Scientific Forecaster Discussion
Area forecast discussion
National Weather Service San Francisco Bay area
502 am PDT Wednesday Sep 17 2014
Synopsis...a cooling trend is forecast through tomorrow as an
upper level low pressure trough over the eastern Pacific
approaches the California coast. Showers and isolated
thunderstorms will be possible this evening through Thursday
afternoon as the system gradually moves inland.
Discussion...as of 3:00 am PDT Wednesday...everything still
appears on track for possible rain for tonight into Thursday as a
storm system currently around 132w continues to track toward
pacnw/norcal region. Before any rain arrives, we should see
generally sunny conditions with a few patches of fog mostly
confined to the coast during the morning hours. As the day
progresses, high level clouds will begin to filter in first across
the North Bay and then more to the south as the evening
approaches. Highs should be cooler today in almost all locations
as the system/longwave trough approach. Look for mostly 60s and 70s
at the coast with 70s and 80s for inland spots. In some areas it
will be a drop of up to 15 degrees compared to Tuesday.
Attention will then switch to rain chances as the system and
associated cold front near our County Warning Area. Latest model guidance is in
fairly good agreement and brings the first chance of rain to the
North Bay starting in the evening hours and then down to as far as
the Monterey/slo border during the overnight hours. Overall
guidance has changed little from 24 hours ago so still appears
that this "event" will feature very light rainfall totals --
mostly under 1/10" -- for most of the County Warning Area. If we do get more than
a tenth anywhere, the favored spots would be the usual spots such
as the northern coastal ranges. Looking back at the records, it's
been nearly 45 days since San Francisco had measurable rainfall
and around 145 since more than an tenth was recorded so even a
small amount will be welcome. Biggest hazard is expected to be
very slick roads due to oils that have built up on them.
Instability will be on the increase during the overnight hours as
the upper level lows gets into northern California. Mosguide
thunderstorm numbers show up to a 25% chance of thunder over the
North Bay for the overnight and morning hours which seems
reasonable given the proximity of the low. Thunderstorm coverage
was slightly expanded for the forecast although it was left out of
all areas south of the North Bay.
Rain chances are expected come to an end by Thursday evening as the upper
level low dives to the southeast near our southern coastal waters.
This should end up being the coolest day of the week.
Temperatures will start to warm back up for Friday into next week as a
ridge of high pressure builds back across our region while 850 mb
temperatures increase. By Sunday most locations will be near normal to a
bit above normal with plenty of sunshine.
Longer range guidance is now in less agreement with some of the
models building a very strong ridge of high pressure across our
area along with offshore winds and very warm 850 mb temperatures. Other
guidance (including the operational Gem and European model (ecmwf) solutions) offer
a different solution and have milder conditions with a longwave
trough off the coast and any ridging much less pronounced. For now
will just advertise warm but not hot conditions for the second
half of next week.
Aviation...as of 5:00 am PDT Wednesday...low clouds remain mainly
confined at the coast this morning with minimal inland intrusion
noted. While is remains difficult to see the low clouds this
morning due to high clouds traversing the area current
observations depict IFR/MVFR ceilings across the Bay area. Across
Monterey Bay area terminals patchy low clouds have resulted in
LIFR conditions over kmry. Winds will remain light with a moderate
and gusty seabreeze expected this afternoon. A chance of rain is
forecast to spread north to south across the district beginning
Vicinity of ksfo...VFR conditions continue over ksfo with MVFR
ceiling expected over koak till 18z this morning. Light onshore flow
will increase this afternoon to around 15 knots. Light rain showers
are possible after 06z tonight.
Ksfo bridge approach...similar as ksfo.
Monterey Bay area terminals...IFR/LIFR ceilings will persist till
around 17z this morning. Light onshore flow will increase this
afternoon to around 10 knots. Light rain showers are possible after
Marine...as of 2:30 am PDT Wednesday...light southerly winds
will persist through Thursday night as a low pressure system
approaches the West Coast. Winds will switch to northwest Friday
as the low moves southeast along the coast. A moderate southwest
swell will continue through at least Thursday.
Public forecast: Bell
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