National Weather Service United States Department of Commerce

Dangerous Heat in the Western U.S.; Flash Flooding Possible Across Portions of the South

High temperatures in the 90s to 100s and warm overnight temperatures will continue across parts of the Interior Northwest, central California, and the Great Basin. Thunderstorms and heavy rain may produce scattered flash flooding across much of the Southern Rockies into the Southwest, particularly over sensitive burn scars in New Mexico, and across the Southeast into the Carolinas. Read More >


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FXUS66 KMTR 212053

Area Forecast Discussion
National Weather Service San Francisco CA
153 PM PDT Sun Jul 21 2024


Issued at 132 AM PDT Sun Jul 21 2024

Another day of temperatures cooling slightly and a deeper marine
layer, followed by another warm up through this week. A Heat
Advisory and Excessive Heat Warning has been issued for inland
areas, in effect Monday morning through Wednesday night.


Issued at 105 PM PDT Sun Jul 21 2024
(This evening through Monday)

The longwave wave pattern over the West today features an upper
low spinning off the PacNW coast, an embedded shortwave trough
rounding the base of the upper low, and an upper ridge over
NV/Desert SW being buffeted by the shortwave. What does this mean
locally? The passing shortwave to the north help to nudge the
ridge eastward cooling the overall airmass and keeping onshore
flow with a marine layer in place. The much talked about cooldown
for Sunday has come to fruition - latest 24 hour trends show even
more cooling along the coast, but also the interior. Interior
East Bay has cool roughly 15 degrees from yesterday to today.
While interior locations had a few 100 plus readings on Saturday,
one will be hard pressed to find a 100 plus reading today. As for
onshore flow, it still persists with SFO-SAC still showing 3+mb.
Still seeing gusts of 30+ mph for East Bay gaps/passes.

Tonight through Monday: The shortwave trough will continue to
push northward into S Canada. Locally, high pressure that was
nudged eastward will build westward. The building ridge aloft will
kick off a marked warming trend across the Bay Area and Central
Coast, especially across the interior. Temperature jump from Sun
to Mon will be roughly 5-15 degrees. At the surface, onshore flow
will remain so the marine layer will bring night/morning clouds.
The lingering marine layer will keep coastal temperatures in check
for Monday as well. Forecast highs for Monday - 60s to mid 70s
along the coast, 70s to near 90 SF Bay, and 90s to lower 100s
interior. The hot weather inland will elevate HeatRisk to moderate
to major. An Excessive Heat Warning remains for far inland areas
of the East Bay, eastern Santa Clara county, and interior Monterey
and San Benito counties. A Heat Advisory remains for the inland
North Bay, Santa Cruz Mtns, Santa Clara Valley, and the southern
Salinas Valley.


Issued at 105 PM PDT Sun Jul 21 2024
(Monday night through next Saturday)

Further strengthening of the ridge will continue through
Wednesday. The airmass associated with the ridge will yield 850mb
temperatures in the upper 20C range. While these values are well
above the 90th percentile and near the daily moving max (per KOAK
sounding climo) they are not the same as what we saw earlier in
July. Not expecting records to break for max temperatures.
Regardless dangerous heat is still expected and heat related
hazards will remain through at least Wednesday. Tuesday will
likely be the hottest day with far inland locations in the
100-108F range with major to extreme HeatRisk. These areas have
the greatest likelihood for heat related illnesses. Not just for
the young/elderly, but everyone who doesn`t take precautions.
Stay hydrated, limit afternoon outdoor activities, and never leave
pets/children in a vehicle. As previously mentioned, the marine
layer will keep the coast cooler, but given the airmass potential
above normal temperatures are expected. The ridge begins to shift
eastward late Wednesday into Thursday as an upper level low moves
northward on the western periphery of the ridge. The shifting
ridge and approaching upper low will initiate a cooling. Thursday
will feature some cooling inland, but more widespread cooling is
expected Friday and into next weekend.

The other item of note will be the influx of upper level moisture
Tuesday into Wednesday as the ridge to the east and upper low to
the west allow for southerly flow aloft. PWATs will increase over
the region as earlier as Tuesday, but peak Tuesday night into
Wednesday at 200% of normal for late July. While there is ample
moisture aloft instability is somewhat lacking. While guidance
shows decent MUCAPE and marginal lapse rates, CINH/inhibition is
also pronounce. Would like to see less CINH before introducing
tstorms over the forecast area. For now, will keep mentioning a
non-zero chance, but more likely for increasing mid-high level
clouds. We`ll be watching this closely as it could have big fire
weather impacts.


(18Z TAFS)
Issued at 1058 AM PDT Sun Jul 21 2024

Gale conditions over the nearshore ocean waters are helping to
generate eddy`s that are keeping low clouds across some of the
main terminals in our forecast area. SFO, OAK, and MRY are all
impacted. Models are now indicating little to no clearing much of
the day. As such, the 18Z TAF package has taken a much more
pessimistic view of the forecast. While there will be breaks at
the terminals, expect them to be short lived. The others, STS,
LVK, SJC, APC, have generally cleared out. However, low clouds
will make a return tonight to STS and APC. As high pressure builds
across the area overnight into tomorrow, this should provide
enough subsidence to keep clouds out of LVK and to some degree SJC
tonight. VFR will prevail at those locations.

Vicinity of SFO...Models are indicating clouds hanging on through
the day at SFO as they continue to filter through the San Bruno
gap. Of course SFO is right on the edge of the clouds. The
forecast now keeps clouds in all day, but in practice there will
be short periods of clearing before the clouds come back in. For
the most part expect MVFR conditions. Winds will become gusty
this afternoon as temperatures warm across the Bay Area. Tonight
is looking to be similar to last night, but as high pressure
builds in, tomorrow we will see clearing occur mid morning.

SFO Bridge Approach...SMB looks to be on the southern end of the
cloud deck that is lingering around. For the most part SMB should
sit in VFR through the day before clouds roll back in this evening
and overnight, bringing it back down to MVFR.

Monterey Bay Terminals...SNS will pop out of the clouds for a bit
midday, otherwise the Monterey terminals (MRY and SNS) will hang
out in the clouds the vast majority of the day. Expect a repeat of
the low clouds and drizzle from last night, tonight. It`s just
going to be one of those days of No Sky July.


(Today through Friday)
Issued at 428 AM PDT Sun Jul 21 2024

Gusty northwest winds continuing with gales over the majority of
the coastal waters. Winds gradually easing over the inner and
southern coastal waters from early to mid week. Short period seas
building to 10 to 13 feet with sea conditions rough today.
Elevated wave heights over the outer waters will persist through


CA...Coastal Flood Advisory until 2 AM PDT Tuesday for CAZ006-506-508.

Heat Advisory from 11 AM Monday to 11 PM PDT Wednesday for

Excessive Heat Warning from 11 AM Monday to 11 PM PDT Wednesday
for CAZ510-514-515-517-518.

PZ...Gale Warning until 9 PM PDT this evening for Pt Arena to Pt
Reyes 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt
to Pt Pinos 0-10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

Gale Warning until 9 PM PDT Monday for Pt Arena to Pt Reyes 10-
60 NM.

Gale Warning until 3 AM PDT Monday for Pigeon Pt to Pt Pinos 10-
60 NM.




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