Flood and Hail in California, January 25, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
946 AM PST Thu Jan 25 2018

.SYNOPSIS...Scattered morning showers will become more numerous
by this afternoon with a slight chance of thunderstorms for the
North Bay during the afternoon. Dry weather is forecast beginning
Friday, through the weekend, and into next week.

&&

.DISCUSSION...as of 8:45 AM PST Thursday... An unstable post
frontal environment is bringing convection in the form of weakly
developed rain showers across the region this morning. Short term
model data suggests these cells will become enhanced by daytime
insolation and become more organized and vertically developed by
this afternoon into evening. Isolated thunderstorms may also be
possible over the North Bay and over the outer waters later today
as the convective potential reaches a peak. Additional rainfall
accumulations will be highly dependent upon the trajectory of the
showers and thunderstorms. On a general scale, generally a quarter
to half an inch of additional accumulations are possible for
areas that fall under these rain showers.

Rain showers taper off later tonight. Clearing skies and cold air
advection aloft will result in cold overnight temperatures for the
area, particularly in extreme inland valley locations. Then,
medium and long term forecast models show a return of dry weather
tomorrow through the next week or more as high pressure builds in
offshore of Northern California. This ridge deflects any incoming
storm system to the north into the PACNW/Southern BC Canada.  As
a result, our water year precipitation deficit will continue into
February, despite January generally running around 125-150% so far
for most locations.

&&

.PREVIOUS DISCUSSION...as of 3:08 AM PST Thursday...Radar scans
are showing that the main rain band has exited our area to the
south and east, and now some showers are still moving onshore.
Shower activity is expected to increase by this afternoon as a
short-wave trough moves through. The best chance for showers, and
even a slight chance of thunderstorms, will be in the North Bay
this afternoon. Shower activity to persist into the evening,
especially near the coast. Highs today will be cool, with 50s most
areas.

High pressure will rebuild over the Eastern Pacific on Friday with
drying conditions returning to the district. The upper ridge will
build over the state and high temperatures will warm into the 60s
and even lower to middle 70s in inland areas by Sunday and Monday.
The dry and mild weather is expected to persist through next week
with even longer-range climate outlooks keeping these trends
going through the first week of February.

&&

.AVIATION...as of 9:46 AM PST Thursday for 18Z TAFs. Dense fog at
KSTS should lift by 19-20z. Latest KMUX radar scan shows
scattered showers occuring and developing across the district,
with a general movement from NW to SE. These showers are
convective in nature and are briefly dropping moderate to heavy
rainfall and possibly small hail in heavier showers. Scattered
showers will continue through the morning and afternoon, becoming
more isolated this evening. There is still a slight chance of
isolated thunderstorms this afternoon, particularly for the North
Bay, but confidence and likelihood was too low to include in TAF.
This will continued to be closely monitored through the day. VFR
expected today, except during showers, where VSBY and CIGs may
drop down to MVFR/IFR briefly.

Vicinity of KSFO...Generally expect VFR conditions to prevail,
with MVFR ceilings associated with scattered showers possible
through the day. Westerly winds increase to around 10 to 15 kt in
the afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR conditions expected, though could
see tempo MVFR ceilings in rain showers. Generally light westerly
winds, around 10 kt this afternoon.

&&

.MARINE...as of 09:27 AM PST Thursday...There is a slight chance
of thunderstorms today over the northern coastal waters as colder,
unstable air moves ushers into the  area. Otherwise, shower
chances will continue for all water zones through this evening.
Heavier showers may contain small hail and gusty winds. Another
long- period swell will arrive through the day,  bringing
hazardous seas in the form of increasing swell heights.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Reyes to Pigeon Pt 0-10 nm from 3 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm from 3 PM
             SCA...Rough Bar Advisory for SF Bar from 3 PM
             SCA...SF Bay from 3 PM

&&

PUBLIC FORECAST: DRP
AVIATION/MARINE: BAM

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