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MAY 1998 MONTHLY REVIEW

A very unusual month with many records scattered throughout the state (most rain in May, most rain/days in month). Most telling is the 15 days of rain, as compared to an average of 2-3 in a normal year, and none in many years. Average rain for May is around .25", so with our 3.58", we received over 14 times (1,400%) of normal rain for May. San Francisco's yearly rainfall is now the second greatest in recorded history with 47.3" YTD. In Pacifica, YTD varies from 64" (Vallemar) to 46" (my house).  The dominant weather pattern featured a series of very cold upper level lows that repeatedly anchored off the West Coast, producing not only prolonged intermittent rain events, but violent weather as well. Tornadoes were observed on three separate days in the Bay Area..the most noteworthy being an anticyclonic F2 doing considerable damage in Sunnyvale on 5/4, with possibly a second one doing lighter damage in Los Altos Hills, NW of Sunnyvale. On 5/12, an F1 touched down outside Fairfield 40 miles inland from San Francisco and funnels were spotted in numerous locations including SFO airport. The main trigger for this almost month-long pattern was a very strong low with cold upper air support establishing itself almost due west of the West Coast then cut-off, carving out a huge trough in the upper atmosphere in the process. This low persisted most of the first 8 days of the month and produced rain each day. By the 10th, a second smaller, but more vigorous low (tracking directly over the Bay Area) pushed in producing another round of violent weather and almost 1" of rain. As the normal East Pacific High slowly started to weakly reestablish itself, GOA impulses moved to our north, leaving us basically cool and cloudy for around 10 days with only some scattered light rain in the area. Then starting on the 25th, rain again fell 5 out of 6 days with around 2" total. This was more typical of "Winter-like" storms with snow levels down to 6,000' and a well defined frontal passage. Up to 2' of snow fell in the Sierra's. While flurries can occur in the high country at any time of year, a snowfall like this is highly unusual. Many ski resorts opened back up for Memorial Day, and some may even have some upper slopes open till July 4. Snow depths at many locations is still over 10'. Sea surface temps in the East Pacific showed signs of rapid cool down, and by months' end, La Nina conditions were starting to develop west of South America. Locally, SST was a few degrees above normal..maybe due to lighter than normal onshore winds and reduced upwelling. Water color had been intermittently purple (thermally stable) spotted with lime green pockets of upwelling. By months end, the purple water appeared to be fading and air temps over water are now near normal at 54-55F.

PACIFICA, CALIF.   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  57.0  62.4   8:00a  54.5   5:30p   6.5   0.0  0.16   7.1  35.0  12:15a     S
 2  58.8  64.5  12:00p  54.9  12:30a   5.3   0.0  0.11   5.9  29.0  12:45p     S
 3  60.2  68.3   2:30p  54.2   2:00a   3.8   0.0  0.05   3.7  21.0   5:30p     S
 4  59.0  65.7   3:45p  54.4  11:15p   5.0   0.0  0.18   2.5  17.0  11:00a     S
 5  57.2  61.0  10:45a  54.2  11:00p   7.4   0.0  0.11   2.2  16.0  11:00a   ESE
 6  55.3  57.8  11:45a  53.7  11:45p   9.3   0.0  0.14   1.8  10.0   4:45p     W
 7  55.0  57.4   3:15p  53.4   9:45p   9.6   0.0  0.03   3.3  15.0   2:30p   SSW
 8  54.1  57.3   1:45p  52.0  11:00p  10.3   0.0  0.04   4.7  24.0   9:00p   WNW
 9  54.0  59.2   1:30p  50.8   3:15a  10.0   0.0  0.00   4.9  24.0   5:15p   WNW
10  55.6  62.4   1:15p  51.8   5:30a   7.9   0.0  0.00   3.6  19.0  12:00m   WNW
11  52.6  54.8   3:30p  50.9  11:45p  12.2   0.0  0.17   2.7  14.0  10:45a   SSW
12  52.1  57.2   1:30p  49.3   4:15a  11.8   0.0  0.73   3.4  24.0   1:45p     S
13  54.3  60.4  11:45a  48.7   3:15a  10.4   0.0  0.01   3.1  18.0   2:00p   SSW
14  55.2  60.8   1:30p  49.3  12:00m  10.0   0.0  0.00   3.0  15.0  11:15a     S
15  56.2  63.5   9:30a  49.2  12:15a   8.7   0.0  0.00   3.3  20.0   4:30p   WNW
16  55.0  60.9   2:00p  50.8  12:00m   9.2   0.0  0.00   4.4  20.0   1:30p     S
17  55.3  63.7   9:00a  47.4   5:45a   9.4   0.0  0.00   2.6  17.0   4:45p     W
18  56.6  68.8   8:45a  47.8   3:45a   6.7   0.0  0.00   2.6  19.0  11:45p   ESE
19  56.0  63.2   1:45p  51.3  12:00m   7.8   0.0  0.00   4.3  19.0   1:30p   WNW
20  56.0  62.9   2:00p  50.6   1:45a   8.3   0.0  0.00   3.4  16.0  11:45a   WNW
21  55.4  62.1  12:30p  51.0   3:15a   8.5   0.0  0.00   4.0  18.0  10:15a     W
22  56.4  62.9  11:15a  52.0   3:00a   7.5   0.0  0.00   3.9  18.0   3:00p   SSW
23  57.9  63.3  11:15a  54.2   4:15a   6.3   0.0  0.00   4.3  20.0  11:15a   SSW
24  59.7  68.6   1:45p  53.9  12:00m   3.8   0.0  0.04   4.2  16.0  12:15p     S
25  56.1  62.9  12:45p  51.3   4:30a   7.9   0.0  0.01   4.0  21.0  12:30p   WNW
26  55.1  62.2  12:00p  47.5   5:30a  10.2   0.0  0.02   4.7  22.0   2:00p     S
27  54.0  57.2   2:30p  51.5   5:15a  10.7   0.0  0.59   6.1  34.0   3:00p     S
28  54.6  57.6   3:15p  51.4   7:30a  10.5   0.0  0.93  11.2  34.0   4:45a     S
29  55.4  59.4   3:45p  51.5   2:45a   9.5   0.0  0.26   6.5  27.0  12:30a     S
30  57.8  65.2   1:15p  50.1   5:15a   7.4   0.0  0.00   2.1  11.0  10:15a     W
31  58.2  65.4  12:00p  52.3  12:30a   6.2   0.0  0.00   2.0  13.0   2:00p   WNW
-------------------------------------------------------------------------------------
                                      HEAT  COOL        AVG
    MEAN                              DEG   DEG         WIND                 DOM
DAY TEMP  HIGH   TIME   LOW    TIME   DAYS  DAYS  RAIN  SPEED HIGH   TIME    DIR

TOT 56.0 68.8 18 47.4 17 257.5 0.0 3.58 4.0 35.0 1 S
Max >= 90.0: 0 Max <= 32.0: 0 Min <= 32.0: 0 Min <= 0.0: 0 Max Rain: 0.93 ON 5/28/98 Days of Rain: 15 (>.01 in) 10 (>.1 in) 0 (>1 in) Heat Base: 65.0 Cool Base: 65.0 Method: (High + Low) / 2