MAY 2000 WEATHER SUMMARY
With periods of wildness punctuated throughout, May was at the least an interesting month overall. Well defined and long-lived weather patterns produced extremes in both rainfall and temperature. The most common weather feature for this time of year is a strong onshore pressure gradient which generally starts evolving by late March or early April, but this year failed to materialize until the last week of May. As a result, the average velocity of NW winds typical for this time of year was markedly reduced and the mean temp of 57.1 was significantly higher than average (55F). Usually, warmer temps this time of year in Pacifica would be a strong indicator of increased Sea Surface Temps - however not so this year....SST's were actually a bit below normal at 51-52F near Pacifica. Lower than normal pressures off the California coast produced N-S and offshore gradient winds, allowing a very warm period to evolve by the middle of the month, with 7 straight days of above average warmth (18th-25th). Interestingly, unusual storminess marked the beginning and end of this pattern. Early in the month, a strong westerly jet in a nearly-perfect zonal flow carried abundent low level moisture that at one time stretched from Asia to the U.S. Midwest , and directly over Cent. California (see Fig.1). Numerous stalled eddy lows (six total in the Northern hemisphere of the Pacific) north and south of the jet, helped pump moisture and energy surges into the stream, and set up another pipeline that has been so characteristic of this La Nina Winter. Between the 7th and 8th, 0.82" of rain in the form of persistant light rain/drizzle fell in Pacifica, with higher amounts (3"+) in orographically favored areas and in isolated pockets where showers amplified rainrates. Because the main flow was almost directly west to east, temps were steady and cool. By the middle of the month, the jet retreated far enough north for lower pressure to develop off the coast which first gave us prolonged offshore wind conditions and much above normal temps, before evolving into a rain producing eddy low directly over the Bay Area. This low on the 23rd, was highlighted by temps in the 90's in the East Bay, and 80's elsewhere, with mid-high level stratus thickening and eventually producing widespread shower activity. At one point around 1700 with light rain falling, fog and temps in low 60's were screeming at an estimated 50+ mph along the immediate coast then inland through the Golden Gate, while2,500' up temps were in the high 70's, and elsewhere balmy (80's-90's) with light winds prevailing. By month's end, moderate onshore conditions returned and typical Summer weather prevailed. (F&LCATCEIN&M)
MONTHLY CLIMATOLOGICAL SUMMARY for MAY. 2000
NAME: HOME CITY: Pacifica STATE: Ca ELEV: 540' LAT: 37.36'N LONG: 122.27'W
TEMPERATURE (°F), RAIN (in), WIND SPEED (mph)
HEAT COOL AVG
MEAN DEG DEG WIND DOM
DAY TEMP HIGH TIME LOW TIME DAYS DAYS RAIN SPEED HIGH TIME DIR
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1 55.7 63.1 3:00p 47.8 1:45a 9.6 0.0 0.00 3.1 17.0 3:15p SSW
2 56.9 62.8 2:15p 51.7 11:45p 7.8 0.0 0.01 2.3 13.0 4:30p SSW
3 56.5 66.5 11:15a 51.2 2:15a 6.2 0.0 0.00 3.5 16.0 2:15p SSW
4 55.7 62.4 2:00p 51.1 6:30a 8.3 0.0 0.00 4.5 20.0 4:45p SSW
5 54.7 61.0 2:30p 50.4 12:00m 9.3 0.0 0.00 4.1 18.0 3:00p SSW
6 53.1 58.9 9:45a 49.0 6:15a 11.0 0.0 0.01 3.6 17.0 1:00p S
7 56.2 59.4 1:15p 52.2 3:45a 9.2 0.0 0.26 6.0 22.0 7:00a S
8 57.4 60.8 4:15p 53.9 11:30p 7.7 0.0 0.56 2.8 14.0 12:30a SSW
9 56.6 62.9 1:15p 52.5 12:00m 7.3 0.0 0.00 3.5 18.0 4:15p SSW
10 52.9 57.7 1:15p 49.1 11:45p 11.6 0.0 0.02 5.2 23.0 2:15p W
11 53.3 62.8 9:30a 47.4 6:00a 9.9 0.0 0.00 4.0 17.0 12:15a W
12 53.0 63.3 9:45a 45.2 5:45a 10.8 0.0 0.00 2.6 13.0 2:45p SSW
13 54.9 63.9 4:00p 47.2 6:00a 9.4 0.0 0.00 2.7 15.0 5:30p SSW
14 56.2 61.4 12:45p 52.5 4:30a 8.0 0.0 0.20 8.3 40.0 4:45p S
15 57.2 62.7 1:15p 52.6 12:00m 7.3 0.0 0.24 5.8 24.0 12:15p S
16 55.0 62.6 1:00p 52.0 3:30a 7.7 0.0 0.00 4.1 22.0 12:30p SSW
17 56.7 63.9 2:15p 50.5 6:45a 7.8 0.0 0.00 2.8 13.0 4:00a WNW
18 58.4 68.9 9:45a 51.1 7:15a 5.0 0.0 0.00 2.7 12.0 6:15p SSW
19 59.6 70.7 9:45a 51.0 4:00a 4.2 0.0 0.00 2.5 11.0 10:45a SSW
20 60.4 72.0 2:30p 52.3 6:45a 2.8 0.0 0.00 2.2 9.0 11:15a SSW
21 66.4 79.6 2:15p 54.2 12:30a 0.0 1.9 0.00 1.4 8.0 3:15p SW
22 62.3 71.9 1:45p 53.5 6:45a 2.3 0.0 0.00 1.9 11.0 1:30p SSW
23 66.5 79.6 1:30p 56.3 3:30a 0.0 2.9 0.00 2.9 27.0 11:00p SSW
24 59.9 71.3 3:15p 53.5 12:00m 2.6 0.0 0.00 6.2 28.0 3:30a S
25 55.6 62.0 2:00p 51.8 5:30a 8.1 0.0 0.00 5.8 28.0 10:15a SSW
26 55.5 60.8 2:30p 52.6 4:15a 8.3 0.0 0.01 4.5 18.0 12:30p S
27 57.9 65.4 3:15p 53.1 6:30a 5.8 0.0 0.02 4.0 16.0 3:45p SSW
28 57.7 64.9 1:45p 51.6 12:00m 6.8 0.0 0.00 4.1 18.0 5:15p SSW
29 55.7 64.8 9:45a 49.3 6:30a 7.9 0.0 0.00 4.4 20.0 1:30p S
30 54.0 61.1 10:00a 48.1 6:00a 10.4 0.0 0.00 4.6 25.0 5:00p S
31 57.3 68.6 3:15p 45.5 4:15a 8.0 0.0 0.00 2.4 11.0 1:15p SSW
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57.1 79.6 21 45.2 12 220.9 4.8 1.33 3.8 40.0 14 SSW
Max >= 90.0: 0
Max <= 32.0: 0
Min <= 32.0: 0
Min <= 0.0: 0
Max Rain: 0.56 ON 5/08/00
Days of Rain: 6 (>.01 in) 4 (>.1 in) 0 (>1 in)
Heat Base: 65.0 Cool Base: 65.0 Method: (High + Low) / 2