Scientific Forecaster Discussion
fxus66 kmtr 261037
Area forecast discussion
National Weather Service San Francisco Bay area
337 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 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 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
Aviation...as of 10:27 PM PDT Monday...offshore winds and dry
conditions support VFR tonight and Tuesday. Light winds except
gusty in the North Bay and East Bay mountains/hills. Easterly
winds may also become locally gusty along the San Mateo coast late
tonight and Tuesday morning.
Vicinity of ksfo...VFR.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR. Light east-southeast winds tonight. Westerly
winds return Tuesday afternoon and evening.
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 3:00 am PDT Tuesday...a thermal trough over the
California interior will shift to the coast resulting in
decreasing winds. An upper level trough will approach the Pacific
northwest Friday increasing the pressure gradients over the area.
... Arena to Pigeon pt 10-60 nm until 3 PM
Public forecast: rww
aviation: west pi
marine: west pi
fire weather: rww
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