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

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000
FXUS66 KMTR 250542
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1042 PM PDT SAT SEP 24 2016

...RECORD HIGH TEMPERATURES OVER THE BAY AREA AND NORTH CENTRAL
COAST SUNDAY AND MONDAY...

...HEAT ADVISORY FOR THE BAY AREA AND NORTH CENTRAL COAST 11 AM
PDT TO 8 PM PDT SUNDAY...

...RED FLAG WARNING FOR THE NORTH BAY MOUNTAINS THROUGH 3 PM PDT
SUNDAY...

.SYNOPSIS...A strong upper level ridge of high pressure over the
eastern Pacific, Pacific Northwest and California will result in
record high temperatures Sunday and Monday. Moderate levels of
mugginess will accompany hot temperatures again Sunday, possibly
Monday as well. Strong offshore flow is also in early stages of
development this evening. Expect offshore flow to persist through
at least Monday with exception of very minor and localized intrusions
of cooler marine air on the immediate coastline. Record warmth
through early next week will be followed by subtle cooling, mainly
on the immediate coast, by the middle to later part of next week.

&&

.DISCUSSION...as of 10:42 PM PDT Saturday...Strong high pressure
for late September /and early October/ time frame is over the
eastern Pacific with overlap onto the Pacific Northwest and
California. 593-595 decameter 500 mb geopotential heights through
Sunday evening are close to the seasonal maximum for all KOAK
soundings back to 1948 per SPC`s sounding climatology page. Based
on model guidance, relatively high sun angle for late September,
and 850 mb temps brought to sea level via the dry adiabat high
temperatures will quite likely climb close to record levels Sunday
and Monday; high temps 90s/lower 100s. Record highs are listed
below.

Temperatures Sunday and Monday will warm primarily through large
scale subsidence coinciding with a period of offshore wind flow.
Also of note, the NAM model and to some degree the GFS model both
have had an ongoing onshore bias with respect to the SFO-WMC
gradient. For the NAM model this onshore bias, upwards of 7 mb to
8 mb, has been present through the summer. The WMC-SFO gradient
will thus likely sustain well up over 10 mb tonight through early
next week, then decrease in strength by the middle of next week.
Also, noting a precipitable water value 0.70" on KOAK upper air
sounding and area dewpoint temps hovering in the 50s this evening;
sort of unusual to see high levels of moist air trapped surface to
lower levels given the developing offshore flow pattern. Origin of
this water vapor is from central Pacific advected latitudinally
northward then southeastward into northern California out ahead of
the eastern Pacific high pressure system. New 00z GFS PW forecasts
show the PW only decreasing to approx 0.60" through early next
week, thus remaining a bit elevated for the given pattern we`re
entering thus expect at least moderate levels of mugginess to hold
in the air Sunday and Monday for locations nearest sea level, much
drier air will be along hill tops and in the mountains.

Made an update earlier this evening. Update was made to increase
the winds somewhat for this evening through Sunday evening for
the North Bay mtns, East Bay hills/mtns, Santa Cruz mtns and Santa
Lucia mtns and along the San Francisco Peninsula coast. High
resolution WRF model indicates increasing E-NE winds developing in
the coastal gaps along San Mateo county by early Sunday morning.
Hot temps peak Sunday and Monday.

&&

.AVIATION...As of 10:12 PM PDT Saturday...VFR. Non-convective low
level wind shear is a possibility late tonight/Sunday at KOAK.
Offshore wind during the period with minor influxes of onshore
flow possible on the immediate coast late Sunday afternoon.

Vicinity of KSFO...VFR. Light winds.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR. Light offshore winds overnight then
light onshore flow again by Sunday afternoon.

&&

.FIRE WEATHER...As of 5:05 PM PDT Saturday...Models continue to
trend towards stronger offshore winds tonight and Sunday morning,
especially in the North Bay mountains. 12Z WRF model forecasts
sustained northeast winds of around 32 kt at 925 mb over Napa and
eastern Sonoma counties late tonight into Sunday morning. This
equates to about 38 mph. Gusts may be as high as 55 mph. These
winds will be following and preceding very warm to hot daytime
temperatures. No nighttime humidity recovery is expected. Ridges
will actually see a RH minimum overnight. As a result of the
strong offshore flow and forecast low RH the red flag warnings
remains in place for the North Bay Mountains (Fire Weather Zone
507). Otherwise, expect the warming and drying trend to continue
through Monday across the entire district. Winds will generally be
light after Sunday. A cooling trend will begin on Tuesday.

In the vicinity of the Soberanes Fire: Portable RAWS stations
above 3000 feet are already showing light offshore flow. The local
WRF is indicating east winds picking up tonight as the thermal
trough offshore strengthens. By late Sunday morning winds in this
area are forecast to decrease slightly then gradually turn to the
south. Temperatures will climb during the day Sunday and again
Monday while RH values drop into the single digits. Monday will
likely be the warmest and driest day, with gradual improvement
expected to begin on Tuesday.

&&

.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 9:01 PM PDT Saturday...Moderate northwest winds
will persist into the evening and then taper off through the night
as the surface pressure gradient weakens over the coastal waters.
Winds are forecast to continue easing through Sunday. A small to
moderate mixed swell will persist through the upcoming week.

&&

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

&&

$$

PUBLIC FORECAST: Canepa
AVIATION: BFG
MARINE: BFG
CLIMATE: BAM
FIRE WEATHER: BFG


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