Fort Lewis, Washington (47 05'N, 122 35'W) Full Year Climatology CONVENTIONS: The spelling of place names and geographical features are those used by the National Imagery and Mapping Agency (NIMA). All distances are in nautical miles (NM) and kilometers (km), except for visibility, which is in statute miles and meters. Elevations are in feet above mean sea level (MSL), with a metric conversion following. Temperatures are in degrees Fahrenheit (F) and Celsius (C). Wind speeds are in knots. Cloud bases are above ground level (AGL) unless otherwise stated; tops are above mean sea level (MSL). Precipitation amounts are in inches, with a millimeter (mm) or centimeter (cm) conversion following. Precipitation values given are liquid equivalent unless stated otherwise. Standard pressure levels are expressed in millibars (mb). Time is reported either in Coordinated Universal Time (UTC) (also known as Zulu or Z), or Local (L). TERRAIN. Fort Lewis, Washington is 5 NM (9 km) southeast of Case Inlet, an estuary at the southern end of Puget Sound. At about 300 feet (90 meters) elevation, Ft. Lewis is in a broad area of river flats between Tacoma (10 NM/18 km north northeast) and Olympia (15 NM/28 km west southwest). There is very little change in elevation in the immediate area. Several small lakes lie just to the north, and the area is dotted with small ponds. Just to the southwest, a small stream, the Nisqually River, flows northward to the Case Inlet. Puget Sound is to the north and northwest with its numerous islands and estuaries, as is the hilly Kitsap Peninsula. The Olympic Peninsula is 30 NM (56 km) northwest of the post, where the Olympic Mountains begin their rise to an average elevation of 5,000 feet (1,500 meters), with several peaks over 8,000 feet (2,400 meters). West of Puget Sound, Chehalis River valley, a gently sloped watershed that ends at the Pacific Ocean 65 NM (120 km) distant. To the southwest of Olympia, low coastal mountains march to the Pacific coast. The massive north-south oriented Cascade Range begins its rise 20 NM (37 km) east and southeast, with an average elevation of 5,000 feet (1,500 meters) and numerous peaks over 9,000 feet (2,700 meters). The highest of these peaks is Mount Rainier (14,410 feet/3,174 meters) 37 NM (68 km) to the southeast. Winter (December-February) General Weather. Fort Lewis experiences a Pacific maritime climate year-round. Two major features control the weather: the Pacific high, a semi-permanent high pressure area in the eastern Pacific Ocean, and the Aleutian low, a semi-permanent low pressure area in or near the Gulf of Alaska. To the east, the Cascade Mountains deflect the prevailing westerlies to a southerly direction most of the year. In winter, the Pacific high retreats to its most southerly position, which allows the Aleutian low to take over as the predominant feature. The Aleutian low spawns frequent weather systems that migrate southeastward to bring Fort Lewis frequent light rain or drizzle and higher than normal wind speeds. A few times a month, heavy rainstorms affect the area with strong gusts. The Aleutian low's exact position determines whether it will affect Fort Lewis with moisture-laden maritime air (the normal conditions) or colder but modified polar air from the Alaskan mainland. On rare occasions, polar continental air intrudes from British Columbia to the north, causing freezing or frozen precipitation. Heavy snowfalls, while rare, can occur. During rainy periods, fog can lower ceiling and visibility, but severe restrictions are infrequent. Sky Cover. The mean cloud cover is broken all season, as maritime air masses combine with frequent frontal systems to produce low ceilings, rain, drizzle, and fog. Ceilings below 10,000 feet occur twothirds of the time. Ceilings below 3,000 feet occur 30-35 percent of the time in December and January, and 20-25 percent of the time in February. Ceilings below 1,000 feet occur 10-15 percent of the time in 1
