Scientific Forecaster Discussion
fxus66 kmtr 241211
Area forecast discussion
National Weather Service San Francisco Bay area
511 am PDT sun Sep 24 2017
Synopsis...high pressure off the West Coast will gradually build
eastward and over California through midweek, maintaining a
warming trend through Wednesday. Slight cooling is expected late
in the week, but temperatures will remain above seasonal averages.
Discussion...as of 3:30 am PDT Sunday...skies remain clear
across our entire forecast area early this morning. Surface
pressure gradients remain weak onshore locally but have trended
offshore across northern California. Dry northerly flow aloft
continues to bring a dry airmass into the region and early morning
relative humidity values are as much as 25 percent lower compared
to this time yesterday morning. Also, the airmass aloft continues
to gradually warm as the upper ridge offshore builds slowly
towards the West Coast. These factors point to a warmer day across
our region today. High temperatures today are forecast to be
anywhere from 3 to 8 degrees warmer than on Saturday. It's
possible we may see a few patches of fog develop along the coast
by sunrise, but any fog will clear by mid morning. The upshot is
that our entire area can expect a sunny and pleasant day with high
temperatures in 70s near the ocean and 80s elsewhere.
Offshore winds have eased in the hills since yesterday, but the
latest WRF model forecasts increasing north winds in the hills
through late morning, especially in the North Bay. Locally
moderate and gusty offshore winds combined with a dry airmass will
continue to produce critical fire weather conditions in the North
Bay mountains. A red flag warning remains in effect for the North
Bay mountains until Tuesday morning (see fire weather discussion
below for more details).
The models have been consistent in maintaining a gradual warming
and drying trend through midweek as the upper ridge offshore
slowly builds over California. 850 mb temperatures are forecast
to increase from about 14 deg c today to nearly 20 deg c by
Tuesday and Wednesday. Meanwhile, light offshore flow will become
better established across the region by Tuesday. High temperatures
by Tuesday and Wednesday will climb into the upper 70s to mid 80s
near the coast...and from the mid 80s to mid 90s inland. If
offshore flow is slightly stronger than currently anticipated,
downtown San Francisco could reach 90 by Tuesday or Wednesday
(instead of current forecast of low to mid 80s). Coastal
temperatures will be the most difficult part of the forecast this
Although daytime temperatures will be very warm by midweek,
longer early autumn nights and a dry airmass will allow for
relatively good cooling overnight throughout the week. Overnight
lows are forecast to mostly be in the 50s at lower elevations.
Overnight cooling will help reduce heat risks which are currently
projected to remain in the low to moderate category through the
Slight cooling is expected later in the week as the upper ridge
axis shifts to our east and light onshore flow develops.
Cooling on Thursday will mostly be confined to coastal areas,
while inland areas remain very warm. More widespread cooling is
likely by Friday. But even with the expected cooling late in the
week, temperatures will remain warmer than normal.
The longer range models agree that an upper ridge will strengthen
over California once again next weekend. However, the European model (ecmwf)
builds the ridge more strongly compared to the GFS and therefore
the European model solution would result in more robust warming
late in the extended.
Aviation...as of 5:00 am PDT Sunday...dry northerly flow over the
area keeping VFR conditions.
Vicinity of ksfo...VFR.
Sfo bridge approach...similar to ksfo.
Monterey Bay terminals...VFR.
Fire weather...as of 3:30 am PDT Sunday...northerly winds have
eased in the hills since yesterday, but models indicate that
north winds will increase this morning and peak during the late
morning hours. Winds are expected to be strongest across the
North Bay, although locally gusty winds are also likely in the
East Bay hills. Relative humidity recoveries have been poor in the
hills overnight, with relative humidity values remaining below 30 percent on the
higher peaks and ridges. Poor overnight humidity recoveries and
periods of moderate and gusty north to northeast winds are
forecast to persist through at least Tuesday morning. The most
critical fire weather conditions will remain over the North Bay
mountains, particularly on Monday night and Tuesday morning when
winds are forecast to be at their strongest. A red flag warning
remains in effect for elevations above 1000 feet in the North Bay
through Tuesday morning.
Marine...as of 5:00 am PDT Sunday...high pressure will continue
off the northern California coast. A thermal trough will shift
towards the coast resulting in decreasing winds for the inner
waters today and the northern outer waters Tuesday.
... Arena to Pigeon pt 10-60 nm
Public forecast: dykema
aviation: west pi
marine: west pi
fire weather: dykema
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