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

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FXUS66 KMTR 302154
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
254 PM PDT FRI SEP 30 2016

.SYNOPSIS... An approaching upper level system will bring cooler
temperatures and breezy conditions into this evening and
overnight. As the main upper level system pushes inland on Sunday,
chances for rain showers will increase with the potential for
thunderstorms, especially across the San Francisco Bay Area. Drier
conditions and a slight warming trend is then forecast by the
middle of next week.

&&

.DISCUSSION...as of 02:40 PM PDT Friday... Satellite imagery
depicts a 547DM upper level trough positioned just offshore of
the Pacific coastline from southern Alaska to central California.
This upper level trough has recently shifted from an unfavorable
positive tilt to a neutral tilt over the previous 24 hours while
digging progressively further south towards our region. A pair of
vorticity lobes embedded in the upper low are now rounding the
base of the trough, which is encourage the trough to dig further
southward, as well as contributing to some weak cumulus
development in the post frontal environment. Ahead of this feature
is a cold dry front which is bringing cooler temperatures,
increased onshore winds, and a thin band of high clouds to the
region today.

So far today, the strongest winds have been observed at the
Altamont RAWS (SW 40 mph), while the largest temperature drops are
primarily being observed in higher terrain of the North and East
Bay, with Mallory Ridge RAWS leading the way (19 degree drop
versus yesterday). 850MB temperatures ahead of the front are
running around 17C, with postfrontal values running closer to
10-12C. Expect to see continued cooling and increased winds across
the area through the remainder of the day as the aforementioned
air mass boundary advances across our region. For tonight, the
marine layer should remain mixed out by the trough, meanwhile,
breezy winds will continue, especially along coastal areas and
over higher terrain.

The dynamics of the upper trough will weaken slightly over our
region by tomorrow as the base of the trough becomes void of
significant vorticity lobe. That said, the upper low is then expected
to become revitalized early Sunday as an energetic vorticity lobe
and core of the upper low approach the region. As a result,
temperatures aloft, and at the surface, will continue to ease over
the upcoming days. 850MB temperatures are anticipated to drop to
around 5-6C on Sunday while 500MB temperatures descend to -24C,
which is an impressive 3.5 standard deviations cooler than the
average. For Sunday, daytime high temperatures will struggle to
push more than a few degrees above the overnight lows, with
widespread upper 50s to 60s for most locations, while the warmest
inland areas could top 70 degrees.

Additionally, the first measurable rainfall for the season may
fall for some locations, especially in the North Bay, where
around a tenth, but locally up to a quarter of an inch may be
possible. Further south in the immediate Bay Area, amounts will
range from a few to several hundredths, while further south, only
a few hundredths, if any rainfall, should be expected. Most of
this rainfall is anticipated to come in the form of rain showers.
Isolated thunderstorms may also develop as the coldest air advects
in aloft and interacts with somewhat favorable upper dynamics
throughout the day on Sunday. Model guidance suggests the North
Bay will be the most likely location to see these stronger storm
cells develop.

The upper low then lifts into the intermountain west on Monday,
leaving behind a cool, transitionary period with northwest flow
for Tuesday. A Pacific high pressure ridge will then build into
the region by mid week next week, leading to gradually warming and
drying conditions for the area. Long term models show another
upper trough descending into the region by late next week.

&&

.AVIATION...as of 10:56 AM PDT Friday...Visible satellite shows
stratus clearing over the Monterey Bay Peninsula, where IFR CIGs
still persist. VFR at all terminals by 1830Z, perhaps even
sooner. Winds are already beginning to increase, especially around
the SF Bay Area. With a dry frontal passage expected this
afternoon, winds will be gusty out of the west/northwest at
times. Have CIGs returning for all terminals later this evening,
but confidence is low on timing and duration due to some questions
on how well the frontal passage will mix out the marine layer.

Vicinity of KSFO...VFR. Sustained onshore winds of 20 to 25 kt
with gusts to 30 kt expected this afternoon and evening. Forecast
models disagree when/if the marine layer will recover by tonight.
If it can, low clouds could move in by 08-10Z. Low confidence.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...IFR CIGs will quickly scatter out between
18-1830Z. Winds increase this afternoon, out of the west, and will
be sustained between 12-15 KT. Still some uncertainty with timing
of CIGs returning tonight, but have IFR CIGs at MRY around 02Z and
by 06Z for SNS. Low confidence.

&&

.FIRE WEATHER...as of 2:54 PM PDT Friday...Two main areas of
concern fire weather wise the next few days will be gusty winds
tonight and rain/tstorm chances on Sunday. A passing dry cold
front will bring cooler and wetter conditions to the region later
today and tonight. More importantly, gusty northwest winds will
develop with gusts to 20-30 mph possible. Do expect better RH
recoveries overnight, but the WRF model continues to show some
very dry air near 850mb. That being said, recoveries could be
early in the night then drop toward sunrise over the higher peaks
of the Soberanes Fire. The Loma Fire should see better recoveries
everywhere. Winds decrease slightly during the day on Saturday,
but ramp up again to breezy on Sunday. An upper level low will
drop south on Sunday bringing a chance of precip and maybe a
thunderstorm or two. Best chance for wetting rains appears to be
north of the Golden Gate with lesser amounts to Monterey. Loma
appears to have a better shot at rain than the Soberanes. LAL of 2
for Sunday afternoon/evening with an unstable airmass.
Thunderstorms most likely north of San Jose, but did include Loma
fire as Santa Cruz mts are on the edge.

&&

.MARINE...as of 01:43 PM PDT Friday...An upper level low
approaching from the north will strengthen winds across the
coastal waters today. Strongest winds are still forecast to be
south of Point Sur through saturday. Winds will ease sunday as
high pressure builds back in over the region. A small mixed swell
with a long period southerly swell will continue through the
period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...SF Bay until 9 PM
             SCA...Mry Bay until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

$$

PUBLIC FORECAST: DRP
AVIATION: BAM
MARINE: BAM
FIRE WEATHER: MM


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