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Area forecast discussion 
National Weather Service San Francisco Bay area
1033 PM PDT Wednesday Oct 22 2014

Synopsis...dry weather conditions and partly cloudy skies are
expected tonight...but then on Thursday the weak tail end of an
approaching Pacific weather system will likely bring some light
rainfall to northernmost portions of our district. Friday looks to
be mostly sunny and mild...but then a somewhat wetter system
will move in on Saturday and spread rain chances across much of
our area. Dry conditions and seasonal temperatures are projected
for Sunday into the first part of next week.

&&

Discussion...as of 8:45 PM PDT Wednesday...temperatures were for
the most part a little milder and a few degrees above normal across
the area today. The only exception to this was over the South Bay
where average temperatures were 1 to 4 degrees fahrenheit below normal
today. Highs ranged from the middle 60s coast to the 70s inland.

Middle to high clouds are drifting in over the Bay area from the
northwest. Winds are mostly light this evening and the rain is
still situated well to the north of the Bay area. Believe it or
not shortwave ridging is still in place this evening over the
forecast area with 500 mb heights over the South Bay still
hovering up near 580 decameters. 500 mb heights will briefly
subside a little around 5 am Thursday morning then bump upwards
again to where they are now by Thursday afternoon. The incoming
frontal system will be on the weak side with only very light
amounts of rain expected...mostly over the northern Sonoma County
coast where up to 1/3" total of rain could fall late tonight and
Thursday morning. The newest 00z NAM model does not predict any
measurable rainfall anywhere else over the area tonight or
Thursday. Dewpoint temperatures are solidly higher this evening
area-wide so periodic light drizzle or light rain is still
possible to the south of Sonoma County as the weakening shortwave
trough squeezes out a little bit of moisture as it moves over the
area. A few areas may see a tiny amount of measurable rainfall
overnight...up to a couple hundredths possibly. The near term fits
perfectly as a low pop low quantitative precipitation forecast forecast. Not planning on making
any adjustments to first or second period pop forecasts.

Once the weak trough ripples east away from the Bay area Thursday
another (more amplified and strengthening) low pressure system near
39n/134w Friday evening moves northeast with the main middle-upper
level low pressure area reaching the Pacific northwest early Saturday
evening. The low pressure system will be trailing a cold front
eastward which will reach northernmost California by early Sat morning and
then the Bay area during the day Saturday. As much as 1" rainfall
is possible over Sonoma...about 1/2"-3/4" Napa and Marin counties
and 2/10" or less over the remainder of the Bay area and north
central coast. By the time the low pressure area gets to California the
strongest jet stream dynamics will have already positioned itself
forward of the trough axis suggesting an acceleration of the low
as well as the heaviest rainfall to the north of the Bay area. If
one is travelling to the Pacific northwest for the weekend expect heavier
steadier rains. Middle level temperatures reach about -18c over the
North Bay by late Saturday morning...not an exceptionally cold system.
The 00z NAM predicts a small pocket...albeit just below 35...of modified
total totals middle level instability by 21z Saturday over northern
Sonoma County. Often times low pressure areas swing over the area a
little faster than models predict. It is quite possible that the middle
level instability arrives in the North Bay area as early as very
early Saturday morning. The NAM depicts the middle level instability
quickly jumping over to Napa County then moving east to the northern
Sacramento Valley Saturday. Slight chance of T-storms was added in
earlier today and see no reason to make any changes except there's a
small possibility the T-storm activity may arrive a little earlier
than presently indicated.

There's much less confidence regarding next week's weather as the models
diverge on solutions. For instance the ecwmf and GFS models want to
bring rain to the area by Halloween but the Gem model is dry. Way too
early to say with any confidence if it'll be wet or dry for trick or
treaters.

&&

Previous discussion...as of 3:05 PM PDT Wednesday...skies are
clear to partly cloudy with middle afternoon temperatures around the
bays generally running in the upper 60s to middle 70s...a couple of
degrees warmer than at the same time yesterday. Tonight...the weak
tail end of a Pacific weather system now moving into the Pacific
northwest will spread in increasing middle and high cloudiness over
more northern portions of our area. Then on Thursday...model
solutions indicate a likelihood of some rain south to near Bodega
Bay...with a slight chance as far south as about San Francisco.
Rainfall totals look to be quite light...except in the far
northern coastal hills of Sonoma County where local amounts up to
the 1/3 to 1/2 inch range are possible.

Any residual North Bay showers should taper off and end Thursday
night with dry conditions then expected district-wide on Friday
as a short wave upper level ridge temporarily builds in westward from
the interior. By late Friday night however...the next Pacific
weather system is expected to rapidly approach from the northwest.
Latest model output continues to indicate it will be both a bit
stronger and wetter than the one on Thursday...with a likelihood
of at least some light precipitation spreading as far south as the
Monterey Bay region by Saturday afternoon. Should also note that
both latest NAM deterministic model output soundings and surface based
cape values...and sref MUCAPE plumes...indicate a sufficient
possibility of convection Saturday afternoon over the North Bay to
warrant inclusion in the forecast of mention of a slight chance of
thunderstorms.

Here again Post frontal showers look to taper off and end pretty
quickly...with dry weather conditions returning across all but
possibly the easternmost margin of our area by Sunday morning. Dry
conditions and seasonal temperatures are expected across the
district through at least Monday night. Thereafter longer range
model solutions begin to diverge...with for example the 12z GFS
bringing in another weak system and light precipitation to the North Bay
on Tuesday while the European model (ecmwf) confines associated rainfall to areas
north of our County Warning Area.

&&

Aviation...as of 10:30 PM PDT Wednesday...VFR conditions will
persist through late tonight with high clouds continue to
streaming across the region. A frontal boundary is forecast to
approach the North Bay tonight and should help increase the
amount of low level cloud cover...especially over the San
Francisco Bay area northward. Light rain or drizzle will be
possible over ksts...yet confidence remains low and have left
precipitation out of the taf for now. Low clouds that do develop
over the region will be slow to burn-off Thursday morning.
Further south...confidence is lower on the return of stratus over
the terminals as boundary layer moisture will be slower to return.

Vicinity of ksfo...VFR conditions will continue through late
tonight with low clouds expected to return after 09z.
Light west winds will continue through tonight. Moderate
confidence.

Ksfo bridge approach...similar to ksfo.

Monterey Bay area terminals...VFR conditions will continue
through late tonight with low clouds expected to return after 09z.
Light west winds will persist becoming light and variable
overnight. Moderate confidence.

&&

Marine...as of 8:51 PM PDT Wednesday...generally light northwest
winds will persist across the coastal waters through Friday...with
exception to the coastal jet south of Point Sur where moderate
winds will persist through Thursday night. Wind speeds will then
increase and become southerly Friday night into Saturday ahead of
an approaching cold front that will also bring chances of rain to
the region. Northwest swell will slowly diminish through Friday
then increase over the weekend.

&&

Mtr watches/warnings/advisories...
... Pinos to pt Piedras Blancas 0-10 nm

&&

$$

Public forecast: CW
aviation/marine: CW

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