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Chapter 09 Appendices REV3 :Layout 1 10/6/08 9:59 PM Page 191

Regional Weather

001 The following are descriptions of typical weather more often in the spring season. patterns in major boating regions of the United States. 007 Gales. Gale force may occur in any season but are most frequent in . Nor’ easters are common and sometimes are of near hurricane New England, force in winter. They are rare in . Mid-Atlantic 008 Hurricanes. West Indies hurricanes are a poten- tial threat until they either move over land well 002 This area is frequently a meeting place for cP to the south (when they may still re-curve and and mT air masses. When cP prevails, the bring flooding rainfall as they move back out to weather is dry. When mT prevails, temperature sea) or until they re-curve at sea and pass south and are comparatively high. and east of 40° N, 70° W. While the official pass along the boundary between the two air season is 1 June to 30 masses and are frequent, especially from November the season is very peaked with most autumn through spring. tropical storms and hurricanes occurring from 003 Most warm fronts move into an area between August through October. south and west; most cold fronts between west and north. East of the Appalachians, however, a 009 Visibility. There is a great deal of advection cold front sometimes moves down from the along the Atlantic coast in spring and summer, northeast. This is known as a back-door front. It from Cape Cod eastward, because of relatively may create a considerable period of adverse cold water inshore. Warm, humid air blows in weather for boating, although severe storms are from the Gulf Stream offshore. Fog produced not common. over the cold coastal water is blown onshore by easterly or southerly breezes. Fog often occurs 004 Temperature & Humidity. Temperatures vary at the mouths of rivers or over partially enclosed widely. In the north, ice, snow, and winter lay-ups harbors along the coast of the northeastern of several months are common. Humidity varies states in the fall. This fog usually burns off by widely, depending on the overlying air mass. midmorning.

005 Barometric Pressure. The pressure changes 010 Water Conditions. Waves on shallow bays and almost continuously. It may vary from 30.65 sounds, such as Long Island Sound, build up inches during clear, dry weather to 28.70 inches rapidly when the increases. These enclosed in stormy conditions. Pressure and tendency are bodies of water have little wave system when air valuable in forecasting when combined with is light, so rising winds have an unruffled surface other data. on which to work. Significant waves can develop in a few minutes. 006 Local Storms. The most intense thunderstorms 011 Over open sea, existing wave and swell sys- occur in connection with fronts and squall lines tems, including cross swells, are almost always from early spring to early summer. Local air- present; These are not easily altered by sudden mass thunderstorms, rare and usually less squalls. The size of the waves increase if severe, are almost entirely a summertime phe- the squall wind comes from the same direction nomenon. Tornadoes are also rare and occur as the waves.

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012 Large changes in coastal and harbor water 019 Squalls. Appearance of a line of cumulonimbus levels are common during hurricanes and clouds on the horizon is commonly a result of onshore gales. Beaches, structures, and moored cooler air under running warmer air. This can boats may be heavily damaged, especially when produce high winds and heavy rainfall. A 180° hurricanes come ashore. Seiches do not occur in wind shift often occurs. Brassy skies in the west this area. or northwest, usually in the late afternoon, may foretell a severe squall bringing within an 013 Wind Conditions. in Spring and hour. Summer are typically from the Southwest across most of New England. In winter, these winds 020 Visibility. Visibility is affected by atmospheric come more predominantly from the west. A char- stability. If distant objects can be seen at 1100, acteristic of New England is a high degree of the atmosphere is unstable and the weather thermally-driven winds also most likely from the could worsen in the afternoon. If visibility is only Southwest. In spring and early summer, these 1 to 2 miles, the air is stable. Haze and smoke thermals may bring warmer, moist air over are stratified near the surface. The weather is colder water leading to advection fog. likely to be good during the day as far as wind and rain are concerned. A pale yellow sky at sun- set may be an indication of rain within 12 hours. The Great Lakes A bright yellow sky indicates a likelihood of good sailing winds. Steam fog is rare, but in early fall 014 This is an area of frequent movement of major it may appear in enclosed harbors until an hour weather systems from the northwest, west, or or two after sunrise. Advection fog is common southwest. Squally conditions and large changes during the spring and early summer. in temperature occur as such systems move through, especially in winter and early spring. 