FXUS66 KMTR 091240 CCA
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
517 AM PDT Thu Apr 9 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1159 PM PDT Wed Apr 8 2026
- Unsettled weather Thursday through the weekend
- Thunderstorms are possible today through Sunday with the best
chances Friday and Saturday
&&
.SHORT TERM...
Issued at 1159 PM PDT Wed Apr 8 2026
(Tonight through Friday night)
A surface low near 38 N, -133 W in the Eastern Pacific Ocean will be
the talk of the short term. Our area narrowly escaped thunderstorms
Wednesday as steeper lapse rates just to our north and east yielded
greater instability. We did have enough lift and moisture however to
record measurable precipitation. With the over performance that
occurred Wednesday, I would expect more of the same Thursday as the
low slowly treks eastward. The best chances for thunderstorms on
Thursday will be in the North Bay, Interior East Bay, and Eastern
Santa Clara County - the same areas that were narrowly missed today.
Rain showers on Thursday will be more efficient as the low-levels
have become saturated; in fact, the April 9th 00Z sounding observed
a PWAT value of 1.04" which is the second highest for the date and
time behind 1.08" from 2016. The greatest chances for thunderstorms
will be Friday as the low enters our waters and continues its
journey inland. The exact location and timing will make all the
difference as the center of the low will offer an area of
convergence. Since it is associated with an upper-level cutoff low,
the independence from the jet stream unfortunately makes the
precision side of the forecast very difficult. Nonetheless, the best
chances are across the Interior Bay Area. Thunderstorm hazards on
Thursday and Friday include: lightning, locally heavy rainfall,
erratic/gusty winds, and small hail. Aside from thunderstorms,
southwesterly winds carrying moist, warm air from the tropics will
increase ahead of the cold front with pre-frontal rain showers
expected in the warm sector. The slow movement and training effect
of the rain bands may lead to localized flooding if a heavier rain
shower/thunderstorm were to develop, especially in poor drainage
and/or urban areas.
&&
.LONG TERM...
Issued at 1159 PM PDT Wed Apr 8 2026
(Friday night through next Wednesday)
An upper-level low in the Gulf of Alaska will dig down the West
Coast Saturday, absorbing the first low as it does so. This second
system is much more dynamic which will yield two noticeable
differences: widespread rainfall instead of just rain showers and
thunderstorms capable of rotating. The three thunderstorm
ingredients of lift, instability, and moisture will be similar if
not better than the first low. A surface low and its attendant cold
front will be the primary lifting mechanism, cold air filtering in
aloft (850 mb temperatures near freezing) will steepen lapse rates
and increase instability as a result, and PWAT values will remain
near the 90th percentile ahead of the cold front. The shear profiles
will be very different between the vertically stacked first low and
the progressive second low. The vertically stacked low will have low
shear while the progressive low will have sufficient low-level shear
to support rotation within thunderstorms. All of this to say,
widespread rainfall with lingering thunderstorm chances are expected
over the weekend with the best chances on Saturday. Moisture will be
a limiting factor behind the cold front, but if the coldest
conditions overlap with ample moisture, snow is possible in the
mountain peaks Sunday morning with snow levels around 4,000 feet.
Rainfall totals between Thursday and Sunday continue to trend higher
with totals on the order of 0.50"-1.50" with up to 3.00" in the
coastal ranges. Commercial and recreational fisherpeople are
encouraged to use extreme caution now through the weekend with the
unsettled weather moving in. Thunderstorms over the waters will be
capable of producing: lightning, locally heavy rainfall, gale force
gusts, small hail, and waterspouts. In the cold front's wake we will
be left with, you guessed it, cold air! Cold starts to the day can
be expected as skies clear and dry air filters in. The warming and
drying trend kicks off Monday and lasts at least until Wednesday as
surface high pressure builds into the West Coast.
&&
.AVIATION...
(12Z TAFS)
Issued at 517 AM PDT Thu Apr 9 2026
Surface pressures are gradually lowering c/o the approaching 500
mb cut-off low and upper level jet stream divergence. Weak cool
air advection continues. All of these processes are now finally
weakening the lower level thermal ridge (temperature inversion)
over our forecast area. Stratus is mixing with fog /IFR-LIFR/
while higher clouds are advancing over the marine layer i.e.,
higher clouds decreasing nocturnal radiative cooling. It may then
mostly be the aforementioned cooling processes resulting in the
air reaching saturation, stratus and fog mix. Webcams show patchy
coastal drizzle as well in the slowly cooling air mass.
High resolution model forecasts indicate stratus and fog mixing
out later in the morning and afternoon. Steady cool air advection
today, tonight and Friday will continue to deepen and weaken the
marine layer temperature inversion (thermal ridge aloft); full mix
out occurs when lower level temperatures are about the same as (or
cooler than) sea surface temperatures. In the current synoptic/meso
pattern it's more complicated, since the incoming low will bring
higher cloud layers and eventually rain. Light rain is introduced
in the 12z TAFs. Periodic light coastal drizzle/light rain today
will increase in coverage by tonight and Friday.
Will also have to watch for potential convection Friday, though
at the moment model forecasts are presenting mixed solutions i.e.
not a high confidence forecast for Friday.
Vicinity of SFO...Stratus /MVFR-IFR/ lifting to VFR by early
afternoon. Low clouds with light rain tonight and Friday morning.
Winds onshore 5 to 15 knots.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Variable conditions at the moment with
stratus and fog /IFR-LIFR/. Patchy light drizzle is possible during
the morning. Conditions improving to MVFR-VFR by late morning and
afternoon. MVFR-VFR in low clouds and light rain then developing
late tonight and Friday morning. Light and variable winds becoming
onshore near 10 knots late morning and afternoon. Light and variable
winds tonight and Friday morning.
&&
.MARINE...
(Today through Tuesday)
Issued at 442 AM PDT Thu Apr 9 2026
A low pressure system approaching the coast from the west today
through Friday will merge with an energetic low from Alaska by
Saturday and Sunday. Expect rain possibly heavy at times and a
chance of thunderstorms Thursday through Sunday. High pressure
builds behind the system with fresh northwest winds developing
Sunday through Monday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Thu Apr 9 06:30:02 PDT 2026
From the National Weather Service
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