Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

Area forecast discussion 
National Weather Service San Francisco Bay area
853 PM PDT Friday Sep 30 2016

Synopsis... an upper level low pressure system approaching from
the northwest will bring cooler temperatures and breezy
conditions into this evening and overnight. As the main upper
level system pushes inland on Sunday, chances for rain showers
will increase with the potential for thunderstorms, especially
across the San Francisco Bay area. Drier conditions and a slight
warming trend is then forecast by the middle of next week.

&& of 8:45 PM PDT Friday...locally windy conditions
have developed this evening as an upper trough deepens near the
West Coast. Wind gusts up to 37 mph have been recorded at San
Francisco Airport, and gusts in excess of 40 mph are occurring
above 1500 feet in the North Bay hills. The windiest spot is in
the hills east of Livermore where local gusts up to 50 mph have
been reported. Winds are expected to gradually decrease overnight,
especially at lower elevations, but locally blustery conditions
will persist in the hills. This upper trough is ushering a much
cooler airmass than was over our area just a few days ago. Today's
highs were mostly in the 60s and 70s and anywhere from 3 to 10
degrees below normal. This is in Stark contrast to the 90s and
lower 100s we saw early this week.

Breezy and cool conditions will continue on Saturday as the
longwave trough position remains just off the West Coast. Dry
weather will persist at our latitude through Saturday night. By
Sunday a potent shortwave, currently forming near the coast of
British Columbia, will drop south and into northern California.
This shortwave is forecast to bring with it enough dynamics and
instability to generate scattered showers across much of the San
Francisco Bay area on Sunday. There may even be enough instability
for isolated thunderstorms by Sunday afternoon and evening,
especially across the northern half of our forecast area where 700
mb temps are forecast to drop as low as minus 5 degrees c.
However, there won't be a lot of moisture available with this shot
of cold air on Sunday and so rainfall amounts are generally
expected to be less than a tenth of an inch. Locally higher
amounts are forecast by the European model (ecmwf) in the North Bay. High
temperatures on Sunday will be as much as 15 degrees cooler than
normal across inland areas.

Sunday's cold weather system will move off to our east by Monday.
But another weather system, this one currently located southwest
of the Aleutian Islands, is forecast to quickly move into our
area by late on Monday. This second system is expected to contain
much more moisture compared to sunday's system. In fact, the GFS
forecasts a plume of precipitable water up around 1.5 inches with
this second system. So although monday's system is forecast to
rapidly weaken as it approaches the California coast, it may
contain enough moisture to generate rainfall in our area late
Monday and Monday night, especially in coastal areas.

Drier and warmer weather is forecast to develop by the middle of
next week with temperatures expected to climb back to near normal
by Wednesday. But the models agree that the weather pattern will
remain active with another system due in over northern California
by late in the week. However, rain with that one is currently
projected to remain north of our area.

&& of 9:00 PM PDT Friday...visible satellite shows
dry frontal system moving through the area. The marine layer has
mixed out and skies are mostly clear this evening. Northwest flow
will provide some low level moisture to allow patchy clouds to
form overnight. This will affect mainly the East Bay hills and the
mry Bay area. Sfo and the approach looks VFR tonight.

Vicinity of ksfo...VFR. West winds to 20 kt decreasing to 15 kt
after 05z.

Sfo bridge approach...patchy clouds may produce cigs over the Bay
but not expected to impact the approach.

Monterey Bay terminals...clear skies this evening but northwest
flow should allow low clouds to form over the southern mry Bay
after 08z.


Fire of 2:54 PM PDT Friday...two main areas of
concern fire weather wise the next few days will be gusty winds
tonight and rain/tstorm chances on Sunday. A passing dry cold
front will bring cooler and wetter conditions to the region later
today and tonight. More importantly, gusty northwest winds will
develop with gusts to 20-30 mph possible. Do expect better relative humidity
recoveries overnight, but the WRF model continues to show some
very dry air near 850mb. That being said, recoveries could be
early in the night then drop toward sunrise over the higher peaks
of the soberanes fire. The Loma fire should see better recoveries
everywhere. Winds decrease slightly during the day on Saturday,
but ramp up again to breezy on Sunday. An upper level low will
drop south on Sunday bringing a chance of precip and maybe a
thunderstorm or two. Best chance for wetting rains appears to be
north of the Golden Gate with lesser amounts to Monterey. Loma
appears to have a better shot at rain than the soberanes. Lal of 2
for Sunday afternoon/evening with an unstable airmass.
Thunderstorms most likely north of San Jose, but did include Loma
fire as Santa Cruz mts are on the edge.

&& of 01:43 PM PDT upper level low
approaching from the north will strengthen winds across the
coastal waters today. Strongest winds are still forecast to be
south of Point Sur through Saturday. Winds will ease Sunday as
high pressure builds back in over the region. A small mixed swell
with a long period southerly swell will continue through the


Mtr watches/warnings/advisories...
... Bay until 9 PM
Small Craft Advisory...mry Bay until 9 PM
Small Craft Advisory...Pigeon pt to pt pinos 0-10 nm until 9 PM
Small Craft pinos to pt Piedras Blancas 0-10 nm



Public forecast: dykema
aviation: west pi
marine: BAM
fire weather: mm

Visit US at www.Weather.Gov/sanfrancisco

Follow US on facebook and twitter at:

National Weather Service Glossary of Abbreviations