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Area Forecast Discussion (AFD)

Issued by San Francisco Bay Area, CA (MTR)

FXUS66 KMTR 212128

Area Forecast Discussion
National Weather Service San Francisco Bay Area
228 PM PDT Sat Sep 21 2019

.SYNOPSIS...Most areas will experience some cooling tomorrow and
Monday as an upper low slides down inland California and into the
Desert Southwest. High pressure will then build over the eastern
Pacific early to mid next week. This combination will bring 
continued warming temperatures, dry offshore flow and increased 
fire weather concerns. A trough of low pressure will move through 
the region late next week resulting in significantly cooler 
temperatures and possible precipitation.

&& of 02:28 PM PDT Saturday...Temperatures across 
the region this afternoon are around 5 degrees warmer than 
yesterday as an upper ridge is stretched over California and 
northward into the Pacific Northwest. Most locations are currently
in the upper 70s to mid 80s with isolated areas in the low 90s. 
Expect a few degrees of additional warming today with temperatures
peaking in the low 70s to low 80s along the coast to mid 80s and 
90s inland. The upper ridge will quickly shift eastward into 
tomorrow and be replaced by an upper trough. The upper trough will
split into a cut off low on Monday that will move southward 
across interior California and Nevada before settling into the 
Desert Southwest Monday night. This will stall warming across the 
area and bring some minor cooling to some locations for tomorrow 
and Monday.

The combination of low pressure to our southeast and high pressure
over the eastern Pacific will create offshore flow across the
region from Monday night into Wednesday morning. This will bring
warm to hot temperatures across the region Tuesday and Wednesday
with offshore flow bringing these temperatures to coastal areas as
well. 850 mb temperatures look similar to the last brief heat
event at around 22-23 deg C. Models did a fairly poor job in the 
extended of capturing the degree of warming expected, particularly
for coastal areas. Given that this event will likely bringing 
stronger offshore flow, went ahead and bumped up temperatures for 
Tuesday. For now, expecting coastal areas in the 80s with low 90s 
possible around the Northern Monterey Bay and along the San 
Francisco Bay Shoreline. Interior locations are currently forecast
in the 90s to low 100s. Uncertainty remains fairly high as 
temperatures will depend on the strength of the offshore flow. As 
we get closer to Tuesday, will look into whether or not heat 
products are necessary. Wednesday looks to be warm as well but a 
few degrees cooler as onshore flow returns. The other concern with
this pattern will be for fire weather (see discussion below). 
Dry/gusty offshore winds and low humidities could bring critical 
fire weather conditions to the North and East Bay Hills Monday 
night through early Wednesday morning. Therefore, a Fire Weather 
Watch has been issued. 

Temperatures will then cool down quickly late this week as a deep
mid/upper trough forms over the West Coast and then digs into 
California. This will bring high temperatures down around 10 to 15
degrees inland from Wednesday to Thursday and leave highs across 
the region well below normal by Friday. There are still differences
between the models as to the positioning and timing of this 
system at the end of the week, but the Central Coast and Bay Area 
could see some precipitation into the start of next weekend. Lots 
of weather changes for the area this week. Stay tuned to the 


.AVIATION...As of 10:20 AM PDT Saturday...for 18Z 
TAFs...Widespread VFR with clear skies. Light winds becoming 
locally breezy onshore by the afternoon, especially near the 
coast. For tonight, short term models indicate enough moistening 
in lower levels (ie +3-5F dew points) to finally allow for return 
of some low coastal clouds overnight Saturday into Sunday. As 
such, latest TAF reintroduces low clouds at predominately 
coastally dominated locations briefly overnight into Sunday. Any 
clouds that do form will likely be compressed below 1000 ft and 
remain near the shoreline or just inland. These clouds will not 
be robust enough to persist beyond mid morning Sunday.

Vicinity of KSFO...VFR. 

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. 


.FIRE of 2:28 PM PDT Saturday...As of yesterday, the
Energy Release Component (ERC) and 100 hour fuel values are right
around normal for the region. An inside slider is still expected 
to bring an offshore flow event Monday night and through early 
Wednesday morning. A Fire Weather Watch has been issued from 9 PM 
Monday to 5 AM Wednesday for the North Bay Mountains and East Bay 
Hills & Diablo Range. Offshore flow will develop on Monday with 
winds peaking Monday night into Tuesday morning. The strongest 
winds thus far are expected in the North Bay Mountains where the 
local WRF model shows gusts up to 45 mph possible overnight 
Monday. Generally expect north to northeast winds 10 to 25 mph 
with gusts 30 to 35 mph. Winds will ease some Tuesday morning, but
will increase and become gusty again in the afternoon and 
overnight. Daytime humidity values are forecast in the teens to 
low 20s with poor overnight humidity recoveries both Monday and 
Tuesday night of 30 to 50 percent. Additionally, very warm to hot 
conditions are forecast for Tuesday with highs in the low to mid 
90s and isolated areas reaching triple digits. This along with low
humidities will contribute to further drying of fuels. 

&& of 08:10 AM PDT Saturday...Light to locally breezy 
northwest winds across the waters through this weekend. Strongest 
winds will in the outer waters north of Point Reyes where 
hazardous squared seas are also possible. Winds will increase 
again by Monday as another low pressure trough drops into the 
Great Basin. A 9 to 10 second northwest swell will be mixed with a
long-period southerly swell. 


     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm from 3 AM




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