Heat in California, July 16, 2023

Area Forecast Discussion
National Weather Service San Francisco CA
141 PM PDT Sat Jul 15 2023

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

.SYNOPSIS...
Issued at 1141 PM PDT Fri Jul 14 2023

Excessive heat will impact inland areas this weekend with high
temperatures reaching the 90s to around 110. Overnight cooling
will be limited, with a moderate to high HeatRisk for the general
population. Gradual cooling will take place through early to mid
week.

&&

.SHORT TERM...
(This evening through Sunday)
Issued at 130 PM PDT Sat Jul 15 2023

Key Messages:
* Dangerous heat this weekend with highs in the 90s to 110 inland
* Limited overnight cooling in the hills with lows of 75 to 85
* Excessive Heat Warning inland areas for Saturday and Sunday
* Low relative humidity values with elevated fire danger

Technical discussion: If one looks at some coastal webcams or the
afternoon visible satellite they may say, "Heat, what heat?" Yes,
there are shallow clouds hugging the coast keeping temperatures
relatively cool, but interior locations are experiencing ample
sunshine with much hotter temperatures. Latest observations show
interior areas that are not being influenced by marine air are
soaring into the 90s and low 100s. Expect these locations to rise
a few more degrees before days end. Given the very hot inland
temperatures the Excessive Heat Warning remains in effect. Similar
to Friday the temperature spread across the forecast area will be
impressive again today with 40-50 degree spread from coast to far
interior.

Tonight: Expect similar to last night. The large area of high
pressure overhead will keep the marine layer compressed. It will
move inland again, but given that it is shallow it won't move
inland very far. Additionally, cannot rule out dense fog along the
coast with mist/drizzle too. Above the marine layer will be very
mild and dry with overnight lows in the 70s to mid 80s.

Sunday: A slight shift in the longwave pattern begins to develop.
The robust upper level high pressure begins to move east as an
upper level trough approaches from the NW. As for the sensible
weather, not much change from Saturday to Sunday. The coast will
be cooler with a lingering marine layer, but dangerous heat
remains across the interior with highs in the 90s to 110 range.
Excessive Heat Warning remains in effect for one more day.

Will continue to convey these heat-related safety tips:

* Stay hydrated and cool (use public AC if needed: malls, libraries,
  movie theaters, etc)
* avoid outdoor activities between 1 to 6 pm, including walking your
  pets
* monitor the elderly, young, sick, pets, and livestock for heat
  sensitivity
* double check your vehicles making sure no people or pets are
  left inside while parked in the sun

&&

.LONG TERM...
(Sunday night through next Friday)
Issued at 135 PM PDT Sat Jul 15 2023

Sunday night into Monday: The pattern shift mentioned above will
set the stage for a low confidence, but potentially higher impact
weather event. As the upper ridge shifts east steering flow aloft
becomes more southerly. The southerly flow aloft will tap into
some sub-tropical moisture (high PWATs). This moisture is
forecast to advect northward over the coastal waters and portions
of the Bay Area. In other words, riding the western periphery of
the retreating upper level high. In addition to the influx of
moisture aloft a weak impulse (vort max at 500mb) rides the
periphery too. This impulse may provide enough instability aloft
to allow for high based thunderstorms. HREF and CAMS support
higher MUCAPE with decent PWATs. Forecast point soundings show the
lower level atmosphere will still be very dry. Therefore, a
greater likelihood for any isolated thunderstorm to be dry.
Confidence on this occuring is 10-15% at this time, but it is in
the forecast for the North Bay 00-12Z Monday. As for temperatures
on Monday the retreating high pressure, lingering onshore flow,
and increased high level clouds will initiate a very subtle
cooling trend. The coast will remain in the 60s to 70s with
interior areas in the upper 80s to low 100s. Even though there is
no Excessive Heat Warning on Monday interior areas will be on day
3 of heat and fatigue may lead to some heat related issues.

Tuesday and beyond:
A slight cooling trend will develop through midweek as the passing
upper level trough to the north moves eastward. Night and morning
clouds clearing to the coast each afternoon will be the story.
Temperatures will be seasonable along the immediate coast with
highs in the 60s to near 80. Interior areas on the other hand will
remain in the upper 80s to near 100. Overnight lows will get a
little better, but thermal belts remain with mild temps in the
hills.

