St Helena, California 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

929
FXUS66 KMTR 110020
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
520 PM PDT Thu Apr 10 2025

...New AVIATION, MARINE...

.SYNOPSIS...
Issued at 320 AM PDT Thu Apr 10 2025

Warm temperatures and light winds will continue today with minor
HeatRisk concerns. Pleasant weather will continue through the
weekend into the beginning of next week.

&&

.SHORT TERM...
Issued at 250 PM PDT Thu Apr 10 2025
(This evening through Friday)

After a summer-like start to the day with low clouds, fog, and
patchy drizzle the sun came out. Despite the afternoon sun, the
much advertised cooldown arrived. A broad look at 24 hour temp
trends shows a few degrees to as much as ten degrees colder than
Wednesday afternoon. High this afternoon will top out in the 60s
to near 70 coast/bays and 70s to mid 80s interior.

Let`s take a look at some other interesting tidbits this
afternoon. The longwave pattern over the West shows a low
pressure system parked near the PacNW with an upper level trough
extending southward. If you happened to look at the radar this
afternoon you`ll notice a few echoes off the coast tracking
eastward. So what`s causing the convective activity? First, there
is a broad moisture plume extending from near HI to the PacNW and
we happen to be on the southern edge. Second, a jet max aloft is
rounding the base of the upper level trough. In fact, our region
is near the right entrance region of the jet. This region favors
vertical ascent. Third, speaking of ascent, Q-Vector (which are
theoretical and not actually in the atmosphere) analysis shows
convergence aloft. Areas of Q-vector convergence shows rising
motion in the atmosphere. Finally, meager instability aloft with
some colder advection aloft. MUCAPE and upper level lapse rates
shows pockets of instability. All that being said, broad lift,
some moisture, and weak instability has led to convective
showers. So what does that mean for the Bay Area this afternoon
through tonight? Mid-high level cumulus/convective clouds
currently on satellite will persist and advect over the region.
CAMS model guidance all suggest some predicted radar reflectivity
over the next 12 hours. The question is, will these reflectivities
reach the ground? Chances are pretty low. Pt soundings show some
mid- level dryness below 700mb. So for now, did up POPs in the
forecast but thinking most of the echoes will be virga. Closer to
the surface and under all of the mid-high level items will be the
shallow marine layer returning. As such, have added patchy fog to
the entire coastal land/sea interface. Added low POPs for a
mention of drizzle as well. Both of those are similar to what we
saw Thursday morning.

Friday: After the AM fog/drizzle end the cooling trend will
continue with an additional drop of 3-5 degrees. Still expecting
some lingering mid-high level convective clouds with possible
virga.

&&

.LONG TERM...
Issued at 330 PM PDT Thu Apr 10 2025
(Friday night through next Wednesday)

For the weekend and into next week - the upper level flow over the
Pacific becomes a little amplified. So amplified, it begins to
look split flow-ism off the West coast. The more noticeable
feature with be a cut-off low developing off CA and then
retrograding slightly. The retrograde will develop subtle ridging
over CA with building 500mb heights. Hi-res models even suggest
some bursts of offshore flow late in the weekend and early next
week. What does this mean for sensible weather? A shallow marine
layer will linger and ebb/flow. As the ridge develops a gradual
warming and drying trend will develop with temperatures returning
to more widespread 80s Sunday and Monday - minor HeatRisk as well.

The ridge breaks down midweek with cooler temperatures returning.
Still no precip (outside of drizzle) next week.

&&

.AVIATION...
(00Z TAFS)
Issued at 520 PM PDT Thu Apr 10 2025

Widespread VFR lasts into the late night. Winds stay moderate to
breezy before easing in the late evening and early night. LIFR and
IFR CIGs build along the coast into the late night and slowly move
inland into Friday morning. These CIGs will affect the SF and
Monterey Bays, KHAF, and the North Bay terminals. In addition to
CIGs, the North Bay terminals look to see moments of fog into the
mid morning. Cloud cover and fog erode into the late morning and
into Friday afternoon as more moderate winds arrive.

Vicinity of SFO...VFR lasts into early Friday. Expect winds to
reduce and become more moderate through much of the night. IFR CIGS
move into the SF Bay into the late night and fill over the terminals
early Friday morning. These CIGs dissipate in the late morning ahead
of more breezy west winds building that afternoon.


SFO Bridge Approach...IFR clouds arrive slightly later than SFO into
Thursday morning.

Monterey Bay Terminals...Moderate winds ease in the mid evening and
become light through the morning late morning. IFR/LIFR CIGs arrive
early Friday and last into the late morning. Moderate west winds
build for Friday afternoon but weaken again into the evening.

&&

.MARINE...
(Tonight through next Wednesday)
Issued at 520 PM PDT Thu Apr 10 2025

A coastal jet will result in locally stronger winds for areas
south of Point Sur causing hazardous conditions for  small craft
through Friday morning. Northwest winds will  strengthen to a
fresh to strong breeze by Friday afternoon with  near gale force
gusts, especially across the norther outer waters. A larger swell
will build over the weekend and early next week. Winds diminish
and seas subside into the middle of the upcoming  week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 3 PM to 9 PM PDT Friday for Mry Bay.

     Small Craft Advisory from 9 AM Friday to 3 AM PDT Saturday 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.

     Small Craft Advisory from 9 AM to 9 PM PDT Friday for Pt Reyes
     to Pigeon Pt 0-10 nm.

     Gale Watch from Friday evening through late Friday night for Pt
     Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 AM PDT Saturday for Pt Pinos to Pt
     Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 AM to 3 PM PDT Friday for Pt Arena
     to Pt Reyes 10-60 NM.

     Gale Watch from Friday afternoon through late Friday night for
     Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...MM
LONG TERM....MM
AVIATION...Murdock
MARINE...RGass

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