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NWS Forecast Area Discussion - San Francisco Area

000
FXUS66 KMTR 151144
AFDMTR

Area Forecast Discussion
National Weather Service San Francisco Bay Area
444 AM PDT Sat Aug 15 2020

.SYNOPSIS...Dangerously hot temperatures with Excessive Heat
Warnings inland and Heat Advisories around the Bay with record or
near record heat possible again today. Little relief along the
coast as the marine layer will be very shallow with only light
onshore breezes. Isolated dry lightning is also possible across
the region late today into early Sunday. The ridge responsible for
the hot weather will stay in place through next week.

&&

.DISCUSSION...as of 3:45 AM PDT Saturday...An oppressive,
prolonged heatwave is now underway not only for our local area but
across much of the western CONUS. Widespread record breaking
temperatures were observed yesterday regionwide and afternoon
temperatures easily exceeded triple digits readings across the
interior with pockets of mid 90s to around 100 even near the
coast. Forecast model data has struggled to keep up with this
oppressive heat along the coastal interface and as a result the
forecast will constantly be reevaluated and action taken as
necessary. An afternoon sea breeze did eventually pick up along
the immediate coast mid to late yesterday afternoon which helped
to bring some limiting cooling, however, many areas away from the
coast or at higher elevations are still struggling to cool
overnight. This lack of overnight cooling relief can lead to
accumulating heat stress and should be taken seriously during
prolonged heat events such as this one.

At the synoptic scale, there are two primary weather systems that
will play a role in our weather through the weekend. The first and
most prominent is the 597dm 500mb high pressure system centered
roughly over Arizona which is slowly expanding into California
from the southeast. Hot stagnant air trapped beneath this dome of
high pressure first encroached on the region Thursday night which
then set the stage for Fridays historic heat which broke
temperature records across the region. This high pressure system
will gradually strengthen to 600dm at 500mb as it retrogrades
towards Vegas and further builds into the local area through next
week, resulting in continued oppressive heat, especially across
the interior through next week. To the west, a 579dm 500mb low
pressure system is colliding with the northwestern perimeter of
the oppressive high pressure ridge. This low pressure system will
bring about a brief cooling trend between Friday and Saturday and
again on Saturday on Sunday as it nudges the perimeter of the hot
dry air under the ridge farther eastward, likely making Sunday
the coolest day throughout the entirety of the prolonged heatwave.
However, as this occurs, a surge of moisture at 700-500mb aloft
will be advected in from the south in the southerly flow between
these features later today, tonight, and into Sunday morning.
Elevated convective parameters meet critical thresholds to spark
the potential for fast moving, high based convection of which some
isolated thunderstorms may develop and move across the area from
south to north. Given the dry fuels and hot, drying air mass in
place, any isolated lightning strikes could lead to new wildfire
starts. In addition, due to both the high based and fast northward
progression, little if any precipitation will accompany lightning
strikes at the surface, suggesting that isolated dry lightning
will be the main outcome of any elevated convection through the
coming days. As a result, in coordination with the neighboring
offices, a fire weather watch has been issued for the potential of
isolated dry lightning Saturday PM to Sunday AM. Those paying
close attention may notice that there are two fire weather watches
for our area, which have been segmented by GACC coverage
(regional fire offices) and likely timing of arrival of highest
MUCAPE/Modified Total Totals/PWAT values.

Lightning risk aside, heat risk will be the broader societal
impact again on Saturday. The excessive heat watches across the
interior (including the urban areas of the East and South Bay
valleys) continue to remain in effect through the prolonged
heatwave with expected peak temperature for today again in the low
to mid 100s, locally in the upper 100s. In addition to the
excessive heat warning across the interior, we plan to issue
targeted daily heat advisories for the nearshore and coastal zones
as needed. Friday showed how oppressive this heatwave can be and
even the highest ensemble members of the hottest forecast models
struggled to encapsulate the full extent of yesterdays record
breaking heat along the coastal to inland interface. Given this
experience and the warm overnight temperatures in place to start
the day today, made several passes on the forecast temperatures
and nudged nearly all of Saturday`s forecast temperatures higher
in the coastally influenced sites that may have been too cool.
Noticeably warmer temperatures are now in place for today along
the San Francisco Bayshore and for the city itself, however,
Saturday is not expected to be nearly as hot as Friday by a margin
of a few to several degrees, though not many days are capable of
such heat. As such, heat advisories are in place for today for all
of the same locations they were in place by the last update
around lunchtime yesterday. This includes, the North Bay valleys
including Santa Rosa and Napa, the San Francisco Bayshore,
including Oakland and Silicon Valley, San Francisco itself, the
Santa Cruz mountains including Scotts Valley, the northern
Monterey Bay including Santa Cruz and Watsonville, and the
southern Monterey Bay including Big Sur. Some locations in today`s
heat advisory may not fall within criteria of the heat advisory
however other portions of a zone will, ie Big Sur Station will
likely be much warmer than Pacific Grove. Likewise, the higher
hills and east to southeastern portion of San Francisco are much
warmer than the west side of town. Overall, these temperatures are
10 to 25 degrees above seasonal normals, depending on the
location.

