Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 082028
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
128 PM PDT Sat May 8 2021

.SYNOPSIS...An early season offshore wind event will result in
critical fire weather conditions across portions of the North and
East Bay through early Monday. A Red Flag Warning is in effect
through 6 am Monday for the North Bay mountains and the East Bay
Hills as well as the East Bay Interior Valleys. Otherwise, expect
a sunny and pleasant weekend with mild temperatures near the ocean
and bays and warm temperatures inland. Offshore winds will ease
by Monday, but high pressure will maintain inland warmth through
midweek. A cooling trend is forecast for the second half of next
week.

&&


.DISCUSSION...as of 01:24 PM PDT Saturday...The current ridging
pattern continues to offer gusty offshore winds especially over
higher elevations in the North and East Bay. Mt St Helena saw some
reduction in winds during the late morning, going from 70+ mph
gusts to gusts around 50 mph, but the winds have become more
widespread around the San Francisco Bay. A few lower elevation obs
around the Bay Area reporting 15 to 25 mph gusts and these winds
look to last into the late afternoon. The dry, offshore flow will
also allow for temperatures to rise to well above average highs
for the afternoon with a few interior locations peaking in the
90s.

By nightfall, winds will focus again on the mid to higher elevations
in the North and East Bay. This combined with poor nighttime
humidity recoveries, continues fire weather concerns. For more
information on the fire weather conditions, please see the "Fire
Weather" section below.

Sunday offers a minor shortwave trough which will weaken the
offshore flow during the day, and may slightly reduce the afternoon
high temperatures. This cooling may only result in a few degrees
difference in most areas compared to Saturday afternoon.
Unfortunately the trough will move South and East into that evening,
allowing for winds to strengthen into the late night again for
higher elevations in the North and East Bay.

The ridging pattern continues for Monday and Tuesday, calling for
both afternoons to see well above average temperatures and dry
conditions. Tuesday, however, sees the ridge move more eastward,
decreasing the offshore flow, and weakening the overnight winds.

Further into the next work week: The ridging pattern begins to
flatten for the Bay Area and the longer term models and ensembles
see this as a start for a cooling trend. Coastal areas will see more
immediate results in this trend but even the most interior locations
look to fall around 5 degrees below average for the next weekend.

As far as precip is concerned, the ECMWF and a fair portion of its
ensemble families lean on a trough developing after the 220 hour
mark in the forecast and bringing in rain chances. The deterministic
model keeps this trough on the narrow side and prevents rain chances
from becoming widespread. Between the amount of time out this
forecast is and how much of a threading of a needle this output is,
there isn`t too much confidence in the ECMWF verifying. Other models
and ensembles follow more of the common/seasonal flow, which keeps
the Bay Area dry.

&&

.AVIATION...As of 11:19 AM PDT Saturday...For the 18z TAFs. Clear
skies with offshore winds through the period. Gusty northeast
winds above 1000 feet will create some LLWS at area terminals.
Winds near the surface to maintain northwest component for
terminals near the coast and bay with northeast winds for Napa and
Livermore.

Vicinity of KSFO...Clear skies with VFR conditions through the
period. Expecting winds at SFO to maintain more of north/northwest
direction given onshore gradient of around 1 mb locally.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with clear skies through the period.
Some light northwest winds for KMRY and KSNS.

&&

.FIRE WEATHER...As of 12:54 PM PDT Saturday...No planned changes
at this time to the ongoing Red Flag Warning. Its in effect for
zones 507/510/511 through 6 am Monday. It may need to be extended
but will leave as is for now while the pattern/event is still
evolving. So far the event has unfolded as expected but if
anything humidity is lower than expected and gusty offshore winds
slightly stronger than models were indicating. General idea was
this would be a moderate early season event. The northerly
gradient from SFO to Arcata peaked around 10 mb last night (thats
strong) and is producing gale force winds over the ocean and the
strong winds across the hills. The offshore gradient from the
Nevada desert was never forecast to be strong during this event
and is currently around 6 mb, so this is more of a north vs
offshore pattern. Often times the marine layer and associated air
can be tough to scour out, especially when the local SFO to Sac
gradient remains onshore. We are holding onto west winds along the
coast and into the Bay. Despite that humidity values are crashing
to some pretty extreme readings with values from 7-9% at places
like Napa/Concord and Livermore. We often will only see single
digit values down across interior Monterey/San Benito. So we
clearly have dry air in place. Hard not to imagine that the
drought and low soil moisture is at least in part leading to such
low humidity. Values this low help to lower the fuel moisture
readings and can exacerbate fire behavior and spotting/ignition
potential.

For this afternoon expect northerly winds to continue across the
Warning area with a gradual decrease this evening but no big
downward trend in wind speeds. Models do indicate some humidity
recovery overnight but that may be overstated. Given current
trends and expected synoptic pattern not expecting much recovery
above 30% for the current warning locations.

Expect another warm and dry day on Sunday with steady/dry
northerly breezes across the region. Latest fuel analysis shows
values at or near record dryness. In this type of pattern a few
days of hot/dry/breezy weather will rapidly dry out the fine
fuels and impact the ERC values adversely.

On Sunday night there could be another uptick in winds, especially
across the Napa hills with gusts in the 45-55 mph range. Will
continue to monitor the current end time of 6 am Monday. Warning
may need to be extended as Monday looks to continue a similar
pattern of persistent offshore winds, warm temps and low humidity.

Latest trends now show possibly another uptick in winds Monday
night into Tuesday as the next shortwave drops into the Great
Basin. Under this pattern we turn more Northeast/east in direction
with continued drying through the day Tuesday.

Pretty high confidence that pattern turns onshore by Weds as the
ridge flattens and shifts eastward. Pattern looks more seasonable
2nd half of the week into next weekend with onshore winds and near
normal temps. Extended range does suggest a trough beyond mid
month with at least some cooler wx. The biggest impact from these
early May heat/drying events can be to really dry the fuels. Ahead
of the June 2008 lightning event we had record heat in mid May
2008.

&&

.MARINE...as of 01:24 PM PDT Saturday...Stationary surface high
pressure over the eastern Pacific will  result in additional
northwest gales over most of the coastal  waters with afternoon
and evening gusty west to northwest winds  over the bays. Seas
dominated by wind driven waves with a small  northwest swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...Red Flag Warning...CAZ507-510-511
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             GLW...Pt Arena to Pigeon Pt 10-60 nm
             GLW...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...SF Bay until 9 PM
             SCA...Mry Bay until 9 PM

&&

$$

PUBLIC FORECAST: Murdock
AVIATION: RWW
MARINE: RWW
FIRE WEATHER: RWW

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