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