021 Water Conditions. On a shallow body of water 015 In the absence of any general weather sys- with no swells or wave trains like those that tem, lake breezes develop close to the shore dur- appear on the open ocean, waves build up rap- ing the day, but the center of a large lake idly with an increase in wind. If a wave system remains calm. already exists and the increasing wind is coming from the same direction as the waves, they will 016 Temperature. Seasonal and daily temperatures grow larger. In a sudden squall, this can happen are variable. Winter cold, ice, snow, and fre- quickly. quent strong winds encourage wintertime lay-up 022 A wind of 15 knots or more, blowing over a during the coldest months. Although the humid- lake with a fetch of some hundreds of miles, will ity is generally lower than in most coastal areas, cause the water to rise along the lee shore. On humid weather occurs when mT air approaches rare occasions this rise may be as much as 10 feet. from the Gulf of Mexico. 023 Seiches, usually due to atmospheric forces, are 017 Barometric Pressure. The barometer shows rapid changes in the level of water along the wide changes: from high to less than 29.00 lakeshore. The change may amount to as much inches, in the deepest systems. Baromet- as 4 feet. Generally the water level falls, but it ric pressure and tendency are useful in predict- may oscillate slowly. Sometimes a series of ing the approach of major weather systems. waves 4 feet high or more accompanies the changing water level. 018 Local Storms. Thunderstorms are largely the result of moving fronts or squall lines and can be severe. Summer air-mass thunderstorms are South Atlantic, not as frequent as in more southern latitudes. These storms are not usually severe but may be Florida, Gulf— hazardous to small boats. Tornadoes and water- General Area spouts occur most frequently from February through August. The tornadoes are generally 024 Weather over these states is dominated much of associated with frontal or prefrontal squall the time by the semi-permanent Bermuda high. lines. The circulation around this high affects not only Chapter 09 Appendices REV3 :Layout 1 10/6/08 9:59 PM Page 193

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inland and coastal areas but also areas well off- 030 Hurricanes. Hurricanes are a threat to all parts shore. of the region. While the official Atlantic Hurri- 025 The area has more atmospheric moisture cane season is 1 June to 30 November the season than the north. Annual is heavy, is very peaked with most tropical storms and ranging from 45 to 65 inches. Amounts are less hurricanes occurring from August through over the extreme western Gulf. Winter October. produce extensive rainfall over the Gulf States, and summer thundershowers are frequent 031 Visibility. All land areas are subject to radiation throughout the region. fog. Advection fog is common in winter when warm, moist air flows inland from the Gulf. The 026 Temperature & Humidity. Comparatively mild waters of the northern Gulf are sometimes temperatures prevail most of the time. However, affected, and these sea tend to be quite per- several times each winter, the westerlies of the sistent. mid-latitudes are displaced southward enough to bring extra-tropical cyclones and cP air to all of 032 Land & Sea Breezes. During the warmer these states. During severe cold waves, freezing months, with a weak pressure gradient, coastal temperatures sometimes reach into southern areas experience a sea breeze from the middle of Florida and extreme southern Texas. However, the morning to shortly after sunset. A weaker boating is a year-round activity. Hot, humid offshore or land breeze usually prevails at night. make spring and fall popular boating seasons. Florida 027 Barometric Pressure. The barometer is a helpful forecast tool, but it is not used as much as in more 033 Movement of Weather. Weather systems usu- northerly latitudes. The diurnal pressure change ally move from west to east over middle and is pronounced, especially in the southernmost higher latitudes. In the latitude of Florida, how- portions of this region, and tends to obscure pres- ever, movement may be from east to west, as sure changes associated with weak migratory easterly winds often extend to high altitudes. An