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NWS Forecast Area Discussion - San Francisco Area

598
FXUS66 KMTR 201624
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
924 AM PDT Fri Jul 20 2018

.SYNOPSIS...Monsoon moisture is expected to spread from south to
north across the region later today, resulting in a slight chance
of high-based showers and thunderstorms. Meanwhile, seasonably
warm daytime temperatures will persist with marine air keeping
conditions cooler near the coast. A more robust warming trend is
then possible next week, especially inland areas, as high
pressure builds over the region.

&&

.DISCUSSION...as of 09:18 AM PDT Friday...Low clouds have quickly
dissipated over most inland areas in response to the shallow
marine layer and surface heating. In addition, do see a bit more
high cloud development with even an isolated high-based shower
that moved over the Delta earlier this morning. Given the latest
cloud development, forecast moisture and instability aloft we`ll
keep in a slight chance of convection across the region today.
While we are only expecting isolated activity at best, we`ll
continue to closely monitor radar and satellite trends through the
day. Otherwise, no changes are anticipated at this time.

&&

.PREV DISCUSSION...As of 2:52 AM PDT Friday...The marine layer
depth appears to be a little more than yesterday at this time with
the Fort Ord profiler indicating a depth of about 1600 feet.
Surface pressure gradients are similarly onshore with 2.8 mb
between SFO and SAC. Current IR satellite imagery shows coastal
stratus has once again spread into inland valleys, although inland
extent does not appear to be as extensive as yesterday morning. A
few surface observations near the coast are indicating patchy
fog. Expect visibilities to lower by sunrise in favored areas like
MRY and STS. High temps today are expected to be mainly similar
to those on Thursday with 60s to lower 70s along the coast, to the
80s to mid 90s well inland. No significant changes to this
temperature pattern are expected through the weekend and into
early next week.

The primary forecast concern in the short-term continues to be
monsoon moisture spreading into California from the southeast.
Water vapor imagery shows most of the moisture flowing into the Great
Basin as well as to our east, over the Eastern Pacific. Models,
at least the NAM12, indicate increasing mid-level moisture today
and tonight over California, thus have left shower/thunderstorm
chances unchanged with slight chance pops spreading north during
the day today.

Drier southwest flow aloft will develop during the weekend, which
will bring an end to precipitation chances. Then, a large upper
high over southwestern United States will build and move slowly
westward by the end of next week. This will bring warmer
temperatures to inland areas by then.

&&

.AVIATION...As of 4:42 AM PDT Friday...LIFR/IFR reported on the
coast, VFR inland; overall marine based stratus and fog coverage
is a little less than Thu/Wed mornings. A limited inland stratus
and fog intrusion in the Bay Area and north Central Coast should
help with bringing VFR back to areas today that have stratus, fog
and any patchy drizzle this morning. Patchy LIFR/IFR is possible
tonight, but coverage may actually be less as southerly flow aloft
becomes a bit more focused ahead of an upper low to the southwest.

Vicinity of KSFO...Good news, given the present marine layer depth
which is compressed down to about 1,200 feet, light westerly wind,
it appears there is a good chance VFR will continue this morning.
Diurnal warming and mixing having more of a foothold on an earlier
start this morning should help mix out stratus and fog presently
entering the Golden Gate to vicinity KOAK airport. Thus if surface
wind at KSFO does shift to light NE briefly this morning a limited
amount of stratus should be advected toward KSFO likely keeping sky
cover few-sct at most /VFR/. Westerly wind gusts to 20-25 knots
possible this afternoon and early evening.

SFO Bridge Approach...San Mateo Bridge is VFR, good chances VFR
holds through this morning into the day. Otherwise similar to KSFO.

Monterey Bay Terminals...LIFR/IFR cigs, patchy drizzle possible
early this morning. Clearing to VFR by late morning, more focused
southerly flow aloft later today into tonight may help keep VFR
going longer into this evening.

&&

.MARINE...as of 09:18 AM PDT Friday...A weak disturbance over the
coastal waters will result in gusty northerly winds over the
northern outer waters and lighter variable to southeasterly winds
elsewhere. Northwesterly winds will redevelop over the coastal
waters early next week. Gusty onshore winds will redevelop north
of the Bay Bridge to the Delta later today and again later
Saturday. A long period southwesterly swell arrives by early next
week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay from 2 PM

&&

$$

PUBLIC FORECAST: RGass/Sims
AVIATION: Canepa
MARINE: MM

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NWS MTR Office Area Forecast Discussion

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