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

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FXUS66 KMTR 250525
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1025 PM PDT Mon Sep 24 2018

.SYNOPSIS...Warm high pressure will last at least through
Wednesday and possibly into Thursday. Cooling trend starts Friday
into Saturday as an upper low approaches. There will be a slight
chance for some showers over the North Bay on Saturday.

&&

.DISCUSSION...as of 9:22 PM PDT Monday...A broad ridge of high
pressure continues to dominates the region resulting in light
offshore flow and warmer temperatures across the northern half of
the district. This strong high pressure system and a relatively
shallow marine layer allowed temperatures to warm to near or
slightly above normal today. Nonetheless, patchy coastal stratus
and persistent onshore flow kept temperatures cool along the coast
and over the southern interior. Most coastal locations reported
little to no change while southern interior locations reported as
much as 3 to 5 degrees of cooling. Highs reached into the 60s
along the coast, 70s to mid 80s around the bays and mid 80s to mid
90s over the warmest inland areas as well as over the hills.

Southerly winds along the coast continue to advect stratus
northward that this hour. The gradient from SMX to SFO remains
over 2 MB. Latest satellite imagery shows stratus has already
reached as far north as Point Arena and continues to move north.
While the relatively shallow marine layer and light offshore flow
will keep the low clouds confined mainly to the coast, the
southerly flow will push low clouds over areas that typically stay
clear and will clear out areas that are typically prone to
clouds. Downsloping over the hills will keep areas such as
Monterey and San Francisco International Airport mainly clear
while areas such as Santa Cruz will remain socked in through the
night.

The forecast remains on track. No update at this time.

From previous discussion...Synoptically speaking surface high
pressure will continue to build across the Great Basin and a weak
trough of low pressure remains at the coast. Aloft we have a Rex
Block type set up with high pressure off the PacNW coast and low
pressure off the CA coast. The overall pattern will not change
drastically through the middle of the week.

Now for the details, given the inland high pressure and coastal
low pressure offshore flow will increase across the region through
tonight. Winds will be strongest over the N Bay Mts and E Bay
Hills with gusts 25-35 mph. The SFO-WMC gradient is projected to
be 10-12mb, which is decent, but not a major offshore flow event.
Offshore flow will keep moderate to poor relative humidity
recoveries in place through mid week as well. The combination of
wind, low relative humidity and dry fuels will result in critical
fire weather conditions. A red flag warning remains in effect
through Tuesday 5 PM. See fire section below for more details.

Temperatures will continue to warm across the region with the
offshore flow set up. Daytime temperatures will warm in the 80s
and 90s across the interior and 60s and 70s near the coast. Given
the lack of a real strong offshore flow component some lingering
night and morning coastal low clouds will remain. Temperatures
near the coast will be kept in check due to clouds. Overnight
thermal belts will be active with mild temps in the hills.

The offshore flow pattern begins to weak by Thursday as upper low
to the west begins to finally move eastward. Expect increased
onshore flow, a return of the marine layer and cooler
temperatures. Latest medium range models continue to advertise
some light showers possible over the North Bay on Saturday.
Introduce a small chance (15-20%) chance for some showers. The
moisture source could be interesting and needs some watching
between now and then. Early indications point to a typhoon
pushing northward through near Japan and drifting northward across
the Pacific. Models diverge on early next week with the GFS dry
and the ECMWF/CAN bringing rain to NorCal. At this point, forecast
will be dry, but watched closely for next Monday.

&&

.AVIATION...As of 10:25 PM PDT Monday...For 06z tafs. Stratus has
begun to fill in along the coast per GOES East satellite imagery.
Given the location of redevelopment, have added back in a tempo
BKN group for KOAK early tomorrow morning. Otherwise not much has
changed. Patchy dense fog for the Monterey Bay taf sites along
with IFR/LIFR cigs with VFR conditions elsewhere. Some patchy fog
also possible in the North Bay in the early morning.

Vicinity of KSFO...VFR, may see some SCT low cigs late
tonight/early tomorrow morning.

SFO Bridge Approach...VFR.

Monterey Bay Terminals...Satellite imagery showing stratus over
the Peninsula and moving down the Salinas Valley. IFR/LIFR cigs
overnight with patchy dense fog.

&&

.FIRE WEATHER...as of 1:25 PM PDT Monday...The biggest story
continues to be the current Red Flag Warning in effect over the N
Bay Mts and E Bay Hills. Last night many higher elevations around
the Bay Area recorded some impressive single digit to lower teen
relative humidity. Winds were not overly strong, but still some
breezy conditions over the highest peaks. Tonight will be a
different story as the offshore flow gradient increases. Overnight
humidity recovery will occur early tonight and then decrease
through sunrise as NE winds increase. The peak period of concern
will be late tonight through early tomorrow, which is common in
these scenarios, with the lowest humidity and highest winds. Winds
of 25-35 mph and RH <30% seems reasonable with most critical
areas - highest peaks of the East Bay and NE Sonoma and N Napa
counties. It should also be noted that other peaks outside of the
Red Flag Warning will see very low relative humidity, but weaker
winds. Those locations will be near critical fire weather
conditions. Winds will decrease through the day on Tuesday and
Tuesday night. That being said, despite the weaker winds relative
humidity remains low keeping heightened fire weather concerns in
place through Wednesday morning.

&&

.MARINE...as of 8:24 PM PDT Monday... Split directional flow with
more southerly winds along the immediate coast and west to
northwest winds over the outer waters through much of tomorrow.
Generally light winds will continue through the rest of the week
across the coastal waters. Light mixed swell, with a predominate
short period northwest swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

$$

PUBLIC FORECAST: CW
AVIATION: AS
MARINE: AS
FIRE WEATHER: MM

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