotted in Fig. 3b, leads to the pressure system exiting the region (as shown development of orographic snow showers. It in Fig. 2a, a daily composite of mean sea is at this point when it becomes critical how level pressure of all 25 upslope cases the mesoscale features develop with time. between 2007 and 2012), with increasing westerly to northerly flow in the low levels. This paper aims to explain how two The interaction of such a flow regime with seemingly similar upslope events can have the topography of Vermont enhances significantly different outcomes. For precipitation, especially where the low level example, an event from 7 December 2010 (surface to 850 hPa) flow pattern is (Fig.4) brought heavy travel-disrupting generally perpendicular to the local terrain. snow to the western slopes, while an event St. Jean et al. (2004) found several specific from 27 January 2010 (Fig. 5) brought the factors important to the development of heaviest snow to the mountains and the significant upslope snow events. These are: eastern slopes. This disparity can be largely attributed to the difference between Blocked Near-saturated conditions from the Upslope Flow and Unblocked Upslope surface to ridge-top level Flow. Strong low level winds (>10 푚푠−1) with significant cross-barrier component 3 Figure 2. Composite daily mean sea level pressure (a) (solid lines, Pa) and composite daily mean 500-hPa geopotential height pattern (b) (solid lines, m) in upslope snow cases between 2007 and 2012 (NOAA ESRL 2012). Figure 3. Composite mean 850-hPa (a) vector wind (arrows ms-1) and wind speed (shaded, ms-1) and (b) relative humidity (shaded, %) in upslope snow cases between 2007 and 2012 (NOAA ESRL 2012). 4 Figure 4. 24 hour snowfall distribution (color-filled, in) ending at 0700 EST 7 December 2010. Re-created from the Daily Climate Maps from National Weather Service Burlington. 5 Figure 5. As in Fig. 4 except for 0700 EST 27 January 2010. b) The Froude Number orientation of that barrier), whether or not that parcel can move over the mountain As discussed, the distribution and depends on three factors: U, the barrier characteristic of the precipitation is highly height (h), and the static stability (on the variable from case to case. It has been found upwind side of the mountain. From the in this study and previously noted studies perspective of energy conservation, the that this variation is due to a number of increase in potential energy corresponding variables, however the most prominent to the increase in altitude in reaching height being the Froude number, and a h must be derived from the motion (kinetic determination if the northwest flow is energy) of the parcel itself. Furthermore, the blocked or unblocked. kinetic energy necessary to offset this increase in potential energy during forced If we consider an air parcel upstream ascent is modulated by a more stable of a mountain barrier with an initial speed stratification will make the parcel more relative to that boundary, U (the component resistant to vertical displacement, requiring of the wind speed perpendicular to the more work. Thus, it follows that slower 6 (faster), more stable (unstable) flow regimes the mountain barrier. When the Froude are more (less) likely to result in orographic number is near or slightly greater than 1, it is blocking. considered a critical level (Fig. 6b) where mountain waves are possible and The Froude number simplified is a precipitation is likely to fall along the ratio of the speed of the barrier- barrier. And when the Froude number is perpendicular wind to the stability of the much greater than 1 (Fig. 6c), the flow is low level air mass (as measured by the considered supercritical, or unblocked, and potential temperature at the surface and at the air will flow freely over the mountain mountain top). The Froude number chain, with the heaviest precipitation likely mathematically is shown in equation 1, falling on the lee side of the barrier. 푈⁄ℎ 퐹푟 = (1) While there have been many studies 푁 completed on orographic precipitation where U represents the wind perpendicular events, including in the Green Mountains of to the boundary, h is the height of the Vermont, there has only been modest boundary (spine of the Green Mountains), research using the Froude number and how and N is the Brunt-Vaisala Frequency, a it plays a role in the characteristics of an result of the Brunt-Vaisala Equation. The upslope precipitation event. More recently Brunt-Vaisala Equation is shown in equation there have been several studies including the 2, Froude number in certain portions of the world, but none in the northeastern United 1 States. g ∂θ 2 푁 = ( ) (2) θ ∂z One main focal point of research has involved the southern Appalachian where g is gravitational acceleration, 휃 is the 휕휃 Mountains, where until recently the potential temperature at the surface, and 휕푧 forecasts of the northwest flow snow events represents the change of potential in this part of the country have struggled.