353
FXUS66 KMTR 182116
AFDMTR

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

...New SYNOPSIS, SHORT TERM, LONG TERM...

.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.

&&

.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...
(18Z TAFS)
Issued at 1034 AM PDT Fri Apr 18 2025

Another morning with a pesky marine layer. Obs, satellite, and
ACARS show depth still 2k feet deep. As such, clearing/thinning
has been slow going. Therefore, many TAF sites were pushed back
30-60 min on clearing times or closer toward 19Z. Some stratus
lurking this afternoon, but trending toward VFR. Tonight is low
confidence as offshore flow develops at 2500 ft, but doesn`t mix
down until Saturday morning. Some compression of the marine layer
is expected with impacts closer to the coast and immediate bay
shoreline. Conf for tafs is medium today and low tonight.

Vicinity of SFO...Cigs will linger through 19z. Some thinning on
cams/satellite, but delayed from prev taf. VFR this afternoon with
onshore flow. Tonight, low cigs sneak through the GG and in the
Gap, but conf is low on impacting SFO. Therefore, SFO will remain
VFR. The same can be said across the Bay at OAK.

SFO Bridge Approach...Thinning quicker over approach and clearing
earlier is expected 1830-19z.

Monterey Bay Terminals...Cigs lingering through 19z. VFR this
afternoon and early return tonight.

&&

.MARINE...
(Today through Wednesday)
Issued at 859 AM PDT Fri Apr 18 2025

Weak high pressure off the California coast will maintain light
to moderate winds today with decreasing seas. Breezy winds build
in the northern outer waters for Friday night with lighter winds
expected for the inner waters. Seas and winds become more elevated
into the weekend into the next work week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 PM this evening to 3 AM PDT Sunday
     for Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Kennedy
LONG TERM....Kennedy
AVIATION...MM
MARINE...MM

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