FXUS66 KMTR 202121
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
221 PM PDT Mon Apr 20 2026
...New SHORT TERM, LONG TERM...
.KEY MESSAGES...
Updated at 1210 AM PDT Mon Apr 20 2026
- Cool temperatures and beneficial rainfall today through
Wednesday
- Isolated to scattered thunderstorms on Tuesday
- Seasonal temperatures and dry conditions for the later part of
the week
&&
.SHORT TERM...
Issued at 200 PM PDT Mon Apr 20 2026
(This evening through Tuesday)
The much advertised wet weather has finally arrive - began with a
few showers last night through early this morning and more
widespread precip this afternoon. A cold front knocking on the Bay
Area's doorstep has led to an increase in precip over the North
Bay stretching south to SF Bay and finally Santa Cruz the last
3-6 hrs. Monterey Bay and points E and SE remain dry at this time.
Decent precip totals over the last 12 hrs with 0.25-0.75 inches
North Bay Coastal Mts and a few hundredths to 0.25" elsewhere. In
addition to the precip, the frontal structure and associated
pressure gradient led to gusty southerly winds. Gusts since
midnight range from 20-40 mph most areas with gust 40-50 mph along
the coast and and higher terrain.
Rest of today through tonight: The front off the coast will
finally makes its way inland with a nudge from an upstream sfc low
and upper level jet. As such, the more stratiform precip will
move through the Bay Area and Central Coast with impacts to the
evening commute. Wet roads and windy conditions. By late this
evening the steadier rainfall will transition to more convective
nature/shower behind the front.
Tuesday: Post frontal regime on Tuesday will be mainly be driven
by the trailing upper level trough moving overhead, cold pool
associated with trough, a phasing jet aloft around 200-250mb, and
a jet max near 850-700mb. Lastly, some increased insolation
during the afternoon due to less clouds. The combo of colder air
aloft and daytime heating will increase sfc and ML CAPE. Factor in
some of the wind fields and there's an increase for late
morning/early evening thunderstorms. While there is some large
0-6km shear values it appears to be more confined to the 3-6km
layer. Would like to see high shear below 3km for
waterspout/tornado concerns. That being said, we'll be watching
closely. Regardless, stronger storms will be capable of producing
gusty winds, heavy rain, and small hail given the temp profile.
SPC is not highlight tor/hail/wind like they were earlier in the
month. They do have a general mention of thunder though.
&&
.LONG TERM...
Issued at 219 PM PDT Mon Apr 20 2026
(Tuesday night through next Sunday)
Once the sun sets Tuesday evening the threat for thunderstorms
quickly fades, but lingering showers will persist through early
Wednesday. We will have to wait for the upper trough and
embedded/trailing weak shortwave to exit before shower activity
ends Wednesday.
Thursday into next weekend. Warm and drier conditions will be
trend. However, that trend will be tempered by persistent
trough/broad upper low settling over the EPAC and CA. Conf with
the forecast decreases for next weekend. Current forecast has a
passing shower late Saturday due to another trough swinging
through CA. Ens members are scattered on amounts, but most members
at least show something. This will be fine tuned over the next
few days, but for now cannot rule out some precip.
&&
.AVIATION...
(18Z TAFS)
Issued at 1149 AM PDT Mon Apr 20 2026
High clouds and scattered mid-level clouds are building as showers
arrive head of the cold front. Shower activity increases through the
afternoon for the North Bay and around the SF Bay, while the East
Bay, South Bay, and Monterey Bay see better chances into the late
afternoon and evening. Expect breezy to gusty southerly winds ahead
and during the frontal passage. Winds reduce slightly into the night
in the post-frontal environment with scattered lighter showers
lasting through the late night. Another band of rain arrives Tuesday
morning, bringing locally breezy winds and widespread light to
moderate showers. Showers will continue to move through the area
behind this band of rain, but will be more scattered. Chances for
storms will be possible into Tuesday afternoon and evening.
Vicinity of SFO...Expect overcast high clouds and building mid-level
cloud cover into the afternoon as breezy south east winds build.
Showers become a line of rain in the mid afternoon, with slight
reductions in visibilities possible. Rain chances and winds reduce in
the late evening, but scattered passing showers will still affect the
area. Another band of rain arrives into early Tuesday with breezy
south winds. Winds turn more southwest in the wake of this rain band
with scattered showers continuing into the afternoon. Chances for
storms will be possible in the afternoon and evening on Tuesday.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Showers are moving through the area, with
better chances for rain arriving into the afternoon. Expect breezy
to gusty south winds to build into the afternoon as rain chances
increase.The main rain band arrives in the evening offering
consistent rain and reduced visibilities. This band exits into the
late night with winds reducing in its wake, but scattered showers
will continue to move through the area. Shower become more
widespread again in the mid morning with some chances for storms.
&&
.MARINE...
(Today through Saturday)
Issued at 1149 AM PDT Mon Apr 20 2026
Breezy to gusty southerly winds are building as a cold front
passes and winds shift to a moderate southwesterly breeze by
Tuesday with some localized, but short lived gale force gusts
possible. The front will also bring steady rain and showers
through the day. Showers become more scattered into the night but
continue into Tuesday with some chances for thunderstorms. As the
weather breaks, winds will gradually shift back to the northwest
Wednesday before increasing to a strong northwest breeze by
Thursday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 PM PDT this evening for 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 Reyes to Pigeon Pt 0-10 nm-SF Bay N
of Bay Bridge.
Small Craft Advisory until 3 AM PDT Tuesday for Mry Bay-Pigeon
Pt to Pt Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10
nm.
Small Craft Advisory from 9 AM to 9 PM PDT Tuesday for Mry Bay.
Small Craft Advisory from 9 AM to 3 PM PDT Tuesday for Pt Pinos
to Pt Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Murdock
MARINE...Murdock
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Prepared by Boulder-Creek.com Weather at: Mon Apr 20 14:30:03 PDT 2026
From the National Weather Service
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