561
FXUS66 KMTR 300817
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
117 AM PDT Mon Sep 30 2024

...New UPDATE, SHORT TERM, LONG TERM...

.SYNOPSIS...
Issued at 1240 PM PDT Sun Sep 29 2024

A fall heat wave kicks off today and will last for several days.
A heat advisory is in effect for the entire Bay Area and Central
Coast through Wednesday night. Moderate and major HeatRisk will
likely lead to numerous heat-related impacts to those sensitive
to hot weather or without access to adequate cooling.

&&

.SHORT TERM...
(Today and tonight)
Issued at 1217 AM PDT Mon Sep 30 2024

The marine layer is quickly compressing from around 1500 ft this
weekend to below 500 ft by mid-day. This is in response to a
building 500 mb ridge extending from subtropical high pressure in
the Eastern Pacific. This higher pressure will cause subsidence
and compressional heating, as well as offshore winds overnight and
through the morning. While the forcing is generally weak, these
winds could be strong at some mountain peaks and passes. In fact,
a few observations in the North Bay Mayacamas range are reporting
near gale force winds and RH in the mid 20s and falling. A few of
the higher peaks are actually meeting red flag criteria, but the
winds are currently localized to the highest peaks.

The temperature will quickly rise under clear skies this morning.
Inland areas will reach 80 between 10 and 11 AM and climb into the
mid 90s shortly after noon. With offshore winds fighting the
marine layer, the coast will also feel the heat today. The warmer
microclimates in San Francisco should reach the mid to upper 80s
today with more heat in the following days. A sea breeze should
bring some late afternoon relief to the coast.

&&

.LONG TERM...
(Tuesday through Sunday)
Issued at 1217 AM PDT Mon Sep 30 2024

By Tuesday afternoon ensemble guidance is suggesting an 85% chance
that the 850 mb temperature will surpass 25 C. While warm weather
is common around here in October, it`s usually driven primarily by
offshore winds. This level of heat at roughly 5,000 ft is rare
in fall. Compared to all October soundings, it ranks in the top
99.8 percentile. The only time consecutive October soundings
measured an 850 mb temp above 25C was between October 1 - October
3 1980. Coincidentally 44 years ago to the day. That heat wave
brought 3 days of 90+ degrees to downtown SF, with a peak of 97
recorded on the 1st. We could very well see similar impacts this
week. Don`t be surprised if some daily records are set during the
peak on Tuesday. Since the offshore wind forcing is relatively
weak, the high afternoon temperatures should trigger a late
afternoon sea breeze, bringing evening relief along the coast.
You`ll certainly feel the sudden change when this happens.

Offshore winds will continue through the night Tuesday into
Wednesday morning. This period will bring the peak of fire weather
concerns, especially at higher elevations. Most terrain above
2,000 ft will stay in the mid 70s all night with very poor RH
recoveries in the 20s. With the stronger than expected winds
reported this evening, it`s not out of the question that red flag
warnings will be issued.

Onshore winds are expected to return Wednesday afternoon, which
should bring relief to coastal areas by Thursday and help
alleviate fire weather concerns. Inland areas are unfortunately
looking hot through Friday. Even late week inland temperatures
will be 15-20 degrees above normal. So when will it end? The
latest guidance shows a gradual cool-down through the weekend and
into the early part of next week. However, the guidance trend
over the last few days has been to extend the duration of this
heat wave.

&&

.AVIATION...
(06Z TAFS)
Issued at 1035 PM PDT Sun Sep 29 2024

VFR across the region tonight, and expected to remain so throughout
the duration of the TAF period for most terminals, with the
exception of Monterey Bay. High pressure is beginning to build, and
light offshore winds overnight will help to erode any moisture and
the marine layer. In the afternoon tomorrow, winds will turn to
become onshore once again, but will generally be out of the NW and
near 10 knots. Winds become more light and variable Monday night.

Vicinity of SFO...VFR through the TAF period. Light offshore
(easterly) winds are expected to develop in the early morning of
Monday, and will last through the early afternoon. Come the
afternoon hours, winds are expected to reverse to become onshore and
out of the NW once again, with speeds up to 12 kts.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR, with VFR expected to last
through the mid morning Monday. Towards the 09-11Z window, stratus
may begin to develop and make an onshore push. Models do seem to
agree in some stratus forming, though the question will be how far
can it penetrate inland before being eroded by light offshore winds.
Currently, only moderate confidence in stratus forming IFR CIGs
briefly Monday morning, given the expectation of light offshore
winds. Will be keeping an eye on this for future updates.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 1035 PM PDT Sun Sep 29 2024

Northwesterly fresh to strong winds over the outer waters tonight
into early Monday. Significant wave heights will continue between
8 to 14 feet over the outer waters and portions of the northern
inner waters into Sunday night, before diminishing on Monday.
Given elevated wave heights and wind gusts, rough seas will
persist over the outer waters through early in the upcoming week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 11 AM this morning to 11 PM PDT Wednesday for
     CAZ006-502>506-508>510-512>518-528>530.

PZ...Small Craft Advisory until 3 AM PDT early this morning for Pt
     Arena to Pt Reyes 0-10 nm.

     Small Craft Advisory until 3 PM PDT this afternoon for Pigeon Pt
     to Pt Pinos 10-60 NM-Pt Arena to Pt Reyes 10-60 NM.

&&

$$

SHORT TERM...Flynn
LONG TERM....Flynn
AVIATION...AC
MARINE...SO

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