&&

.AVIATION...
(18Z TAFS)
Issued at 1024 AM PDT Sat Jul 15 2023

Stratus clearing out across the SF Bay region and near Monterey Bay,
with the coastal deck lingering through the TAF period. Relatively
light onshore flow develops this afternoon, with speeds up to 10-12
knots, with stratus returning this evening. Highest confidence in
stratus development near Monterey Bay, with lower confidence across
the SF Bay Area and APC. Considered changing SCT cloud groups to BKN
for tonight based mainly off persistence from the last couple of
mornings, but opted against it. Beyond the TAF period, monitoring
a low confidence for elevated convection Sunday and Monday.

Vicinity of SFO...VFR through the day with westerly winds
approaching 20 knots this afternoon and evening. Have retained SCT
cloud group over SFO for tonight, despite persistence edging towards
at least a temporary ceiling over the terminal Sunday morning. Winds
turn more southwesterly on Sunday with afternoon gusts up to 15
knots.

SFO Bridge Approach...If stratus forms tomorrow, moderate confidence
that stratus impacts the approach path, but not SFO terminal.
Otherwise, similar to SFO.

Monterey Bay Terminals...VFR expected at the terminals today,
although IFR stratus might linger at MRY for another hour or so.
Relatively light northwest flow at up to 10 knots this afternoon,
with LIFR stratus developing again this evening around 2-3Z,
persisting overnight until late Sunday morning. Not enough
confidence in clearing time at MRY to put into the TAF at this time,
although model output does suggest clearing to VFR before the end of
the TAF period.

&&

.MARINE...
(Today through Thursday)
Issued at 1024 AM PDT Sat Jul 15 2023

Breezy to locally gusty northwest winds continue over the outer
waters through the weekend, with gale force gusts developing in
the far northern outer waters on Sunday. The SF Bay and Delta
will continue to see breezy conditions each afternoon before winds
diminish into the night. These stronger winds will result in
steep wind waves and hazardous conditions to small craft. Mixed
seas of light northerly and moderate period (8-10 seconds) swell,
and light southerly swell continue through the weekend. Steeper
wind waves expected next week.

&&

.FIRE WEATHER...
Issued at 845 AM PDT Sat Jul 15 2023

Triple digit heat across the interior combined with daytime
minimum humidity readings in the upper single digits to teens will
help to dry out any moisture in the finer fuels. Weak to moderate
onshore flow will also persist leading to some breezy afternoon
winds in the valleys, gaps, and passes. Therefore, hot, dry, and
breezy conditions will lead to elevated fire weather concerns
through the weekend. Overnight recovery will be greatest below
1,000 feet through the weekend as night/morning clouds will be
possible. Above the marine layer will see little to no overnight
recovery with active thermal belts (mild and dry conditions).
With elevated fire weather concerns fuels will be more receptive
to new starts and could easily spread in the finer fuels. The
downward trend noted on the 100 hour fuels charts will continue
through early next week at a minimum given the current weather
pattern. Additionally, the forecasted ERC charts show the the Bay
Area will reach the 90th percentile as well.

Finally, one additional fire weather concern we're monitoring
closely will be high based convection Sunday in Monday. While the
confidence is low at this point the impact could be high following
this round of triple digit heat. Confidence is about 10-15% for
convection Sunday night into Monterey. Stay tuned...

&&

.CLIMATE...
Issued at 908 PM PDT Mon Jul 10 2023

Climate Site        July 15 (Sat) Record/Year

Santa Rosa          107 (degrees F) in 1972
Kentfield           105 in 1972
Napa dwntn          109 in 1972
Richmond            91 in 2009
SF dwntn            93 in 1888
SFO                 93 in 1995
Redwood City        110 in 1972
Half Moon Bay       74 in 1957
Oakland dwntn       88 in 1998
Livermore           107 in 1948
San Jose            105 in 1972
Gilroy              115 in 1972
Salinas AP          86 in 1998
King City           107 in 1926

Climate Site        July 16 (Sun) Record/Year

Santa Rosa          99 (degrees F) in 1935
Kentfield           103 in 1988
Napa dwntn          103 in 1995
Richmond            84 in 2017
SF dwntn            89 in 1988
SFO                 87 in 1988
Redwood City        100 in 1988
Half Moon Bay       71 in 2014
Oakland dwntn       88 in 1988
Livermore           108 in 1935
San Jose            97 in 2017
Gilroy              107 in 1995
Salinas AP          89 in 1935
King City           108 in 1935

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
CA...Excessive Heat Warning until 11 PM PDT Sunday for CAZ502>504-506-
     510-512>518.

     Heat Advisory until 11 PM PDT Sunday for CAZ528.

PZ...Small Craft Advisory until 9 PM PDT this evening for PZZ530-560.

     Small Craft Advisory until 3 AM PDT Sunday for PZZ540-570.

&&

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

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