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

Current Version |  Previous Version |  Text Only |  Print | Product List |  Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
281
FXUS66 KMTR 210612
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1112 PM PDT Sat Jul 20 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 210 AM PDT Sat Jul 20 2024

A bit cooler this weekend as a weak system moves through. Warm up
still slated for early next week with moderate to major HeatRisk.
Potential for a more substantial cool down towards the end of next
week.

&&

.SHORT TERM...
Issued at 230 PM PDT Sat Jul 20 2024 (This evening through
Sunday)

Afternoon satellite imagery shows clouds over the coastal waters.
As noted on the morning update, clouds are struggling to clear
for the southern half of Monterey Bay - aka No Sky July.

The bigger weather story for the nearterm and for the rest of the
weekend will be the relative cooldown. Latest 24 hour trends
shows many locations running 5 to 15 degrees. Max temps this
afternoon will top out in near 60 to lower 70s coast/bays and 80s
to upper 90s inland. Even with the overall cooldown this afternoon
a few pockets of 100 plus will be possible far interior N Bay and
S Monterey/San Benito. The reason for the cooldown? Two fold:
stout onshore flow and a passing upper level shortwave trough over
NorCal. The current SFO-SAC gradient is near 4mb and some short
term guidance even has it exceeding 5mb later this afternoon. The
stronger onshore flow will also result in some gusty winds this
afternoon/evening, especially gap/passes (like Pacheco and
Altamont). Finally, the passing shortwave will lower 500mb heights
and lower 850mb temps briefly. An additional cooling of 5-10
degrees will possible on Sunday with another cloudy start to the
day with marine stratus.

&&

.LONG TERM...
Issued at 300 PM PDT Sat Jul 20 2024 (Sunday night through next
Friday)

Weather impacts for the long term will be focused on a multi-day
heat event and a non-zero chance for thunderstorms.

The passing shortwave trough will ride northward Sunday/Monday to
the PacNW and BC. Behind it, the high pressure currently over S NV
and the Desert SW will build westward. The building high will
bring rising 500mb heights and much warmer 850mb temperatures.
850mb anomalies of 4-8 or values in the upper 20C range. Upper
20C range would definitely rival the daily max for OAK upper air
sounding site. Despite the building ridge the marine layer will
remain, at least some form of it, next week. Therefore, coastal
areas will once again be spared from the big heat. Monday will be
the ramp up day temperature wise with Tuesday/Wednesday being peak
heat and moderating Thursday. Temperature spread will be wide
from the coast to far inland, typical in these scenarios with no
offshore flow component. Temperatures will generally be in the mid
60s to near 80s coast, 80s to lower 90s bays, and 90s to low 100s
interior. Highest potential over far inland areas could reach up
to 110. Overnight lows will be mild for lower elevations, but warm
in the hills with some spots staying in the 70s. Therefore, given
the multi-day scenario with hot daytime temps and mild nights
have issued a mix of Heat Advisories and Excessive Heat Warnings.
Interior N Bay, S Bay, Santa Cruz Mts, and S Salinas Valley are in
a Heat Advisory Monday through Wednesday, which aligns with
moderate to pockets of major HeatRisk. Interior E Bay, Interior
San Benitio, S Interior Monterey are in an Excessive Heat Warning,
which aligns with major to pockets of extreme HeatRisk. May need
to extend heat products through Thursday, but confidence is lower
at this time. Cooler weather arrives late next week and next
weekend.

Now for the non-zero threat of thunderstorms. Not only is there
high pressure building westward from the Desert SW there is also
an upper low spinning off the CA coast early next week. The upper
level steering flow will help to advect some lingering Monsoon
moisture over the region Tuesday into Wednesday. Longer range
guidance does show marginal upper level instability, which
overlaps the 700-500mb moisture. If this scenario pans out some
high-based convection will be possible. Not all guidance is sold
on this scenario right now, but something we`ll be watching
closely over the coming days.

&&

.AVIATION...
(06Z TAFS)
Issued at 1112 PM PDT Sat Jul 20 2024

The marine layer depth varies from 1,000 feet to 1,300 feet per
recent profiler data and surface observations. Coastal stratus and
fog /LIFR-IFR-MVFR/ will continue to develop and gradually move
inland on onshore breezes overnight, far inland it`ll remain VFR.
Improving conditions to MVFR-VFR will occur back to the coastline
by late Sunday morning.

Vicinity of SFO...MVFR ceiling overnight and Sunday morning, improving
to VFR by 19z. West wind prevailing during the period, becoming gusty
to near 30 knots Sunday afternoon and evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...LIFR-IFR tonight and Sunday morning, improving
to MVFR-VFR by late Sunday morning. Winds mainly onshore 5 to 15
knots.

&&

.MARINE...
(Tonight through next Friday)
Issued at 1029 PM PDT Sat Jul 20 2024

Fresh to strong northwest winds continue with gale force gusts
continue over the majority of the coastal waters. Winds continue
to strengthen and spread over the waters through the end of the
weekend before gale force winds weaken for all areas, excluding
the northern outer waters, by the early work week. Gale force
winds continue over the northern outer waters through mid week
before weakening to a fresh to strong breeze. Short period seas
will build to 10 to 13 feet with conditions expected to
deteriorate and become very rough by Sunday. Elevated wave heights
over the outer waters will persist through midweek.

&&

.FIRE WEATHER...
Issued at 900 AM PDT Sat Jul 20 2024

...Near critical fire weather conditions continue through
Saturday due to hot and dry conditions inland...

Just finished conference calls with fire agencies and partners.
Biggest near term concerns will be near critical fire weather
conditions today, especially later this afternoon/early this
evening. While a marine layer developed overnight, it is rather
shallow and dry conditions persist above it (1,000 feet).

While today will be a few degrees cooler, far interior locations
will remain hot and dry. In addition, onshore flow persists and
recent guidance suggests rather strong onshore flow for about 3
to 6 hours late this afternoon. This will allow for a period of
time with stronger bursts of wind (30-40 mph) with low RH. Any
fire starts will likely spread quickly. Areas of greatest concern
will be the gaps/passes of the East Bay with very flashy fuels.

Further cooling is expected on Sunday and thankfully weaker winds.

Looking farther down the road, hot and dry conditions rebuild
early next week. Will also be keeping a close eye on some upper
level moisture, which at the moment has a non-zero for high-based
thunderstorms.

MM

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
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
     CAZ503-504-506-512-513-516.

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

PZ...Gale Warning until 3 AM PDT Sunday for Pigeon Pt to Pt Pinos 10-
     60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Pinos to Pt Piedras
     Blancas 0-10 nm.

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

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

&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...Canepa
MARINE...SO

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea