FXUS66 KMTR 210719
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1219 AM PDT Tue Apr 21 2026
...New SHORT TERM, LONG TERM...
.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...
(06Z TAFS)
Issued at 939 PM PDT Mon Apr 20 2026
The cold front has reached the coast, with prefrontal rain
ongoing across the majority of the terminals. This light to
moderate steady rain will continue through the night before a
break in the early morning hours. This won't last long, however,
as a band of stronger showers and possible thunderstorms is
expected to roll through late morning through early afternoon.
After this finale, showers will become much more isolated as the
sun comes back out.
Vicinity of SFO...Prefrontal rain will continue through most of the
night with moderate southerly winds. There will likely be a break
in the rain from around 12-18Z as the southerly winds start to
increase. The main band of showers and possible thunderstorms is
expected to reach the terminal around 18Z and continue for 2-4
hours. In addition to the heavy rain, lower visibility, and
lightning potential, winds will be gusty and erratic during this
window. Conditions will improve in the early afternoon, but there
is a chance for the atmosphere to recharge for another round of
showers in the late afternoon or early evening.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Rain has finally arrived at the southern
terminals. Impacts will be similar to what's described above,
just delayed by an hour or two.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 939 PM PDT Mon Apr 20 2026
Moderate to fresh SW breeze will continue through the night before
gradually shifting to westerly through the day Tuesday. The first
band of light to moderate rain is ongoing, with a break expected
in the early morning before showers return later in the morning.
Some of these showers will be strong, and there is a 20-30% chance
of thunderstorms as well. 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 3 AM PDT early this morning 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 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...Flynn
MARINE...Flynn
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Prepared by Boulder-Creek.com Weather at: Tue Apr 21 02:30:02 PDT 2026
From the National Weather Service
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