NWS Forecast Area Discussion - San Francisco Area

FXUS66 KMTR 221322

Area Forecast Discussion
National Weather Service San Francisco Bay Area
522 AM PST Fri Nov 22 2019

.SYNOPSIS...Seasonable temperatures along with dry weather
conditions will persist through the upcoming weekend. Colder
conditions and unsettled weather will develop over the region as
early as Tuesday of next week and continue through Thanksgiving,
likely impacting holiday travel.


.DISCUSSION...As of 9:00 PM PST Thursday...Temperatures were very
isothermal across our region today, with nearly all locations
reporting high temperatures in the 60s. Light southerly flow
brought increased low level moisture into our area and patchy low
clouds persist this evening, mainly near Monterey Bay.

No significant changes are anticipated through Friday. Light
southerly flow will continue through the morning hours, with the
flow forecast to turn to the east and northeast by late
afternoon, resulting in low level drying. This drying probably
won`t occur early enough in the day to have much impact on
temperatures, and tomorrow`s highs will likely be similar to
today, or perhaps a few degrees warmer.

Light offshore flow will persist into the weekend while the
airmass warms over our area in response to an upper ridge building
across northern California. These developments will result in a
modest daytime warming trend through the weekend, with lower 70s
becoming more common. Drier air and mostly clear skies will also
mean cooler nighttime temperatures, particularly in the valleys
where overnight lows will drop into the 30s. Patchy early morning
valley frost is likely through the weekend.

A pattern change is expected to get underway early next week. On
Monday the upper ridge is forecast to shift offshore while a
shortwave trough skirts by our area to the northeast. Passage of
this shortwave trough will trigger increasing northwest winds
across our area on Monday afternoon. Winds are then expected to
veer to the north in the hills on Monday night and become locally
gusty. The brief period of offshore winds in the hills on Monday
night is not expected to be as strong as the most recent offshore
wind event. However, humidity will be low and fuels will still be
very dry. Thus, fire weather concerns will be somewhat elevated
from late Monday through early Tuesday.

Models continue to indicate widespread rainfall across our area by
late Tuesday. A system dropping southeast from the Gulf of Alaska
is forecast to tap into moisture from another system north of
Hawaii. Models agree in spreading precipitation from northwest to
southeast across our area on Tuesday afternoon and evening. Rain
rates are expected to be moderate, and perhaps even briefly heavy
late on Tuesday, as a robust cold front sweeps across our area. A
cold upper trough is then forecast to settle over California
through at least Thanksgiving, and possibly into Friday as well,
maintaining a cool and unsettled weather pattern. Preliminary
precipitation totals from Tuesday through Friday range from
0.25-0.75" and locally up to an inch. Across the northern part of
our area, the bulk of the precipitation is expected to fall with
the cold front on Tuesday, with not much more thereafter. In
southern areas, precipitation looks to be most widespread late on
Tuesday, but with scattered showers then adding to precipitation
totals through through Thanksgiving. Next week`s system will be
cold with 850 mb temperatures forecast to drop as low as minus 3
deg C by Wednesday night. The coldest air will arrive after most
precipitation falls, but we could see snowfall accumulation on our
higher mountains next week, particularly on the Santa Lucias in
Monterey County. Highs next next Wednesday and Thursday are
forecast to be held in the 50s in most areas, with colder
temperatures expected in the hills.

&& of 5:20 AM PST Friday...Light easterly wind over
the Bay Area from the low pressure system over the southwest.
Winds will switch to a light onshore this afternoon as the
influence of the low diminishes. Low clouds along the coast
skirting the Peninsula and the MRY Bay Area and the Salinas Valley
this morning. Models indicate less low level moisture tonight and
Saturday morning even though the flow is onshore so VFR expected

Vicinity of KSFO...SCT-BKN clouds through 17Z with bases around
2000 ft. Light easterly wind this morning becoming onshore to 10
kt late in the day.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Clouds in the Salinas Valley are lower
than the coastal clouds. MVFR at MRY but IFR at SNS with vsbys
down to 3-5 miles. Clearing after 16Z.

&& of 05:20 AM PST Friday...Low pressure over the
southwest will move east. Easterly winds will switch to north to
northwest tonight. A long-period northwest swell will arrive today
and continue through the weekend. Winds will increase late Sunday
or Monday as a strong storm approaches.


     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm from 9 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm from 3 PM




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NWS MTR Office Area Forecast Discussion