The most important weather shift to watch for today will be subtle
shifts in wind direction as afternoon sea breezes are expected to
develop once again this afternoon. As with many previous heat
events, coastally influenced communities may hit their maximum
daytime temperature as early as 1130am before winds shift onshore
and cooler air from the nearshore waters advects in and begins to
counterbalance the warmer air inland. This occurred yesterday at
Carmel, for example, which peaked in the mid 90s before noon when
the wind shifted. For today, watching for wind shifts towards the
west will be key in determining maximum temperatures for the day
and is one key reason days like yesterday are difficult to nail
both for the models and human forecasters, as even the slightest
of onshore breezes result in significant temperature shifts over
the course of an hour or two.

Looking ahead, the upper low off the coast weakens and is ejected
out of the area late Sunday into early Monday which allows the
high pressure system to further strengthen and recenter itself
north of Vegas over the Great Basin. Consequently, high pressure
will be in total control of the weather pattern and hot stagnant
air will result in increasing temperatures from Monday into mid
week next week, likely the next peak in the prolonged heat wave.
This forecast shift nudged up the forecast maximum temperatures
somewhat for the Tuesday to Wednesday timeframe however expect
further increases as we near this time frame. Medium to long term
models suggest a potential for this high pressure to remain the
primary weather driver across the western CONUS through next
weekend. We will continue to closely monitor both the heat and
fire weather risk over the coming week and add or update any
watches, warnings, and advisories as the data requires it.
Generally after a few days of extreme heat, we see hints of a
southerly wind reversal and return of a shallow but vigorous
marine layer first over the Central Coast and then into Bay Area,
however, still nothing of that sort expected in the nearterm.

&&

.AVIATION....as of 04:42 AM PDT Friday...for 12Z TAFs. VFR
through the TAF period. Winds are currently light, but Saturday
afternoon will see traditional onshore breezy conditions, before
subsiding at night. As monsoonal moisture continues to move over
the region on Saturday, high clouds are expected across the sky.
There is a slight chance for an isolated elevated shower Saturday
afternoon, but there is low confidence and should mostly be over
the ocean or far enough to the east.

Vicinity of KSFO...VFR. Light, onshore winds, will begin to
increase and become breezy Saturday afternoon. High clouds are
expected over the terminal for much of the day, but remaining VFR
throughout the TAF period with subsiding winds Saturday evening.
Low confidence in the chance for storm development, but VCTS was
added to KLVK as models are showing some development in the
eastern portion of the region in the late afternoon. Storm
development will be monitored and removed if necessary.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR with calm winds. As moisture from the
south moves over the area, expect increasing number of high
clouds, but the terminals will remain VFR through Saturday. Winds
will increase and become breezy Saturday afternoon. With the
influx of moisture, models have indicated a chance for isolated
storms popping up, but there is low confidence in it developing. A
VCTS group was added to KSNS for Saturday afternoon, and will be
monitored/removed if storms end up not forming.

&&

.CLIMATE...
Record High Temperatures with the upcoming heat event.
    August 15   August 16
-----------------------------------------
Santa Rosa103, 2019;100, 2019;
Kentfield101, 2019;101, 2015;
Napa102, 1906;104, 2015;
Richmond 91, 2019; 94, 2015;
Livermore108, 1967;105, 1951;
San Francisco 87, 1950; 90, 2015;
SFO 94, 2019; 90, 2015;
Redwood City100, 2019;101, 2015;
Half Moon Bay 71, 1984; 72, 2015;
Oakland Dtown 93, 2015; 92, 2015;
San Jose 99, 2019; 97, 2015;
Gilroy104, 2019;107, 2019;
Santa Cruz101, 2015; 94, 1983;
Salinas 93, 2015; 94, 1933;
Monterey 90, 2015; 83, 2015;
King City104, 1950;103, 2015;

&&

.MARINE...as of 02:31 AM PDT Saturday...Winds are currently
light, but locally stronger afternoon and evening winds are
expected along coastal gaps, particularly near Santa Cruz and
along the Big Sur Coast south of Point Sur. There is a small
chance of scattered showers late on Saturday, mostly over the
southern waters. Winds will increase slightly early next week as
high pressure starts to build off the California coast. Mixed seas
will prevail with a persistent small period northwest swell and a
longer period southerly swell.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...Excessive Heat Warning...CAZ507-510-511-513-516>518
             Fire Weather Watch...CAZ516>518-528-530
             Heat Advisory...CAZ006-506-508-512-528>530
             SCA...Pigeon Pt to Pt Pinos 0-10 nm from 3 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 3 PM

&&

$$

PUBLIC FORECAST: DRP
AVIATION: DK
MARINE: DK

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