December and January and 5-10 percent of the time in February. Ceilings below 200 feet occur 5 percent of the time during the morning hours, when fog is enhanced by radiative cooling. Visibility. Frequent winter rain and drizzle is nearly always accompanied by fog. Rain and drizzle alone can restrict visibility during afternoon hours, but fog becomes the predominant restriction to visibility overnight and in the mornings. Haze, dust, snow, and smoke restrictions are rare. Fog restricts visibility below 7 miles (11,000 meters) an average of 20 days per month in December and January, and 17 days per month in February. Visibility below 11,000 meters occurs 35 percent of the time in December and January, and 25 percent of the time in February. Conditions are worst during mornings. Visibility below 3 miles (4,800 meters) occurs 15-20 percent of the time. Visibility below 1 mile (1,600 meters) occurs 5-10 percent of the time. Visibility below 1/2 mile (800 meters) occurs 5 percent of the time. Winds. Prevailing winds come from the south or southwest all season, as the Cascade Range deflects the maritime westerlies northward. Winds are normally light, averaging less than 5 knots, and calms are reported over one-third of the time. Exceptions occur 1) just before and after frontal passage, and 2) briefly during thunderstorms, when 25-35 knot gusts can occur. Gusts above 45 knots are rare, but nearby Puget Sound area stations have experienced gusts as high as 75 knots. Precipitation. The mean monthly precipitation, nearly all light rain and drizzle, is 5.9 inches (150 mm) in December, 5.8 inches (147 mm) in January, and 4.5 inches (114 mm) in February. Precipitation occurs on an average of 16 days per month all season. Heavy rain (greater than 0.5 inches/13 mm) occurs 2 days per month. Snowfall occurs infrequently. Significant snowfalls have been recorded at the site, but there have been entire winters with no measurable snowfall. Thunderstorms do not occur. Snow cover is rare. When snowfall does occur, any accumulated snow melts rapidly, as the mean temperature is well above freezing all winter. Temperature. The mean highs are 46 to 50F (8 to 10C) all season, with the coolest highs in December and January. The extreme highs were 66F (19C) in December, 62F (17C) in January, and 71F (22C) in February. The mean lows are 34 to 35F (1 to 2C), with the coldest lows in January. The extreme lows were 1F (- 17C) in December, 5F (-15C) in January, and 6F (-14F) in February. The temperature falls to or below freezing an average of 12 days per month in December and January, and 10 days per month in February. The average relative humidity is 80 percent. Spring (March-May) General Weather. The Pacific high begins to migrate northward during these transitional months. Migratory lows from the Aleutian low still affect the area frequently, and steady light rain and fog are common, especially during March and April. Ceilings and visibility are somewhat restricted during rainy periods, but severe restrictions are uncommon. As the Aleutian low weakens, migratory lows that reach Fort Lewis are weaker and milder, and by March freezing/frozen precipitation is rare. Thunderstorms are infrequent and weak, as the Pacific high maintains a relatively stable atmosphere. Sky Cover. The mean cloud cover is broken all season, as maritime air masses combine with frequent frontal systems to produce lowered ceilings in rain, drizzle, and fog. Conditions are at their worst in 2
March and gradually improve during the season. Ceilings below 10,000 feet occur an average of 50 percent of the time in March, and 45 percent of the time in April and May. Ceilings below 3,000 feet occur an average of 15-20 percent of the time in March, and 10-15 percent of the time in April and May. Ceilings below 1,000 feet are infrequent during afternoon and evening hours, but occur 5-10 percent of the time during the morning hours because of fog. Ceilings below 200 feet can occur during morning hours but are rare. Visibility. Fog, normally with frequent rain and drizzle, creates the most frequent restriction to visibility. Night cooling enhances fog and lowers visibility during the early mornings. Visibility restrictions in snow, smoke, haze, and dust are rare. Visibility below 7 miles (11,000 meters) occurs on an average of 14 days in March, 9 days in April, and 5 days in May. Visibility below 11,000 meters occurs 15 percent of the time in March, 10 percent of the time in April, and 5 percent of the time in May, with the worst conditions during mornings. Visibility below 3 miles (4,800 meters) occurs only between 00-10L 10 percent of the time in March, 5 percent of the time in April, and rarely