FXUS66 KMTR 151633
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
933 AM PDT Mon Sep 15 2025

...New UPDATE...

.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 
   Thursday and Friday

&&

.UPDATE...
Issued at 907 AM PDT Mon Sep 15 2025

Near term weather concerns were focused on patchy dense fog
impacting a small area along coastline from Half Moon Bay 
southward to Big Sur, including Monterey Bay Region. A few 
sensors and cams showed dense fog being reported. That being said,
recent trends are already showing rapid improvement. In fact, 
latest satellite shows some weak low level offshore flow kicking 
in, which is "peeling" the clouds off the coastline and into the 
coastal waters. 

Current forecast looks on track.

Fine tuning two impactful scenarios with afternoon forecast: 
HeatRisk concerns Monday through Wednesday with interior temps in
the 90s to near 100. Additionally, tropical moisture surging 
northward as early as Wednesday afternoon/evening. Impacts would 
be lightning and potential fire weather concerns.

MM

&&

.SHORT TERM...
Issued at 344 AM PDT Mon Sep 15 2025
(Today and tonight)

A building ridge has begun to shift onshore overnight. Stratus will 
continue to hug the coastline as the marine layer compresses and 
weak offshore flow develops by sunrise, with low stratus mixing out 
entirely by mid/late morning. Once the clouds clear, temperatures 
will begin to rise above normal. Inland temperatures, especially 
higher elevations will see highs ranging from the mid-80s to upper 
90s, with temperatures along the immediate coast in the 70s to low 
80s. Although a Heat Advisory is not necessary today, areas of 
Moderate HeatRisk will make being outside for an extended period of 
time uncomfortable for working or recreation. Remember to remain 
hydrated with plenty of water and electrolytes, limit time outdoors 
during peak heating if possible, and never leave children or pets 
unattended in vehicles.

With little to no marine layer influence tonight into early Tuesday 
morning, we'll see poor RH recovery away from the direct 
coastline, especially at higher elevations. The warmer overnight 
lows and drier airmass will lead to accelerated drying of finer 
fuels (grasses/brush) over the next few days, which will increase 
fire weather concerns later this week, with more details in the 
fire weather section below.

&&

.LONG TERM...
Issued at 344 AM PDT Mon Sep 15 2025
(Tuesday through Sunday)

The warmest temperatures and largest areal coverage of Moderate 
HeatRisk (and small pockets of Major HeatRisk) will occur on Tuesday 
with only slightly lower temperatures on Wednesday. The timing and 
locations of a potential Heat Advisory for Tuesday will be made with 
the afternoon update later today. While the warmer temperatures 
persist through the afternoon on Wednesday, an upper level 
disturbance off the coast of SOCAL begins to lift north toward the 
Central Coast, bringing a large plume of moisture associated with 
Tropical Storm Mario with it. As the upper level low tracks 
north, we'll see an increase in PWATs that continue to approach 
200% of normal on Thursday.

As of this update, the best initial window for elevated dry 
thunderstorms appears to be after midnight early Thursday through 
late Thursday morning, primarily across the Central Coast in 
Monterey County and southern San Benito County. Mid-level lapse 
rates approach 7C/km with 30kts of shear Thursday morning, however 
thermal instability is limited with MUCAPE values <100 J/kg and some 
CIN to overcome. There is plenty of time between now and Thursday 
for thermal and kinematic instability to better align. Given the 
warm and dry conditions over the next few days leading into the 
potential for isolated elevated convection early Thursday into 
Friday, the risk for lightning induced ignition and rapid fire 
spread is certainly non-zero and should be monitored closely by 
our partners. Although Thursday morning appears to be the best 
window attm, chances for elevated convection continue into Friday 
and farther north over a larger portion of our forecast area. 
We're not expecting a widespread wetting rainfall, with overall 
rain amounts not expected to exceed a quarter of an inch for those
locations that are fortunate enough to receive any rain at all. 
Chances for light showers continue into Saturday with low 
confidence in the synoptic pattern for Sunday and beyond.

&&

.AVIATION...
(12Z TAFS)
Issued at 344 AM PDT Mon Sep 15 2025

Patchy stratus this morning leading to localized IFR and FG along
coastal areas. Marine layer becoming very shallow through today 
due to light offshore flow and building high pressure aloft. This 
will aid in quick clearing to VFR this morning across most of the 
area. Some coastal terminals may hold on to stratus a bit longer, 
but anticipating VFR across the board by mid-morning. This shallow
marine layer pattern persists through the rest of the TAF period,
so not expecting any widespread stratus Tuesday morning. Only 
local FG impacts along coastal areas.

Vicinity of SFO...IFR cigs around the Bay this morning with 
intermittent impacts at the terminal. Anticipated stratus to mix out 
rather quickly this morning during the 17Z hour, then VFR through 
the rest of the TAF period.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Patchy dense FG this morning thanks to a 
compressed marine layer. Expecting these conditions to linger into 
the 17Z hour before clearing to VFR. Another round of patchy FG is 
possible tonight into Tuesday morning, but largely anticipating VFR 
through the rest of the period.

&&

.MARINE...
(Today through Saturday)
Issued at 344 AM PDT Mon Sep 15 2025

Winds continue to subside 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 Galiban 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.

&&

.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...JM
LONG TERM....JM
AVIATION...Behringer
MARINE...Behringer

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Prepared by Boulder-Creek.com Weather at: Mon Sep 15 10:30:03 PDT 2025