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

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FXUS66 KMTR 290605
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1105 PM PDT WED SEP 28 2016

...LIGHT RAIN POSSIBLY RETURNS FOR NORTHERN PARTS OF OUR AREA
STARTING ON SUNDAY...

.SYNOPSIS...Cooling trend will continue into the weekend as the
marine layer and onshore flow become better established with a
trough along the West Coast. In addition, a dry cold front will go
through on Friday which will lead to breezy conditions. A system
will drop down from the north over the weekend which will bring
rain to our area especially over the North Bay.

&&

&&

.DISCUSSION...as of 9:08 PM PDT Wednesday...A cooling trend with
onshore winds is well underway. Close to the coast and Bays high
temperatures peaked in the 60s to 70s today while inland valleys
and over the higher terrain above the marine layer peaked in the
80s and 90s.

As discussed by the day shift we are at the start of a substantial
change in the pattern. A pattern change with a period of much cooler
temperatures on NW flow will develop as a long wave trough axis
between 140W and 130W gradually shifts eastward reaching the west
coast this weekend and persists into Monday as a progressive northern
hemispheric pattern nudges the long wave ridge (that resulted in
record high temps Sunday and Monday) downstream to the Midwest.
Energy rotating through the long wave trough will tend to amplify,
dig and slow the forward progression of the overall pattern this
weekend. Cooler and possibly wet weather is of course great news
for fire-fighting efforts, and with respect to area climatology
we are still just ahead of the peak offshore flow and hot/dry
season of early October so this is of great help/timing. However,
it`s also important to note that a dry cold frontal passage and a
period of gusty NW-N winds later Friday into early Saturday negates
some of this benefit early on. Ongoing critical fire weather
conditions with the backdrop of long term drought, low to very
low fuel moistures and a period of strong and gusty winds upcoming
later Friday into Saturday morning all combined may necessitate a
fire weather watch; greatest potential for wind is over the coastal
terrain including the north Bay mtns, Santa Cruz mtns and possibly
the Santa Lucia mtns.

QPF forecasts for this weekend from this morning`s 12z ECMWF run
and the 18z GFS have both gravitated toward what yesterday`s GFS
was forecasting, approx 1/3" north Bay to 1/10" Santa Cruz mtns.
Locally higher amounts of QPF are possible due to convective
showers with the cold pool of air aloft.

After a chillier than normal summer season and a roller coaster
ride in temperatures throughout September, October will open the
door to much cooler temperatures as well as a chance for rain this
weekend and early next week. Model forecasts at this time indicate
that mid-late next week the flow pattern entering the western U.S.
tends back toward a zonal (much less amplified, west-east) flow.

Forecast looks good, updates are not anticipated this evening.

&&

.PREV DISCUSSION...as of 2:30 PM PDT Wednesday...As expected
temperatures have taken a tumble today for many inland locations
due to a return of onshore flow, the ridge heading to the east,
and a building marine layer. A few spots are sitting more than 15
degrees cooler than Tuesday at this time.

This is the start of a substantial change in our synoptic pattern
which will feature the ridge continuing to shift farther to the
east as a system associated with a longwave trof dives down the
west coast. Out ahead of that an associated dry cold front will
move through our area late on Friday. As the front approaches
winds will steady increase from the west and northwest through the
day with locally gusty winds to over 30 MPH possible. This will
produce an elevated concern for fire weather. Winds will abate on
Saturday. Along with the stronger winds the shift in the pattern
will lead to cooler temperatures. By Saturday all locations should
be just in the 60s to mid 70s.

On Sunday a system that will be dropping down from the north will
move into Northern California. Rainfall associated with a frontal
system now appears likely to move into the North Bay on Sunday
and continue to progress to the south through the day. Models are
now in good agreement that the rain will make it at least as far
as San Francisco with less agreement to Monterey Bay. Rainfall
amounts will range from 1/3" in parts of the North Bay to a tenth
or less for San Francisco and southward. Instability will also be
on the increase with the frontal passage which could trigger a few
thunderstorms. Will take a closer look at that tonight before
adding it to the grids. Although totals will be light, due to the
amount of oil that has collected on roads, diving conditions could
be quite tricky with the onset of rainfall. Worth noting that the
GEM has up to 1/2" down to Monterey Bay. At this time, that is an
outlier and appears to be an unlikely solution.

Behind the front a secondary piece of energy may go through Monday
and Monday night. Models are in less agreement with this feature
for both coverage and amounts. That piece will likely produce less
than 1/10" in most spots. Temperatures will continue to be cooler
than normal on Monday.

A ridge of high pressure will start to rebuild back to our region
by the middle of next week. This will lead to a warming trend
along with dry conditions.

&&

.AVIATION...As of 10:45 PM PDT Wednesday...Low clouds have moved
in across the region this evening under an 1800 ft marine layer.
IFR to LIFR cigs expected overnight across most area terminals
with clearing anticipated around 18z Thursday morning. Light winds
will continue overnight.

Vicinity of KSFO...IFR cigs filling in over the bay are expected
to spread over KSFO after 08z tonight with clearing anticipated
around 19z Thursday morning. West winds will gradually ease
overnight.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...LIFR/IFR cigs expected through tonight.
Clearing expected between 18z-19z Thursday. Light winds.

&&

.FIRE WEATHER...As of 2:45 PM PDT Wednesday...Marine layer will
continue to deep over the Bay Area the next few days leading to
better recoveries and cooler temperatures. The most notable
weather concern over the next 48-72 hours will be increasing
winds on Friday afternoon. A passing dry cold front will bring
gusty northwest winds to the coast and higher terrain. Models
continue to show gusts in the 20-30 mph will be possible Friday
evening into Saturday.

&&

.MARINE...as of 9:00 PM PDT Wednesday...High pressure building
off the Northern California coast will bring increasing northwest
winds to the northern waters through tonight. The high will
weaken Thursday as an upper level trough deepens off the west
coast.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm until 3 AM

&&

$$

PUBLIC FORECAST: Canepa/Bell
AVIATION: CW
MARINE: CW
FIRE WEATHER: MM


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