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

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738
FXUS66 KMTR 251613
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
913 AM PDT SUN SEP 25 2016

.SYNOPSIS...Strong high pressure over the region combined with
offshore flow will result in record warm temperatures today and
Monday. Modest cooling is then forecast by Tuesday with a more
significant cooling trend expected during the second half of the
week as an upper level trough deepens along the West Coast.

&&

.DISCUSSION...as of 09:03 AM PDT Sunday...The day is setting up to
be very warm to hot across the entire region as clear skies
prevail along with weak offshore flow. Temperatures at the coast
are already warming into the lower 70s, warmer than even some
inland spots at this hour. The warming trend will continue near
the coast through the late morning before seeing some slowing of
the warming trend if/when the sea-breeze develops by early
afternoon. Regardless, expecting many coastal areas to reach into
the upper 80s to near 90 this afternoon. Meanwhile, areas away
from the coast/bays will warm through the afternoon hours with
widespread 90s likely and 100-105 degrees in the warmest inland
locations. The ongoing forecast remains on track and no updates
are needed at this time. Please see the previous discussion below
for additional details.

&&

.PREV DISCUSSION...As of 3:35 AM PDT Sunday...Skies are clear
across our entire forecast area early this morning as subsidence
under a strong high pressure ridge aloft continues to warm and dry
the airmass. In addition, surface high pressure has strengthened
over eastern Oregon and Nevada, resulting in a robust offshore
surface pressure gradient of nearly 12 mb from WMC to SFO. Winds
remain light near sea level, but offshore breezes have begun to
pick up in the hills above a shallow nocturnal inversion.
Northeast winds in the North Bay Mountains are currently gusting
as high as 35 mph. The combination of a very warm airmass over our
region and offshore flow will push temperatures up into record
warm territory today. In fact, high temperatures today for many
locations are forecast to be the warmest so far this year
including in Downtown San Francisco, San Jose, the Oakland Museum,
and Salinas. The heat is forecast to continue unabated into
Monday, and according to most model guidance high temperatures on
Monday will be a few degrees warmer than today. More record highs
are likely on Monday. These hot temperatures over the next two
days will create stress on anyone sensitive to heat. One
mitigating factor to the heat during the day will be the
relatively long autumn nights which will allow homes without air
conditioning to cool down. Even so, conditions during the day will
be oppressive, particularly for those living in coastal areas who
are not acclimated to such warm conditions. A heat advisory is in
effect from 11 am to 8 pm both today and Monday for our entire
forecast area.

A shortwave trough approaching the Pacific Northwest on Tuesday
will weaken the upper ridge over California and it`s expected
that light onshore flow will develop across our area. Thus we
should see at least modest cooling on Tuesday, particularly in
coastal areas where high temperatures may drop by as much as ten
degrees. Inland areas will remain very warm however. It`s not
until Tuesday night that the models indicate a good inland influx
of marine air and a return of coastal low clouds and fog.
Widespread and significant cooling is then forecast for Wednesday
and Thursday as an upper trough in the Gulf of Alaska drops southeast
towards California. On Wednesday high temperatures are forecast to
be anywhere from 15 to 25 degrees cooler compared to Monday. For
example, the forecast high in San Francisco on Monday is 90 and
on Wednesday 65.

Precipitation with the upper trough late in the week and into next
weekend is expected to remain well to our north, but it`s worth
noting that the 00Z GFS develops rain as far south as Mendocino
County by Saturday.

&&

.AVIATION...As of 4:45 AM PDT Sunday...Strong upper level high
over the area with offshore flow will keep VFR clear skies.
Non-convective low level wind shear is a possibility Sunday near
the East Bay Hills.

Vicinity of KSFO...VFR. Light winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR.

&&

.FIRE WEATHER...As of 3:30 AM PDT Sunday...Northeast winds have
increased in the hills since midnight, particularly at elevations
above 1500 feet in the North Bay, gusting locally as high as 35
mph at 2000 feet in Sonoma County. At the same time, relative
humidities have not recovered and in some cases RH values have
fallen below 20 percent overnight. A red flag warning is currently
in effect for the North Bay Mountains. Latest model output
continues to show gusty offshore winds across mainly the higher
elevations of northern Sonoma and Napa counties through the
morning hours, with winds then expected to gradually diminish in
the afternoon. The Red Flag warning for the North Bay Mountains is
in effect until 3 pm this afternoon. Winds are expected to remain
light from late this afternoon onward. However, hot and dry
conditions will persist through Monday with only slight cooling
expected on Tuesday. Much more significant cooling is forecast
beginning Wednesday and continuing through the second half of the
week.

In the vicinity of the Soberanes Fire: Portable RAWS stations
indicate mostly light winds below 3000 feet early this morning
with northeast winds of 10 to 15 mph with gusts to 20 mph above
3000 feet. This is consistent with latest WRF model wind guidance.
The WRF indicates easterly winds will diminish by sunrise and then
turn to the south and southwest by midday. Southwest winds are
then forecast to increase somewhat in the afternoon hours,
probably up to around 10 to 15 mph. Meanwhile, hot and dry
conditions are forecast to continue into Monday, although winds
tonight and Monday are expected to be lighter. Slight cooling is
projected for Tuesday, mainly along lower coastal slopes, with
very warm to hot conditions continuing for most other areas. Will
have to wait until Wednesday before widespread cooling occurs.

&&

.CLIMATE...Here are the record highs for September 25 and
September 26.

                   September 25                September 26
                   Record/Year                 Record/Year
Bay Area

Kentfield.............100/1964.....................99/1921
San Rafael............100/1964.....................99/1963
Napa..................101/1960....................105/1963
San Francisco..........93/1954.....................94/1992
SFO....................91/1970.....................95/1958
Oakland Museum.........95/1978.....................93/1973
Oakland Airport........94/2010.....................97/1958
Richmond...............95/1964.....................95/1963
Livermore.............101/1978....................103/1952
Mountain View..........91/1978.....................95/1999
San Jose...............97/1952.....................98/1963
Gilroy................106/1978....................104/1963

Monterey Bay Area

Monterey...............95/1954.....................98/1970
Santa Cruz............100/2010....................101/1970
Salinas................97/1960....................100/1970
Salinas Airport........99/1937.....................99/1970
King City.............105/1975....................105/1963

&&

.MARINE...as of 09:03 AM PDT Sunday...A high pressure ridge covers
the eastern Pacific and the Great Basin while a thermal trough
lies along the California coast. The trough will move into the
California interior on Tuesday. Winds will become light today and
Monday then increase Tuesday and Wednesday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Heat Advisory...Entire San Francisco Bay Area and North
                             Central Coast from 11 AM to 8 PM PDT

&&

$$

PUBLIC FORECAST: RGass/Dykema
AVIATION: W Pi
MARINE: BAM
CLIMATE: BAM
FIRE WEATHER: Dykema


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