systems. It is often more important to note the example is the “easterly waves” that appear 24-hour change than the change during the past over the Caribbean. These sometimes affect the few hours. Pressures substantially above or lower Florida peninsula and the Keys. below normal for the season warrant investiga- tion. Unusually high readings indicate strong 034 Fronts. Nearly all frontal action comes from , which produce above-normal winds early October to mid-May. Most cold fronts that and seas over extensive areas surrounding the reach Florida extend from cyclones far to the benign center of the high. In general, a pressure north. The typical weather sequence described of 1010 millibars (29.8 inches) or less is cause for in texts is rarely experienced. Many cold fronts concern and may indicate that some kind of produce no rain at all. Often the rain that does weather disturbance is near. occur is associated with a prefrontal squall line. The front is then dry but can be distinguished 028 Local Storms. Thunderstorms occur frequently from the squall line by lower dew points and a over land on summer afternoons, usually in a more persistent veering of the wind. spotty pattern, owing to the abundant supply of 035 After passage of a relatively strong cold moist, conditionally unstable air. Upper winds front, the weather usually turns clear and cool may carry these storms offshore in late after- for a few days, and then gradually warms up noon or evening. Nocturnal thunderstorms often until the next front arrives. Many fronts dissi- occur over the warm waters of the Gulf Stream pate south of Florida. Those that do return as and the Gulf of Mexico. warm fronts are usually quite weak, and the typ- 029 Tornadoes are seen in all seasons, but the ical pre-warm front cloud sequence is rarely most destructive ones usually occur in late win- observed. Occluded fronts rarely if ever occur ter or spring, with severe thunderstorms along over the peninsula of Florida. squall lines ahead of cold fronts. Tornadoes are more common in the Gulf States than elsewhere 036 Upper Troughs & Convergence Lines. Troughs in the region. in the upper air, not indicated by the surface Chapter 09 Appendices REV3 :Layout 1 10/6/08 9:59 PM Page 194

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pressure pattern, may cause areas of cloudy, north of Florida and is either stationary or slow- showery weather with possible thunderstorms. moving. The result can be several days of small- These are best located by satellite photos. craft advisories, especially along the east coast Recent satellite pictures are shown on nearly all of Florida. The most likely months for these TV weather programs. windy periods are October, March, and April. 037 Showers and thunderstorms also form along October nor’ easters can cause considerable lines where two wind streams are converging at beach erosion. an angle that may be much less than 45°. These are difficult to forecast. Be alert to their devel- opment by watching for lines of convective cloud Gulf States development and by monitoring NOAA Weather Radio broadcasts of radar reports. 043 Boating is a year-round sport. Winter lay-up is a rarity. Sailors often find better winds for racing 038 Local Storms. Florida has more thunderstorms or cruising in the cooler part of the year than in per unit area than any other state. The vast mid-summer. majority of these are afternoon storms over land, from mid-May to November. Winter thun- 044 Fronts. Cold fronts moving down the Mississippi derstorms are usually associated with fronts Valley may go as far as extreme southeast and squall lines. Nocturnal thunderstorms Texas. Owing to the Bermuda high, cold fronts develop over the Gulf Stream and, in perhaps sometimes tend to stall and move slowly into the half the cases along the southeast coast, drift Gulf of Mexico to become stationary, returning inland during the early morning hours (0500- over land as a warm front when a developing low 0900). Morning thundershowers are less fre- progresses eastward and northward from the quent from Palm Beach northward. The most central Gulf states. frequent months are August and September. Morning storms tend to be less severe than the 045 Local Storms. The Gulf Coast has severe thun- afternoon variety. derstorms that occasionally spawn tornadoes, 039 A number of tornadoes are reported annu- although they are not so frequent or as severe ally, however, these are usually small and less as those in “ Alley” (Northwest Texas, severe than those of the Mississippi Valley and Oklahoma, and Kansas). Watches and warnings Plains states. are issued as needed, usually for a period of one 040 are numerous in the Florida to a few hours. Monitor severe thunderstorm