-- Discussion containing changed information from previous version are highlighted. --
217
FXUS66 KMTR 222014
AFDMTR

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

...New SYNOPSIS, SHORT TERM, LONG TERM...

.SYNOPSIS...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

.SHORT TERM...
-- Changed Discussion --
(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.
-- End Changed Discussion --

&&

.LONG TERM...
-- Changed Discussion --
(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.
-- End Changed Discussion --

&&

.AVIATION...
(18Z TAFS)
Issued at 1042 AM PDT Sun Sep 22 2024

Tricky forecast earlier this morning with amendments and
adjustments to many TAFS. The reason? A compressing marine layer
through sunrise allowing for expansion of the marine layer, but
also patchy fog/reduced visibility. As of the 18Z TAF package
issuance, vis satellite shows rapid thinning of stratus layer
yielding VFR all terminals by 18Z or shortly thereafter. VFR this
afternoon/this evening with low stratus hugging the coastline. Do
expect the lurking stratus to move inland again tonight. However,
building high pressure will leading to further compression.
Extent of stratus will not be as much as this morning.
Additionally, NE flow begins to develop late night just above the
marine layer. Earlier clearing of stratus for Monday AM rush.

Vicinity of SFO...Clearing over terminal by 1815Z. Stratus
lingering SE and E of runways. Onshore flow developing later this
afternoon. IFR to LIFR late tonight.  Earlier clearing tomorrow.

SFO Bridge Approach...Stratus layer clearing from the
edges...therefore some stratus will linger longer over the
approach than SFO.

Monterey Bay Terminals...VFR this afternoon. CIGS return early
this evening.

&&

.MARINE...
(Today through Friday)
Issued at 853 AM PDT Sun Sep 22 2024

High pressure over the Eastern Pacific and low pressure over
California will lead to breezy northwesterly flow over the coastal
waters through at least Monday. Winds will be strongest over outer
waters north of Point Reyes. The locally stronger winds will
result in a steeper fresh swell. Northwesterly breezes decreasing
and seas abating Tuesday. Seas rebuilding to become rough in the
outer waters towards the end of the forecast period.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

$$

SHORT TERM...DialH
LONG TERM....DialH
AVIATION...MM
MARINE...MM

Visit us at www.weather.gov/sanfrancisco

Follow us on Facebook, Twitter, and YouTube at:
www.facebook.com/nwsbayarea
www.twitter.com/nwsbayarea
www.youtube.com/nwsbayarea