FXUS66 KMTR 081121
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
421 AM PDT Wed Apr 8 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 1111 PM PDT Tue Apr 7 2026
- Above normal temperatures and dry conditions with a marine
layer Wednesday
- Unsettled weather returns as early as Thursday and continues
through the weekend with light to moderate rainfall
- Thunderstorms possible Friday through Sunday
&&
.SHORT TERM...
Issued at 1111 PM PDT Tue Apr 7 2026
(Tonight through Wednesday night)
Wednesday's weather is brought to you by upper-level shortwave
ridging. The aforementioned ridge's axis across the state and a
cutoff low right behind it is supporting onshore flow. A modest
marine layer of 1,500 feet was observed on the 00Z April 8th
sounding - this may actually undergo some deepening tonight as
heights ever so slightly begin to fall ahead of the approaching
cutoff low. Nonetheless, a cloudy start can be expected along the
coast and in the valleys with temperatures warming to 5-10 degrees
above normal by the afternoon.
&&
.LONG TERM...
Issued at 1111 PM PDT Tue Apr 7 2026
(Thursday through next Tuesday)
The aforementioned cutoff low near 38 N, -138 W in the Eastern
Pacific Ocean will kickoff our pattern change on Thursday. Due to
its independence from the jet stream, its exact location and timing
is difficult to say with certainty. Thus, this is a boom or bust
forecast by nature which is evident in the spread between the 10th
and 90th percentiles. For example, 72-hour accumulation for Santa
Rosa between now and 5 PM Friday is forecast to be 0.22" with a 10th
percentile of 0.03" and a 90th percentile of 0.40". No matter how
you slice it, it's not a lot of rain; however, this provides insight
that rainfall totals will be highly dependent on where rain
showers/thunderstorms develop. The trend has been for an even slower
progression of the surface low to the east. While rain and
thunderstorms on Thursday cannot be completely ruled out with the
outer rain bands, the best thunderstorm chances are now expanding
from Friday into Sunday. Point forecast soundings illustrate that
the profile will be characterized by conditionally unstable lapse
rates yielding low CAPE, high 0-6 km shear, and PWAT values near the
90th percentile. It also shows that thunderstorms will likely be
diurnally driven by surface heating with the most unstable parcel
being the surface parcel. Thunderstorm hazards include lightning,
locally heavy rainfall, erratic/gusty winds, and small hail. Due to
the slow steering motion and training effect of cutoff lows,
localized flooding will be possible if a rain shower/thunderstorm
were to develop. There is high confidence that the cutoff low gets
absorbed with the help of a digging upper-level longwave trough from
the Gulf of Alaska late Friday, but global ensemble clusters diverge
by Saturday on the depth, strength, and type of troughing across the
state. Deterministic ECMWF and GFS are both advertising a surface
low digging down the West Coast and cutting inland in Northern
California. The close proximity to the low would maintain rain and
thunderstorm chances over the weekend. Rainfall totals on the order
of 0.50"-1.00" are expected with up to 2.00" in the coastal ranges.
Heights will begin to rise Monday, kicking off at least a brief
warming and drying trend.
&&
.AVIATION...
(12Z TAFS)
Issued at 421 AM PDT Wed Apr 8 2026
Areas of coastal stratus /MVFR-IFR/ continue to form and winds
are light onshore. The highest probability of VFR will be inland
today. Coastal stratus /MVFR-IFR/ increases and moves inland
tonight and Thursday morning.
Vicinity of SFO...VFR. West wind near 10 knots increasing to 15
knots in late morning and afternoon. West wind 10 knots tonight
and Thursday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Stratus /MVFR-IFR/ until late morning
then VFR for the afternoon. Stratus /MVFR-IFR/ redevelops and
moves inland tonight and Thursday morning. Mainly onshore winds 5
to 15 knots.
&&
.MARINE...
(Today through Monday)
Issued at 412 AM PDT Wed Apr 8 2026
Winds will shift to southwest today through Thursday ahead of a
low pressure system. This system will merge with another low
pressure system arriving from the northwest and result in showers
and a chance of thunderstorms Friday through Sunday. A cold front
with the newly arriving system will move across the coastal waters
and bays Saturday and Saturday night. Northwest winds will develop
behind the cold front late in the weekend.
&&
.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: Wed Apr 8 06:30:02 PDT 2026
From the National Weather Service
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