Keys- requiring constant vigilance. Many warnings and keep a visual watch for threaten- appear in relatively benign weather—that is, ing convective activity. Such storms can pose light wind along a shower-free portion of a line severe hazards for small boats. of building cumulus clouds with no thunder- storms in the area. Fortunately, most water- 046 Squall Lines. Squall lines frequently cause spouts are small, and the alert skipper can take severe thunderstorms and tornadoes. Occurring evasive action by steering a course at right usually in late winter or early spring, these angles to, and away from, the forward move- storms are brought about by the clash between ment of the vortex. warm, moist Gulf air and colder air from the north. Houston, Texas, is one popular boating 041 Visibility. Fog, the main restriction to visibility, area over which these storms are notable. occurs mainly during winter and early spring. 047 During the unstable periods when squall Fog is quite prevalent along the northeast coast lines are in the area, cumulonimbus clouds and and on the west coast from Cedar Key to Pen- waterspouts often appear. If you sight such phe- sacola during this time. Occasionally it may last nomena, be prudent and take shelter. all day. Fog is rare in south Florida except for 048 The temperature difference between North local ground fog that is of little danger to boating. Houston and Galveston Island is normally 5 to 10 degrees; such a temperature difference does 042 Small-Craft Advisories. Advisories in summer not necessarily imply a difference of air masses. are nearly always brief unless a tropical storm is involved. A prolonged windy period is usually 049 Hurricanes. While the official Atlantic Hurri- due to a large that is centered well cane season is 1 June to 30 November the season Chapter 09 Appendices REV3 :Layout 1 10/6/08 9:59 PM Page 195

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is very peaked with most tropical storms and 055 Storms. Storms are most frequent on the Wash- hurricanes occurring from August through ington coast in the winter months. Extratropical October. cyclones are less common toward the south and are infrequent in southern in any sea- 050 Visibility. The chief restriction to visibility is fog. son. Severe storms are rare anywhere on the Most fogs occur during the winter and early West Coast during the summer months. spring months. From Pensacola west to Port 056 Because of the protection afforded by the O’ Conner, fog during these seasons are some- mountains, major storms from the east or north- times severe, forcing shipping, boating, and air- east are rare. Most storms move in from the craft activities to halt. northwest, with south to southwest winds pre- ceding the front and west to northwest winds fol- lowing it. 057 Because the air is abruptly lifted as it is The West Coast— forced over the coastal mountains and the pres- General Area sure gradients steepen as the storm system approaches the coast, the south to southwesterly 051 West Coast weather is strongly influenced not wind flow can be strong, with gale-force winds only by normal meteorological patterns but also and heavy precipitation. by topography, the Pacific high (also called the 058 Thunderstorms and squall lines, typical of Hawaiian high), and the cold California Current. cold front passage on the East Coast, are rela- tively rare on the West Coast. 052 Topography. The entire West Coast is moun- tainous, rising abruptly from the sea in most 059 Local Winds. The coastal mountains normally places. Storms moving in from the Pacific act as a barrier to winds from the continental deposit most of the precipitation between the interior. However, the pressure pattern occa- coast and the inland mountain ranges. These sionally creates a strong gradient between the ranges protect the coastal areas from most of interior and the ocean, with strong easterly (off- the severely cold air that sweeps over the rest of shore) winds developing. These winds, which fol- the country from Canada in the winter months. low the valleys to the ocean, exhibit regular pat- terns and are given special names by local 053 Pacific High. The vast North Pacific is domi- residents. The air flow is downslope to the sea so nated by a huge high-—the compression heating occurs. Pacific high. It intensifies during the summer 060 These winds are characterized by low rela- months and weakens during the winter. In sum- tive humidity and clear skies (except for the dust mer, the Pacific high is centered approximately stirred up). The most prominent local winds are 1,200 to 1,500 miles west of and is so those that flow through the Strait of Juan de dominant that it forces most lows well to the Fuca, between Washington