Castro Valley, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

622
FXUS66 KMTR 301757
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
957 AM PST Mon Dec 30 2024

...New AVIATION...

.SYNOPSIS...
Issued at 1203 AM PST Mon Dec 30 2024

A new weather pattern will bring much colder temperatures to the
Bay Area this week. The next couple weeks look much drier than
recent trends.

&&

.UPDATE...
Issued at 859 AM PST Mon Dec 30 2024

Lingering fog and low stratus remains around the interior East and
South Bay down to Holister, the southern Salinas Valley, and
around the Watsonville area, with the rest of the region remaining
generally clear. Low temperatures this morning were somewhat
warmer than the forecast, and the operations team will take a
closer look at forecast lows for the next couploe of days with the
next update, particularly with regards to any impacts on the
Freeze and Frost products to be issued.

DialH

&&

.SHORT TERM...
(Today and tonight)
Issued at 1203 AM PST Mon Dec 30 2024

The big question over the short term is how cold will it get this
morning. Much drier air has arrived in the post frontal
environment. The precipitable water has dropped from around 1.3"
during the rain yesterday, to around 0.37" as measured by GOES 18.
We`ve been expecting this dry air to cause much colder
temperatures tonight. The forecast challenge stems from the fact
that the temperature can`t drop below the dew point. If the
temperature drops to the dew point, the air is saturated and any
further cooling will condense water vapor into liquid (cloud or
fog formation). This process actually releases heat back into the
air, and the clouds restrict radiational cooling to space, both
putting a big damper on further cooling. While the precipitable
water (water vapor through the entire troposphere depth) is
behaving as expected, the surface dew points are still elevated.
The recent rounds of rain have led to elevated soil moisture. USGC
soil moisture gauges across the region are showing around 25-30%
water content in the soil (we get worried about landslides around
35%). As the dry air moves in, this water is evaporating, which
increases the water vapor content and therefore the surface dew
point. Now this process is overshadowed by the plummeting dew
points in the new air mass, but it does work to slow the fall.
Many areas are starting to report mist starting to form and the
temperatures are still widely in the mid 40s at this time. I used
a new forecast blend (33% NBM, 33% NBM ensemble mean, 33% bias
corrected consensus of short term guidance) that warmed the
temperature a couple degrees this morning, but somehow the
confidence on a 6 hour temperature forecast is still rather low. I
decided not to make any changes to the frost advisory for now. If
things continue, the Santa Clara Valley may have to be removed.
Other than the cold temperatures in the morning, it will be a
beautiful brisk day getting into the upper 50s with lots of
sunshine and only a gentle breeze.

&&

.LONG TERM...
(Tuesday through Sunday)
Issued at 1203 AM PST Mon Dec 30 2024

Tuesday morning will be the coldest of the week. Temperatures are
expected to drop into the low to mid 30s for many inland
locations and lower 40s along the coast. While it`s becoming
increasingly rare to break daily low temperature records (22
degrees at San Jose from 1895), the last day of 2024 will bring
the coldest temperatures of the year for much of the Bay Area and
Central Coast. Radiational cooling is the dominate reason. Long
nights, calm winds, clear skies and dry air will enhance this
process. With zonal flow, the actual air mass isn`t that cold. The
850 mb (roughly 5,000 ft) temperature will be around 41 F, which
is near the Dec 31 average of 43 F. It`s much easier to forecast
large air masses than shallow, boundary layer processes. A little
bit of wind could mix warmer air down to the surface, for
instance. As such, it`s not quite clear exactly how cold it will
get Tuesday, but the bike commuters out there will want to wear
gloves for sure.

Wednesday low temps will jump up a few degrees as a plume of
moisture from the subtropical Pacific takes aim at far northern
California. This warming will be gradual, and there will very
likely still be some frost advisories for the typical cold spots
Wednesday morning. The moisture plume will slide south through
Friday, when we may pick up some light rain. There`s an equal
chance that it just brings some clouds, but the impacts will be
limited either way. More dry air moves in over the weekend and
into next week as the jet stream becomes meridional, and a more
stable ridge anchors over the Eastern Pacific. While there may be
a day of light rain Fri, the next couple weeks look dry enough to
put the cushions back on the patio furniture to me.

&&

.AVIATION...
(18Z TAFS)
Issued at 957 AM PST Mon Dec 30 2024

Patchy fog is lingering in the vicinity of LVK but satellite shows
that fog coverage is actively diminishing and is expected to
dissipate by 19Z at the latest. Winds generally remain light through
the period with NW winds persisting at most sites through the
afternoon before becoming more N to E/NE during the
evening/overnight. Generally VFR through the TAF period but fog
becomes more likely again overnight for STS, APC, and LVK. LAMP
probability guidance and ensemble guidance point towards a higher
likelihood of fog developing after 10/11Z but visibilities will
start dropping as early as 09Z. Conditions should start to improve
around 17-18Z tomorrow with fog dissipating close to the end of this
TAF period/beginning of the next TAF period.

Vicinity of SFO...VFR through the TAF period. Light winds, generally
out of the NW, with high level clouds.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR through the TAF period but slight
decreases in visibility are possible at SNS overnight. Winds
generally remain light with more easterly winds expected during the
early morning/overnight hours and more northwesterly winds expected
during the afternoon. Ensemble guidance shows some fluctuations in
visibility overnight at and in the vicinity of SNS with most keeping
visibilities above 6SM. Moderate confidence that visibilities will
not drop below 6SM at SNS but temporary lower fluctuations may
occur.

&&

.MARINE...
(Today through Saturday)
Issued at 859 AM PST Mon Dec 30 2024

Fresh northwest winds will prevail across the coastal waters into
the New Year. Breezy winds and rain will arrive ahead of a cold
front for the northern waters. The cold front will move eastward
taking the rain with it as well later Friday. Moderate to long
period swell will continue to run underneath the wind driven
waves.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Frost Advisory from 8 PM this evening to 10 AM PST Tuesday for
     CAZ502-503-505-508-510-512-513-515-529-530.

     Freeze Warning from 8 PM this evening to 10 AM PST Tuesday for
     CAZ504-506-514-516>518-528.

PZ...Small Craft Advisory until 3 AM PST Tuesday for Pigeon Pt to Pt
     Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Arena to Pt
     Reyes 10-60 NM-Pt Pinos to Pt Piedras Blancas 0-10 nm-Pt
     Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 9 AM PST Tuesday for Pigeon Pt to Pt
     Pinos 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Kennedy
MARINE...Murdock

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