Area Forecast Discussion
Issued by NWS San Francisco Bay Area, CA

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FXUS66 KMTR 230302
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
802 PM PDT Fri Mar 22 2024

...New UPDATE...

.SYNOPSIS...
Issued at 121 AM PDT Fri Mar 22 2024

Look for unsettled weather for Friday into the weekend with breezy
winds, rain showers, and even a few thunderstorms. Drier
conditions by early next week with rain chances returning for mid
to late week.

&&

.UPDATE...
Issued at 746 PM PDT Fri Mar 22 2024

Rain continues across the Bay Area. Around 715 PM, winds at SFO
shifted from S to WNW as the front passed. Due to the distinct
wind shift, the radar velocity data clearly shows the exact cold
front location, which is roughly halfway between the leading and
trailing edges of the rain band. At the time of writing the front is
approaching the South Bay and continuing through the East Bay.
Rain totals have over performed a bit in portions of Sonoma County
with Healdsburg reporting 1.06". The rest of the Bay Area is on
track so far, generally reporting 0.1"to 0.25", with several
hours of rain left. There looks to be a dry period overnight
before scattered showers pick back up Saturday. Some of these
showers could be strong and there is a decent (30% or so) chance
for thunderstorms. The Sacramento Valley had a few lighting
strikes a few minutes ago, which is a sign that the convection may
become more organized tomorrow afternoon.

&&

.SHORT TERM...
(This evening through Saturday)
Issued at 140 PM PDT Fri Mar 22 2024

Current situation:
Picturesque satellite imagery off the West Coast this afternoon.
Visible satellite imagery clearly shows a wound up circulation
spinning off the NorCal Coast. This circulation is an area of low
pressure. Attached to this feature are a band of clouds that
extend into the PacNW coast then southward along the coast to the
North Bay. The band of clouds are associated the surface front
approaching the coast. That`s the view from above, but at the
surface KMUX radar continues to pick up shower activity over the
coastal waters. The number of showers over coastal waters has
increased in coverage since this morning.

Rest of today through Saturday:
The overall trend of the approaching cold front has been much
slower. Previous model guidance had way more precip this afternoon
over the North Bay and the heaviest band is just now forming off
the coast. Latest timing from ensemble guidance puts the front
over the North Bay late this afternoon and just entering SF Bay by
4-5 PM this evening. As noted on previous , biggest impact will be
to the evening commute. Thereafter, the cold front and associated
precip will move S and E through the rest of the forecast area
through early tonight. Late tonight into early Saturday steadier
precip will transition from steady to more scattered in nature.
Behind the front, a secondary surface trough will swing through on
Saturday. The passing trough will be aided by some colder upper
level air. The combination of daytime heating, cold air aloft, and
a weak surface feature will set the stage for a more convective
environment. Therefore, hit or miss showers will be possible
through the day Saturday. Given the more unstable atmosphere on
Saturday cannot rule out a thunderstorm or two 15-30%. SPC still
has the entire region a general mention of thunderstorms. While
convective parameters don`t look overly dynamic stronger cells may
produce brief heavy rain and small hail on Saturday. Speaking of
colder air aloft, snow levels will lower during the day Saturday
4-5K. Therefore, snow will be possible if showers line up just
right over the highest peaks. It goes with out saying that max
temps on Saturday will be much colder than Friday. Max temps on
Saturday will reach the mid 50s to lower 60s and upper 30s to 40s
for higher peaks. That`s a 5-15 degree temp drop from Friday to
Saturday. Total rainfall through Saturday evening hasn`t changed
much: 1.00-2.00" for the North Bay Coastal Ranges and Santa Cruz
Mountains, 0.5-1.00" for the Bay Area/Monterey Bay/Santa Lucia
Mts, 0.25" or less for interior Central Coast.

In addition to precip, the passing front will bring an uptick in
wind. Latest mesonet observations indicate gusts already reaching
the 30-45 mph range along the coast and higher peaks. This will
be the case this afternoon through early tonight with ahead and
with the fropa. Not strong enough for a wind advisory, but weaker
branches could come down.

&&

.LONG TERM...
(Saturday night through next Thursday)
Issued at 152 PM PDT Fri Mar 22 2024

While the cold front will be old news at this point the lingering
upper level trough and associated upper low will maintain widely
scattered showers and thunderstorms (10-15%) Saturday night
through Sunday afternoon. Given the scattered nature of precip
additional rainfall amounts will vary, but in general an
additional few tenths of an inch will be possible.

Monday: While the overall upper level pattern shows weak ridging
over the EPac, cannot rule out a passing shower or two thanks to
a weak disturbance traversing the PacNW.

Dry weather and slightly warmer temperatures for Tuesday.

More widespread rain returns to the region Wednesday and Thursday
as a weak cold front slides through the region. At this point it
appears to be more beneficial rain than really impactful.

&&

.AVIATION...
(00Z TAFS)
Issued at 437 PM PDT Fri Mar 22 2024

Frontal rain is moving through the area this evening, bringing
periods of MVFR conditions and strong southerly winds for the next
several hours. Rain will taper off tonight, with a window of dry
conditions before Saturday morning, when rain will return in the
form of convective showers. Overall, CIG and VIS are expected to
remain borderline VFR for the majority of the TAF period, but the
strong, shifting wind this evening will bring aviation impacts.

Vicinity of SFO...Borderline VFR-MVFR conditions will prevail
through 05Z or so as the rain moves through. During this time, the
wind will shift about 30 degrees to the W-SW and gradually
decrease. There will be a window of moderate winds and no rain
overnight. The TAFs keep a broken ceiling around, but this may be
pessimistic as the satellite shows pretty clear conditions behind
the front. Scattered showers return mid-morning Saturday, and may
some may become strong in the afternoon. The wind will pick up
again Saturday, but should be weaker than today and from a more
westerly direction.

SFO Bridge Approach...Similar to SFO

Monterey Bay Terminals...Still waiting on the rain to arrive.
Running a time-speed-distance on the leading edge of the radar
loop shows an arrival time of 0145Z or so at MRY and 0200Z at SNS.
The TAFs are about an hour slower than this as the front is
decelerating. Steady rain should stick around for 4 or 5 hours
tonight before a dry period. Scattered showers return mid-day
Saturday.

&&

.MARINE...
(Tonight through next Wednesday)
Issued at 437 PM PDT Fri Mar 22 2024

Breezy southerly winds continuing through Saturday before
transitioning northwesterly late Saturday into Sunday. Light to
moderate rain along a cold front is moving over the waters with
southerly gusts between 25 to 30 mph possible across most marine
zones. Shower activity will continue into early Sunday with
increasing thunderstorm chances Saturday afternoon and continuing
through early Sunday morning.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening 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-SF Bay N of
     Bay Bridge.

     Small Craft Advisory from 3 PM Saturday to 3 AM PDT Sunday for
     Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.

     Small Craft Advisory until 9 PM PDT Saturday for Pt Arena to Pt
     Reyes 10-60 NM.

     Small Craft Advisory from 9 AM Saturday to 3 AM PDT Sunday for
     Pigeon Pt to Pt Pinos 10-60 NM.

&&

$$

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

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