FXUS66 KMTR 041139
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
439 AM PDT Mon May 4 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1210 AM PDT Mon May 4 2026

 - Cooler temperatures through Tuesday with a chance for drizzle
   and light rain, mainly along the coastline and the Bay Area

 - Warmer and drier conditions return mid to late week

&&

.SHORT TERM...
Issued at 151 AM PDT Mon May 4 2026
(Today and tonight)

The 500 mb low center we've been monitoring is currently drifting
and wobbling around approx 100 miles southwest of Monterey. It's 
temporarily quasi-stationary with the polar jet stream winds on the
back side of the low while supported by subtropical jet stream winds
at the base of the low. The geopotential height of the low center is
~ 559 decameters, for reference that's near the 10th percentile 
compared to Oakland upper air long term climatology for early May.
Higher surface pressures extend underneath the upper low corresponding
to chilly to cold lower level temperatures (925 mb and 850 mb temps).
The cold air intrusion lifted the temperature inversion and ultimately
did cause deeper vertical mixing to partially clear the sky for sunny
breaks Sunday thanks to the May sun angle (heat energy into the system
i.e. the Earth-atmosphere system). 

And it's chilly across the area with temperatures in the 50s, 40s and
a few upper 30s at higher elevations. Partial cloudiness continues
through this morning. 

With the low moving slowly today and tonight, warm air advection 
to the north of the low center will have a chance to interact with
the colder air aloft resulting in light to moderate stratiform rain
beginning later today over the North Bay then spreading across much
of the Bay Area tonight. Rainfall forecasts have increased. Several
hundredths to two to three tenths of an inch of rain (possibly slightly
higher amounts may occur) are forecast across much of the Bay Area.

On first glance, one may think the counter-clockwise wind flow to
the north of low center (east to northeast winds) from the Sierra
Nevada would produce downsloping, a drier atmosphere and near zero
or zero rain chances. And it is true it most certainly can do this.
However, this low is in a different synoptic pattern. The low is in
a temporary quasi-balance long enough for the warm air advection to
arrive while most likely in near balance because of near surface 
higher pressures (surface pressures are lower over the Great Basin,
meaning no offshore winds) and the net forces due to the upper level
winds around the low. This is a great example as to how complex the
atmosphere is, with numerous modes of circulation, etc.

&&

.LONG TERM...
Issued at 151 AM PDT Mon May 4 2026
(Tuesday through Sunday)

The subtropical jet stream at the base of the 500 mb low will 
move the low eastward away from California by mid-week. After this,
high pressure will move in from the west and northwest bringing 
with it dry conditions and a warming trend. In general because of 
the strengthening long wave troughing on each side of the high 
pressure ridge, meridional (north-south) displacements will also 
amplify/strengthen the ridge, with greater compressional warming 
and surface warming resulting in above to well above normal high 
temperatures in our forecast area by late week and weekend. Far 
inland locations especially by next weekend will likely reach the 
80s and 90s.

&&

.AVIATION...
(12Z TAFS)
Issued at 438 AM PDT Mon May 4 2026

Scattered showers are expected to move into the Bay Area today with 
the highest confidence in showers impacting the North Bay. Given the 
more mixed out marine layer ceilings are higher with VFR conditions 
across the board. There have been temporary dips into MVFR territory 
overnight but these have been short lived with CIGs quickly rising. 
Moderate confidence that VFR conditions will last for much of the 
TAF period with MVFR conditions returning first to the North Bay 
this evening and spreading into the rest of the Bay Area overnight. 
Rain chances increase around 00Z for STS and APC with chances 
increasing closer to 09-12Z for the rest of the Bay Area. Not 
currently expecting showers to reach the Central Coast.

Vicinity of SFO...VFR with MVFR conditions becoming more likely 
overnight as showers reach SFO. Periods of MVFR CIGs are possible 
this morning but they are expected to be short lived with ceilings 
predominantly >3000 ft. Breezy onshore winds are expected this 
afternoon with gusts to around 18 knots. Showers are likely to reach 
SFO around 09Z but could arrive as early as 06Z. CIGs are expected 
to lower as showers return with MVFR CIGs returning tonight into 
tomorrow morning.

SFO Bridge Approach....Similar to SFO.

Monterey Bay Terminals...VFR with increasing chances for MVFR to 
return towards the end of the TAF period. Periods of MVFR CIGs are 
possible this morning but they are expected to be temporary. Breezy 
onshore winds return during the afternoon/evening before winds ease 
overnight. Isolated showers may impact MRY/SNS today into tonight 
but confidence is low that widespread showers will reach the Central 
Coast. 

&&

.MARINE...
(Today through Saturday)
Issued at 438 AM PDT Mon May 4 2026

A light to gentle breeze continues over the waters with winds to
become more moderate by Tuesday. A weak upper level disturbance
lingers over the region through midweek. This disturbance will
bring light, scattered showers and a non-zero chance of
thunderstorms Monday into Tuesday. Northerly winds and seas
strengthen Thursday into next weekend as high pressure rebuilds.

&&

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

$$

SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...Kennedy
MARINE...Kennedy

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Prepared by Boulder-Creek.com Weather at: Mon May 4 10:30:03 PDT 2026