and British Colum- north of Washington. This high weakens rapidly bia, and down the Santa Ana River Valley in through October and November, allowing a pro- , including the Los Angeles gression of low centers to affect the weather in area. Washington and Oregon. Continuing to lose strength during December, the Pacific high 061 Fog. The large temperature difference between weakens enough by January to allow lows to warm offshore water and cold inshore water sets penetrate into northern California and, occa- the stage for advection fog. It develops when the sionally, as far south as southern California. By moist, relatively warm air lying over the off- March and April, the trend reverses; the pres- shore water is gently transported across the cold sure rises gradually and the Pacific high water near shore—provided there is no cloud resumes its blocking effect. system overhead. The air is cooled to its dew point and fog results. 054 California Current. The prevailing westerly 062 There are two reasons why the water is winds are relatively warm and moist. The cool- much colder near the coast. One is the California ing effect of the California Current encourages Current. The other is the northwesterly winds, advection fog as the wind crosses the colder which cause upwelling of cooler water from the water. ocean depths close to the coastline. Chapter 09 Appendices REV3 :Layout 1 10/6/08 9:59 PM Page 196

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063 Advection fog is most common in late sum- Northern and mer and early autumn. This is the suggested procedure for forecasting this type of fog: Central California 1. If the wind is predicted to come from west or 069 The stretch of coast from Cape Mendocino to Pt. southwest with high relative humidity, fog is Arguello is bordered by a lesser mountain range probable. not as close to the coastline, running northwest- 2. If the wind is from the northwest and the rela- southeast. A large desert, the California Central tive humidity is high, fog is unlikely, but possi- Valley lies inland. Farther inland, along the Cal- ble. ifornia-Nevada border, is the 3. If the wind is north or northwest with low rel- mountain range. ative humidity, the likelihood of fog is remote. 070 The Sierra Nevada range generally blocks the flow of intensely cold Canadian cP air, pre- 064 Most West Coast harbors are located in bays venting it from overflowing from the central and formed by the mouths of major river systems northwestern United States into the California that drain the interior. Nocturnal radiational coast zone. The northwest-southeast orientation cooling sends a flow of cold air down these val- of the coastal range, and the fact that this area is leys. A shallow layer of radiation fog forms in the more frequently under the domination of the early morning hours, and generally dissipates Pacific high, create conditions similar to the before noon when the sea breeze returns. Occa- Washington-Oregon coast but much less severe. sionally, the nighttime radiation fog is sup- planted by advection fog carried in on the daily sea breeze, and fog persists for several days. Southern California

065 Boating Weather. West Coast boating centers 071 The stretch of coast from Point Conception to are strongly influenced by local peculiarities. the Mexican border runs predominantly west- Study the topography, ocean currents, air east, trending more southeast below Los Ange- drainage, and historical weather patterns for the les, and backed by a high west-east mountain area in which you intend to operate your boat. range a few miles inland. The area is protected by the mountains and is also shielded from frontal activity by the blocking effect of the Washington, Pacific high, so much so that true frontal pas- sages are restricted to occasional winter storms. Oregon Rain is infrequent and requires a strong air flow from the south or southwest to be moderate or 066 The stretch of coast from Canada to Cape Men- heavy in amount. This happens only when the docino is closely bordered by a high north- is weak or far to the west. mountain range, which causes strong orographic 072 The area inland (Southern California, lifting and wrings rain out of almost any appre- Nevada, , North-Mexico) is a large ciable air flow from the south-southwest to desert—hot, dry, and usually dominated by a north-northwest. thermally induced low-pressure trough. 