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

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FXUS66 KMTR 200143

Area Forecast Discussion
National Weather Service San Francisco Bay Area
643 PM PDT Tue Mar 19 2019

.SYNOPSIS...A cooling trend will begin today as cloud cover
increases over the region ahead of an approaching weather system.
Rain showers will develop over the region tonight into Wednesday
with an unsettled pattern likely to continue through late week.
While dry conditions are possible this weekend, unsettled weather
will return heading into next week.

&& of 02:40 PM PDT Tuesday...A negatively tilted
trough continues to slowly approach California from the west
this afternoon. This storm system is aligned nearly parallel with
the coastline from Point Arena through Santa Barbara and has
already brought increased cloud cover, cooler temperatures, and
higher humidities to the region thus far today.

Southerly winds will progressively strengthen ahead of the cold
front through the remainder of the day, becoming gusty (peak
gusts up to 40-50mph in the windiest spots) along the coast and
over the higher terrain with breezy winds elsewhere.

Latest KMUX radar imagery is detecting light reflectivity returns
ahead of the leading edge of a cold frontal boundary approximately
60 miles off of the coast. Despite the close proximity to the
area most models delay the onset of precipitation moving inland
until much later this evening, owing to the negative tilt and
sluggish speed of the parent trough. The latest model output has
decreased the overall precipitation intensity and coverage versus
previous runs. Storm total precipitation will range from 1.25-2.25"
for the higher terrain of the Santa Cruz and Big Sur mountains
while urban areas will generally range from 1/5" to 1/3", with
rain shadowed areas on the lower end of the spectrum. HRRR model
output confines precipitation mostly in broken bands and scattered
showers wrapped closely on the eastern flank of upper low as it
lifts northeastward across the region. Previous model runs showed
somewhat more organization and strength than the current runs and
have reduced overall QPF as a result.

Models generally agree that precipitation should begin to spread
inland between 10pm and midnight tonight, however, a few precursor
showers could arrive in time for this evenings outbound commute.
Rain intensity and coverage should scale upwards after midnight as
the upper low shifts inland and lifts northeastward over the San
Francisco Bay Area. Isolated thunderstorms will be possible
overnight tonight through Wednesday morning due to an increasingly
unstable environment advecting into the region. The overall
weaker model output from the latest runs may increase the
potential for thunderstorms Wednesday as a result increased cloud
breaks leading to more surface heating and further destabilization
of the atmosphere.

A secondary vorticity pulse on the rear flank of the broader
trough will descend into the local area later Wednesday into
Wednesday night and extend the possibility of a few lingering
showers potentially into early Thursday. Confidence is relatively
low regarding how this second disturbance will interact with the
initial disturbance with some models introducing an increased
chance of showers over the higher terrain into Thursday morning.

A transitory high pressure ridge will develop by midday Thursday
allowing for a brief warming/drying trend, however, another storm
system is set to descend into the region from the Gulf of Alaska
by Friday morning. Fridays storm system will move from north to
south and bring the bulk of any precipitation to the North Bay.
Precipitation along the North Bay coastal range could approach
1.00" while the North Bay valleys pick up 1/2 to 2/3". Much less
precipitation is expected south of the Golden Gate, with 1/4 to
1/3" in the immediate SF Bay Shoreline, and progressively less
farther southward.

Drier weather will prevail through most of the weekend with a
slight warming trend into Sunday but a third storm system will
begin to advance on the region early next week. Forecast models
and ensemble members continue to pepper later Monday through early
Wednesday as wet time periods, however, there is still a bit too
much noise to pin down exactly when, where, and how much to


.AVIATION...As of 6:43 PM PDT Tuesday...Increasingly strong and
gusty winds through the evening, low level wind shear developing
nearest the coast with some overlap into the South Bay, possibly
extending to the East Bay and North Bay later this evening.
Showers arriving later in the evening into Wednesday. Terminals
reporting VFR thus far.

An amplifying and negatively tilted 500 mb height trough is approx
200 miles southwest of Monterey county and moving north and will
sweep over the area through the evening and overnight. Showers on
Wednesday. NAM model shows fairly rapid cold air advection through
06z, near 7 Celsius cooling at 925 mb and 850 mb levels and
thermal troughing embedded with the 500 mb height trough indicating
this trough has a fair amount of dynamics and lift with it. Expect
rapidly changing weather conditions at least through the evening.
Clouds looking convective southwest of SLO county, but not seeing
lighting yet; echo tops near 20 thousand feet. There`s a slight
chance of thunderstorms mainly late tonight into Wednesday morning.

Vicinity of KSFO...Increasingly strong and gusty winds through the
evening. Wind flow tending to downslope, dry out any chance of
showers in nearest term. Low level wind shear developing by mid to
late evening and lasting into early Wednesday morning. Showers
arrive overnight. VFR decreasing to MVFR ceilings late tonight
into Wednesday morning. VFR Wednesday with a few showers.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR, increasing winds. Wind flow is
downsloping and drying in lee of the Santa Lucia mountains drying
out any chance of showers in nearest term. Expect rapidly changing
weather through the evening including low level wind shear through
the evening into early Wednesday morning. Showers expected to
arrive overnight into Wednesday. MVFR ceilings in showers. VFR
Wednesday and lingering showers.

&& of 5:18 PM PDT Tuesday...Southerly winds will continue
tonight ahead of an approaching low pressure system and cold front
moving in from the west. Winds will increase overnight and tomorrow
before winds veer to the west to northwest later tomorrow. A long
period west swell will continue into this evening increasing
shoaling on bars and harbor entrances. An additional long period
west to northwest swell will arrive towards the end of the week.


     .Tngt...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...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...SF Bay from 3 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm from 3 AM
             SCA...Pt Arena to Pt Reyes 0-10 nm from 3 AM
             SCA...Rough Bar Advisory for SF Bar from 9 PM
             SCA...Mry Bay




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