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

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FXUS66 KMTR 170256
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
756 PM PDT Mon Oct 16 2017

.SYNOPSIS...Look for another warm day regionwide today. A gradual
cooling trend is forecast for Tuesday and Wednesday. The pattern
will then change as a system drops down from the north and brings
generally light rainfall to parts of the area late Thursday into
Friday.

&&

.DISCUSSION...as of 8:00 PM PDT Monday...Unseasonably warm
temperature and dry conditions prevailed across most of the
forecast area earlier today under the apex of a broad moderate
amplitude ridge. Temperatures were generally 8 to 12 degrees above
normal. Onshore winds have begun to cool the coastal areas as of
late this afternoon/evening. For example, Monterey cooled from the
mid 80s to mid 60s and humidities rapidly increased in the course
of around an hour this evening when the winds shifted to become
onshore. This goes to show the importance of wind direction on
temperature and humidity.

Updated the afternoon forecast package to include a mention of a
slight chance of dry thunderstorms primarily over Monterey, Santa
Cruz, and San Benito county for Tuesday afternoon. There is a
surge of moisture around 600mb coupling with some elevated
instability on the southwestern flank of the exiting ridge.
Satellite imagery already shows an area of cooler cloud tops in an
area the size of Texas that the National Hurricane Center gives a
30% chance of forming into a tropical cyclone within the next 48
hours. Should this complex strengthen into a cyclone, we would
likely need to make updates to the forecast, so please keep close
attention to any updates regarding this.

A regionwide cooling and dampening trend will begin Tuesday and
peak on Thursday with increasing onshore flow and the arrival of
the upper trough in Northern California. This upper trough will
also spread light amount of precipitation across much of the
greater San Francisco Bay Area, with the highest amounts of rain
forecast over the areas that need it most, the North Bay hills and
mountains in a period from late Thursday through early Friday.
Some very light warm sector precipitation will precede the
heavier band of cold frontal precipitation earlier in the day
Thursday.

Despite some run to run model consistency issues regarding
precipitation amounts, data suggests wetting rains should fall
over most of the North Bay fires. Wetting rains are those that are
sufficient in helping to suppress fires, generally with
accumulations in excess of a 1/10" of rain. The latest forecast
includes a range of 1/4" to 1/3" for the coastal and mountains
regions of the North Bay which surpass this thresholds. Valley
locations in the North Bay are forecast to receive a little less
than the surrounding hills and mountains, generally from around
1/6" to 1/4", with similar amounts in the coastal Santa Cruz
mountains to Ocean beach. Elsewhere, precipitation amounts will
taper off with distance inland and southward. San Francisco Bay
Shoreline areas (ie downtown SF, Oakland, San Jose) can expect
closer to 1/10"-1/6", with a few to several hundredths of an inch
anticipated for locations in the far East Bay and Monterey Bay.

In the wake of the trough, high pressure will build back into the
region heralding in a return to warmer temperatures, drier
weather, and less onshore influence.

&&

.AVIATION...as of 10:38 AM PDT Monday...For 00z TAFs. FU/HZ will
continue to impact North Bay terminals through tonight and likely
into Tuesday. Aside from MVFR conditions due to the FU/HZ at the
aforementioned terminals, look for VFR conditions to prevail for
the remainder of the region through the forecast period. High
clouds will also continue to move over the region along with weak
onshore flow. Onshore winds are forecast to increase Tuesday
afternoon.

High confidence.

Vicinity of KSFO...VFR conditions to persist with light winds
becoming W/NW late this evening before once again diminishing
overnight. More predominate W/NW winds will return Tuesday
afternoon with winds at or around 10 KT.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions to persist with light to
moderate SW winds through the remainder of the afternoon. Winds
become light and variable late this evening and continue through
Tuesday morning. More predominate W/NW winds will return Tuesday
afternoon.

&&

.FIRE WEATHER...As of 3:30 AM PDT Monday...Fairly light winds can
expected today across our entire region along with warmer than
normal temperatures. High pressure will weaken starting on Tuesday
which will lead to cooler temperatures plus a return of onshore
flow. This will lead to an improvement in humidity values
especially by Wednesday. For Thursday into Friday wetting rain is
likely for part of our region as a system drops in from the Gulf
of Alaska. Winds will be breezy as it moves across, however the
combination of higher humidity readings and cooler temperatures
should help to keep the fire weather risk relatively low.

&&

.MARINE...as of 01:57 PM PDT Monday...High pressure over the
eastern Pacific will gradually weaken and  move southward this
week. A cold front from the Gulf of Alaska will arrive in the
California coastal waters later Thursday along with rain spreading
southeastward across the waters and bays. Moderate to long period
southwesterly swell may temporarily arrive Tuesday mixing in with
northwesterly swell. A very large northwesterly swell train, the
largest of this season so far, is then forecast to arrive Thursday
night through Friday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...None.

$$

PUBLIC FORECAST: DRP
AVIATION: RGass
MARINE: BAM
FIRE WEATHER: Bell

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