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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
------------------------------------------------------------------------------------
 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
-------------------------------------------------------------------------------------
    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