Heat Advisory 7, NWS San Francisco Bay Area, CA

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1041 AM PDT Wed Aug 30 2017

.SYNOPSIS...An upper level trough approaching the Pacific Northwest
will maintain near seasonal conditions across our region today. A
significant warming trend is then forecast to begin on Thursday and
continue into the upcoming weekend with oppressive heat likely for
interior and higher elevation locations, especially on Friday and
Saturday.

&&

.DISCUSSION...as of 8:45 AM PDT Wednesday...Final day of cooler
temperatures across our region for quite awhile as all signs point
to very hot weather for a good portion of our CWA later in the
week. Right off the bat did a very minor update to the grids to be
more in line with the clouds at the coast and inland. Skies should
go mostly sunny by the afternoon with highs in the 60s and 70s at
and near the coast with 80s to lower 90s for many inland spots.
Satellite does show convection southeast of our CWA and at this
time it is forecast to remain out of our region. Main focus will
be on the upcoming heat with advisories and warning likely to be
issued.

.PREVIOUS DISCUSSION...Low clouds this morning are not as
widespread compared to yesterday as the marine layer compressed
late yesterday evening to around 1,000 feet in depth. However,
have seen trends on the Fort Ord Profiler that the marine layer is
again deepening to around 1,200 feet or so with low clouds
beginning to spread inland over the past few hours. Do expect this
trend to continue through sunrise before clouds begin to burn-off
over inland areas late in the morning. Some guidance would also
suggest clearing even at the coast by late this afternoon as the
marine layer mixes out. OVerall, do expect the marine influence to
be strong again today with temperatures near or only slightly
above seasonal averages for most urban areas. With that said, hot
conditions will still persist in the higher elevations, especially
across the East Bay Hills and in the mountains of Monterey and
San Benito County.

A more robust warming trend will then begin on Thursday as high
pressure from the Desert Southwest builds toward the coast. By
Friday and Saturday, forecast models show 850 MB temperatures
warming to around 30 deg C, translating to very warm to hot
temperatures at the surface. The region may experience some of the
warmest temperatures it has seen in a while as oppressive heat is
forecast for both Friday and Saturday. To give a few examples, the
GFS and ECMWF show temperatures at Livermore in excess of 110 deg F,
105 deg F in Santa Rosa, and 100 deg F in San Jose both Friday and
Saturday afternoons. Onshore flow is also forecast to weaken these
days and will help bring warmer temperatures closer to the coast
and/or bays as well. Looking for 90 to 95 deg F readings in and
around the San Francisco Bay Shoreline and Salinas Area while the
Monterey Peninsula and City of San Francisco could potentially warm
into the mid/upper 80s. With this, have expanded the Excessive Heat
Watch for all areas away from the immediate coastline from Thursday
afternoon into Monday evening. While onshore flow is forecast to
increase slightly on Sunday and Monday, interior areas will continue
to see hot to very hot daytime temperatures. With all this said,
overnight lows will also struggle to cool given the very warm air
mass aloft. Many higher elevation areas may only cool into the
mid/upper 70s to mid 80s, only exacerbating the threat for heat
related illnesses.

Along with the heat risks through the Labor Day holiday weekend,
fire weather concerns will also become elevated given the very hot
and dry conditions combined with dry fuels. While the models
generally keep wind speeds light, the excessive heat, low relative
humidity values and state of the fuels would support potentially
explosive fire growth. Thus, Fire Weather Watches and/or Red Flag
Warnings may be needed in the coming days.

The upper level ridge is then forecast to shift eastward early next
week with an increase in onshore flow moderating temperatures
slightly. Meanwhile, look for conditions to remain above seasonal
averages across the interior and higher elevations both Monday and
Tuesday. Southeast flow in the mid/upper levels is also possible
early next week as the ridge shifts to the east. This may bring some
monsoon moisture into portions of California and bring the potential
for high based convection. At this time, the threat remains low
across our region and will continue to maintain a dry forecast.
Something work keeping an eye on however.

&&

.AVIATION...As of 10:40 AM PDT Wednesday...Some lingering stratus
along the coast will mix out soon, then VFR everywhere. A building
high pressure ridge will further compress marine layer for
tonight. This will result in lower ceilings, patchy dense fog
along the immediate coast, and a shorter duration stratus event
(mainly over by 16Z THU) with only a few fingers of stratus
protruding inland by sunrise. Monterey Bay will see more stratus
than the San Francisco Bay on Thursday morning. Little to no
stratus will push inland by Friday morning.

Vicinity of KSFO... West winds to 20 kt after 22Z. Brief SCT/BKN
around sunrise THU, otherwise VFR.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...Lingering stratus along lip of Monterey
Bay will mix out shortly, then VFR through the afternoon. Even
lower ceilings and ground fog will be possible late tonight/early
THU.

&&

.MARINE...as of 10:32 AM PDT Wednesday...Moderate northwest winds
will persist through this evening before weakening tomorrow.
Predominately light winds prevail through the weekend. Light swell
today with some steeper fresh swell mixed in due to the moderate
winds.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Reyes to Pigeon Pt 0-10 nm
             SCA...Pigeon Pt to Pt Pinos 0-10 nm
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Mry Bay from 1 PM

&&

PUBLIC FORECAST: Bell/RGass
AVIATION: DRP
MARINE: DRP

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