Los Altos Hills, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

645
FXUS66 KMTR 281134
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
334 AM PST Sat Dec 28 2024

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 1227 AM PST Sat Dec 28 2024

Some light rain in the forecast through Saturday, but mostly in
the North Bay. A cold front will bring heavier rain and strong wind
Sunday morning, followed by clear skies and cold morning
temperatures starting Monday and persisting through most of the
week.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1227 AM PST Sat Dec 28 2024

Any remaining rain is very light tonight. A triple point
(occluded, cold and warm front intersection) is making land fall
near the CA/OR border, with most rain confined to far northern
California and wind in the Pacific Northwest. That boundary will
meander north of us through the day Saturday, keeping most of the
light rain confined to the North Bay. Most of the area will
actually be under the influence of high pressure anchored SW of
southern California for the next 24 hours. This will keep the
weather fairly benign in terms of wind and rain, but is already
doing what high pressure does after a rainy day - supporting dense
fog. The impacts are currently focused on the Central Coast where
the winds are calm. Despite the high cirrus, temperatures have
dropped to the mid 40s inland and low 50s along the coast, with
further cooling expected. The visibility has dropped to 1/4 mile
from Monterey to Paso Robles and should only get worse as the
night wares on. With the long winter nights, these foggy mornings
can really drag on, and the dense fog advisory extends through 11
AM for the southern Monterey Bay and Salinas Valley zones.

&&

.LONG TERM...
(Sunday through Friday)
Issued at 1227 AM PST Sat Dec 28 2024

It`s going to rain Sunday morning as the ridge moves inland and a
negatively tilted, low amplitude trough brings a cold front to
the area. This system will move through quickly, which means the
rain and wind may be strong at times, but won`t stick around all
day. Ample moisture will be available from a narrow plume of
subtropical water vapor. PWAT vales expected to be in the 1.1-1.3"
range ahead of frontal passage, or about 200-250% of normal.
Dramatic drying will follow the front as northerly winds finally
return and the skies clear.

These clear skies will usher in our next forecast challenge of the
week. Cold mornings starting Monday. With long nights, northerly
winds, dry air and clear skies, radiational cooling will be
optimized. The actual air mass doesn`t look too cold, with 850
temps only dropping to 2-5 C, or the 25-40th percentile for this
time of year. As a result, this cooling will be dominated by
radiational cooling, and confined mainly to the boundary layer.
Since this surface layer is so shallow, the forecast is a
challenge. The deterministic NBM is several degrees warmer than
the NBM ensemble mean. This is likely due to the 60 day running
bias correction applied to the deterministic NBM. December has
actually been quite warm. The minimum temperature has been 2
degrees F above normal at downtown San Francisco and 4.4 degrees
above normal at San Jose. The problem is that this new air mass is
totally different than the string of warm rain events with ample
cloud cover, high moisture and SE winds. The bias is not
calibrated for this new pattern. As such, I blended the NBM
deterministic and ensemble mean minimum temperatures from Monday
through Thursday. If my theory is correct, why not just use 100%
ensemble mean and abandon the deterministic NBM? Honestly it`s a
hedge, I`m not certain my theory is correct and the NBM is hard to
beat on the whole. We may lean more and more into the ensemble
mean on subsequent updates, but we`ll wait and see what the higher
resolution models come up with over the next couple runs. Either
way, inland areas will surely drop into the 30s next week.

The zonal pattern becomes pretty stable next week, with a low
amplitude ridge remaining to our north. That will bring pretty
benign weather, outside of the cold mornings and some fog here and
there. The next potential rain maker is on Friday, but even if
that happens, it looks drier than normal the following week.

&&

.AVIATION...
(12Z TAFS)
Issued at 334 AM PST Sat Dec 28 2024

Low clouds and fog, including patches of dense fog will continue
this morning. Light rain mainly in the North Bay today and tonight.
MVFR-LIFR during the morning, slowly improving to VFR-MVFR during
the day though conditions may be slow to lift per HREF showing
moderate-high probability of IFR continuing through morning.

Vicinity of SFO...MVFR and light southeast through morning, light
southwest wind in the afternoon and early evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...MVFR-LIFR and light winds. Patchy dense
fog this morning.

&&

.MARINE...
(Today through Thursday)
Issued at 328 AM PST Sat Dec 28 2024

An approaching cold front will cause gusty winds and choppy seas
tonight into Sunday. Long to moderate period swell will continue
to run through the waters for the next several days.

&&

.BEACHES...
Issued at 1227 AM PST Sat Dec 28 2024

A High Surf Advisory is in effect through 4 AM Sunday. Large NW
swell will persist through late Saturday night to early Sunday
morning. The waves are beginning to abate, but will still be in
the 18-25 ft range through the day. Inexperienced swimmers should
stay out of the water, wave watchers should maintain a safe
distance, and no one should ever turn their back on the ocean.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...High Surf Advisory until 4 AM PST Sunday for CAZ006-505-509-529-
     530.

     Dense Fog Advisory until 11 AM PST this morning for CAZ516-528-
     530.

PZ...Small Craft Advisory from 3 AM to 3 PM PST Sunday for SF Bay N
     of Bay Bridge.

     Small Craft Advisory until 3 PM PST this afternoon for Mry Bay.

     Small Craft Advisory until 9 PM PST Sunday for Pigeon Pt to Pt
     Pinos 0-10 nm-Pigeon Pt to Pt Pinos 10-60 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.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...Canepa
MARINE...Canepa

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