in May. Visibility below 1 mile (1,600 meters) is uncommon in March, and rare thereafter. Visibility below 1/2 mile (800 meters) is rare. Winds. Prevailing winds come from the south and southwest all season. Winds are generally under 5 knots, and calms are reported over 25 percent of the time. The exception occurs just before and after frontal passage, when winds can gust to 25-35 knots. Winds over 40 knots are possible but rare. Precipitation. The mean monthly precipitation, nearly all light rain and drizzle, is 4 inches (102 mm) in March, 3.2 inches (81 mm) in April, and 2 inches (51 mm) in May. Precipitation occurs on an average of 17 days in March, 14 days in April, and 12 days in May. Heavy rain (greater than 0.5 inches/13 mm) occurs 1-2 days per month. Snowfall can occur in March but is rare and without accumulation. Thunderstorms do not occur. Temperature. The mean highs are 50 to 59F (10 to 15C) all season, with the coolest highs in March. The extreme highs were 73F (23C) in March, 85F (29C) in April, and 97F (36C) in May. The mean lows are 35 to 40F (2 to 4C), with the coldest lows in March. The extreme lows were 12F (-11C) in March, 25F (-4C) in April, and 27F (-14F) in May. The temperature falls to or below freezing an average of 7 days per month in March, 3 days per month in April, and never in May. The average relative humidity is 75 percent. Summer (June-August) General Weather. The Pacific high reaches its greatest strength and most northerly position. The mean storm track is far north of Fort Lewis, so few if any migratory lows affect the area. Thunderstorms are infrequent under the stable atmosphere of the Pacific high. Variability of weather conditions during the summer depends on wind direction, which, in turn, depends on the position of the Pacific high. Although precipitation is infrequent during the summer, calm or onshore (westerly) winds can lower ceilings and visibility in fog, whereas easterly winds from the land generally produce drier, warmer conditions. A thermal low that builds northward from interior California and Oregon can occasionally produce very warm conditions. Sky Cover. The mean cloud cover is scattered to broken. The Pacific high suppresses convective storms, but its stability enhances low ceilings with early morning fog. Ceilings below 10,000 feet occur 3
45-55 percent of the time. Ceilings below 3,000 feet occur 15 percent of the time overall, but 25 percent of the time during mornings. Ceilings below 1,000 feet occur 5-10 percent of the time during morning hours and infrequently the rest of the day. Ceilings below 200 feet are rare. Visibility. With the loss of migratory weather systems and the strengthening of the Pacific high, the primary visibility restriction in summer is fog. Fog may be associated with rain (especially in June), but maritime air and stable conditions in July and August create a favorable environment for low clouds and radiation fog. As in other seasons, fog conditions are worst during mornings. Visibility restrictions due to haze and smoke are infrequent. Restrictions due to dust never occur. Visibility below 7 miles (11,000 meters) occurs on an average of 5 days per month. Visibility below 11,000 meters occurs 5-10 percent of the time. Conditions are worst during mornings. Visibility below 3 miles (4,800 meters) occurs infrequently, and then only during morning hours. Visibility below 1 mile (1,600 meters) occurs rarely. Visibility below 1/2 mile (800 meters) is very rare. Winds. The prevailing wind direction is variable. In June and August, south and southwesterly winds prevail. On warm days, and especially in July, a sea breeze from Case Inlet often generates afternoon northwesterly winds. Winds are generally under 5 knots, with calms reported over 25 percent of the time. Wind gusts can reach 20-30 knots under rare summer thunderstorms. Precipitation. The mean monthly precipitation, nearly all light rain and drizzle, is 1.8 inches (47 mm) in June, 0.8 inches (20 mm) in July, and 1.2 inches (30 mm) in August. Precipitation occurs an average of 9 days per month in June, and 5 days per month in July and August. Heavy rain (greater than 0.5 inches/13 mm) is rare. Thunderstorms occur about once a month. Temperature. The mean highs are 70F (21C) in June, 76F (24C) in July, and 77F (25C) in August. The extreme