FXUS66 KMTR 211646
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
946 AM PDT Thu May 21 2026

...New UPDATE, MARINE...

.KEY MESSAGES...
Updated at 1201 AM PDT Thu May 21 2026

 - Diurnally driven winds, with onshore winds during the afternoon
   and early evening

 - Warm and dry weather today leads to elevated fire weather
   concerns for the interior Bay Area and Central Coast

 - Gradual cooling trend begins Friday with seasonal temperatures
   for the upcoming weekend

&&

.UPDATE...
Issued at 945 AM PDT Thu May 21 2026

Low clouds have mostly retreated to the coastline as of this 
writing. That said, the coast is expected to remain in the upper 
50s to middle 60s (thanks to the marine influences) while the 
interior warms into the lower 80s to lower 90s under sunny skies. 
That said, no large scale changes are anticipated for this 
morning. Please see more from both the short/long term sections 
below. 

RGass

&&

.SHORT TERM...
Issued at 1201 AM PDT Thu May 21 2026
(Today and tonight)

Coastal stratus lingers across the Central Coast and has filled in 
on the western coast of San Mateo County. A decaying Otter Eddy 
within Monterey Bay combined with a dry slot paralleling the Big Sur 
coast is making the stratus forecast somewhat uncertain with a 
chance that the immediate vicinity of Monterey remains clear 
overnight. Confidence in inland stratus development, particularly 
away from the immediate Bayshore or south of Salinas, is low. 
Stratus should generally clear back to the immediate coast in the 
hours after sunrise.

The large scale pattern over central California is a balance between 
a ridge over the northeast Pacific and edging into the Pacific 
Northwest and a trough lingering over the Rocky Mountains with the 
axis extending into southern California and Baja California. This 
pattern keeps the upper level winds offshore, if light, but at the 
surface, a thermal trough over the Central Valley keeps the winds 
generally onshore with coastal breezes developing across the region 
during the afternoon and evening. 

Low temperature this morning range from the upper 40s to low 50s 
near the coast, lower to middle 50s in the inland valleys, and the 
middle 50s to lower 60s in the higher elevations. The NBM model 
output is slightly overshooting the high temperatures, especially in 
the immediate coastal areas under the marine layer influence, so I 
have tamped down today's coastal highs to the upper 50s to the 
middle 60s. These high temperatures are liable to be an 
underestimate of as much as 5 degrees or so if stratus clears out 
earlier than the current forecast, or indeed never develops at all. 
Away from the immediate coast, temperatures remain near to well 
above the seasonal average, with highs in the 70s to the lower 80s 
near the bays, while across the inland valleys, high temperatures 
range from the middle 80s to the middle 90s. The continuing hot and 
dry conditions will contribute to another day of elevated fire 
weather concerns in the interior regions, and indeed a couple of 
grass fires were reported across the region, one in Contra Costa 
County and one in San Benito County. For those living or visiting 
those hot and dry regions, keep the following in mind:

* Properly dispose of cigarettes and matches
* Do not use a lawn mower on weeds or dry grass
* Avoid using equipment that creates sparks
* Make sure campfires are completely put out
* Make sure no vehicle parts drag on the ground, including towing 
  chains
* Obey burn bans & only burn with a permit

&&

.LONG TERM...
Issued at 1201 AM PDT Thu May 21 2026
(Friday through Wednesday)

A gradual cooling trend begins on Friday with the upper level ridge 
retreating westward, allowing for more zonal upper level flow across 
the West Coast and a gradually thickening marine layer. By Saturday, 
temperatures should be near the seasonal average with highs in the 
middle 70s to the middle 80s inland. By the early part of next week, 
a cold front associated with a deep upper level low is likely to 
sweep through the state, which would cause highs to drop into the 
70s across the inland valleys. Close analysis of the ensemble model 
clusters reveals a small, but significant fraction of the ensemble 
members (around 10-20%) keeping some level of ridging over 
California. There is even a slight chance of light rain or drizzle 
associated with an incoming front, but it's a low confidence 
forecast at this point with around a 20-30% probability of seeing 
any water falling out of the sky, and that is tilted towards the 
coastal regions where topography would tend to enhance the rainfall 
amounts. CPC extended guidance does show a slight lean towards 
precipitation totals above seasonal averages into the beginning of 
June, with the caveat that for the last weeks of May, just a 
couple hundredths of an inch will tie or exceed the daily average 
totals for downtown San Francisco.

&&

.AVIATION...
(12Z TAFS)
Issued at 436 AM PDT Thu May 21 2026

Shallow marine layer developed initially in Monterey Bay before
spread up the Salinas Valley and then northward along the
coastline. Given the shallow nature dense fog/mist has been
reported. At this time impacts look to be confined to SNS, HAF,
and at times MRY. Clouds will hug this coast today, but do expect
VFR conditions at all terminals at some point this afternoon.
Marine layer increase slightly in coverage tonight, but still not
a big inland intrusion.  Overall conf is moderate.

Vicinity of SFO...VFR. Onshore flow this afternoon. Tonight CIGs 
move through GG and head toward KOAK, but guidance suggest CIGs do
remain north of terminal. THerefore, VFR tonight as well.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals... Low CIGs and fog at SNS, but local wind
flow at MRY has led to in-out stratus pushes. VFR by this
afternoon, but early return this evening. 

&&

.MARINE...
(Today through Tuesday)
Issued at 945 AM PDT Thu May 21 2026

Fresh to strong northerly winds continue across the far northern
outer waters, resulting in hazardous conditions and rough seas
through Thursday. Moderate northwesterly breezes and moderate seas
will prevail for the rest of the waters. Conditions improve into
the weekend as seas and northwesterly breezes become moderate with
the occasional fresh to strong breeze possible across the far
northern outer waters.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 AM PDT Friday for Pt Arena to Pt 
     Reyes 10-60 NM.

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...MM
MARINE...Kennedy

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Prepared by Boulder-Creek.com Weather at: Thu May 21 10:30:03 PDT 2026