836
FXUS66 KMTR 222245
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
245 PM PST Wed Jan 22 2025

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

.SYNOPSIS...
Issued at 235 PM PST Wed Jan 22 2025

Chilly to cold conditions will continue tonight with a gradually
moderating temperatures. A slightly stronger push of offshore
winds are forecast on Saturday with gusts averaging between 30
and 40 mph with across the higher terrain of the North and East
Bay regions. Opportunities for measurable rainfall are confined
largely to the Central Coast this weekend with a non-zero chance
for an isolated lightning strike or two. Dry conditions are
forecast into next week.

&&

.SHORT TERM...
Issued at 235 PM PST Wed Jan 22 2025 (This evening through
Thursday)

While record breaking cold and a little snow continues across the
eastern US, largely quiescent conditions are forecast tonight and
into Thursday across the Bay Area and Central Coast. We will
likely have to contend with at least another night of chilly to
cold conditions across the North Bay, East Bay, and Central Coast
interior. We`ll roll with another Frost Advisory where
temperatures fall into the mid 30s tonight into Thursday morning.
A few locales in far southern Monterey County have around a 70-80%
or greater chance of dipping below 32 degrees, but the small
spatial extent doesn`t warrant a separate freeze product. Regardless,
take preparations to mitigate impacts due to cold for people,
pets, plants, and sensitive infrastructure.

Thursday is shaping up to be another pleasant and dry day with
diurnal wind patterns. This means largely offshore during the
overnight and early morning hours, with onshore flow returning
late in the afternoon. High temperatures are anticipated to
average between the 50s and 60s across most regions, except across
the Salinas and Santa Clara Valleys where temperatures will climb
into the 70s.


&&

.LONG TERM...
Issued at 235 PM PST Wed Jan 22 2025
(Thursday night through next Tuesday)

Key Messages
*Stronger offshore flow regime Saturday
*20-30% chance for showers across the Central Coast on Saturday
*20-30% chance for a rain/snow mix at or above 4,000` in the Santa
 Lucias on Saturday
*10% chance for very isolated thunderstorms


Thursday Night into Friday morning will feature weaker offshore
flow during the nighttime hours. This should translate to a
slightly stronger onshore or even calm wind pattern in some
locations and help keep temperatures in check. Cloud cover will
also invade portions of the North Bay. With less optimal
conditions for radiational cooling due to clouds and less in the
way of offshore flow, most areas are forecast to remain above
32-34 degrees. The exception will be some of the sheltered/low-
lying regions away from the coasts. At this time, cold weather
headlines may not be needed, but future shifts will examine
trends and latest NWP.

The increasing cloud cover noted above will precede our next
upper storm system. The general synopsis is that an upper low will
dive southward through the Central Valley late Friday and into
Saturday. Some deterministic guidance advertises in excess of 150
meter 12 hour height falls with this feature. These 12 hour height
tendencies are a sign of robust ascent. The GFS and its
associated ensemble members advertise a much lower 500mb height
anomaly (so a stronger storm system) compared to the EPS and GEPS
ensemble members. The deterministic NBM forecast does output
mentionable PoPs (perhaps influenced by the GEFS suite of
guidance) largely across the Central Coast late Friday and into
Saturday. This seems to be a pretty decent starting point for PoPs
given these types of tricky patterns where the low levels remain
dry, but ascent is strong. Forecast soundings do show increased
moistening (RH > 70%) within the 800-900mb layer with lapse rates
approaching 7 C/km atop that. With the aforementioned ascent, it
won`t take much to generate widespread shower-type activity. The
radar scope will likely appear very illuminated Saturday, however,
as forecast soundings indicate dry low levels. This will
translate to virga for *most* locations. Outside of some minor
tweaks for consistency, we`ll advertise 20-30 PoPs as a result.
The greatest opportunity for measurable precipitation is forecast
across southern Monterey and San Benito County as well as across
the higher terrain. If lapse rates steepen further, a lightning
strike or two cannot be ruled out. NBM probability of thunder
amounts around 10% seem reasonable, and we`ll use more of the
high-res output as we venture closer to fine tune the thunder
forecast. It should be noted that these types of patterns can
overproduce in terms of the coverage of precipitation (including
small hail or graupel) and/or lightning.

Precipitation amounts, will remain largely below 0.10" as the
boundary layer will remain parched after an extended period of
offshore flow. Snow levels do fall down to around 4,000`, but at
this juncture, the dry low levels precludes anything above trace
accumulations of snow. Again, if this system overproduces (e.g,
stronger ascent or more moist low levels than expected) then we
may need to entertain adding measurable snow amounts across the
Santa Lucias.

Finally, with respect to the wind headlines. Overall, it appears
that wind gusts of 30 to 40 mph or greater will be confined to the
higher terrain of the North Bay and East Bay on Saturday. These
wind prone areas have a tendency to exceed the model consensus,
potentially due to the combination of momentum transfer, terrain
influences, and isallobaric forcing as high pressure builds across
the Great Basin. An addition of the higher NBM percentile NWP
into the grids appears appropriate to account for these factors
and it`s not out of the question that wind headlines for a small
portion of the Bay Area may be needed by this weekend.

Sunday afternoon and onward should see a return to tranquil
conditions across the area. However, after a slightly stronger
push of offshore flow, cold hazards may resurface, especially on
Monday morning when ideal radiational cooling conditions set up.
For now we`ll monitor trends and consider appropriately messaging
this next round of cold hazards as we progress through the
weekend.

&&

.AVIATION...
(18Z TAFS)
Issued at 1106 AM PST Wed Jan 22 2025

High confidence in VFR through the TAF period. The afternoon sea
breeze will be followed by light offshore flow tonight.

Vicinity of SFO...High confidence in VFR through the TAF period.
Light NW seabreeze will return this afternoon with a return to
light offshore flow tonight.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...High confidence in VFR through the TAF
period. The sea breeze will return to both terminals this
afternoon with easterly flow prevailing overnight.

&&

.MARINE...
(Today through Monday)
Issued at 355 AM PST Wed Jan 22 2025

Moderate to fresh northerly breezes and moderate seas are expected
through Thursday. Seas become rough to very rough by Friday with
gale force gusts expected Friday and Saturday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory from 11 PM this evening to 9 AM PST Thursday for
     CAZ506-510-516-518.

PZ...None.
&&

$$

SHORT TERM...Bain
LONG TERM....Bain
AVIATION...Behringer
MARINE...Behringer

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