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Area forecast discussion
National Weather Service San Francisco Bay area
902 PM PDT Mon Oct 21 2019

Synopsis...a warming and drying trend will continue through
the week as high pressure strengthens over California. Offshore
flow will continue through midweek and strengthen Wednesday night
into Thursday morning. This will result in warm temperatures
reaching all the way to the coast, with the warmest temperatures
expected on Thursday. Locally gusty offshore winds are likely at
times in the hills, especially in the North Bay hills on Wednesday
night. In the longer range, cooling is forecast during the
upcoming weekend, although another round of dry offshore winds
is possible by late in the weekend and into early next week.

&&

Discussion...as of 09:01 PM PDT Monday...this afternoon ended up
being very warm region-wide with widespread 70s near the coast to
80s inland. Even a few of the warmer Interior Valley locations in
the North Bay topped out in the lower 90s. All of this was under
mainly clear sky conditions.

Looking for the warming trend to continue through midweek before
peaking on thursdays and/or Friday as offshore flow increases
region-wide. This should push temperatures into the 80s at the
coast with Lower/Middle 90s more widespread inland on Thursday.
With this said, the ongoing forecast remains on track this evening
with no updates needed at this time. Please see the previous
forecast discussion and fire weather section below for complete
details on the warm up and midweek critical fire weather concerns.

&&

Previous discussion...as of 02:05 PM PDT Monday...clear skies continue
over California this afternoon as high pressure aloft builds into
the West Coast. Temperatures have warmed quite a bit since
yesterday with many locations already surpassing yesterday's high
temperature as of 1 PM this afternoon. Highs today will end up
5-15 degrees warmer than yesterday, with temperatures in the 70s
and 80s. Offshore flow continues aloft with north to northeast
winds continuing in the hills.

The upper trough will continue to build over the region through
Tuesday. On Wednesday an upper trough from Canada will surge
south and move into the intermountain west and northern Great
Plains. The trough will be broad enough to bring a weak boundary
through area on Wednesday. It will also push the upper ridge near
California slightly to the west and the ridge will become centered
near 135w Wednesday afternoon. The warming trend will resume on
Thursday as the upper ridge moves back to the east and becomes
centered overhead. Surface high pressure will amplify over the
Great Basin while surface low pressure along the coast
strengthens.

As a result of the aforementioned weather pattern, highs on
Tuesday will be a couple degrees warmer than today as temperatures
aloft continue to warm and offshore flow continues. The warming
trend will largely halt on Wednesday before ramping up again on
Thursday. As offshore flow increases and the ridge builds
overhead, Thursday will likely be the hottest day this week with
highs warming well into the 80s near the coast and upper 80s to
low 90s inland.

Fire weather concerns will increase this week as offshore flow
persists and becomes strongest Wednesday night and into Thursday
morning. Gusty winds will develop tonight and again on Tuesday
night in the higher terrain of the north and East Bay. The
limiting factor for fire weather concerns through Wednesday
morning will be the humidity. Models project relative humidity at most elevated
locations to recover to around 30-50% the next two nights.
A much drier airmass will arrive late in the day on Wednesday and
continue to advect into the region on Wednesday night and
Thursday. The drier airmass will coincide with the period of
strongest offshore winds and lead to critical fire weather
conditions late Wednesday through Thursday.

Offshore winds will subside during the day on Thursday with
lighter winds continuing through Friday. However, it will remain
very dry and temperatures will continue to run very warm, with
highs only a few degrees cooler than Thursday. Cooler temperatures
are expected over the weekend as an upper trough approaches the
region from the north. Models have been inconsistent with their
handling of the trough as it drops into the Great Basin. There is
still the potential for a stronger, more widespread offshore wind
event late in the weekend, but confidence is low as the trajectory
of the trough will largely control the strength and timing of the
offshore winds. Stay tuned to the latest updates as the forecast
continues to be refined.

&&

Aviation...as of 4:45 PM PDT Monday...offshore flow over the area
will keep clear VFR conditions through Tuesday.

Vicinity of ksfo...VFR. West winds to 15 kt decreasing after 04z.

Sfo bridge approach...similar to ksfo.

Monterey Bay terminals...VFR.

&&

Fire weather...as of 2:22 PM PDT Monday...first and foremost a
Fire Weather Watch has been issued for the North Bay mts, East Bay
hills and Santa Cruz. The watch initially begins Wednesday
afternoon North Bay and East Bay/Santa Cruz Wednesday evening
through Thursday afternoon.

Now for the details - models have been rather consistent with an
offshore flow event beginning today and peaking Wednesday into
Thursday (most critical). NE winds ramp up Tuesday night, but it
will take some time for the humidity to drop. Tuesday night can be
best characterized as a near critical fire weather concerns with
moderate humidity recovery and breezy/gusty north winds. Much drier
air begins to work southward into the area Wednesday afternoon
across the North Bay and subsequently southward elsewhere Wednesday
evening/overnight. Greatest concerns will be late Wednesday
night/early Thursday morning with poor humidity recovery with the
strongest winds. Winds will diminish through the day on Thursday,
but humidity values will be Bone dry with readings in the upper
single digits to teens.

Conditions gradually improve by Friday. Looking down the Road,
another offshore flow event is possible late in the weekend into
early next week. Models have been wavering on exact
timing/strength, but regardless fire weather concerns return.

&&

Marine...as of 04:36 PM PDT Monday...high pressure building over
the eastern Pacific will maintain moderate northwesterly winds
across the coastal waters through the first half of the week.
Strongest winds are expected across the northern outer waters. As
high pressure builds in over the Great Basin later this week, a
thermal trough will develop along the coast this will result in
light offshore flow across the coastal waters through the later
half of the week. A moderate northwest swell will continue through
mid week.

&&

Mtr watches/warnings/advisories...
... Arena to Pigeon pt 10-60 nm

&&

$$

Public forecast: rgass/St
aviation: west pi
marine: CW
fire weather: mm

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