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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 181034
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
334 AM PDT Sat Sep 18 2021

.SYNOPSIS...Increasing clouds and cool today as an early season
front approaches the North Bay with some light rain showers. The
front will weaken and pass through the Bay Area overnight with
some light rain showers, mainly near the coast. Warming and drying
trend starts Sunday afternoon with offshore winds developing
Sunday night. Offshore winds along with lowering humidity and
warm temperatures Monday and Tuesday will elevate fire weather
concerns. Continued seasonably warm and dry through next week.

&&

.DISCUSSION...as of 3:15 AM PDT Saturday...We`ve got some
interesting weather to focus on for the next several days. In the
short term mostly cloudy skies across much of the region as a
combination of marine clouds has pushed inland while mid and high
level clouds associated with incoming front are starting to reach
the North Bay. The frontal boundary will bring significant rain to
Oregon and far Northwest California which will help immensely
with the McCash, Monument and River Complex fires that have been
burning since a late July lightning event. IR satellite shows the
front pushing into Oregon at this time with TPW values well in
excess of 1.5 inches. Looking at the atmospheric river tools we
see very high integrated vapor transport values as the boundary
reaches Cape Mendocino later today but then rapidly dissipates as
the moisture plume falls apart over the North Bay and south of the
Golden Gate.

In terms of sensible weather all areas will see much cooler
temperatures today due to a combination of cloud cover and cooling
aloft with 60s around the bay and mainly 70s inland. Dewpoint
values will be near 60 degrees this afternoon across the Bay Area
so it`ll feel fairly muggy by Bay Area standards. Any winds off
the ocean will be moderated by our mild sea surface temps in the
lower 60s as well. As the moisture boundary reaches the North Bay
the mesoscale models suggest there`ll be enough moisture and lift
to generate some light rain showers or sprinkles today, especially
for coastal Sonoma county.

For tonight the surface front will move into the North Bay and we
do expect some legitimate light rain for northwest Sonoma county
where values could locally approach 0.25-0.50 for the coastal
Sonoma hills. Even coastal Marin and around Mt Tamalpais we could
see 0.10-0.20 inches of rainfall by Sunday morning. Cross sections
show the moisture is confined mainly below 850 mb with the main
forcing from southwesterly upslope flow perpendicular to the
coastal range, meaning little or no rain for interior Napa county.
As the boundary passes over the Golden Gate tonight we expect a
few tips in the rain bucket from San Francisco proper down the
peninsula but with little or no precip for the East/South Bay
region. The front will completely wash out by the time it reaches
the Monterey Bay region with only a hundredth or so by Sunday
morning.

Plenty of lingering clouds and moisture Sunday morning then rapid
warming by afternoon as high pressure begins to build.

The next focus will then be on developing offshore winds. A strong
pressure gradient sets up over the Sierra on Sunday night that
will drive some moderate northeast offshore winds across the North
and East Bay hills late Sunday night into Monday. A Fire Weather
Watch starts at 11 pm Sunday night for the North/East Bay hills
with the models showing an initial burst of winds above the
ridgetops around this time. The airmass looks to stay somewhat
moist behind the weekend weather system so the initial shot of
offshore winds will start to rapidly dry the airmass. Will also
need to see just how much or little rain falls over the next 24
hours before properly assessing the upgrade of the Fire Weather
Watch.

On Monday expect rapid warming and drying as north winds work
their way down the Sacramento valley. North and East Bay valleys
should rapidly warm into the upper 80s and 90s or about 20 degrees
warmer than todays forecast. RH values will lower significantly
down into the teens by Monday afternoon. Little or no humidity
recovery Monday night with light but persistent offshore winds
that will lead to another unseasonably hot and dry day on Tuesday.

A weak weather system may pass to our north around midweek with
another potential offshore wind event late next week as high
pressure builds into next weekend keeping temperatures
unseasonably warm and dry. Enjoy the cool temps and any precip
that falls the next 24 hours as things look to revert back to
seasonably warm and dry through the rest of Sept.

