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

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000
FXUS66 KMTR 130008
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
508 PM PDT Wed Aug 12 2020

.SYNOPSIS...A warming trend will continue through late week as
high pressure builds toward the coast. Some tropical moisture
arriving from the south will bring a chance for a few showers and
thunderstorms Thursday and Friday. By Friday and moreso over the
weekend hot weather returns to inland areas with triple digit heat
likely continuing into early next week while a shallow marine
layer persists along the coast.

&&

.DISCUSSION...as of 2:00 PM PDT Wednesday...Fort Ord Profiler put
the depth of the marine layer this afternoon around 1500 ft, with
most of the low stratus hugging the coast. Of interesting note, a
notable temperature gradient is evident across San Francisco
County, with the west side of the county running 5-7 degrees
cooler than than east side of the county, owing to the low stratus
deck which has been sitting over the west side of the county all
day.

The axis of the longwave trough is currently over BC, with the
PAC NW on the divergent side. To our south, a shortwave trough is
expected to continue moving towards our CWA. Tropical storm Elida
is due west of Baja and gradually weakening as it moves to the
northwest. Expecting its remnants to be absorbed by the shortwave
and advected northward into our CWA Thursday through Friday.
Model guidance is currently suggesting that the tropical moisture
will move into our southern CWA sometime around late Thursday
morning. Theta-e tongue can be observed on the NAM-12km runs for
today, while Total Totals >30 are apparent on both the GFS and
ECMWF runs over southern Monterey County and fairly close to the
Delta. Not expecting widespread thunderstorms and showers, nor is
there a significant probability for them Thursday and Friday.
Nevertheless, should any convection develop, expect the most
likely areas for their development to be down in Southern
Monterey/San Benito counties on Thursday. Moreover, PWAT values
on both the GFS and ECMWF runs today stay relatively over an inch
across that area, suggesting that if any convection does develop
then we should expect for it to be wet convection. Nevertheless,
going to have to keep an eye for lightning. Theta-e values
increase through Friday across the central part of the CWA as the
tropical moisture continues tracking northward. As such, the
slight chance for possible showers and thunderstorms will move
north and westward on Friday. NAM- 12km runs have kept most of
that risk offshore, however, so currently keeping inland chances
fairly low.

Friday onwards, expecting the warm up across the region to really
get going as the high over the Desert Southwest continues to
build. Models suggesting heights >590dm 500hPa across most of our
eastern CWA, while 850hPa temps are expected to be in the upper 20
C range. Models have continued to trend towards this setup, and if
it continues to trend this way, then high confidence will exist
for widespread 90s and low 100s to be expected across the
interior this weekend, with some isolated locations like Pinnacles
and Lake Berryessa potentially getting into the 100 to 108 degree
range. Not much in the way of relief is expected through the
weekend, with poor humidity recoveries and hot conditions to
influence most of our CWA. Sharp temp gradients are expected
between the coast and areas just a few miles inland this weekend
as a result. Even along the coast, expect weekend max temps in the
mid-70s and even the low 80s. All that being said and after
coordinating with neighboring offices will maintain an Excessive
Heat Watch beginning Friday afternoon continuing through Sunday
evening for interior East Bay, Santa Clara Valley (recently added),
and Southern interior of Monterey/San Benito.

&&

.AVIATION....as of 5:08 PM PDT Wednesday...for 00z TAFs. Marine
layer has compressed to just over 1000 feet. This will keep low
clouds out of SF Bay for this evening but shallow cigs will move
into Monterey Bay terminals before sunset. Given the shallow
marine layer, northerly gradient and lighter onshore winds will
not be expecting stratus to be as widespread Thursday morning.
Should be an early mix out for any terminals that get cigs due to
shallow nature. Will be expecting increasing mid and high clouds
during the day Thursday, impacting the Central Coast first and
then spreading northward later into Thursday evening.

Vicinity of KSFO...VFR this evening with no cigs forecast
overnight. Any patchy clouds Thursday morning should mix out by
16z with some increasing high clouds by afternoon. Light afternoon
seabreeze on Thursday.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Low cigs to impact KSNS shortly and then
KMRY overnight. Shallow cloud deck should mix out by 16z Thursday.

&&

.MARINE...as of 3:00 PM PDT Wednesday...Strengthening northwest
winds across the waters, especially north of Point Reyes where
hazardous conditions for small craft exist as a result of the
winds and seas. In addition, locally gusty onshore winds of up to
around 25kt are expected on the south and east sides of Angel
Island in the Northern San Francisco Bay this afternoon. Northwest
winds decrease later in the week. Seas will remain mixed with a
short period northwest swell and a longer period southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM
             SCA...SF Bay until 9 PM

&&

$$

PUBLIC FORECAST: RGass
AVIATION: RWW
MARINE: DRP

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