Weather Service Report


844 
FXUS66 KMTR 230625
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1125 PM PDT Wed Sep 22 2021

.SYNOPSIS...Offshore flow will develop overnight while the marine
layer stays near the coast. Expect rapid warming and drying trend
Thursday as offshore flow develops across the region bringing a 
return of widespread readings in the 80s and 90s. A few degrees of
cooling as onshore winds return Friday afternoon. A seasonable 
weather pattern this weekend with a shallow marine layer along the
coast while clear and warm weather continues inland. Will be 
watching for a frontal passage by Tuesday that should bring cooler
temperatures and perhaps some showers to the North Bay.

&&

.DISCUSSION...as of 9:07 PM PDT Wednesday...No significant short 
term updates this evening. Temperatures cooled today, most notably
near the coast where the marine layer returned while readings 
still reached 90 plus for the East Bay interior valleys. The 
marine layer is around 1200 feet deep this evening with a modest 
2.4 mb onshore gradient that is ushering in cooler air into SF Bay
as some low clouds are now up into the Berkeley hills. SFO is 
reporting west winds at 17 mph but the marine air hasn't reached 
beyond the East Bay hills with Concord still a warm 77 degrees. 
Conditions above the inversion layer remain warm and dry with 
readings in the upper 70s above 1500 feet with low humidity values
in the 30-40% range this evening. (For those looking for RAWS data
that server is currently down, hopefully up by morning).

All attention turns to another round of light offshore winds that
will develop overnight. Northerly gradients now up to 5 mb and
increasing while the SFO-WMC has reversed to 1 mb offshore from
the deserts as the shortwave begins to dive into the Great Basin.
Surface pattern is pretty favorable for a light offshore wind
event in the hills that will translate into another round of
fairly hot temperatures for the Bay Area on Thursday. Northeast
winds should push into the Napa hills around midnight and then
slowly spread over the North and East Bay hills by sunrise
Thursday. Wind speeds generally in the 6-12 mph range with some 
gusts 20-30 mph for the highest peaks with RH values generally 
above 30 percent overnight. Similar to Monday expect rapid warming
and drying during the day Thursday as offshore flow attempts to 
wipe out the marine layer. Temps should warm rapidly into the 
lower 80s by lunchtime then into the 90s by afternoon. The pre- 
existing marine layer and generally light offshore pattern may 
have a hard time completely wiping out the marine layer so it 
shouldn't get quite as hot in the City on Thursday, generally 
upper 70s to around 80.

Light offshore flow to continue in the hills Thursday night
keeping fire weather concerns elevated due to poor humidity
recovery, warm temps and persistent offshore winds. The coast may
see a return of a shallow marine layer by Friday morning.

Only modest cooling Friday, from hot to warm as onshore winds
gradually return by Friday afternoon bringing some relief coast
and bays while seasonably warm temps continue beyond the East Bay
hills.

The pattern aloft turns zonal over the weekend producing tranquil
weather with seasonable temps for the weekend. Extended forecast
will focus on a possible frontal passage by around Tuesday with
ensemble data increasing North Bay PoPs into the 20-30 percent
range. That will become the forecast focus after we get through
tomorrows heat and offshore wind fire weather concerns.

&&

.PREV DISCUSSION...as of 01:35 PM PDT Wednesday...A mid/upper level 
short-wave trough pushing into the Pacific Northwest and the 
return of a shallow marine layer have resulted in a region-wide 
cooling trend this Wednesday. Many coastal areas are as much as 15
deg F cooler compared to 24 hours ago while some interior and 
higher elevation locations have noticed little change. With low 
clouds hugging the entire coastline, look for low clouds to spread
back inland overnight as the marine layer deepens lightly in the 
next 12 to 24 hours. 

The cooling trend will be short-lived however as the trough drops to 
the southeast and into the Intermountain West through late week. 
This will result in brief offshore flow over the higher elevations 
of the Bay Area from tonight into Thursday morning. While elevated 
fire conditions are possible, winds are not anticipated to be strong 
enough to create widespread critical fire weather conditions. 
Additionally, high pressure will build in from the eastern Pacific 
through late week helping to aid in another warming trend. That 
said, marine influences should keep conditions cooler near the coast 
while interior areas warm back into the upper 80s to middle 90s both 
Thursday and Friday. 

The ensembles and deterministic models are in a bit better agreement 
heading into the upcoming weekend with brief zonal flow developing 
aloft. This should allow for temperatures to cool slightly Saturday 
into Sunday as onshore flow also persists and temperatures aloft 
cool. Additional cooling looks likely early next week as well in 
response to an approaching mid/upper level trough. Ensembles are 
also indicating the potential for light precipitation across 
portions of the North Bay early next week with the deterministic 
ECMWF showing precipitation as far south as the Monterey Bay region. 
Having said that, we'll continue to monitor trends in the models 
over the coming days as confidence remains low at this time with 
respect to precipitation across the greater San Francisco Bay Area 
and Central Coast.

&&

.AVIATION...as of 11:16 PM PDT Wednesday...For the 06z TAFs. Stratus
and fog /LIFR-IFR/ returning inland with onshore flow tonight into
Thursday morning. Terminals farther inland should remain VFR
overnight such as KSJC and KLVK. TAFs with cigs overnight should 
lift to VFR late Thursday morning.

Vicinity of KSFO...VFR for both KSFO and KOAK, but FEW-SCT005 
will likely turn into BKN-OVC by 08Z with IFR conditions. Light 
winds overnight. Expect clearing by 15-16Z. Onshore winds of 
10-15kt return Thursday afternoon and evening.

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...VFR at KMRY, but it's on the edge of stratus and
will likely get cigs by 08Z. KSNS already at IFR. Both sites
should start lifting to VFR around 17Z.

&&

.MARINE...as of 11:14 PM PDT Wednesday...Gusty northwest winds
over the northern outer waters tonight into Thursday morning.
Winds then decreasing by late week. A longer  period northwest
swell mixing in with the weaker southerly swell. The next chance
for new longer period swell is Sunday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...None.

$$

PUBLIC FORECAST: RWW
AVIATION: Bingaman
MARINE: Canepa

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Western Regional Climate Center, wrcc@dri.edu