&&

.AVIATION...as of 10:55 PM PDT Friday...For the 06z TAFs. Mostly
MVFR conditions tonight for all terminals. Chances for IFR
ceilings at KSTS/KAPC from 12Z-18Z. Moderate confidence on -DZ/-RA
for KSTS/KAPC as well stating around 12Z this morning, reducing
visibilities to around 5SM. As a FROPA approaches the North Bay
tomorrow afternoon, intermittent -DZ will continue for KSTS
through the period, becoming -RA/RA towards the end of the period
(02-06Z) when the FROPA is forecasted to move southward down the
central CA coast. Slight chance for -DZ at KSFO/KOAK this morning
from 12-15Z, with visibilities remaining around 5-6SM. MVFR
ceilings will hang around the Bay Area, inland, and Monterey
terminals until around 18-20Z, with SCT-BKN 050-070 skies in the
afternoon and evening hours. MVFR ceilings should remain around
the North Bay terminals through the period. Winds will remain
light through the period as the FROPA itself is weaker.
Southwesterly winds for coastal terminals Saturday afternoon,
veering to westerly by the end of the period.

Vicinity of KSFO...MVFR ceiling then developing by 08z, light
drizzle reducing surface visibility 11z-18z Saturday morning.
Drizzle could be borderline light rain, expect wet runways. A
steady deepening of the marine layer will lift ceilings to MVFR-
VFR by late Saturday morning and afternoon. Model output leaning
toward more light rain nearing KSFO just after 06z (Saturday
evening).

KSFO Bridge Approach...Similar to KSFO.

Monterey Bay...MVFR ceilings tonight and Saturday morning, light
drizzle chances through 18Z this morning. Ceilings lifting to
MVFR- VFR by late Saturday morning and afternoon. Skies fill in
with MVFR ceilings by 02-03Z.

&&

.FIRE WEATHER...As of 3:34 AM PDT Saturday...Fire Weather Watch
in effect for North and East Bay hills from 11 pm Sunday night
through 11 am Tuesday. As noted above in the short term we do
expect some wetting rains for coastal Sonoma county. Wetting rains
for Napa and East Bay hills look unlikely at this time though a
few hundredths is certainly possible. Did look at the updated
fuels charts that literally just updated. For the North Bay the
ERC readings are down from record highs but still running near the
97th percentile (though thats obviously not accounting for any
precip we receive the next 24 hours). The Bay Area marine zones
that are continuously impacted by the marine layer and thus higher
humidity are actually now running below normal for this time of
year. Finally the Diablo/Santa Cruz mountains are around the 90th
percentile but will likely drop slightly over the next 24 hours.

Anyway, we`ll be monitoring rainfall totals closely through Sunday
morning. High confidence for warming and drying to start Sunday
afternoon but the airmass being replaced is very moist as noted by
the high PW values. Watch start time of 11 pm Sunday still looks
good with latest NAM model showing an abrupt increase in northeast
wind around 925 mb between 06-09z. Given the stability profile
would expect most of the winds to stay at or above 2000 feet
with gusts generally in the 30-40 mph range (50 mph Mt Saint
Helena). RH values look to stay above 30% into Monday morning
which may be a limiting factor. In addition should wetting rains
occur across much of the North Bay that could put us out of
Red Flag criteria for 24 hours after the rain ends.

High confidence for rapid warming and drying Monday as temps will
quickly rebound into the upper 80s and 90s with humidity dropping
into the teens as persistent but light offshore winds continue.
The entire Bay Area will dry out Monday night into Tuesday but
wind speeds look light as the strongest pressure gradient stays
over the northern Sierra.

General trend looks to be above average temps and continued dry
next week though the models are grappling with a potential cut-
off low. The more likely scenario is a transient midweek system
passes to our north with another potential offshore wind event
late in the week followed by building high pressure and
unseasonably warm and dry wx through the end of the month.

&&

.MARINE...as of 3:00 AM PDT Saturday...Breezy to light west winds
ahead of an approaching frontal system today. Gusty winds down
the Big Sur Coast this afternoon lasting through tonight. These
gusts could create hazardous seas for smaller vessels. Light rain
is expected over the waters as front passes through from north to
south starting this afternoon. Stronger northwest winds establish
behind the front starting Sunday afternoon across most of the
waters. Seas remain mixed with a short period northwest swell and
a longer period weak southerly swell.

&&


.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM

&&

$$

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

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