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

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FXUS66 KMTR 240552

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1052 PM PDT Mon Apr 23 2018

.SYNOPSIS...A cooling trend is forecast from Tuesday through the
end of the week as an area of low pressure offshore slowly
approaches the coast. This low pressure may produce scattered
showers from late Friday into Saturday, mainly across the North

&& of 9:26 PM PDT Monday...No significant grid or
forecast updates planned for this evening. As we are getting
deeper into April some of the microclimates are starting to show
up with the stronger spring time sun angle. We had high
temperatures this afternoon ranging from 60 at Half Moon Bay to
91 in the southern Salinas Valley after a morning low of 39 in
Carmel Valley.

In addition our old friend the marine layer is starting to come
back home. The Ft Ord profiler shows a well defined layer to
around 1400 feet now but with signs of deepening to as much as
2000 feet. Low clouds are already inundating the entire Monterey
Bay region and now surging through the Golden Gate and bumping
into Berkeley. Strong northerly gradients 24 hours ago have
collapsed and now the onshore from sfo to sac is dominating in
excess of 2 mb. However were not seeing strong winds yet out in
the Delta so for the most part gusty onshore breezes are confined
to the coast and bay. Yet in the last hour Concord is reporting a
13 kt west wind, a sure sign that cool marine air is spreading

A slow moving upper low will approach the coast through midweek.
Impacts will be minor but it will keep sustained onshore winds in
place with gradual airmass cooling and at this point it would
appear the marine layer will be trying to re-establish itself in
this onshore environment. By Thursday cooling aloft would be
sufficient to mix the inversion out but the airmass aloft will be
unseasonably cold. Main theme from Thursday afternoon through
Saturday will be well below normal afternoon highs with even usual
warm interior valleys of the East Bay struggling to reach the 70s.

Will continue to honor slight chance pops, especially for the
North Bay later Friday and Saturday but really precip chances
look meager (with better chances Mendo northward). Main forecast
theme for incoming upper low will be the below normal temps (and
some snow showers for the Sierra).

We should be on the backside of the upper trough by Sunday
afternoon but the euro drops reinforcing cold (but mainly dry)
shortwaves through the base of the longwave trough keeping
temperatures on the cool side of normal through the end of the


.PREV of 01:27 PM PDT Monday...Sunny skies prevail
across most of the Bay Area this afternoon. The sunshine and light
winds have allowed for well above normal temperatures away from
the coast. Afternoon temps will likely top out in the 70s to mid
80s. Just like during the summer months, different story at the
coast. Weak pressure gradients N-S and E-W have kept low clouds
along the coast, mainly San Mateo Coast southward. Made some late
day adjust to the forecast to lower temps around the Monterey Bay
and keep clouds through the afternoon.

Otherwise, only minor tweaks were made to the forecast. High
pressure will continue to weaken as an upper low currently
spinning off the West Coast slowly tracks eastward. Night and
morning low clouds will continue through Wednesday and possibly
mix out completely by Thursday/Friday. The lower 500mb heights and
cooler 850mb temps will translate to a slight downward trend in
daytime high temperatures. Temps by Wednesday will be closer to
normal (60s coast, 70s to lower 80s inland) and possibly below
normal by Thursday (60s/70s).

The track of upper lows are always difficult to forecast,
especially in the medium to long range. The 12z model run didn`t
shed any new light on the rain potential Friday into Saturday.
Model consensus brings an upper low into far NorCal with some
precip possible across the North Bay. Confidence is low and will
maintain the slight chc of showers SF Bay northward. QPF looks to
be minimal at this point.


.AVIATION...As of 10:52 PM PDT Monday...A stratus, fog intrusion
is taking place transported inland by onshore winds, the SFO-SAC
pressure gradient is up to 2.8 mb. The southerly SMX-SFO pressure
gradient has settled back to 1.2 mb, but should be relatively
short-lived as southerly gradients strengthen again Tuesday into
Tuesday night. The marine layer has deepened since this afternoon
per profiler data. Thus cigs and visibilities forecasts for Bay
Area tafs is a little lower confidence while stratus and fog fills
in along the coastline; greater areal coverage is likely Tuesday
evening into Wednesday morning.

Vicinity of KSFO...VFR, W-NW wind with a few more gusts to 20 knots
possible this evening. Near high confidence VFR holds this evening
then lowering confidence on cig forecast as the marine layer has
deepened. 06z taf indicates VFR for the overnight hours for now.
Radiative cooling improves tonight with a clear sky, near surface
cooling is a little more pronounced Tuesday morning which may result
in a patch or two of fog or low stratus.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR/LIFR for the overnight hours, late
morning mixing should help result in a return of VFR for the area
terminals. Stratus and fog are likely to roll back inland early
Tuesday evening.

&& of 9:01 PM PDT Monday...Northwest winds will remain
light to moderate through most of the week as an area of low
pressure approaches the coast. Onshore winds will remain strong
and gusty north of the Bay Bridge this evening, possibly gusty
into the overnight hours. Light to moderate seas will prevail
through the forecast period with a mixed southerly and
northwesterly swell developing late this week.





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