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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 300144 CCB
AFDMTR

Area Forecast Discussion...Corrected Aviation Discussion
National Weather Service San Francisco Bay Area
537 PM PST Tue Nov 29 2022

.SYNOPSIS...Cool and quiet weather conditions will prevail
through Wednesday with frost and freezing temperatures across the
interior each morning. Breezy winds and widespread rainfall will
occur on Thursday. Colder temperatures will follow with another
around of rainfall expected for the upcoming weekend.

&&

.DISCUSSION...as of 01:30 PM PST Tuesday...The proverbial calm
before the storm, but that doesn`t mean there won`t be any weather
impacts in the near term.

The dry cold front that passed through the region yesterday is now
washing out over SoCal this afternoon. In its wake, wall to wall
sunshine blankets the Bay Area and Central Coast this afternoon.
Despite ample insolation this afternoon the post frontal regime is
keeping temps down. Latest 24 hour trend is showing many
locations running several degrees colder this afternoon than
Monday afternoon. Max temps this afternoon will top out mainly in
the 50s with a few spots eclipsing 60 degrees.

Near term weather impacts will continue to be temperature based.
Similar to this morning Wednesday morning will be chilly to cold.
Clear skies and relatively light winds will lead to radiational
cooling. A few more temps in the 30s will be possible near the
coast and bays tonight, but the colder temperatures will still be
interior valleys. Coldest interior spots could even dip into the
mid 20s tonight. There will be a few climate sites that have a
20-40% chance of setting a record low, but current forecast is
just above record vales. Regardless of records, patchy frost will
be possible with concerns for the vulnerable population,
especially those with out adequate shelter.

Wednesday will feature another seasonably cool day with temps in
the 50s. Mid-high level clouds will increase through the day as a
storm system approaches from the north. By Wednesday evening a
cold front will enter far NorCal. This front is still in its
infancy off the PacNW Coast this afternoon.

As the front drops southward Wednesday night it will receive a
boost from some upper level support in the way of a digging upper
jet. As the Bay Area awaits the cold front it will be in the warm
sector with more moisture SW flow off the Pacific. Scattered
showers will be possible initially over the North Bay then
progress southward.

Thursday - Now for the main precip event. Timing hasn`t wavered
too much. A more pronounced and potent cold front will enter the
North Bay during the morning commute and then march southward
through the day. The Thursday evening commute will also be
impacted by the fropa, but mainly South Bay and locations
southward. Hi-res CAMs have been suggesting a NCFR with the fropa
and today is no different. This is further supported by various
ensembles as well. As with NCFRs do expect a period of moderate
to locally heavy precip with gusty winds. The question as of now
is thunderstorms? Some low level instability will be present along
the front, but conf isn`t high enough to add thunder to the
forecast yet. This may be added on later shifts. Steadier precip
will quickly transition to showers from N to S behind the front
Thursday afternoon and Thursday night. As this happens colder air
will arrive dropping snow levels so a transition to frozen precip
over the higher terrain is possible Thursday night into Friday.
Atmospheric River guidance put this system in a weak AR category.
This seems reasonable as the moisture fetch doesn`t stretch all
the way back to the sub-tropics. Storm total QPF hasn`t changed
much either from Wednesday afternoon through early Friday. N
Bay/Coastal Mts 1.5-3", Bay Area 1-2", Interior Monterey/San
Benito 0.75-1.25".

Friday morning looks to be down right cold behind the front. A
lot will depend on how much clearing is actually seen and how week
the winds get. For now, still predicting temps in the upper 20s
to mid 30s interior and 30s to near 40 along the coast/bays.
Patchy black ice will be possible.

A second, but much weaker, system is still on track for the
weekend with cool temps and scattered precip.

&&

.AVIATION...as of 5:37 PM PST Tuesday...For the 00Z TAFs. Near
high confidence VFR except patchy late night and morning fog and
freezing fog /VLIFR-IFR/ North Bay valleys. Guidance shows calm
valley winds too thus some spots late tonight and Wednesday
morning may have frost again in lieu of fog. Otherwise dry air
(continental air) is in place per 24 hour trends showing lower
surface dew point temperatures i.e. lower amount of water vapor
across much of the cwa.

Surface winds are a mix of late day onshore winds 5 to 15 knots
mainly near along the coast and bays, inland winds are NE to E
near 5 knots. Winds surface and aloft shift to southerly to
southwesterly with warm air advection aloft increasing later in
the day Wednesday, a stable atmospheric profile increasing the
chances of low level wind shear developing during Wednesday evening
and night; model output show 925 mb southwesterly winds nearing 35
knots Wednesday evening and night ahead of a cold front arriving
Thursday morning. Added in low level wind shear to the 30 hour
TAFs KSFO and KOAK, will add in low level wind shear remaining
Bay Area TAFs for the 06z issuance, in addition to developing
and lowering ceilings and rain.

Vicinity of KSFO...VFR forecast, a late day influx of westerly
wind has locally bumped up humidity a little, fog not forecast
currently, but with radiatively cooling temperatures under a
clear sky tonight and Wednesday morning will monitor satellite and
surface observations for potential nearby patchy fog development.

KSFO Bridge Approach...Similar to SFO.

Monterey Bay...VFR, light and variable winds becoming easterly to
southeasterly cold air drainage winds 5 to 15 knots tonight and
Wednesday morning.

&&

.CLIMATE...

Daily Record Low Temperatures Nov 29-30

                 11/29 Record Low   Record Year    Forecast Low

Santa Rosa coop        25               2019            30
Kentfield              28               1931            39
Napa State Hospital    28               2015            34
Richmond               37               1906            40
Livermore coop         25               1975            35
SF downtown            40               1906            42
SFO                    36               1954            43
Redwood City           28               1931            40
Half Moon Bay          34               2006            39
Oakland Museum         36               1975            40
San Jose Airport       22               1896            39
Gilroy                 28               1976            35
Salinas Airport        27               2004            38
King City              23               2004            31

                 11/30 Record Low   Record Year    Forecast Low

Santa Rosa coop        26               1933            29
Kentfield              26               1954            39
Napa State Hospital    27               1936            33
Richmond               36               1954            39
Livermore coop         25               1969            34
SF downtown            41               2004            41
SFO                    31               1954            41
Redwood City           29               1954            38
Half Moon Bay          32               2004            38
Oakland Museum         38               2004            39
San Jose Airport       25               1896            37
Gilroy                 28               1969            33
Salinas Airport        29               1933            35
King City              22               1954            28

&&

.MARINE...as of 3:46 PM PST Tuesday...Stronger northwest winds
linger making for hazardous seas, especially for smaller crafts
but these winds look to reduces further into the day. Expect
stronger to winds return Thursday and switch to the south ahead of
the next cold front that will bring rain over the waters for the
end of the week. While a fresh northwest swell continues through
the waters, a new northwest swell will arrive on Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...NONE.

&&

$$

PUBLIC FORECAST: MM
AVIATION: Canepa
MARINE: Murdock

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