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

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FXUS66 KMTR 251551
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
851 AM PDT Mon May 25 2020

.SYNOPSIS...Hot temperatures are forecast for inland areas
through Thursday. Temperatures in coastal areas will be relatively
mild by comparison, but still much warmer than seasonal averages
through midweek. Temperatures will likely peak near the coast on
Tuesday and across inland areas on Tuesday and Wednesday. Near
record to record temperatures are possible, especially for inland
areas Tuesday and Wednesday. A Heat Advisory is in effect for
interior portions of both the San Francisco Bay Area and Central
Coast through Thursday. Temperatures are forecast to cool in all
areas from Friday into next weekend. There is also a potential
for showers across the northern portion of the Bay Area on
Saturday.

&&

.DISCUSSION...as of 08:50 AM PDT Monday...Sunny sky conditions
prevail over the entire region this morning with stratus 80 to 160
miles off of the coast out over the Pacific. Additionally calm
winds are being reported at most regional airports with
temperatures 5 to 10 deg F warmer compare to 24 hours ago. This
said, the advertised warming trend is well underway with
widespread 90s likely across the interior with 70s to lower 80s
near the coast by this afternoon. Look for an afternoon sea-breeze
to develop which will keep coastal areas from warming as much as
those inland. Those with outdoor plans are urged to take necessary
precautions related to the very warm to hot daytime temperatures,
especially for those inland areas. No forecast updates are
anticipated this morning as the forecast remains on track. For
complete details on this week`s heat event, please see the
forecast discussion below.

&&

.PREV DISCUSSION...As of 3:15 AM PDT Monday...Very warm to hot
temperatures for inland areas will be the primary weather impact
over the next four days. The primary forecast challenge is
determining just how warm temps will be and how close to the coast
the heat will extend.

Early morning temperatures are running as much as eight degrees
warmer than 24 hours ago as subsidence warming continues under a
building upper ridge that is currently centered over the northern
California coast. The ACV-SFO surface pressure gradient is nearly
7 mb, more than enough to significantly reduce onshore flow. High
temperatures today are expected to be anywhere from 5 to 10
degrees warmer than yesterday, which will mean widespread 90s for
inland areas, with a few of the hottest inland valley locations
likely to reach triple digits. The SFO-SAC pressure gradient
remains weakly onshore and the latest local WRF model indicates
weak seabreeze winds near the coast for the entire day, except
perhaps a few hours this morning. Thus, temperatures close to the
ocean are forecast to remain mostly in the upper 60s and 70s, with
80s expected close to the bays. Given today`s forecast
temperatures, heat risks are projected to be moderate to locally
high across most inland areas, and low to moderate near the coast.
A Heat Advisory goes into effect for interior portions of our
forecast area at 11 am.

The upper ridge is forecast to strengthen a bit more over
California by Tuesday and surface gradients will remain very weak
onshore. Thus, high temperatures are expected to increase a few
additional degrees in most areas on Tuesday. Heat Risk will
remain elevated across inland areas, with expanding areas of high
risk due to increasing daytime temperatures as well as warmer
nighttime temperatures which will not allow for as much overnight
relief. Because offshore flow is not expected at the surface,
coastal areas should remain in the low to moderate heat risk
category. The Heat Advisory continues for inland areas on Tuesday.

Model consensus indicates that temperatures in coastal areas will
peak on Tuesday and then cool a bit on Wednesday as the upper
ridge axis shifts inland and onshore flow increases slightly.
However the airmass aloft is forecast to continue warming into
Wednesday with 850 mb temps projected to climb to 24-25 deg C.
Thus, inland temperatures will likely be as hot on Wednesday as on
Tuesday, or even slightly warmer. The Heat Advisory will continue
on Wednesday for inland areas.

By Thursday the upper ridge axis is forecast to shift farther to
the east and be centered over Nevada. The airmass will still be
very warm over our area, but onshore surface flow will continue to
gradually increase and the GFS indicates low level flow will begin
to turn to the south near the coast by afternoon. Temperatures
are expected to cool modestly near the coast and in the coastal
valleys. Only the interior valleys and hills will remain hot.
Even if there is slight cooling inland on Thursday, the Heat
Advisory will continue in large part because of the cumulative
effect of several days of heat.

Friday will be a transition day as an upper low approaches from
the southwest and a deep southerly flow develops over our area.
This southerly flow will help transport cooler marine air well
inland and so even inland areas are expected to cool considerably
on Friday.

NBM forecast temperatures are generally warmer than MOS guidance
from today through Thursday. Since MOS guidance often under-
forecasts temperatures during heat events, the warmer NBM temps
are preferred. But on Friday, the NBM temperatures appear to be
much too warm, especially inland, and have thus trended our
forecast temps for Friday closer to MOS guidance.

Beyond this heat event, our next focus will be on the upper low
that is forecast to reach the northern California coast early next
weekend. Shower chances are in the forecast for the northern
portion of our area late Friday night and Saturday. We will also
need to consider the potential for thunderstorms. If thunderstorms
do develop and rainfall is limited, the concern will be for dry
lightning and potential fire starts.

&&

.AVIATION...as of 4:34 AM PDT Monday...For the 12z TAFs. VFR is
forecast for the period due to a compressed marine layer under a
strong upper level ridge.

Vicinity of KSFO...VFR. West wind increasing to 15 to 25 kt in the
afternoon and evening.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...A few patches of low clouds possible over
the Monterey Bay early this morning otherwise VFR. West winds near
10 knots in the afternoon and early evening.

&&

.CLIMATE...

Record highs        May 25            May 26            May 27
-----------------------------------------------------------------
Santa Rosa          96/1951           96/1974           94/1984
Kentfield           92/1992           99/1951           99/1919
Napa                98/1951           98/1896          102/1984
Richmond            92/1951           87/1996           98/1984
San Francisco DT    87/1951           91/1896           85/1933
SFO Airport         87/1951           89/1974           97/1984
Redwood City        99/1982           98/1974           98/1984
Half Moon Bay       78/1975           78/1951           72/1982
Oakland DT          90/1975           94/1974          100/1984
San Jose            95/1982           95/1951          101/1984
Gilroy              98/1982           98/1979          101/1974
Santa Cruz          95/1896           94/1896           93/1984
Salinas             85/1984           94/1974           84/2003
King City          102/1951          104/1974          104/1984

&&

.MARINE...as of 08:50 AM PDT Monday...Weakening high pressure
over the eastern Pacific will allow northwesterly winds to ease
across much of the coastal waters. Locally breezy conditions will
persist over the northern outer waters through much of the week.
These winds will generate steep fresh swell resulting in hazardous
seas conditions for smaller vessels. Mixed seas will continue
with a short period northwest swell and a longer period southwest
swell. A longer period northwest swell will arrive later this
week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Heat Advisory...CAZ506-507-510>513-516>518-528
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm from 3 PM

&&

$$

PUBLIC FORECAST: RGass/Dykema
AVIATION: Canepa
MARINE: AS

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