FXUS66 KMTR 090701
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
1201 AM PDT Thu Apr 9 2026

...New SHORT TERM, LONG TERM...

.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...
(06Z TAFS)
Issued at 1001 PM PDT Wed Apr 8 2026

IFR stratus has developed at the coast and will expand inland 
through the night, as moderate onshore winds become light through 
Thursday morning. Timing of stratus impacts is low to moderate 
confidence as the pace of expansion has been slower than the model 
output has depicted. Stratus will lift and retreat to the coastal 
regions through the course of Thursday morning, with mid- to high 
level clouds across the region as a system approaches, causing 
southwest winds across the region on Thursday afternoon. Some 
isolated showers are possible in the coastal region and North Bay 
valleys, but most terminals should not see rain until Friday past 
the end of the TAF period. Also beyond the TAF period, expect 
scattered thunderstorms to develop late Friday morning and continue 
through the day.

Vicinity of SFO... Moderate northwest winds will diminish overnight 
with IFR stratus developing in the next few hours and lasting 
through late Thursday morning. Exact stratus formation time is 
uncertain owing to poor model depictions of the present situation. 
Ceilings will lift late Thursday morning into the early afternoon, 
with moderate winds resuming from a southwesterly direction in 
advance of a system that will bring MVFR-IFR ceilings and some 
isolated showers to the region on Thursday evening into Friday 
morning, but with most of the rain expected to arrive after the end 
of the TAF period. Also beyond the TAF period, expect scattered 
thunderstorms to develop late Friday morning and continue through 
the day.

SFO Bridge Approach... Similar to SFO.

Monterey Bay Terminals... It appears that large-scale southwest flow 
is already impacting the stratus pattern at the terminals with the 
stratus deck deflected to the north of MRY and SNS as of the latest 
satellite imagery. Expecting this deck to inch its way southward as 
light winds continue at the terminals, but timing of impacts is 
uncertain as models are not capturing this situation well. Stratus 
will scatter out through the course of Thursday morning, leaving a 
layer of mid- to high level clouds with moderate onshore winds 
through the afternoon. Some isolated showers could arrive Thursday 
evening but most impacts, including scattered thunderstorms, should 
arrive beyond the TAF period on Friday.

&&

.MARINE...
(Tonight through next Tuesday)
Issued at 1001 PM PDT Wed Apr 8 2026

Gentle to moderate northwest winds across the coastal waters are
shifting to the south and southwest tonight as a low pressure
system approaches the coast. This system will result in showers
and a chance of thunderstorms Thursday through Sunday, with SW
winds increasing to a strong breeze on Saturday. As high pressure
builds behind the system, fresh northwest winds will develop
Sunday through Monday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&

$$

SHORT TERM...Sarment
LONG TERM....Sarment
AVIATION...DialH
MARINE...DialH

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Prepared by Boulder-Creek.com Weather at: Thu Apr 9 04:30:03 PDT 2026