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

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FXUS66 KMTR 200554
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1054 PM PDT Tue Mar 19 2019

.SYNOPSIS...Rain showers will develop overnight into Wednesday
with a noted cooling trend. Isolated showers possible Thursday but
most places stay dry. Brief high pressure on Friday will keep
things dry. Next front passes through Friday night into early
Saturday with a period of light rain. Dry conditions Sunday with
rain chances returning later Monday through the middle of next
week.

&&

.DISCUSSION...as of 9:28 PM PDT Tuesday...Disorganized upper low
is starting to bring some light rain bands onshore along the
Central Coast. Would like to see some hints of cold air in the IR
satellite but not seeing those signals right now. Not seeing any
bucket tips yet but a few light showers will become likely over
the Monterey Bay region shortly. Latest model trends seem to be
trending a little drier with incoming rain chances. Nonetheless
expect most areas to receive some light rainfall overnight into
Weds morning. Enough to impact the commute with wet roads as the
core of the upper low looks to rotate over the heart of the Bay
Area early Weds morning. The air aloft will be much cooler which
in combination with cloud cover will lead to a noted cooling trend
for many places with temps only in the upper 50s and 60s for Weds
afternoon.

Trend with secondary shortwave later Weds night into Thursday has
been to stay offshore so only isolated precip chances into
Thursday and most likely focused Central Coast.

Higher confidence for a brief drying and warming trend Friday as
shortwave ridging develops. However, next front will be fast
approaching with some light rain chances Friday night into early
Saturday. Looks dry Saturday afternoon into Sunday with another
chance of rain later Monday into Tuesday.

&&

.PREV DISCUSSION...as of 2:40 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
expect.

&&

.AVIATION...As of 10:54 PM PDT Tuesday...Increasingly strong and
gusty winds through the evening, low level wind shear developing
nearest the coast with overlap extending north and east across the
remaining cwa 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
100 miles southwest of Monterey county and moving north and will
sweep over the area through the evening and overnight. NAM model
shows fairly rapid cold air advection, 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
through the evening into the overnight hours. No lightning seen yet
in our cwa, but there`s a slight chance of thunderstorms mainly late
tonight into Wednesday morning.

Vicinity of KSFO...Increasingly strong and gusty winds through
tonight into early Wednesday morning. Low level wind shear through
15z Wednesday. Showers arrive overnight, some heavy. VFR-MVFR late
tonight into Wednesday morning. VFR Wednesday with a few showers,
gusty winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR-MVFR, low level wind shear otherwise
winds occasionally reaching to the ground via momentum transfer.
Strong and gusty SE winds in the Salinas Valley into Wednesday
morning. Showers arriving overnight into Wednesday morning, heavy
at times. Lingering showers and VFR Wednesday.

&&

.MARINE...as of 9:06 PM PDT Tuesday...Gusty southerly winds
continuing tonight ahead of an approaching low pressure system and
cold front moving in from the west. Winds will shift back to the
west and northwest later tomorrow. A long period west swell
gradually diminishes late tonight and Wednesday. An additional
long period west to northwest swell will arrive towards the end of
the week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .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
             SCA...Mry Bay

&&

$$

PUBLIC FORECAST: RWW
AVIATION/MARINE: Canepa

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