highs were 93F (34C) in June, and 99F (37C) in July and August. The temperature rises to or above 90F (32C) on an average of 1-2 days per month all season. The mean lows are 50 to 53 F (10 to 12C), with the coldest lows in June, and the warmest in July and August. The extreme lows were 32F (0C) in June, 40F (4C) in July, and 39F (4C) in August. The temperature never falls below freezing. The average relative humidity is 70 percent. Fall (September-November) General Weather. Frontal systems become more active, and precipitation more frequent, as the Pacific high begins its annual retreat southward and the Aleutian low strengthens. Still, September is almost a continuation of summer, and quiet conditions prevail. October brings the first significant weather to the area as the mean storm track slides southward. Light, steady rain and fog become more frequent under cloudy skies. Radiation fog becomes slightly more common during October. By November the winter pattern is completely reestablished, with frequent frontal systems that bring stronger winds and light, steady rain or drizzle. Heavy rain and gusty winds occur several times in November. Sky Cover. The mean cloud cover is broken all season, mainly due to rain drizzle and fog; November has the worst conditions. Ceilings are lowest when early morning fog thickens. Ceilings below 10,000 feet occur 40 percent of the time in September, and 50-60 percent of the time in October and November. Ceilings below 3,000 feet occur 15 percent of the time in September, but 20-25 percent of the time in October and November. Most of these incidences occur in the mornings. Ceilings below 1,000 feet are infrequent most of the day, but occur 15-20 percent of the time during mornings. 4
Ceilings below 200 feet are rare except in September, when they occur 5 percent of the time in the mornings. Visibility. Migratory weather systems return during fall, with accompanying rain, drizzle, and fog. As in other seasons, fog conditions are worst during mornings. Visibility restrictions due to snow, dust, haze and smoke are rare. Visibility below 7 miles (11,000 meters) occurs on an average of 5 days per month. Visibility below 11,000 meters occurs 5-10 percent of the time. Conditions are worst during morning hours. Visibility below 3 miles (4,800 meters) occurs infrequently, and then only during morning hours. Visibility below 1 mile (1,600 meters) is rare. Visibility below 1/2 mile (800 meters) does not occur. Winds. Prevailing winds come from the south and southwest all season. Maritime westerlies are deflected northward by the Cascades. Winds are generally under 5 knots, and calms are reported over 25 percent of the time. The exceptions occur just before and after frontal passage, when winds can gust to 25-35 knots. Winds over 45 knots are possible but rare at Fort Lewis. Other stations in the Puget Sound basin have reported gusts as high as 75 knots. Precipitation. The mean monthly precipitation, nearly all light rain and drizzle, is 1.8 inches (46 mm) in September, 3.4 inches (86 mm) in October, and 6.0 inches (152 mm) in November. Precipitation occurs on 8 days per month in September, 11 days per month in October, and 18 days per month in November. Heavy rain (greater than 0.5 inches/13 mm) occurs 4 days per month in November. Snowfall is rare. Thunderstorms occur only in September, and they are rare. Temperature. The mean highs are 72F (22C) in September, 62F (17C) in October, and 51F (11C) in November. The extreme highs were 99F (38C) in September, 89F (32C) in October, and 71F (22C) in November. The temperature rises to or above 90F (32C) on an average of 1 day in September and not at all the rest of the season. The mean lows are 50F (10C) in September, 44F (7C) in October, and 39F (4C) in November. The extreme lows were 31F (-1C) in September, 21F (-6C) in October, and 5F (- 15C) in November. The temperature falls to or below freezing an average of 7 days in November, but never in September or October. The average relative humidity is 75 percent. POINT OF CONTACT: Address questions and comments pertaining to this narrative to AFCCC/DOPA, 151 Patton Avenue, Room 120, Asheville, NC 28801-5002. Telephone: DSN 673-9001 or Commercial (828) 271-4218. E-mail DOPA@afccc.af.mil Approved for public release: distribution is unlimited August 2000 5