067 The prevailing pressure gradient between 073 The southward flowing California Current, the and the Pacific high keeps an combined with upwelling from the ocean depths a almost continuous westerly flow going and sends short distance offshore, brings cold water close to a succession of cold and occluded fronts beating the coastline. A light wind from southwest to west into the coastal mountains. carries moist air across this cold surface and 068 These fronts vary seasonally in severity and causes fairly frequent fog and low stratus clouds timing, from almost daily gale-force winds in the in the coastal zone. This is a time to expect fog winter months to a summertime two- to four-day since the southwest-to-south breezes are most cycle of cloudiness. This is followed by light rain, common at night and in the early morning hours. clearing for a day or two, and then repeating. They are burned off by the sun or dissipated as the sea breeze develops. 074 The inland desert heats up rapidly each day, Chapter 09 Appendices REV3 :Layout 1 10/6/08 9:59 PM Page 197

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establishing the that draws in air does not appear to be the same cold air that from the ocean. The sea breeze generally starts overlies the inland areas. The major channel for as a light flow of 4 to 8 knots but increases rap- this flow is through Cajon Pass and down the idly to 15 to 25 knots from 1200 to 1400. It dimin- Santa Ana River Valley, whence Santa Ana ishes rapidly after sunset. This sea breeze devel- winds get their name. ops a considerable chop over coastal water, 079 These generally arise in creating hazardous conditions for outboard the afternoon (after the sun’s thermal effect boats and some discomfort to small cruisers. over the desert areas begin to diminish) and 075 A dominating mountain range lies west-to- blow most strongly during the late afternoon, east from Pt. Arguello past Los Angeles, then evening, and nighttime hours. curves toward the southeast. This explains why 080 Santa Ana winds can be accurately forecast many storms appear to change characteristics by watching for higher pressures and lower tem- when entering the southern California zone. For peratures in the desert areas. The zones affected example, a west wind sweeps upslope and causes are those below the mountain valleys. clouds and rain all the way from the Canadian 081 You may get a warning of half an hour to an border to Pt. Arguello. But below Pt. Arguello, hour by observing the advancing wall of dust storms coming from the northwest or west bring and dirt that outlines the leading edge of the considerably less precipitation and cloudiness. Santa Ana coming from inland. The main dan- 076 The Pacific high generally blocks the gers to boaters from these winds are their sud- advance of cold and unstable mP air into South- den onslaught and the fact that they blow off- ern California. This high must be weak or far to shore. the west or southwest to permit a really vigorous 082 Storms are almost unheard of in southern frontal passage in Southern California. California waters from June through Septem- ber. They are most frequent during January, 077 Forecasting Hints. Strong winds can arise with- February, and March, and even then they are out much visual warning or any advance indica- unusual. For a storm to be severe the Pacific tion by the barometer. The main cause is the high must usually retreat and a strong flow of steep pressure gradient that develops when any unstable, moist air must advance from the north. firm circulation (whether a low or a high) Since this phenomenon is well reported as it pro- presses in from seaward against the coastal gresses down the coast, the main problem is to mountain range. In such a situation, the wind decide whether the storm will “turn the corner” direction varies from northwest to south-south- at Pt. Arguello and head into this area. A prior west and the speed varies from 20 to as high as strong flow from south to southwest increases 60 knots. Lows generally have extensive cloud the probability of a storm. systems, and several times each year produce 083 Tropical storms or hurricanes that move in a conditions unstable enough to cause heavy rain. northwest direction from the Mexican coast 078 When highs advance on the southern Cali- develop the largest swells affecting the southern fornia coast from inland, they obliterate the California coastline. This is because the generat- usual thermal low over the desert area. This ing area is from the quadrant to the right of the occasionally happens when a mass of cold air storm’s path. surges down from Canada into the Great Basin. 084 Therefore, the swells form and move in a This cold air spills though the mountain passes direction parallel to the storm. This becomes a and rushes down the valleys toward the ocean at continuous generating region of strong winds. speeds of 30 to 50 knots or more. The air is The huge swells from these tropical storms often heated by compression as it descends and thus result in breakers 6 to 8 feet high.