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

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FXUS66 KMTR 261728
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1028 AM PDT Tue Sep 26 2017

.SYNOPSIS...Breezy in the hills this morning then turning sunny
and warm to hot again by midday. Another shot of gusty winds again
this evening for the North Bay and East Bay hills. Continued warm
and dry on Wednesday which may be the hottest day this week but
with winds easing. Onshore flow gradually returns by Thursday with
a few degrees of cooling followed by more noted cooling Friday
and Saturday as a trough passes to our north. Dry high pressure
rebuilds Sunday into next week.

&&

.DISCUSSION...as of 08:29 AM PDT Tuesday...Modest warming trend
will continue over the region today with temperatures running a
few to as much as 7 degrees warmer (at Santa Rosa Airport) than to
24 hours ago. With this, look for coastal locations to warm into
the upper 70s to lower 80s with mid 80s possible around Santa
Cruz. Inland areas and locations around the San Francisco Bay will
see afternoon temperatures in the upper 80s to near 90 while
inland areas warm into the mid/upper 80s to lower 90s in the North
Bay and East Bay. Overall, the very warm temperatures will be
mostly uniform through this heat event. Have made a few minor
changes to the forecast this morning, mainly nudging up
temperatures for today and tomorrow as the MOS guidance output is
favored for most locations. Otherwise, clear skies will prevail
region-wide through at least midweek before onshore flow returns
late in the week brining slightly cooler temperatures back inland.
For additional details, please see the previous forecast
discussion below.

&&

.PREV DISCUSSION...as of 3:37 AM PDT Tuesday...Skies remain mostly
clear this morning with mild overnight lows around 60 most areas.
Surface gradients currently show a 6 mb pressure gradient down the
coast form Arcata to SFO which is a dry northerly flow. The large
scale offshore from the deserts is around 7-8 mb which is only
moderate. This is translating to some gusty winds in the north and
east bay hills with Hawkeye in northern Sonoma currently gusting
28 mph with 19 mph on Mt Tamalpais. This all sets the stage for
rapid warming again today as the sun comes up and the dry airmass
quickly warms up under adiabatic compression off the coastal
hills. The official forecast for today has widespread 80s and 90s
across the Bay Area.

Models show another shot of east/northeast winds in the hills this
evening which will keep fire weather concerns in place due to poor
humidity recovery, mild temps and continued dry offshore winds.
Winds will finally ease on Wednesday but another warm to hot day
is forecast across the region.

Some coastal cooling should become obvious by Thursday as onshore
flow gradually returns to area but inland areas will remain above
normal once again.

The ECMWF nows brings a dry cold front over the Bay Area by
Friday. This should induce some gusty onshore winds and a possible
return of the marine layer into Saturday. The pattern looks
progressive with high pressure rebuilding by Sunday into early
next week with another thermal trough setting up at the surface.
This should translate too another round of sunny and mild days for
early next week before the next trough knocks temps back down a
few degrees.

&&

.AVIATION...As of 10:30 AM PDT Tuesday...For 18z tafs. Another
clear day with prevailing VFR conditions expected through the
period. Generally light offshore flow expected this morning before
winds turn westerly this afternoon to around 10 kt.

Vicinity of KSFO...VFR. Generally light winds. Afternoon
westerlies 10-15 kt.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR prevailing with clear skies through
the period.

&&

.FIRE WEATHER...As of 3:37 AM PDT Tuesday...Red Flag Warnings
remain in place for the North and East Bay hills into Weds.
Humidity remains below 30 percent early this morning with dry
north winds showing up in the hills. Winds forecast to ease
through the daytime hours today but another warm and dry day.
Models show another shot of gusty east/northeast winds by this
evening and overnight, strongest in the Napa hills. Latest fuels
analysis shows that fuels are around normal in terms of seasonable
dryness but of course this is the driest time of year. However
trend of warmer and drier will continue to dry out the fine and
live fuels. Winds will finally ease on Wednesday but that may be
the hottest and driest day this week. Will need to watch for
return of onshore winds Thursday afternoon for the interior
valleys. Breezy onshore winds Friday and Saturday then warming and
drying again Sunday into early next week.

&&

.MARINE...as of 08:29 AM PDT Tuesday...Moderate northwesterly
winds will continue into this afternoon over the northern outer
waters. A thermal trough over California will shift towards the
coast this afternoon decreasing winds across coastal the waters.
Expect winds to pick up again heading  into the weekend as an
upper level trough approaches the Pacific  Northwest Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Arena to Pigeon Pt 10-60 nm until 3 PM

&&

$$

PUBLIC FORECAST: RGass/RWW
AVIATION: AS
MARINE: AS
FIRE WEATHER: RWW

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