FXUS66 KMTR 201202
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
502 AM PDT Mon Apr 20 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 1210 AM PDT Mon Apr 20 2026

 - Cool temperatures and beneficial rainfall today through
   Wednesday

 - Isolated to scattered thunderstorms on Tuesday

 - Seasonal temperatures and dry conditions for the later part of
   the week

&&

.SHORT TERM...
Issued at 1210 AM PDT Mon Apr 20 2026
(Today and tonight)

High clouds continue to flow over the Bay Area and Central Coast 
tonight, an outgrowth of the cold front that will make Monday a 
pretty rainy day -- more on that a little bit later on. KMUX radar 
is showing returns from the first rain band off the coast of the Bay 
Area and Central Coast, and just now coming ashore in the 
northwestern corner of Sonoma County. Southerly winds are prevalent 
across the region as the front approaches, leading to some pretty 
abnormal nocturnal drying across the Silicon and Santa Clara 
valleys, where the southerly winds downslope from the Santa Cruz 
Mountains into the region. To give just one example, at 4 PM Sunday 
afternoon, San Jose reported a dew point of 48 degrees, a relative 
humidity of 40%, and a light northwest breeze. By 9 PM, San Jose 
reported a dew point of 8 degrees, a relative humidity of 11%, and a 
gentle southerly wind, with gusts reaching 20-24 mph over the 
previous couple of hours. Low temperatures this morning should hover 
around the middle 40s to the lower 50s, downslope warming effects 
notwithstanding, with the interior valleys and mountains dipping 
into the middle 30s to lower 40s.

Looking into the big picture, a deep and large upper level low 
drives a massive circulation across the eastern Pacific with a 
pronounced and highly noticeable comma cloud wrapping up the 
California and Oregon coastlines before diverting away from 
Washington state into the Pacific. The 03Z, or 8 PM, WPC surface 
analysis shows a closed circulation that spans the ocean off the 
Central Coast all the way to offshore Washington State, and two 
surface lows embedded in the comma cloud, one located around 600 
miles west of Cape Mendocino and one around 550 miles west of where 
the Columbia River empties into the Pacific Ocean, on the border 
between Washington and Oregon. This is the system that will approach 
the region through the night, increasing the cloud cover and placing 
a few scout showers into the North Bay overnight before light to 
moderate rain comes into the North Bay close to the end of the 
morning commute on Monday. The rain will start to spread south and 
east through Monday as the cold front approaches, with a messy 
evening commute expected as rain spreads across the SF Bay Area and 
Monterey Bay regions.

As the front approaches, southerly flow should ramp up as the 
pressure gradient tightens, with the winds increasing across the 
coastal regions through the morning before they spread inland during 
the afternoon and evening hours. Winds are not expected to be strong 
enough for a Wind Advisory, but wind gusts of 25 to 35 mph, possibly 
up to 40 mph in favored coastal or ridgeline areas, are expected. 
Given the southerly flow, initial rain chances may be delayed across 
downslope and rain shadowed areas -- think the Petaluma region, the 
Santa Clara Valley, or the interior Salinas Valley. 

Highs in the North Bay will drop into the middle 50s to lower 60s in 
the lower elevations, to the upper 40s to middle 50s across the 
higher elevations, as the cold front passes through. South of the 
Golden Gate, temperatures remain relatively mild with the continuing 
warm sector influence and, for some areas, including the Monterey 
area, enhancement from the southerly downslope flow. Highs range 
from the middle 60s to the lower 70s in the SF Bayshore and Monterey 
Bay regions, to the lower to middle 70s in the inland valleys. 

