Wildfire in California, June 17 to 26, 2020

Area Forecast Discussion
National Weather Service San Francisco Bay Area
1026 AM PDT Wed Jun 17 2020

.SYNOPSIS...Much warmer and drier conditions are forecast today
through Thursday as onshore flow weakens considerably and high
pressure builds over California. Slightly cooler temperatures will
develop near the coast late in the week and into the upcoming
weekend, while warm conditions persist inland.

&&

.DISCUSSION...As of 3:40 AM PDT Wednesday...The upper trough that
brought cooler than normal temperatures to inland areas over the
past few days, as well as breezy conditions to much of our
forecast area, has moved off to our east early this morning.
Northwest winds, which were locally gusty late yesterday, have
since subsided as the trough has shifted eastward. Meanwhile, in
the wake of the departing upper trough, surface high pressure has
been building rapidly across Oregon and northern Nevada. The ACV
to SFO surface pressure gradient has increased to more than 7 mb
and this steep gradient is producing localized strong and gusty
winds at the highest elevations of the North and East Bay. At
Mount Saint Helena in the North Bay (4300') northerly winds are
currently gusting in excess of 50 mph, while winds atop Mount
Diablo (3850') in the East Bay are currently gusting at around 40
mph. Otherwise, wind gusts in the hills are generally around 25
mph or less. These north winds are beginning to bring drier air
into the region, but thus far humidity values in the hills have
mostly been holding steady overnight and the latest local WRF
indicates that the airmass won't see significant drying until the
daytime hours. Gusty north winds are expected to become somewhat
more widespread in the hills by daybreak and then remain locally
gusty through midday. These developments are concerning from a
fire weather perspective but only in isolated higher elevation
locations for now. Will need to continue to monitor closely
through the day. Fortunately, winds in the hills are forecast to
subside by this afternoon when humidity values drop more
significantly and the airmass warms up. Northerly winds are
forecast to increase in the hills once again tonight, but are not
expected to be strong enough, or widespread enough, for
significant fire concerns.

An upper ridge currently centered offshore along 135W is shifting
eastward towards California early this morning. Subsidence warming
under this ridge over the next two days will result in a robust
warming trend through Thursday. Temperatures today are forecast to
be anywhere from 5 to 15 degrees warmer than yesterday. Many
inland areas are expected to warm to the lower 90s today and then
into the mid and upper 90s by Thursday. At least light onshore
surface flow will keep coastal temperatures from rising out of the
70s today, except around Santa Cruz where 80s are likely. Lower
80s may become a little more widespread near the coast on
Thursday, but without significant offshore surface flow, coastal
temps likely won't rise much higher than that. Early morning
satellite shows clear skies across our entire region and there is
little evidence of a marine layer this morning. Based on the local
WRF model, patchy fog could develop near Monterey Bay by late
Thursday, but not likely elsewhere along the coast until early
Friday.

Although the upper ridge axis is forecast to be centered over
California on Friday, cooling is likely near the coast due to the
development of light southerly surface flow. In fact, if
southerly flow develops as forecast, temperatures will cool in the
coastal valleys as well. Only the inland valleys and hills will
remain warm on Friday. The NBM appears to be too warm on Friday in
coastal areas and so downward adjustments were made to coastal
max temps that day. Temperatures are forecast to hold nearly
steady through the weekend, with inland areas about 5 to 8 degrees
warmer than seasonal averages, while coastal areas likely see
temps near normal or just slightly above normal.

More widespread cooling is likely early next week as the upper
ridge shifts off to the east and a weak trough approaches the
coast.

&&

.AVIATION...As of 10:26 AM PDT Wednesday...For 18Z TAFs. VFR
conditions are forecast to prevail through the TAF period as the
region remains influenced by dry, northerly flow. Onshore winds
will develop along the immediate coast this afternoon, then
subside after sunset.

Vicinity of KSFO...VFR conditions are forecast to prevail over the
next 30 hours. West/northwest winds will increase this afternoon
around 15-20 kt with gusts near 25 kt. Winds will then subside
after sunset.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions are forecast to prevail
with onshore winds developing this afternoon.

&&

.MARINE...as of 8:37 AM PDT Wednesday...Moderate and gusty
northwesterly winds will prevail over the waters today and
tonight. The strongest winds are expected over the northern outer
waters and along the Big Sur Coast south of Point Sur where near
gale-force gusts will remain possible. These winds will generate
steep fresh swell resulting in hazardous seas conditions,
particularly for smaller vessels. Winds will begin to diminish
over the southern and inner waters tonight and tomorrow as breezy
conditions persist over the northern outer waters into late week.
Mixed seas will continue with short period northwest waves and a
long period southerly swell.

&&

.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...Pt Arena to Pt Reyes 0-10 nm
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm
             SCA...Mry Bay from 1 PM

&&

PUBLIC FORECAST: Dykema
AVIATION: Rowe
MARINE: Rowe

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