150
FXUS66 KMTR 190523
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1023 PM PDT Fri Apr 18 2025

...New AVIATION...

.SYNOPSIS...
Issued at 207 PM PDT Fri Apr 18 2025

Seasonal to slightly above normal temperatures in the mid to upper
70s are expected through Tuesday. Cooler temperatures and some
potential for light rain return mid next week.

&&

.UPDATE...
Issued at 845 PM PDT Fri Apr 18 2025

The forecast remains on track this evening with no changes. Drier
boundary layer air has gradually spread south across our area today.
Little to no stratus, fog or drizzle is expected for the North Bay,
South Bay and most of the East Bay. Low level onshore flow will keep
coastal areas from San Francisco south past the Big Sur with a
better chance of impacts from low level stratus/fog overnight into
Saturday morning, but will remain confined mostly to the immediate
coastline. A gradual warm up will continue tomorrow, with most areas
seeing high temperatures a few degrees warmer than what was realized
this afternoon.

&&

.SHORT TERM...
(This evening through Saturday)
Issued at 207 PM PDT Fri Apr 18 2025

While not quite as deep as yesterday, the marine layer was deep
enough to support a fairly impressive layer of cloud cover across
the Bay Area and Central Coast. Cloud cover is rapidly dissipating
with most sites expected to clear by early this afternoon. As of 1PM
most sites are in the mid to upper 50s but temperatures should start
to rise rapidly as cloud cover clears out. High temperatures today
are forecast to peak in the upper 60s to mid 70s. As we head into
the evening/overnight hours, the marine layer will compress to
around 500 ft as high pressure builds. This will help to keep any
stratus that is able to develop confined to coastal regions, to the
Monterey Bay, and to the San Francisco Bay shoreline. With clearer
skies forecast overnight across the interior the question becomes,
what about fog? Well it all comes down to moisture. High
resolution guidance is showing much lower PWAT values across the
region (generally less than 0.5") tonight in comparison to the
past few nights. The combination of lower PWAT values and a
compressed marine layer suggests that any moisture will be
confined to the coast and not extend into the interior Bay
Area/Central Coast. The two main factors contributing to drier
interior overnight conditions would be our building high
pressure/ridging and continued offshore flow Friday evening into
early Saturday morning. This offshore flow will be light, but, it
should serve to transport drier, continental air into the Bay
Area overnight thus decreasing moisture across the interior and
inhibiting widespread fog development.

Diving a little deeper into offshore flow, generally light offshore
winds have been observed across the region. The exceptions are
the highest peaks of the North Bay Interior Mountains where Mt.
St. Helena West (elevation 4340 ft) reported a peak gust of 57 mph
early this morning. Elsewhere, winds have generally been calmer
with peak winds between 20-35 mph observed. The synoptic setup
remains progressive with the positively tilted trough set to
weaken and eject eastward while high pressure continues over the
West Coast. Winds will then transition from offshore back to
onshore again by early to mid Saturday morning (remaining light)
as ridging becomes more zonal/less positively tilted over the West
Coast.

&&

.LONG TERM...
(Saturday night through next Thursday)
Issued at 207 PM PDT Fri Apr 18 2025

Saturday night into Sunday, a weak shortwave trough will move
through the region before zonal flow and ridging redevelop Sunday
through Tuesday. The main impact of this shortwave will be to deepen
the marine layer Saturday into Sunday and bring more widespread
overcast conditions and some potential for fog to both the coast and
interior regions. By Sunday night, zonal flow will again dominate
with the marine layer expected to compress below 500 ft.
Temperatures will remain fairly seasonal through Tuesday with highs
in the mid to upper 70s across the interior and upper 50s to mid 60s
along the coast with overnight temperatures in the 40s to low 50s.
Minor HeatRisk, primarily impacting those incredibly sensitive to
heat, is forecast across urban areas through mid next week. If you
are sensitive to heat, remember to take breaks while outdoors and
drink plenty of water.

The pattern changes mid week when an upper level trough is set to
deepen and move into the West Coast. Cluster guidance has come into
slightly better agreement (in comparison to yesterday) that
troughing will develop mid week but there is still variance as to
how deep the trough will be. Temperatures are expected to drop,
becoming below seasonal normals, with highs in the upper 50s to low
60s, as this trough moves inland Wednesday and Thursday. Ensemble
guidance indicates some potential for light rain as this system
arrives but confidence is low as to the exact amounts. The CPC has
below normal temperatures and leaning above normal precipitation
continuing from the middle of next week through the end of April.

&&

.AVIATION...
(06Z TAFS)
Issued at 1023 PM PDT Fri Apr 18 2025

Satellite shows low ceilings have pushed inland over Monterey Bay
and along the coast causing IFR/MVFR conditions over KSNS, KMRY and
KHAF. Most models show agreement of MVFR conditions building over
most of the terminals (with the exception of KLVK) but there is some
uncertainty as some offshore winds should keep things a bit on the
drier side and keep the marine layer confined to the coast. VFR
expected to return by 18Z for all terminals, but MVFR conditions are
anticipated to return by Saturday night.

Vicinity of SFO...VFR currently, but low stratus has developed along
the coast. Models vary with bringing MVFR ceilings in tonight.
Therefore, moderate confidence on MVFR building near 10Z, as there
is a chance that a low SCT group may prevail through the night. If
MVFR conditions occur, expect VFR near 17-18Z. MVFR ceiling will
return by late Saturday night.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR/IFR conditions will prevail until 18-
19Z, where VFR is expected to return. Timing is a bit uncertain as
conditions may clear closer to 20Z. MVFR conditions may prevail
through most of KMRY TAF period. Expect both terminals to see a
return of MVFR by 03Z.

&&

.MARINE...
(Tonight through next Thursday)
Issued at 845 PM PDT Fri Apr 18 2025

Northerly flow increases going into early Saturday, especially
over the northern outer waters, as a weak cold front approaches
from the north. The increasing winds will result in a building
northerly fresh swell. Stronger northerly winds will last into the
next week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 PM PDT Sunday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...SO
MARINE...SO

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NWS MTR Office Area Forecast Discussion