Mountain View, California Weather Forecast Discussion

241
FXUS66 KMTR 160518
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
1018 PM PDT Mon Sep 15 2025
...New AVIATION...
.KEY MESSAGES...
Updated at 344 AM PDT Mon Sep 15 2025
- Above normal temperatures today through Wednesday
- Widespread Moderate HeatRisk today through Wednesday
- Pattern shift brings chances for elevated dry thunderstorms
   Wednesday Night through Friday
&&
.UPDATE...
Issued at 806 PM PDT Mon Sep 15 2025
Today marked the first of 3 warm days. Max temperatures were
broadly in the 90s inland and in the 70s along the coast and mostly
clear skies through the day. In other words, it was about 10
degrees warmer than normal across the interior, and closer to 5
degrees warmer than normal along the coast. The forecast captured
these temperatures well today, increasing the confidence in
tomorrow`s forecast. The 00Z sounding measured the 850 mb temperature
at 18.35C, which was not too far from the median, and about a
degree lower than expected. This could have been a timing error as
the benchmark quickly increases tonight under building high
pressure. We`ll have a better sense with the 12Z sounding, which
should approach 20C.
There is still a lot of uncertainty regarding the moisture push
later this week. Tropical Storm Mario strengthened more than
expected earlier today, which means the system was deeper and it
was being steered more by upper level winds. This caused the track
to drift right of the forecast with each update. Mario has since
moved over much cooler water and is quickly losing structure. As
it falls apart, the remnant spin and moisture will move north.
It`s important to realize that tropical cyclones are much harder
to forecast than the typical long wave pattern that brings most of
our weather. As such, we can`t say with any real certainty how the
moisture plume will impact the Bay Area and Central Coast. To
illustrate this uncertainty, the LREF PWAT above San Jose on
Thursday night has a (10%-90%) spread of 0.84" - 1.57". That`s
between the 75th percentile and 99th percentile. While there is
certainly a chance for a high impact event, it`s important to look
at the entire suite of guidance rather than the high end impacts
alone.
&&
.SHORT TERM...
Issued at 158 PM PDT Mon Sep 15 2025
(This evening through Tuesday)
The Bay Area and Central Coast warm up is upon us. The
compressed/shallow marine layer that brought coastal fog/clouds
quickly vanished by mid morning. Weak low level offshore
developed as noted on visible satellite showing clouds eroding
over land and "rolling" westward over the waters. Latest 24 hour
trends show a solid 5-10 degrees warmer than Sunday afternoon as
max temps soar into the 90s across the interior. Temperatures are
a tad cooler at the coast, but still above normal with max temps
in the 70s and 80s. Parts of San Francisco may even have a shot at
hitting 90 degrees today.
So what`s it look like the rest of the short term? More of the
same as high pressure overhead and a mild airmass both contribute
to above normal temperatures. Latest hi-res models do suggest
another narrow strip of low level moisture along the immediate
coastline Monday night. Therefore, Tuesday will start off with
pockets of clouds and fog, which will quickly vanish by mid
morning. Expecting active thermal belts tonight as well. Even with
the narrow strip of moisture higher terrain will remain dry and
mild with lows in the 60s and lower 70s. The temperature dial gets
turned up on Tuesday as 850mb temps jump 2 celsius. Current
forecast is leaning toward the warmer end of model guidance with
more widespread 90s across the interior. Latest prob guidance
gives a 30-50% chc of the hottest interior locations breaking 100
degrees. Thermal belts will be active again Tuesday night into
Wednesday with not a lot of relief.
&&
.LONG TERM...
Issued at 158 PM PDT Mon Sep 15 2025
(Tuesday night through next Sunday)
Tuesday night into Wednesday is the time period where the
forecast begins to get really interesting. Wednesday will likely
be the hottest day of the bunch as 850mb temperates peak. If
we`re going to break 100 across the interior Wednesday will be the
day to do it with probs now up to 50-70%. Still "cooler" toward
the coast, but highs well above normal and in the 70s to near 90
degrees. HeatRisk values Monday through Wednesday to increase and
become more widespread Moderate. Even a few pockets of Major
HeatRisk on Wednesday, but given the item of year will not issue
any heat related products. That being said, if you have outdoor
plans on Wednesday take heat safety precautions with hydration and
plan accordingly. Wednesday temperatures could be exacerbated by
increasing tropical moisture making it muggy and "feel" warmer.
This is where the forecast gets interesting.
