FXUS66 KMTR 201138
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
438 AM PDT Mon Jul 20 2026

...New AVIATION, MARINE...

.KEY MESSAGES...
Updated at 148 PM PDT Sun Jul 19 2026

 - Near or slightly below normal temperatures and higher humidity
   through the first half of the upcoming week.

 - The marine layer and mid to upper level cloud cover will
   continue to help keep temperatures cooler under building high
   pressure.

 - Approximately 5% chance of elevated showers with isolated 
   thunderstorms Tuesday afternoon through Wednesday evening.

&&

.SHORT TERM...
Issued at 158 AM PDT Mon Jul 20 2026
(Today and tonight)

A 581 decameter geopotential height low remains located 500 miles
west of San Francisco. The southern branch of the polar/subtropical
jet stream is just forward of this low with jet stream winds to our
southwest through northeast. These jet stream winds have helped 
quickly transport high and mid level clouds away from TS Elida in 
the last 24 hours or so. GOES-West Geostationary Lightning Mapper
showed lightning approx 600 miles southwest of Point Conception 
earlier Sunday, but since Sunday evening it's been quiet. This 
lightning activity was closer to the northward moving and weakening 
center of TS Elida.

KMUX has occasionally shown widely scattered high based showers 
over our forecast area, but there have been no measurable rain or 
lightning reports so far. A band of light high based rain is 
developing over the coastal waters as well. We're in a little bit 
of unknown territory here with mainly minor energy transformations
taking place, complexities that occur whenever tropical moisture 
travels northward and interacts with the mid-high latitude synoptic
pattern. At the moment, the high based showers/band of light rain
correspond to an area of weaker 500 mb positive vorticity that is
forecast to continue moving northward of our forecast area along 
with the aforementioned upper level jet stream. So in a way, the 
combination of parameters and the steady diffusing of mid and high
level clouds (water vapor) with minimal to no organization so far
appear to be self limiting, leaning the pattern toward stability,
keeping convective potential at a minimum.

Cloud cover will be a mix of stratus and mid and high level clouds
today. The sun may break out over the southern interior today,
possibly some sunny breaks farther north to the South Bay as well.
Daytime highs today will be about the same it was on Sunday, varying
from the mid to upper 60s along the coast to the 70s bayside to the
80s and 90s well inland. Tonight lows will cool to the 50s except in
the 60s in the hills and mountains.

&&

.LONG TERM...
Issued at 158 AM PDT Mon Jul 20 2026
(Tuesday through Sunday)

With generally decreasing 500 mb positive vorticity Monday this 
will help keep convective potential at a minimum with exception of
the North Bay Monday afternoon to early evening when there may be
a brief uptick in positive vorticity. However, even the 00z NAM 
output for this timeframe shows minimal convective potential along
with convective inhibition. The HRRR, RRFS and GFS show essentially
no convective potential. 

For Tuesday, 500 mb positive vorticity is essentially non-existant
including in the 00z NAM cross sections. Residual mid to high level
clouds may still be around, but there'll likely won't be a lifting
mechanism left to destabilize an atmospheric layer or air parcels.
The westerlies, wind shear and drier air is destructive to tropical
origin convective development, but there can be the exception to 
this when a steep east to west moisture gradient helps temporarily
destabilize a lifted layer or air parcels. Given model trends and
further dissipation of TS Elida, this is unlikely Tuesday when the
tail end of the moisture gradient moves across the forecast area.
Stay tuned to the latest updates.

Otherwise it'll be summertime dry weather, warm to hot inland and
coolest at the coast and bays mid to late week. The high pressure 
system over the interior west will gradually shift southward then
slowly move to the Four Corners by the weekend.

&&

.AVIATION...
(12Z TAFS)
Issued at 436 AM PDT Mon Jul 20 2026

Currently a mixed bag of LIFR-VFR. Patchy stratus with mid-level 
clouds streaming in from the southwest is making for a difficult 
forecast. Low confidence on stratus expanding this morning with high 
confidence in LIFR-IFR conditions continuing at the coast with the 
help of coastal drizzle. Elevated rain showers moving in from the 
southwest will bring virga to the region. Widespread VFR and diurnal 
winds will prevail by this afternoon. High confidence in the return 
of IFR conditions to coastal terminals tonight with low confidence 
elsewhere due to the shallow nature of the marine layer.

Vicinity of SFO...Currently VFR with northwesterly flow. Confidence 
is decreasing in the terminal getting a ceiling this morning based 
on current conditions and camera observations. If a ceiling does 
develop, it'll likely be IFR and clear by 16Z. Low confidence on 
the return of an IFR ceiling tonight. 

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently LIFR with westerly flow at MRY 
and VFR and calm at SNS. Since SNS had an IFR ceiling earlier in the 
night, they have a TEMPO through 16Z for it returning. Stratus will 
retreat to the coast by late morning with brief VFR expected at the 
terminals this afternoon. Moderate (SNS) to high (MRY) confidence
on IFR ceilings returning tonight.

&&

.MARINE...
(Today through Saturday)
Issued at 436 AM PDT Mon Jul 20 2026

Moderate northwesterly breezes will prevail through Wednesday,
increasing to become moderate to fresh Thursday into Friday.
Slight to moderate seas will prevail. Tropical cyclone activity in
the Eastern Pacific Ocean will spur several rounds of moderate
period southerly swell; hazardous conditions from mixed seas may
result. The same activity will yield 5% chances for elevated rain
showers through Wednesday. The hazards would be gusty and erratic
outflow winds that would be capable of generating steep seas.

&&

.FIRE WEATHER...
Issued at 148 PM PDT Sun Jul 19 2026

Near to slightly cooler than normal temperatures will continue 
through the week with typical diurnal intrusion of the marine layer 
each night and retreat to the coast each morning as onshore flow 
persists. However, above 1500-2000ft we'll see relatively weak 
thermal belting the first half of the week with continued warm 
overnights and poor RH recovery at higher elevations.

Guidance is trending toward an increasing potential for elevated 
showers and isolated thunderstorms Tuesday afternoon through 
Wednesday evening. Chances are below 5% in the official forecast but 
may increase in subsequent updates tonight and Monday. The 
primary concern is dry lightning strikes across the North Bay, 
with lesser chances farther south into the Bay Area.

&&

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

$$

SHORT TERM...Canepa
LONG TERM....Canepa
AVIATION...Sarment
MARINE...Sarment

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Prepared by Boulder-Creek.com Weather at: Mon Jul 20 08:30:03 PDT 2026