FXUS66 KMTR 211137
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
437 AM PDT Tue Apr 21 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1214 AM PDT Tue Apr 21 2026
- Steady rain this morning gives way to isolated to scattered
thunderstorms in the afternoon and evening, lingering showers
through Wednesday
- Seasonal temperatures and dry conditions for the later part of
the week
- Monitoring a passing shower this Saturday and potential
rainfall early next week
&&
.SHORT TERM...
Issued at 1214 AM PDT Tue Apr 21 2026
(Today and tonight)
As of midnight tonight, the main frontal rain band continues to make
its way into the interior regions of Monterey and San benito
Counties, with the steady rainfall extending across the majority of
the San Francisco Bay Area, the Monterey Bay region, and the Santa
Lucia range. The back edge of the steady rainfall has passed through
the western half of Sonoma County and the Point Reyes area, behind
which some scattered showers have developed offshore. The steady
rain band should continue to pass through overnight, and should move
east of the SF Bay Area and Monterey Bay regions by the morning
commute. Low temperatures tonight range from the middle 40s to the
middle 50s, while today's highs range from the upper 50s to the
middle 60s in the lower elevations, down to the middle 40s to lower
50s in the higher elevations.
The main forecast issue today is the chance for convective activity
across the Bay Area and Central Coast. To discuss this, let's take a
look at the ingredients for convection, namely instability,
moisture, and lift. At the 200-250 mb level (around 35000-40000 feet
above sea level), the polar and subtropical jet streams are phasing,
meaning that they are aligned so that their troughs are roughly
placed along the same line of longitude. This enhances the upper-
level low and enhances its cold pool, and this system will pass over
the region, while scattering skies behind the frontal rain band will
enhance surface warming, both effects destabilizing the atmosphere
for the afternoon and evening hours. Most unstable CAPE values from
the models are peaking around 500 J/kg, near the 99th percentile
value for pretty much any time of year in the long-term records for
the OAK radiosondes. Some moisture will continue to be advected into
the region through the morning on the back side of the front, but
most of it will be left behind from the passing of the front earlier
in the day. That leaves lift, and there are a couple of sources for
lift across the region. First, and most intuitively, the wind can
force air to condense and form clouds or storms when it flows across
our mountain ranges (we call this "orographic" lift). The second
source requires a fair bit of explanation. Back at the 200-250 mb
level we see a jet streak, or some hints of one, passing over the
state around the upper level low. Some of the model output hints
that we may very well see the left exit region of the jet streak,
where due to the dynamics of the atmospheric flow, divergent flow
aloft will induce convergent flow at the surface, thus promoting
atmospheric lift across the region in question.
Chances for thunderstorms top out around 30 to 40% across the region
late this morning into the early evening, while K-Index values of
around 25-28 degrees Celsius suggest a possibility for isolated to
widely scattered thunderstorms. If thunderstorms do develop,
lightning, locally heavy rainfall, and small hail are the primary
threats. Not expecting waterspouts or weak tornadoes with this
system. Model output values for the 0-6 km shear look impressive,
but most of that shear is confined to the 3-6 km layer, whereas you
would want higher shear between 0-3 km to note a higher risk of
rotating cells. We will keep monitoring the forecast evolution
through the day. The SPC continues to mention a general risk of
thunderstorms across the region, but is likewise backing off of
any severe thunderstorm chances.
&&
.LONG TERM...
Issued at 1214 AM PDT Tue Apr 21 2026
(Wednesday through Monday)
Some lingering showers may hang on through Wednesday afternoon as a
shortwave trails behind the upper level trough as it departs into
the Intermountain West. As the frontal system passes, temperatures
will warm slightly into a rather dry latter part of the week, close
to or slightly below the seasonal averages as highs in the lower
elevations range from the lower 60s to lower 70s. Ensemble model
cluster analysis points to continued troughing across the state
which will help modulate the warming trend through the 7-day
outlook. On Saturday, another passing trough will bring a chance
for a passing shower to the region, and although the ensemble
members continue to show more scattered precipitation total
forecasts, most members continue to show some rainfall coming into
the region. Model output is even putting rain chances in the
early part of next week and extending beyond that, but it is
still too early to say anything definitive at this point in the
forecast.
&&
.AVIATION...
(12Z TAFS)
Issued at 421 AM PDT Tue Apr 21 2026
Multiple rounds of light to moderate showers will continue to move
across our area through the morning hours into the afternoon.
There will be breaks at individual terminals at times between
rounds. North Bay terminals should be the first to see widespread
showers come to an end, especially KSTS by mid-afternoon. KAPC
may be far enough south to see shower activity extend into late
afternoon. There continues to be a 30-40% chance of thunderstorms
after 8AM this morning through the afternoon hours. Expect gusty
erratic winds and moderate to heavy rain with the strongest
showers/thunderstorms.
Vicinity of SFO...MVFR to VFR is expected to prevail through the
TAF period with multiple rounds of scattered showers continuing
through the morning and afternoon, with some breaks in shower
activity at times. There is a chance of isolated thunderstorms as
early as 8AM this morning through late morning into the afternoon.
Expect gusty erratic winds and moderate to heavy rain with the
strongest showers/thunderstorms.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR is expected to prevail with periods
of MVFR as several rounds of showers move across the peninsula
this morning and afternoon. There's a chance of isolated
thunderstorms late this morning into the afternoon, expect gusty
and erratic winds and moderate to heavy rainfall with the
strongest showers/thunderstorms. MVFR ceilings are possible later
this evening into the overnight hours.
&&
.MARINE...
(Today through Sunday)
Issued at 421 AM PDT Tue Apr 21 2026
Moderate to fresh westerly breeze will develop today. Scattered
light showers will increase in coverage later this morning into
the afternoon. Some of these showers will be strong with a 20-30%
chance of thunderstorms. Once the weather breaks by late Wednesday
afternoon, winds will become moderate out of the northwest, then
increase to strong out of the northwest on Thursday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM this morning to 9 PM PDT this
evening for Mry Bay-Pigeon Pt to Pt Pinos 0-10 nm.
Small Craft Advisory from 9 AM this morning to 3 PM PDT this
afternoon for Pt Pinos to Pt Piedras Blancas 0-10 nm.
&&
$$
SHORT TERM...DialH
LONG TERM....DialH
AVIATION...JM
MARINE...JM
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Prepared by Boulder-Creek.com Weather at: Tue Apr 21 06:30:02 PDT 2026
From the National Weather Service
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