Late Tuesday night and Wednesday focus quickly turns southward as
left over tropical moisture from Mario begins advecting
northward. The longwave pattern shows the ridge that brought heat
begin to shift eastward as a robust trough approaches from the
west. Much farther south is the left over pieces of Mario. The
moisture/left over circulation are in a bit of squeeze between the
departing ridge and approaching trough. As such, the
transport/steering flow is right into Central CA. 12Z model suite
vary on details, but still paint a similar picture. PWATs rapidly
increase off the SoCal and Central Coast Wednesday (150-200% PON).
There is a little bit of instability over the coastal waters as
well. As the day progresses more moisture and instability
increase across the coastal waters and Central Coast. Chances for
showers and thunderstorms will increase by Wednesday afternoon
and Wednesday night over the Central Coast before spreading
through the rest of the Bay Area Thursday. Point soundings show a
classic inverted v profile indicating dry lightning potentially at
the onset before transitioning to wetter storms Thursday. Pretty
impressive PWAT values being kicked around for Thursday with
values exceeding 1.5 inches. Highest confidence continues to be on
the amount of moisture. Lacking confidence will be extent of lift
and instability. Those two are more nuanced with subtle
indicators in the way of 1.5PVU anomalies/weak shortwave trough
and lower end lapse rates. MUCAPE is pretty solid, as the CAMS are
getting more in focus, but need more than just MUCAPE. Convective
threat eases by late Thursday with lingering showers Friday and
into early Saturday. Simply put, this has the potential to be a
higher impact event with initial dry lightning changing to wetter
storms, especially for the Central Coast. Lastly, wet vs dry. As
mentioned, initially dry,but hard to ignore the PWAT. As such, QPF
has increase substantially over the Central Coast with vales
above Big Sur reaching over 0.5". Amounts quickly taper to less
than 0.10" to the north.
&&
.AVIATION...
(06Z TAFS)
Issued at 1017 PM PDT Mon Sep 15 2025
Winds are reducing, and IFR/LIFR CIGS build along the coast,
affecting HAF and the Monterey Bay terminals. Fog is expected in
those areas further into the night and into the morning. Moments of
IFR Cigs look to affect OAK into Tuesday morning, but don`t fill
over the site consistently. Cloud cover retreats into the late
morning on Tuesday clearing for all but HAF which sees LIFR CIGs
turn to IFR levels that afternoon then a brief period of scattering.
Moderate to breezy winds arrive Tuesday afternoon and look to linger
into that evening. LIFR CIGs return to HAF Tuesday evening as winds
reduce.
Vicinity of SFO...VFR through the TAF period. Winds are reducing and
will become light and variable through the night and Tuesday
morning. Moderate northwest build into Tuesday afternoon, but reduce
into that night.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...IFR/LIFR CIGs are building around the the
bay as winds become light. CIGS lower and fog builds around the
Monterey Bay into the late night. SNS clears into the mid morning,
while MRY sees some improvements in visibilities, but holds LIFR
CIGs until clearing in the late morning. Expect VFR and moderate
west winds for Tuesday afternoon. Winds reduce that evening and into
the night with CIGs returning beyond the TAF period.
&&
.MARINE...
(Tonight through next Sunday)
Issued at 526 PM PDT Mon Sep 15 2025
Expect light winds through the mid week before more moderate to
breezy winds arrive in the northern outer waters by the end of the
work week. Moderate seas continue, especially offshore, and will
subside through midweek before building again late week. Tropical
remnant moisture is anticipated to enter the region by midweek.
Isolated showers and thunderstorms are possible beginning late
Wednesday through Thursday.
&&
.FIRE WEATHER...
Issued at 806 PM PDT Mon Sep 15 2025
The remnants of Tropical Storm Mario will bring a robust push of
moisture from S-N starting late Wednesday. This moisture will
initially arrive in the upper atmosphere. With dry air remaining
in the lower levels, this set up brings a chance for high-based, dry
thunderstorms. The initial threat is to the Central Coast
Wednesday night before spreading north Thursday and Friday. The
ignition chances are elevated since the dry lightning threat succeeds
a period of warm, dry weather. As the event unfolds, any storms
should transition from dry to wet, with a better chance of
beneficial rain reaching the ground. Thunderstorms (both dry and
wet) also bring localized erratic, gusty winds that can cause
rapid spread of new or existing fires.
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...None.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...Murdock
MARINE...Murdock
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NWS MTR Office Area Forecast Discussion