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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 231151
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
451 AM PDT Fri Jun 23 2017

.SYNOPSIS...Cooling trend starts today for most areas as onshore
flow returns. The far interior valleys will remain seasonably
warm through this evening. The overall trend this weekend and into
next week will be a return to more seasonable temperatures along
with more extensive night and morning low clouds followed by sunny
afternoons.

&&

.DISCUSSION...as of 3:34 AM PDT Friday...All signs point to a
significant cooling trend today and for the coastal valleys these
transitions are rarely gradual. The marine layer has deepened to
around 1200 feet around Monterey Bay and trending deeper with a
southerly surface wind component as the southerly wind reversal is
now approaching Cape Mendocino. 24 hour trends for the hills show
as much as 10-20 degrees of cooling at this hour due to a
deepening marine layer and onshore winds. The gradients from
Arcata to SFO have gone flat while the SFO to Sac is 3.3 mb
onshore at this early hour. Wind out in the Delta are already
gusting to 25 kt as cooler marine air oozes inland. Given all
this will maintain the current heat advisories for some of the far
interior zones and hills. This is mainly being done for
consistency of message and lingering heat impacts for heat
sensitive populations. Its expected that any remaining heat
headlines for our office will be allowed to expire by 9 pm this
evening if not earlier.

Looking at the 06z nam graphical and mos output numbers confidence
is growing that the overall trend this weekend will be continued
cooling back to or even slightly below normal by Sunday afternoon.
In general it looks like a more normal summer time weather
pattern this weekend across the Bay Area with night and morning
low clouds pulling back to the coast and followed by clear and
warm afternoons.

Latest ecmwf keeps a zonal or slightly cyclonic flow aloft off the
coast for next week implying near normal temperatures with dry
weather forecast. Longer range solutions later next week or
towards the end of the month show more pronounced ridging with
potential for another round of weak offshore flow and associated
warming.

&&

.AVIATION...As of 4:55 AM PDT Friday...For 12z Tafs. Compressed
but relatively deeper marine layer offshore pushing further
inland this morning bringing VLIFR-IFR cigs to low lying areas
along the coast this morning. Expect to see further expansion of
marine stratus/low clouds through the early morning hours before
retreating in the 17Z hour. Winds generally light today, except
breezy onshore winds through coastal gaps, including near KSFO, by
this afternoon and evening.

Vicinity of KSFO...Stratus in the Bay Area will need to wrap
clockwise around the Bay before reaching KSFO. More likely to see
low clouds near the bridge/KSMB rather than the tower/KSFO this
morning. Low confidence this morning with forecast counting on
VFR conditions at KSFO but possible impacts near bridge.


SFO Bridge Approach...Similar to KSFO with higher chance of LIFR-
IFR cigs at bridge versus tower.

Monterey Bay Terminals...VLIFR TO IFR cigs through 17Z hour. Early
return of low clouds/stratus tonight.  Generally light winds.

&&

.MARINE...as of 04:43 AM PDT Friday...Generally light southerly
winds over the coastal waters today. Onshore winds increase later
today and tonight through coastal  gaps such as the Golden Gate
gap and across the northern San Francisco Bay. Northwest winds
return this weekend. Generally decreasing seas through the coming
days.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Heat Advisory...CAZ510-511-517-518 til 9 pm
             SCA...SF Bay from 2 PM

&&

$$

PUBLIC FORECAST: RWW
AVIATION: DRP
MARINE: DRP


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NWS MTR Office Area Forecast Discussion

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