Scientific Forecaster Discussion

Return to Local Conditions & Forecast
Area forecast discussion 
National Weather Service San Francisco Bay area
202 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.

&&

Discussion...as of 2:00 PM PDT Thursday...active weather through
the entire forecast period with rain...thunderstorms...poss patchy
frost Saturday morning and the first real warm up of the season
next week.

A mix of sun and clouds over the Bay area this afternoon...but
just upstream rain is falling. A cold front continues to slide
south toward the County Warning Area and will impact the region later tonight. In
the meantime...pleasant temperatures with many locations in the 60s with a
few 70s in the southern County Warning Area this afternoon.

Short term(tonight-sunday)...
change is literally on the horizon. The aforementioned cold front
is still on track to move through the Bay area tonight and early
Friday. The evening commute today looks to be dry...but as the
fronts moves into the North Bay light rain will gradually spread
southward. There is still some uncertainty as to the southern
extent of the rainfall...but for now will have a wet commute
Friday morning for much of the area. Not a total wash out...but
some light precipitation around the region. Behind the front cold air
aloft will fill in as a jet maximum takes aim at the Bay area. The
convective threat still seems plausible for Friday with several
parameters showing unstable airmass(no cin, decent cape, minus
lifted indice's and steeper lapse rates). Given the cold air aloft (-28c at
500mb) storms that develop will be capable of producing some small
hail. Nothing severe is expected tomorrow...but something to
monitor on the radar. Storm Prediction Center expanded their
general mention of thunderstorms to include much California north of San
Francisco. Total rainfall from this evening through Friday night
will generally be a a few hundredths up to a tenth or two.

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.





&&

Aviation...as of 10:00 am PDT Thursday...tricky forecast this
morning with clouds clearing our earlier and then clouds
returning. Satellite shows plenty of lower clouds to the west
however loop clearly shows clearing now happening in the Bay. Feel
that VFR will be at all spots by 19z. Approaching system will
bring some showers to the area tonight into Friday although latest
runs have back off on coverage. Therefore, converted all -shra
just to vcsh. Locally breezy this afternoon into the evening.
Overall just moderate confidence.

Vicinity of ksfo...look for VFR to occur shortly and continue
through at least 02z. Westerly breezes kick up for the afternoon
with gusts into the middle 20s. MVFR conditions forecast after 03z
with a chance of showers after 08z. Moderate confidence.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...VFR through at least 02z for both
kmry and ksns. Breezy at sns with westerly gusts to around 20 knots
this afternoon. MVFR returns overnight with a chance of showers
overnight. Confidence is high.

&&

Marine...as of 02:00 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...
... Francisco Bay north of the Bay Bridge until 8 PM
Small Craft Advisory...San Francisco Bay south of the Bay Bridge until 8 PM
Small Craft Advisory...Monterey Bay until 8 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 8 PM
Small Craft Advisory...pt pinos to pt Piedras Blancas 10-60 nm until 5 am
Small Craft Advisory...pt pinos to pt Piedras Blancas 0-10 nm

&&

$$

Public forecast: mm
aviation: Bell
marine: Bell

Visit US at www.Weather.Gov/sanfrancisco

Follow US on facebook, twitter, and youtube at:
www.Facebook.Com/US.Nationalweatherservice.Sanfranciscobayarea.Gov
www.Twitter.Com/nwsbayarea
www.Youtube.Com/nwsbayarea