FXUS66 KMTR 200901
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
158 AM PDT Mon Jul 20 2026
...New SHORT TERM, LONG TERM...
.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...
(06Z TAFS)
Issued at 1000 PM PDT Sun Jul 19 2026
High clouds are obscuring low level stratus across the region but
observations and cameras indicate it is starting to move inland.
Pushed back the timing of stratus arrival for Bay Area sites
slightly with the thinking being high clouds are slowing down the
arrival of low level stratus. Confidence is mixed as to if stratus
will reach SJC and LVK so have included a tempo for stratus at
SJC but not at LVK. Stratus is forecast to start out MVFR but will
compress to IFR overnight. LIFR conditions and fog are possible
along the coastline. Conditions improve by late morning with the
exception of the coast where some models indicate stratus could
linger through the entire day. The marine layer compresses to
around 1000 ft tonight (and again tomorrow night) so we can expect
stratus to return late tomorrow evening for most sites. Diurnally
breezy onshore winds continue.
Vicinity of SFO...High clouds are streaming into the Bay Area with
cameras showing stratus approaching SFO. Highest confidence in
stratus making it to SFO around 08Z but it may arrive as early as
06Z. Most likely MVFR CIGs but IFR CIGs may temporarily develop
during the early morning hours. Clearing will take place by the
late morning and remain VFR through the rest of the day. Models
indicate some potential for stratus to return tomorrow night but
confidence is mixed this far out given the compressing marine
layer and high level clouds.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...LIFR conditions to persist through the
late morning with some potential for reductions in visibility and
fog overnight. Highest confidence in reduced visibilities to
around 2-3SM but the LAMP model does show dense fog developing
during the early morning hours. Clearing will take place by late
morning with SNS around 17Z and MRY around 19/20Z before stratus
returns early that evening. The MRY TAF is on the more optimistic
side in terms of clearing with LAMP guidance keeping stratus over
the site through the entirety of tomorrow.
&&
.MARINE...
(Tonight through next Saturday)
Issued at 1000 PM PDT Sun Jul 19 2026
Slight to moderate seas and a gentle to moderate northwesterly
breeze will persist through the upcoming workweek. Long period
southerly swell is forecast to increase due to tropical cyclone
activity across the Eastern Pacific. Southerly swell heights are
anticipated to remain in the slight category but mixed seas may
result in hazardous boating conditions through the week.
Additionally, there are non-zero chances for high based showers
which could result in pockets of gusty outflow winds and
subsequent steep seas now through mid-week.
&&
.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...Kennedy
MARINE...Kennedy
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Prepared by Boulder-Creek.com Weather at: Mon Jul 20 04:30:03 PDT 2026
From the National Weather Service
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