635
FXUS66 KMTR 230412
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco CA
912 PM PDT Sun Sep 22 2024

...New UPDATE, MARINE...

.SYNOPSIS...
Issued at 1209 PM PDT Sun Sep 22 2024

Temperatures peak on Monday and Tuesday with a Heat Advisory in
effect for the interior East Bay, the South Bay, and the Santa
Cruz Mountains. Temperatures return to near the seasonal average
on Wednesday through the rest of the week.

&&

.UPDATE...
Issued at 905 PM PDT Sun Sep 22 2024

No significant updates needed in the evening forecast.

Remember: Heat Advisories go in effect tomorrow. Be sure to find
ways to beat the heat.

&&

.SHORT TERM...
(This evening through Monday)
Issued at 1209 PM PDT Sun Sep 22 2024

The warming trend continues today as an upper level ridge
continues to build into the Pacific Northwest, although with a
developing shortwave trough helping moderate the influence over
California. The latest RTMA analysis shows inland temperatures
running around 5 to 10 degrees higher than this time yesterday,
and high temperatures across the inland valleys are expected to
reach the lower to mid 90s. Elsewhere in the region, the Bayshore
and coastal Santa Cruz County will see highs reach into the mid
70s to the low 80s, while the Pacific coast sees highs in the low
to mid 60s. Low temperatures on Monday morning will generally
hover in the mid 50s to lower 60s across the lower elevations,
with some lower 50s in the Sonoma and Marin County valleys, while
the higher elevations see lows reach the mid 60s to lower 70s.

The warming trend peaks on Monday, when the upper level ridge
crosses into the Pacific Northwest and the shortwave trough spins
off a weak upper level low. A Heat Advisory has been issued for
the interior East Bay, South Bay, and the Santa Cruz Mountains, as
highs reaching the mid 90s to near 100, or around 15 to 20
degrees above seasonal averages, create widespread Moderate
HeatRisk (heat that impacts sensitive individuals, especially
those without adequate hydration and/or cooling) with some patches
of Major HeatRisk (heat that impacts anyone without adequate
hydration and/or cooling). The Bayshore will see highs reach the
mid 80s to lower 90s, coastal Santa Cruz and downtown San
Francisco see highs near 80, while locations along the Pacific
coast see a slight uptick to the mid to upper 60s. While some
offshore flow is possible during the overnight and morning hours,
onshore winds are expected to prevail during the afternoon and
evening hours.

The stratus deck has retreated to the immediate coastal regions,
with some limited inland development overnight into Monday
morning, especially in the Monterey Bay region into the Salinas
Valley, but overall stratus coverage will be limited by the
influence of the upper level high.

&&

.LONG TERM...
(Monday night through next Saturday)
Issued at 1209 PM PDT Sun Sep 22 2024

A slight cooling begins on Tuesday with temperatures dropping a
couple of degrees across the region. The Heat Advisory continues
through Tuesday in the interior East Bay, where potentially
dangerous heat lingers. Elsewhere there is some uncertainty over
how severe the heat impacts will be. The NBM ensemble products
are showing significant spread in the low temperature forecast
along and adjacent to the coastal ranges and San Francisco Bay on
Tuesday morning, with the spread between the 25th and 75th
percentile forecasts reaching 5 to 10 degrees across the coastal
ranges. The main forecast issue is with the evolution of the
marine layer which would enhance onshore flow and thus the
overnight cooling wherever it sets up, whereas weakly offshore
flow could keep temperatures warm above the marine layer inversion.
Colder low temperatures will limit the risk of heat- related
illnesses, as the body is able to "cool down" better overnight
than in a scenario where low temperatures remained above seasonal
averages. For now, the low temperature forecast has kept the
region near the cooler side of the forecast range, but this is
something to monitor over the next day, and should confidence in a
warmer scenario increase, Heat Advisories may need to be extended
into the Bayshore region.

More significant cooling is expected on Wednesday, as an upper
level trough moves into the West Coast and temperatures return to
around or somewhat below the seasonal average. The current
forecast calls for a slight warming closer to the end of the week,
and CPC outlooks suggest a lean towards temperatures above
seasonal averages through the first week of October.

&&

.AVIATION...
(00Z TAFS)
Issued at 446 PM PDT Sun Sep 22 2024

Currently VFR at all terminals. The TAFs were constructed using
persistence and a reasonable worst case scenario as guidance/models
are not conclusive on what is going to happen tonight. My educated
guess as to why this could be is due to offshore flow developing
overnight, and while it is not expected to be strong, it could be
just enough to keep clouds closer to the coast and increase dew
point depressions if the surface is able to dry out. The marine
layer is currently at 1,000 feet and is expected to further compress
through the TAF period as high pressure continues to build. This
will result in the ceilings that do develop being LIFR in addition
to sub-VFR visibilities. Widespread VFR will prevail by late-morning
tomorrow.

Vicinity of SFO...Currently VFR with westnorthwesterly flow. Low to
moderate confidence on a ceiling developing through the TAF period.
Winds will begin backing early tomorrow morning to become offshore
before the afternoon sea breeze prevails tomorrow afternoon.

SFO Bridge Approach...Similar to SFO.

Monterey Bay Terminals...Currently VFR with northerly flow at MRY
and VFR with northwesterly flow at SNS. High confidence in LIFR
conditions at both terminals this evening through late-morning
tomorrow.

&&

.MARINE...
(Tonight through next Saturday)
Issued at 905 PM PDT Sun Sep 22 2024

The pressure gradient between subtropical high pressure over the
Eastern Pacific Ocean and low pressure over the West Coast will
support strong northwesterly breezes and rough seas in the outer
waters through Monday. Northwesterly breezes decrease and seas
abate beginning Tuesday. Northwesterly breezes increase and seas
rebuild to become rough in the outer waters by Thursday.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Heat Advisory from 11 AM Monday to 8 PM PDT Tuesday for CAZ510-
     515.

     Heat Advisory from 11 AM to 8 PM PDT Monday for CAZ512>514.

PZ...Small Craft Advisory until 9 AM PDT Monday for Pt Arena to Pt
     Reyes 10-60 NM.

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...Sarment
MARINE...Sarment

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