San Carlos, California Weather Forecast Discussion

413
FXUS66 KMTR 152153
AFDMTR
Area Forecast Discussion
National Weather Service San Francisco CA
253 PM PDT Mon Sep 15 2025
...New SHORT TERM, LONG TERM...
.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
&&
.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...
(18Z TAFS)
Issued at 1034 AM PDT Mon Sep 15 2025
VFR conditions have returned to all TAF sites this morning and are
likely to remain that way through much of the afternoon and evening.
However, coastal sites such as KHAF remain sub-IFR but are forecast
to scattered out by around 19Z. The marine layer will remain shallow,
generally less than 500 feet in depth with moderate to high
confidence for fog/low ceilings to impact coastal areas such as
KMRY, KSNS, and KHAF this evening and continuing into Tuesday
morning. For the Bay Area terminals, looking for the potential for a
few hours early Tuesday morning for IFR conditions, moderate
confidence. Any low ceilings or visibilities will clear by around
15Z-17Z Tuesday and will return to VFR.
Vicinity of SFO...VFR. Onshore winds are forecast to increase this
afternoon before becoming light/variable early Tuesday morning.
Moderate confidence for IFR ceilings to develop over the terminal
between 12Z-16Z Tuesday before VFR conditions return and onshore
winds increase.
SFO Bridge Approach...Similar to SFO.
Monterey Bay Terminals...VFR. Onshore winds are forecast to increase
this afternoon before easing after sunset. There is moderate to high
confidence for IFR ceilings/visibilities to return this evening and
lower to LIFR late in the night and into Tuesday morning.
&&
.MARINE...
(Today through Saturday)
Issued at 1034 AM PDT Mon Sep 15 2025
Winds continue to diminish across the waters through today while
fair weather persists. 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 139 PM PDT Sun Sep 14 2025
The fire weather threat will elevate this week for a few reasons.
1) Despite last weeks precipitation, our area has NOT had a season
ending rainfall event. 2) While Energy Release Components (ERCs)
have rebounded significantly, the above normal MaxTs and MinTs
will serve to rapidly cure some of the finer fuels and
subsequently ERC values will support some fire spread. The above
normal warmth will also translate to poor overnight humidity
recovery (RH below 35%) across the Mayacamas, Santa Cruz
Mountains, as well as the Diablo, Santa Lucia, and Gabilan Ranges
this week. Afternoon RH will also fall into the 20-35% range for
most areas, except along the immediate coast. 3) There is an
opportunity for isolated thunderstorms as early as Thursday and
into Friday. The areas with the greatest chances (around 20-25%)
for isolated thunderstorms, are across the Central Coast, Eastern
Santa Clara Hills, and East Bay. At this time, rain amounts are
anticipated to remain largely below one-tenth of an inch and as a
result, ignitions due to dry lightning may transpire Thursday and
into Friday.
Bain
&&
.MTR WATCHES/WARNINGS/ADVISORIES...
CA...None.
PZ...Small Craft Advisory until 3 PM PDT this afternoon for Pt Arena
     to Pt Reyes 10-60 NM.
&&
$$
SHORT TERM...MM
LONG TERM....MM
AVIATION...RGass
MARINE...RGass
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NWS MTR Office Area Forecast Discussion