Technical NOAA Weather Discussion

FXUS66 KMTR 210552

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1052 PM PDT Tue Mar 20 2018

.SYNOPSIS...The first in a series of storm systems is now
exiting to the southeast. Scattered showers will continue across
the area tonight and Wednesday, along with possible afternoon and
evening thunderstorms tomorrow primarily inland areas. The next
storm system will bring another round of widespread rainfall
Wednesday night into Thursday. Showers then look to diminish from
the north Thursday afternoon and evening, but with unsettled
weather conditions and cooler temperatures for the end of the
week and into the weekend. By the end of the weekend and as we
head into next week, dry conditions look to prevail districtwide.

&& of 09:20 PM PDT Tuesday...Radar and satellite
imagery along with surface obs show the main frontal precip band
now exiting our area to the southeast. Deep, mild and moist flow
out of the southwest persists however, and thus the continuing
scattered showers tonight (and more persistent rain on the
upslope sides of the coastal hills). Largest rainfall totals noted
thus far are around 1.5 inches, in the Santa Lucia Mountains
above Big Sur. Our official site in Downtown San Francisco has
received 0.35 inches, and amounts taper down to under 0.10 inches
in rain-shadowed valley locations like San Jose and Morgan Hill.
Also of note were some very strong winds that developed late this
afternoon and early evening in the northern Salinas Valley and
Hollister areas. The ASOS at Salinas Airport reported a peak gust
of 51 mph at 4:59 pm PDT.

New 00Z model output just coming in and remains basically
consistent with present forecasts. Scattered showers will continue
overnight and through the day on Wednesday, with thunderstorms
possible in the afternoon especially inland. Then the next more
focused plume of subtropical moisture streams rapidly in from the
southwest Wednesday evening and Thursday. Continues to look like
focus and heaviest precip will be south of our area, but this
appears both stronger and wetter than today`s and moderate to
locally heavy rain is possible, especially in the Big Sur area. If
rates and totals trend towards higher-end possibilities, will
raise concern over possible localized flooding due to saturated
ground from today`s precip and additional heavy rain. The greatest
potential for small stream flooding and mudslides, rock slides
and debris flows will be along the Big Sur coast and adjoining
coastal slopes.

Southerly winds will again pick up as this it approaches and moves
through, but presently look to remain below wind advisory levels.
Rainfall will diminish gradually during the day Thursday from the
northwest as a cold front begins to move through from the
northwest. Thunderstorm chances will return Thursday afternoon,
primarily in the southern half of the CWA.

Rainfall totals with this next system look to range from as much
as 4-6 inches wettest portions of the southern Big Sur coastal
mountains to 2.5-3 inches wettest locations in the Santa Cruz and
North Bay Mountains. Presently expect lower-elevation totals
around the Bays to generally be in the 0.5 to 1.0 inch range.

Dry conditions appear on tap districtwide Thursday night as the
subtropical moisture plume shifts inland and drier westerly flow
aloft develops. But then unsettled weather conditions quickly
return, this time from the north, beginning Friday as the first
in a series of cool upper level disturbances moves in from the
Gulf of Alaska. Showers will be possible at times through
Saturday night, along with unseasonably cool temperatures.

On Sunday however, the large scale upper level trough axis looks
to finally begin shifting inland with large scale upper level
ridge development over the west coast and dry weather for us as we
head into next week.


.AVIATION...As of 10:45 PM PDT Tuesday...Scattered showers still
moving into the SFO Bay Area this evening with MVFR cigs. Models
indicate shower activity decreasing after 14Z with cigs rising
into VFR. Daytime heating will provide a little better chance of
showers and even possibly a thunderstorm during the day Wednesday.
Nothing organized is expected until the cold front arrives late
Wednesday night.

East to southeast winds will through Thursday. Surface winds have
been 10 kt or less except for gusty winds to 30 kt in the Salinas
Valley and 25 kt in the Santa Clara Valley that will affect SJC
and SNS. Winds up to 35 kt above 925 mb could produce LLWS
through early Wednesday morning.

Vicinity of KSFO...MVFR conditions expected through Wednesday as
showers continue to move through. Shower activity decreasing after
15Z with cigs rising to 4000-5000 ft. Confidence is low as cigs
could remain lower. Stronger winds above the surface may cause
LLWS through 15Z. Daytime heating will allow more cumulus clouds
to form in the afternoon with a slightly higher chance for
showers. Thunderstorms possible but most likely will be east of
the area.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR for cigs remaining above 5000 ft.
Low clouds could form along the coast overnight and bring MVFR
cigs to MRY and SNS but confidence is low on this happening.
Shower activity should be limited to the Big Sur coast and the SFO
Bay Area.

&& of 10:38 PM PDT Tuesday...Southerly winds will
increase Wednesday as a low pressure system associated with a
frontal system approaches the area. These winds will generate
southerly swell which will mix with a northwest swell to creaste
rough and hazardous seas through at least Thursday. Winds will
switch to northwest Thursday morning after  the front passes.
There is a slight chance of thunderstorms Wednesday and Thursday
afternoon mainly near land.


     .Tngt...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
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm




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Prepared by Weather at: Wed Mar 21 00:30:03 PDT 2018
From the National Weather Service