St Helena, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

487
FXUS66 KMTR 270805
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1205 AM PST Fri Dec 27 2024

...New SYNOPSIS, SHORT TERM...

.SYNOPSIS...
Issued at 1204 AM PST Fri Dec 27 2024

Multiple rain systems will move across northern California through
the weekend, bringing periods of rain to the region. Dangerous
surf and beach conditions are expected into the weekend. Dry
weather should arrive for the early part of next week, which could
lead to some cold morning temperatures.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1204 AM PST Fri Dec 27 2024

Stratiform rain extends across the Bay Area and edges into the
Central Coast, with the rain expected to taper off later this
morning. This current rainmaker is expected to be the most intense
of the series of systems coming through northern California,
especially for locations south of the North Bay. The higher
elevations could expect up to 1-2 inches of additional rain from
this system, while the lower elevations of the Bay Area are
generally forecast for around 0.5-1" of rain, except in the rain
shadowed Santa Clara Valley, where up to 0.25" is expected. On the
Central Coast, the Monterey-Salinas region sees around 0.25" of rain
while the interior valleys could see a few sprinkles. Gusty winds
also continue through the morning, with gusts as high as 30-40 mph
along the coast, at the ridgetops, and through gaps and passes,
before tapering off later in the morning and afternoon.

Mainly dry conditions are expected for the day, with some lingering
showers possible at the higher elevations, before another round of
rain comes into the North Bay later tonight into Saturday. This rain
system is expected to be lighter, with rainfall totals 0.5-0.75" in
the North Bay mountains and up to 0.25" in the North Bay valleys,
and more concentrated, with a few sprinkles possible in the coastal
and Bayshore SF Bay Area and nothing expected in the interior Bay
Area and south of Santa Cruz.


&&

.LONG TERM...
(Friday night through next Wednesday)
Issued at 136 PM PST Thu Dec 26 2024

Zonal flow through the early portion of the weekend will let
additional disturbances pass over us, keeping our weather wet. An
upper level trough barrels through the PacNW Sunday into early
Monday, which should be the last hurrah for rain for us as ridging
builds behind it. This period of ridging will let colder air nestle
into the region, leading to some chilly mornings for the start of
the work week. Will need to keep an eye out for any sort of frost
advisories for the North Bay Valleys and interior Monterey and San
Benito counties as temperatures look to fall into the 30s. Beyond
that, ensemble and model guidance start to differ around the New
Year / beginning of January. Kept NBM guidance in for now as it
advertises light rain for the North Bay counties and San Francisco
peninsula.

&&

.AVIATION...
(06Z TAFS)
Issued at 1037 PM PST Thu Dec 26 2024

Warm frontal rain and gusty winds will move through late tonight
and early Friday morning, with a wind shift to southwesterly taking
place Friday morning. Lower level frontogenesis may help keep gusty
winds aloft (including in the hills and mountains) overnight, resulting
in marginal low level wind shear tonight, up until the wind shift line
with greater vertical mixing arrive early Friday morning. Warm air
advection and vertical lift have mainly been keeping cloud layers
MVFR-VFR, however the probability of IFR will increase in moderate
to heavy rain up through the passage of the wind shift line early
Friday morning. BKN-OVC ceilings Friday with diminishing showers
and winds Friday morning. Mainly weak 850 mb warm air advection
returns to the Bay Area (stronger warm air advection to the north)
Friday night with MVFR-VFR ceilings and light rain redeveloping.

Vicinity of SFO...MVFR-VFR except tempo IFR 08z-10z in heavy rain.
Gusty southwest wind developing by 10z Friday, gusts nearing 30 knots.
Southwest wind easing between 12z-14z. Conditions improving to MVFR
during the day. Light southeast wind, light rain and BKN009 OVC030
developing mid Friday evening.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...VFR except tempo IFR KMRY in rain, wind
becoming gusty with the wind shift line arriving ~ 11z Friday
morning, greater chance of a gusty southeast wind developing in
the Salinas Valley. Rain and wind easing later in the morning,
MVFR-VFR Friday.

&&

.MARINE...
(Tonight through next Wednesday)
Issued at 1003 PM PST Thu Dec 26 2024

A warm front with gusty southwesterly winds and rain will move  to
the east across the coastal waters and bays tonight and early
Friday morning. Another warm front will reach northernmost
California Friday evening. On Sunday a cold front will arrive from
the northwest bringing additional wet weather and gusty winds,
winds shifting to northwest Sunday afternoon. A high pressure
system will move across the coastal waters and bays bringing dry
weather early next week.

&&

.BEACHES...
Issued at 1000 PM PST Thu Dec 26 2024

A High Surf Advisory is now in effect through 4 AM Sunday. Large
NW swell will persist through late Saturday night to early Sunday
morning. Swell will be roughly 15 feet with a 15 second period,
translating to 20-30 foot breaking waves, depending on the beach
orientation and bathymetry (beach slope). The waves will peak Friday
and begin to abate through the day Saturday. While these waves are
smaller than the monster waves we got on Monday, the coast is still
dangerous. 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. Let`s get through this weekend without another
ocean fatality.

&&

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

PZ...Small Craft Advisory until 9 AM PST this morning for SF Bay N of
     Bay Bridge.

     Small Craft Advisory until 9 AM PST Saturday for Mry Bay-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 Pinos to Pt Piedras Blancas 0-
     10 nm-Pt Reyes to Pigeon Pt 0-10 nm.

     Gale Warning until 3 AM PST early this morning for Pt Arena to
     Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...DialH
LONG TERM....KR
AVIATION...Canepa
MARINE...Canepa

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