FXUS66 KMTR 261227
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
524 AM PDT Thu Mar 26 2026
...New AVIATION, MARINE...
.KEY MESSAGES...
Updated at 911 PM PDT Wed Mar 25 2026
- Above normal temperatures persist into the weekend
- Offshore winds across the North Bay Interior Mountains thru Friday
- Pattern change with chances for largely beneficial rain next week
&&
.SHORT TERM...
Issued at 911 PM PDT Wed Mar 25 2026
(Tonight through Friday)
Generally quiescent conditions are forecast during the short term
period, with the exception being across some of the higher terrain
of the North Bay Interior Mountains as well as parts of the East Bay
Hills tonight, but especially pre-dawn Friday morning.
A dry/cool frontal boundary continues to slide southward through
portions of the North Bay this evening, with a pretty discernible
northerly wind shift evident on observations upstream across Lake
and Mendocino Counties. This frontal boundary will usher in some
cooler air and as noted previously, min temperatures should be a
touch cooler (at least across the North Bay), but sunrise Thursday
compared to the past few days. Overall, not anticipating a
potential for widespread frost/freeze conditions, though a few
spots may dip down into the upper 30s across extreme northern
Sonoma and Napa Counties.
925mb flow does increase some on Thursday morning, in the
immediate wake of the front, but the real potential for any type
of mountain wave activity is greater on Friday morning. This will
be due to building high pressure in the wake of the front through
the day on Thursday. As the MSLP gradient tightens beneath a
largely stable regime, flow will become trapped and forced
around the complex terrain. Areas such as Mt. St. Helena and other
peaks across the North Bay Mountains may see winds gust as great
as 45 mph pre-dawn Friday morning. The SJSU/PG&E WRF as well as
our in- house model highlight this potential well.
Otherwise, mid/upper level clouds will continue to invade from
the southwest resulting in partly sunny skies. Despite largely
filtered sunshine, high temperatures will remain 10 to 15 degrees
above normal. This trend will continue through the weekend.
&&
.LONG TERM...
Issued at 911 PM PDT Wed Mar 25 2026
(Friday night through next Wednesday)
The weekend is anticipated to be quite pleasant. With the
magnitude of offshore flow weakening slightly, this may allow for
the afternoon sea-breeze to bring MaxTs down a few degrees
compared to the previous days. Still highs are expected to remain
5 to 10 degrees above normal.
Our ridge pattern that has resulted in our well above normal heat
will yield to more pronounced troughing as we venture into next
week. There do remain some temporal and spatial differences in
the placement of more vigorous shortwaves in the longwave trough
and we'll continue to examine the latest guidance as it arrives
for next week.
Some of the synoptic scale progs from the deterministic guidance
advertise swift mid-level flow extending from the southwest with
favored large scale ascent beneath the exit region of a 50 knot jet.
This will equate to cooler conditions next week. At this time,
Monday appears largely precipitation-free and this is a slight
change from previous forecast cycles due to a slower evolution of
the upper trough as it amplifies across the West Coast. Overall,
NWP is in fair to good agreement with this feature. After a few
days of troughing, a corridor of 180-200% of normal precipitable
water air will stretch from the Central Pacific eastward toward
California. Both GEFS and EPS renditions of the probability of
Integrated Vapor Transport (a useful tool for quantifying moisture
transport) above 250 kilogram/meter/second have gradually
increased from less than 10% 72 hours ago to near 50% in the
latest guidance. There are some spatial differences among model
guidance with regard to the exact location of this plume of rich
moisture and thereby the placement of the greatest rainfall and
we'll continue to iron out these details through the weekend.
Current forecast rain amounts appear respectable for this time of
year, with the greatest potential for at least 1" of rainfall across
the North Bay from late Monday into Wednesday. Chances for at least
2" of rainfall (which would be the higher end rain amount) are
around 10% across the Western Sonoma Hills, Santa Cruz Mountains,
and the Diablo Range. As noted above, once we get closer, we'll be
able to refine these rain amounts.
With actual upper level dynamics, surface/low level cyclogenesis
appears that it'll give some of the higher terrain and Pacific Coast
regions around a 40-70% chance for SW'ly wind gusts near/above 40
mph. Increased onshore flow and subsequent cloud cover will also
equate to temperatures being 5 to 15 degrees below normal by the
time we get into next week with daytime highs ranging between the
mid 50s to mid 60s.
&&
.AVIATION...
(12Z TAFS)
Issued at 524 AM PDT Thu Mar 26 2026
There's patchy coastal stratus /IFR-MVFR/ this morning, otherwise
VFR is forecast. The northerly ACV-SFO pressure gradient is strong
at 8.0 mb, UKI-STS is 2.0 mb and the SFO-SAC gradient is onshore
1.7 mb.
Later today, tonight and Friday morning the WMC-SFO gradient rapidly
steepens to ~ 10 mb with a strong zonal i.e. west-east jet stream
on the synoptic scale across the northern CONUS. Mentioning this
for multiple reasons: 1) the pattern will be dry in our forecast
area supporting VFR, 2) there'll be a temporary period of low level
wind shear favoring the North Bay tonight and Friday morning, 3) the
synoptic pattern has been very energetic as we have seen either
side of the now deamplified, flatter yet still strong CONUS long
wave ridge, 4) the WMC-SFO gradient rapidly weakens by Saturday,
5) rarely does the jet stream take a west-east route when the
WMC-SFO steepens like this, instead it's often north-south while
advancing eastward across the Pacific Northwest and 6) suspect the
rapid motion of the surface high with origins at higher latitudes
i.e. exiting the Bering Sea two-three days ago is related to the
negative PDO i.e. corner of the Pacific where sea surface temps
remain colder than normal (favoring more swiftly moving weather
systems).
Vicinity of SFO...VFR. West to northwest wind shifting to light
northeasterly this morning then northwest 10 knots in the
afternoon and evening. Light wind shifting to northeasterly late
tonight and Friday morning.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR. Light and variable winds becoming
onshore 10 to 15 knots today, winds decreasing to light easterly
to southeasterly tonight and Friday morning.
&&
.MARINE...
(Today through Tuesday)
Issued at 421 AM PDT Thu Mar 26 2026
Northwesterly gales will persist through today mainly across the
northern outer waters. Surface high pressure will build over the
Great Basin today, tonight and Friday morning resulting in winds
and seas easing across the coastal waters tonight, Friday. Onshore
winds will then prevail through the weekend. A wet weather pattern
develops next week.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 9 AM PDT this morning for Pigeon Pt
to Pt Pinos 0-10 nm-Pt Arena to Pt Reyes 0-10 nm-Pt Reyes
to Pigeon Pt 0-10 nm.
Small Craft Advisory until 9 PM PDT this evening for Pt Pinos to
Pt Piedras Blancas 0-10 nm.
Gale Warning until 9 PM PDT this evening for Pt Arena to Pt
Reyes 10-60 NM.
Gale Warning until 9 AM PDT this morning for Pigeon Pt to Pt
Pinos 10-60 NM.
&&
$$
SHORT TERM...Bain
LONG TERM....Bain
AVIATION...Canepa
MARINE...Canepa
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Prepared by Boulder-Creek.com Weather at: Thu Mar 26 08:30:03 PDT 2026
From the National Weather Service
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