Milpitas, California Weather Forecast Discussion

093
FXUS66 KMTR 230439
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
939 PM PDT Mon Sep 22 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 138 PM PDT Mon Sep 22 2025
- Warming trend continues through Tuesday, areas of Moderate and
   Major HeatRisk on Tuesday
- Weak offshore flow through Tuesday in the higher terrain
- Rain showers and isolated thunderstorm chances return Tuesday
   night and lingering through midweek
&&
.UPDATE...
Issued at 912 PM PDT Mon Sep 22 2025
Widespread Moderate HeatRisk with pockets of Major HeatRisk
remains in the forecast for Tuesday. A Heat Advisory has been
issued for the East Bay, San Francisco Bay Shoreline, Santa Clara
Valley and the city of San Francisco. Micro-climates will play a
roll in how the heat is felt and perceived, especially for the
city of San Francisco (where the eastern half of the city may see
temps climb into the mid/upper 80s, but the western half stays
mostly in the 70s).
Once we get past the above normal temperatures we`ll see
increasing rain chances that begin across much of the Central
Coast Tuesday night, then spread north during the day on
Wednesday. By late morning Wednesday through the early evening
hours, locations that receive rain showers will also have a
chance of isolated thunderstorms. Rain chances shift east of our
area by Thursday morning with gradually clearing skies away from
the coast Thursday afternoon.
&&
.SHORT TERM...
Issued at 138 PM PDT Mon Sep 22 2025
(This evening through Tuesday)
Another very warm afternoon expected away from the immediate
coastline with Moderate HeatRisk across the interior where
temperatures warm into the low to mid 90s. Temperatures will begin
to cool after sunset this evening and lower into the mid 50s to
lower 60s across most coastal and valley locations. However, the
higher terrain will remain warm with temperatures only dropping into
the mid 60s to lower 70s overnight. Offshore winds are likely in the
peaks and ridges across much of the region through Tuesday afternoon
with minimum relative humidity values between 15-25%. However, winds
speeds are not forecast to be greater than 15 mph with gusts up to
about 25 mph in the peaks and ridges. Expecting low clouds and/or
fog once again near the coast and around the Monterey Bay region as
the marine layer is forecast to be anywhere between 250-750 feet in
depth.
Tuesday is expected to be the warmest day of the week as
temperatures soar into the mid-to-upper 90s and may approach 100
degrees F across the warmest interior spots across the region.
Downtown San Francisco has about an 80% chance of exceeding 85
degrees F and a 10% chance of exceeding 90 degrees F. This is when
more widespread Moderate HeatRisk spread to coastal locations with
pockets of Major HeatRisk across the East Bay Valleys and portions
of Santa Clara Valley.
&&
.LONG TERM...
Issued at 138 PM PDT Mon Sep 22 2025
(Tuesday night through next Sunday)
The much advertised sub-tropcial/monsoonal moisture has once again
been delayed as by 6-12 hours as the cut-off mid/upper level low
pressure lingers off of the southern California coast. As this
feature lifts northward toward the Central Coast on Tuesday evening
and into Wednesday, it will advect PWAT values between 1.00"-1.40"
northward across the Central Coast and then by Wednesday afternoon
and evening across the Bay Area and North Bay. The greatest
potential for thunderstorms remains across the Central Coast where
MUCAPE values range between 100-400 J/kg.
The potential for rain showers and isolated thunderstorms will
extend northward into the North Bay by Wednesday afternoon and may
linger into early Thursday morning before the upper level feature
shifts inland and then meanders across or just off of the southern
California coast.
There is also the potential for stronger wind/wind gusts in the
favored gaps, passes, and in the higher terrain from Wednesday
morning into Thursday morning depending exactly where the cut-off
low pressure moves inland. Wind gusts could exceed 45 mph in these
areas if the cut-off low moves inland over the Central Coast as
currently expected.
Dry conditions are anticipated by Thursday afternoon in wake of the
exiting cut-off low. A gradual cooling trend, to near seasonal
averages, is expected late week and the upcoming weekend as
troughing lingers over the West Coast. Be sure to keep up to date on
the latest forecast information as we near this upcoming pattern
shift.
&&
.AVIATION...
(06Z TAFS)
Issued at 937 PM PDT Mon Sep 22 2025
Currently VFR at all terminals with not a cloud in the sky. There
has been decreasing confidence in LIFR ceilings returning to HAF,
MRY, and SNS tonight due to warmer and drier conditions, lack of
current cloud cover, and a very shallow marine layer - if it does
happen, it will likely be of "sunrise surprise" type i.e. right
around the 12Z timeframe. Winds will be light and likely variable
overnight, generally having an offshore component. High clouds
will lower through the TAF period with rain showers beginning to
approach the Monterey Bay terminals towards the end of the TAF
period.
Vicinity of SFO...Currently VFR with westerly flow. High confidence
in VFR through the TAF period. Rain showers will approach towards
the end of the 30-hour TAF period.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...Currently VFR and calm at MRY and VFR with
northwesterly flow at SNS. Low to moderate confidence in LIFR
ceilings returning to the terminals tonight.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 912 PM PDT Mon Sep 22 2025
Isolated rain showers and thunderstorms are expected Tuesday
afternoon through Wednesday, especially over the southern waters.
Thunderstorms will pose the risk of lightning, locally heavy
rainfall, and erratic/gusty winds. A gentle to moderate breeze
will prevail through Wednesday, increasing to become fresh to
strong in the outer waters Thursday into Friday. Moderate seas
will prevail through Wednesday, building to become rough to very
rough in the outer waters Thursday into Friday.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 11 AM to 7 PM PDT Tuesday for CAZ006-508-510-
     513-515.
PZ...None.
&&
$$
SHORT TERM...RGass
LONG TERM....RGass
AVIATION...Sarment
MARINE...Sarment
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NWS MTR Office Area Forecast Discussion