Heat in California, September 25, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1059 AM PDT Wed Sep 25 2019

.SYNOPSIS...Dry offshore flow and gusty winds will result in
critical fire weather concerns for the North Bay Mountains through
midday. Very warm to hot daytime temperatures will also continue
through this afternoon and evening. Onshore flow will return to
the region on Thursday resulting in a region-wide cooling trend
that will extend into the upcoming weekend.

&&

.DISCUSSION...as of 8:58 AM PDT Wednesday...Warm start to the day
for much of the San Francisco Bay Area as another hot afternoon
is in store for the region. The temperature at sunrise in Downtown
San Francisco was in the middle 60s -- the warmest start to the
day since September 13. Temperatures will warm to the 90s along
the Bay Shoreline and rise as high as the low/middle 100s for the
hottest locations in the far interior North and East Bays.
Consequently, a Heat Advisory is in effect for a second
consecutive day for much of the region, including the City/County
of San Francisco.

Locally strong north to northeast winds in the North Bay Mountains
continue to present fire weather concerns again this morning.
Wind observations in some of the highest peaks in Sonoma/Napa
Counties -- notably one of the PG/E weather stations at Mount
Saint Helena -- registered isolated sustained winds above 40 mph
with gusts above 50 mph. The Red Flag Warning continues for the
North Bay Mountains through 11 am PDT.

Present short-term forecast remains on track. Main challenge
through the day will be monitoring the temperature trends. For
additional details, please refer to the previous discussion
section.

&&

.PREV DISCUSSION...as of 3:14 AM PDT Wednesday...Clear sky
conditions prevail over the region early this morning as a warm,
dry air mass persists aloft. This is a result of high pressure
building inland over the region as a cut-off upper level low is
positioned over northern Baja California. This pattern has kept
very warm temperatures in the region's higher elevations where 70s
to even lower 80s are being reported at this hour. Meanwhile,
temperatures have fallen into the 60s for most urban and coastal
locations.

A few passing high clouds will move across the region from the
northwest today as a weak disturbance rotates around the eastern
periphery of the ridge of high pressure. However, these clouds will
do little to affect temperatures across the region with another very
warm to hot day forecast. Overall, look for daytime temperatures to
be similar to those yesterday with widespread 90s inland with the
warmest interior valley locations reaching between 100 to 104 deg F.
Meanwhile, coastal locations will warm into the middle 80s to lower
90s. Thus, a Heat Advisory remains in effect from 11 AM to 7 PM
today for most interior valley locations, around the San Francisco
Bay Shoreline and the city itself as well as coastal locations of
the Central Coast. While offshore winds are forecast to diminish in
the region's higher elevations later this morning, strong and gusty
winds combined with critically dry conditions will persist through
midmorning over the North Bay Mountains (primarily in far eastern
Sonoma County and those in Napa County). Thus, a Red Flag Warning
continues through 11 AM for the aforementioned areas while the
warning has been dropped early for the East Bay Hills/Mountains. For
more information, please see the Fire Weather Discussion below.

While the ridge aloft will linger into Thursday, forecast models are
in good agreement with onshore flow developing near the coast. This
should allow for a region-wide cool down by 10 to 15 degrees as the
ridge becomes suppressed southwestward. In addition, a southerly
surge is forecast to develop late Thursday which will bring the
potential for low clouds and fog back to the coast. A more
widespread cool down is forecast for Friday with temperatures
falling below seasonal averages in response to marine influences
spreading further inland. As the marine layer rapidly deepens early
Friday morning, patchy drizzle will be possible for many coastal and
adjacent areas. The cooling trend will extend into the upcoming
weekend as a mid/upper level trough deepens and drops southward
across the western portion of the country. Seasonably cool
conditions are forecast to continue into early next week as the
ensembles indicate troughing over the region. It also appears likely
that chances for widespread precipitation will remain well to the
north and inland.

&&

.AVIATION...As of 11:00 AM PDT Wednesday...For 18z tafs. VFR will
prevail at the terminals through the afternoon and evening hours.
A few high clouds will pass through the area this afternoon,
otherwise skies will be mostly clear. Light and variable to
offshore winds this morning will become light onshore this
afternoon. A southerly surge is expected to develop and begin to
progress up the Big Sur coast this afternoon, and into the rest of
the coastline this evening and overnight. Stratus off the Santa
Barbara coast is patchy, and models depict only patchy stratus
along our coastline tonight. Thus, most of the terminals should
remain in VFR through Thursday morning, with some patchy low
clouds possible around SF Bay and the North Bay valleys. The
Monterey Bay terminals could see more low clouds develop tonight
as the surge will impact this area first.

Vicinity of KSFO...VFR. Light offshore wind in the morning
becoming west with sustained speeds near 15 kt and gusts to 20 kt
this afternoon around 21-22z. A few patchy low clouds are possible
late tonight but the terminal is expected to remain VFR.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR through the afternoon and evening.
Generally light onshore winds at KMRY with 10-15 kt winds at KSNS.
Patchy MVFR/IFR cigs could develop late tonight and early
Thursday morning.

&&

.FIRE WEATHER...as of 3:14 AM PDT Wednesday...Red Flag conditions
persist over the higher elevations of the North Bay, mainly
interior Sonoma County and in the mountains of Napa County. Gusty
winds of 40 to 55 MPH are being reported around Mount Saint Helena
with humidity values ranging from 35% to 45%. Meanwhile, wind
speeds have diminished over the East Bay Hill including the Diablo
Range where the Red Flag Warning has been dropped early. Short-
range, high-resolution models indicate winds will diminish for all
areas by late morning which will help to ease fire weather
concerns. Regardless, dry and very warm to hot conditions will
persist through this afternoon with moderate to locally breezy
offshore winds. Onshore flow and increased humidity will return to
the region beginning late tonight into Thursday further reducing
fire weather concerns.

&&

.MARINE...as of 8:46 AM PDT Wednesday...Gusty northwest winds
continue over the northern outer waters through tonight. Northwest
winds along the coast will decrease today and switch to southerly
by tonight. Moderate southerly winds can be expected near Point
Pinos and in the northern San Francisco Bay Thursday afternoon and
evening. A moderate northwest swell will continue through
Thursday before subsiding. This will create hazardous conditions
for small crafts.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Heat Advisory...CAZ006-506>508-510-512-513-516-528>530
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm

&&

PUBLIC FORECAST: Rowe/RGass
AVIATION: ST
MARINE: ST
FIRE WEATHER: RGass

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