Scientific Forecaster Discussion

Return to Local Conditions & Forecast

Without Abbreviations
With Abbreviations

fxus66 kmtr 200357 
afdmtr

Area forecast discussion
National Weather Service San Francisco Bay area
857 PM PDT Sat Oct 19 2019

Synopsis...a warming and drying trend will begin on Sunday and
continue through Thursday as high pressure strengthens over
California. In addition, offshore flow will develop by Monday, and
increase by midweek, which will result in warm temperatures
reaching all the way to the coast. Locally gusty offshore winds
may develop at times in the hills, especially in the North Bay
hills on Wednesday night and Thursday morning. In the longer
range, cooling is forecast next weekend.

&&

Discussion...as of 8:55 PM PDT Saturday...a weak frontal
boundary tracked from north to south across our region today. In
most areas, the boundary only produced cloudiness, but
precipitation also occurred in some areas. Light rain fell mainly
in Sonoma County this morning, where as much as a tenth of an
inch accumulated in coastal areas. Very light precipitation made
it as far south as San Francisco this afternoon, with 0.01" of
drizzle reported in the Sunset District.

Low level residual moisture from the dissipated front remains over
our area this evening, resulting in lingering low clouds south and
east of San Francisco. Drier air will begin to filter in from the
north overnight, and much of this cloudiness will clear by Sunday
morning.

A broad upper ridge over the eastern Pacific is forecast to
amplify and shift eastward over the next few days, resulting in a
warming and drying trend in our area. Warming on Sunday will
partially be due to more sunshine and a drier airmass. More
significant warming is forecast for Monday. By Monday, 850 mb
temperatures over our area will have warmed by as much as 10 deg
c. In addition, light offshore flow is forecast to develop by
Monday and Tuesday, which will give an extra boost to coastal
temperatures.

A trough over western Canada is forecast to drop southeast and
over The Rockies on Tuesday night and Wednesday. In response, the
ridge over California will temporarily retreat a bit offshore,
and the temperature of the airmass aloft will cool slightly on
Wednesday. However, the trough digging to our east will enhance
surface high pressure over the Great Basin which in turn will
result in stronger onshore flow across our area around midweek.
Thus, the warming and drying trend will likely continue on into
Wednesday and Thursday, particularly in coastal areas.

The primary weather concern for the week ahead is the potential
for critical fire weather conditions. Warm and very dry air will
be present across our region for much of the week, and models
indicate potentially gusty offshore winds in the hills at times.
Locally gusty winds may develop in the North Bay hills as early as
Tuesday night, but the greatest potential for winds that may
produce critical fire weather conditions is on Wednesday night.
This is when the GFS increases the wmc-sfo gradient to 17 mb and
forecasts 925 mb north to northeast winds of 45 knots across the
North Bay. However, it should be noted that the European model (ecmwf) has
consistently placed the location of the midweek upper trough
farther to our east and thus forecasts lighter offshore winds. For
example, the 12z European model (ecmwf) forecasts only 20 knots of offshore winds
at the 925 mb level above the North Bay on Wednesday night. But
even that much wind may produce critical fire weather conditions
considering how dry the airmass will be. See fire weather
discussion below for more details.

In the longer range, the ensemble mean forecast from both the GFS
and European model (ecmwf) show another trough dropping south out of western
Canada next weekend. But this trough is forecast to be farther
west than the midweek trough, and thus likely to bring cooler
temperatures to California next weekend, along with more
widespread gusty winds.

&&

Aviation...as of 4:45 PM PDT Saturday...weak frontal passage
bringing low clouds to the sfo Bay area this afternoon. As the
front slides south clearing is expected over the sfo Bay area
after 03z while clouds move into the mry Bay area. There is the
possibility of lingering low clouds in the Bay area tonight but
models indicate air from aloft drying out the lower levels.

Vicinity of ksfo...MVFR cigs through 03z with bases 1500-2000
feet. Becoming scattered conditions after 03z. West winds to 15 kt
decreasing after 08z.

Sfo bridge approach...clearing from north to south beginning after
03z.

Monterey Bay terminals...increasing clouds from the northwest
after 02z becoming MVFR cigs. Clearing takes place after 08z as
front washes out.
&&

Fire weather...as of 3:55 PM PDT Saturday...heightened fire
weather concerns are expected next week as periods of offshore
flow combine with gusty winds at times to create critical to near
critical fire weather conditions. Locally breezy northwest to
north winds are expected in the hills tonight and Sunday, but
relative humidity values will remain somewhat elevated. A warming
and drying trend will get underway on Sunday, and especially
on Monday, as high pressure builds over California and offshore
flow develops. Offshore flow will mostly be light through Tuesday
night, except locally moderate and gusty at times in the hills.
Relative humidity values will drop into the teens by Monday
afternoon and very poor overnight humidity recoveries are expected
in the hills starting Monday night. Warm and very dry conditions
will then persist for the remainder of next week. The period of
greatest concern is Wednesday night and Thursday morning when some
models indicate the potential for strong and gusty north to
northeast winds in the hills, mainly across the higher elevations
of the North Bay. Fire weather watches and/or red flag warnings
may be issued over the next few days if critical fire weather
conditions are deemed likely to occur.

&&

Marine...as of 8:55 PM PDT Saturday...high pressure off the
central California coast will keep moderate winds over the
southern waters through Sunday. Winds will be lighter over the
northern waters as a frontal system passes north of the area. The
high builds north early next week resulting in increasing winds
for the northern waters and lighter winds over the southern
waters.

&&

Mtr watches/warnings/advisories...
... Pinos to pt Piedras Blancas 0-10 nm
Small Craft Advisory...rough bar advisory for sf bar

&&

$$

Public forecast: dykema
aviation: west pi
marine: drp
fire weather: dykema

Visit US at www.Weather.Gov/sanfrancisco

National Weather Service Glossary of Abbreviations