Scientific Forecaster Discussion

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Area forecast discussion 
National Weather Service San Francisco Bay area
1041 PM PDT Thursday Apr 24 2014

Synopsis...a storm system will bring cooler temperatures and
chances for precipitation to the region late tonight through
Friday. Another weaker system could bring rain chances Saturday
night and Sunday. Above normal temperatures are expected next week
as high pressure builds over the region.

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Discussion...as of 8:00 PM PDT Thursday...a cold front stretched
across northern California will gradually slide south bringing
rain to the region late tonight and Friday. There exists a
possibility of thunderstorms Friday as the an upper level low and
the accompanying unstable airmass move directly across the County Warning Area.

Currently temperatures remain in the 60s across most locations
with moderate and locally gusty westerly winds. Ceilings are
gradually lowering from north to south across the district. The
chance of rain will increase through the night with a wet commute
expected Friday morning across the County Warning Area. Rainfall totals will not
be very large with a few hundredths to a few tents expected. The
hills and mountains will receive the highest amounts. The main
concern for Friday will be the threat of thunderstorms in response
to the alignment of the jet maximum and the unstable airmass that will
accompany the upper low. Buffer sounding data depicts cape in
excess of 500 j/kg which is significant in this area.
Thunderstorms that do develop will be capable of producing gusty
winds and small hail and will need to be monitored carefully.

From previous discussion...Storm Prediction Center expanded their
general mention of thunderstorms to include much of California north of
San Francisco.

Precipitation chances and thunderstorm threat diminishes quickly after
00z Saturday as the best forcing moves east and drier air aloft
fills in from the west. Dry weather is expected for much of
Saturday...but below normal temperatures are being forecast. Highs
on Saturday will be in the upper 50s to middle 60s...high elevations
in the 40s to 50s.

Another weak system will move in Saturday night into Sunday
morning. Low confidence for this potential precipitation event...but
given recent model output decided to introduce light precipitation late
Saturday night/Sunday morning. Persons with outdoor activities
planned Sunday morning...especially along the coast...should at least plan
for the very low chance of precipitation. Rainfall amounts will be very light.

Long term(monday-thursday)...
once the precipitation ends Sunday afternoon...much drier weather is
expected for the entire long term. Medium range models still
indicate building high pressure and developing offshore flow by
Tuesday. The offshore flow will be strongest from Monday night
through Wednesday night. The daytime will be mild and very dry
with highs in the 80s to middle 90s inland...70s at the coast.
Overnight lows will see thermal belts with mild temperatures in the hills
with moderate to poor humidity recoveries. That being said...fire
weather may become more of a concern next week.

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Aviation...as of 10:30 PM PDT Thursday...approaching upper
trough has pushed stratus into the sfo Bay area early. A few
sprinkles have been reported in the past couple of hours. Latest
GFS brings a front through the area late tonight with rain
spreading across the area while NAM continues to be drier and
much slower. Given the rain that is already occurring the latest
tafs have pushed up the timing of the rain up. Southwest winds
will turn more southerly by daybreak as a frontal system
approaches. This front should help mix out the low ceilings by late
morning.

Vicinity of ksfo...showery with MVFR ceilings. Southwest winds to
5-10 knots through 12z then becoming southerly.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...MVFR. Showers Friday morning.

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Marine...as of 10:30 PM PDT Thursday...west to northwest winds
will prevail along the central coast into the weekend. A mixed
swell will enter the southern coastal waters on Saturday featuring
a 7 to 9 foot 14 second westerly swell and a 3 to 4 feet 19 to 21
second southerly swell. The southerly swell will produce
potentially hazardous conditions for south-facing beaches in
Monterey and Santa Cruz counties through Sunday.

&&

Mtr watches/warnings/advisories...
... Pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm

&&

$$

Public forecast: west pi
aviation/marine: west pi

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