&&

.LONG TERM...
Issued at 1210 AM PDT Mon Apr 20 2026
(Tuesday through Sunday)

With the frontal passage passing the Bay Area and Central Coast 
Monday night through Tuesday morning, the stage is set for a rather 
convective Tuesday afternoon and evening. The cold pool behind the 
frontal passage and the arrival of the upper level low will 
contribute to a destabilized atmosphere, while the placement of a 
left exit region from a jet streak (a pattern that typically results 
in upper level divergence) and potential low level shear provide 
sources of lift. The latest SPC outlooks continue to place the Bay 
Area and Central Coast in a general mention of thunderstorms for 
Tuesday. If any storms do develop, lightning, localized heavy rain, 
and small hail are the primary threats. Some lingering showers may 
hang on through Wednesday afternoon as a shortwave trails behind the 
upper level trough as it departs into the Intermountain West.

In total, rainfall totals today through Wednesday remain generally 
beneficial, half an inch to an inch and a half in the lower 
elevations, up to 3 inches in the coastal ranges and the North Bay 
interior mountains. While there is no threat for widespread river
flooding, minor flooding is possible in urban and poor drainage
areas should particularly heavy showers or thunderstorms develop.

As the frontal system passes, temperatures will warm slightly into a 
rather dry latter part of the week, close to or slightly below the 
seasonal averages as highs in the lower elevations range from the 
lower 60s to lower 70s. Ensemble model cluster analysis points to 
continued troughing across the state which, if nothing else, should 
help moderate the warming trend, particularly towards the latter 
part of the 7-day outlook. Extended guidance from the Climate 
Prediction Center leans towards temperatures and rain totals above 
seasonal averages for the last days of April into the first days of 
May. 

&&

.AVIATION...
(12Z TAFS)
Issued at 502 AM PDT Mon Apr 20 2026

It's VFR across the forecast area with exception KHAF reports 
MVFR. The low pressure system offshore will approach the forecast
area with rain and breezy southerly winds today. As the cold core
low gets closer tonight and Tuesday it'll result in occasionally 
heavy rain showers and a chance of thunderstorms. It's still early
to try to narrow the timing of thunderstorms, for now will wait 
placing a mention of thunderstorm(s) in the 12z TAFs.

Vicinity of SFO...It's VFR, the cloud ceiling will gradually 
lower to MVFR with rain developing during the morning. Expect MVFR
in occasionally heavy showers tonight and Tuesday. South-southeast
winds 10 knots shifting to south-southwest in the afternoon then 
light southeast wind tonight and Tuesday.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...It's VFR and with increasingly south- 
southeast winds. Downsloping drying winds will occur while cloud 
layers and rain with the eastern Pacific low advance to the coast.
Frontal convergence however will catch up to the coastline and 
eventually diminish the downsloping drying, resulting in MVFR in 
occasionally heavy showers later today, tonight, Tuesday.

&&

.MARINE...
(Today through Saturday)
Issued at 441 AM PDT Mon Apr 20 2026

Southerly winds will increase to a strong breeze through morning
before the cold front passes and winds shift to a moderate
southwesterly breeze by Tuesday. The front will also bring steady
rain today and showers and a chance of thunderstorms on Tuesday.
As the weather breaks, winds will gradually shift back to the
northwest Wednesday before increasing to a strong northwest breeze
by Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory from 9 AM this morning to 9 PM PDT this 
     evening for SF Bay N of Bay Bridge.

     Small Craft Advisory from 9 AM this morning to 3 AM PDT Tuesday 
     for Mry Bay.

     Small Craft Advisory from 9 AM to 9 PM PDT Tuesday for Mry Bay.

     Small Craft Advisory until 9 PM PDT this evening for Pigeon Pt 
     to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 0-10 nm-Pt Arena 
     to Pt Reyes 10-60 NM-Pt Reyes to Pigeon Pt 0-10 nm.

     Small Craft Advisory until 3 AM PDT Tuesday for Pigeon Pt to Pt 
     Pinos 0-10 nm-Pt Pinos to Pt Piedras Blancas 0-10 nm.

     Small Craft Advisory from 9 AM to 3 PM PDT Tuesday for Pt Pinos 
     to Pt Piedras Blancas 0-10 nm.

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Canepa
MARINE...Canepa

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Prepared by Boulder-Creek.com Weather at: Mon Apr 20 06:30:02 PDT 2026