Wildfire in California, September 1, 2019

Area Forecast Discussion
National Weather Service San Francisco Bay Area
203 PM PDT Sun Sep 1 2019

.SYNOPSIS...Warm temperature will continue under mostly sunny
skies. Warmest temperatures will be inland while persistent light
onshore flow will keep coastal areas mild. Temperatures will cool
3 to 5 degrees across the region on Labor Day as an upper trough
nears the coast. A gradual cooling trend is in store for the
remainder of the week as the marine layer deepens and onshore flow
strengthens.

&&

.DISCUSSION...as of 2:00 PM PDT Sunday...Latest satellite imagery
shows a narrow strip of stratus persists along much of the coast
at this hour. The marine layer remains compressed at under 1000 ft
and both the northerly and onshore gradients have strengthened
from 24 hours ago. At this hour the onshore gradient from SFO to
SAC has increased by half a mb to 1.4 mb and the northernly
gradient from ACV to SFO has increase by 1 mb to 3.9 mb. While the
onshore gradient will help push low clouds to the coast, the
shallow marine layer will keep them confined mainly to the
immediate coastline. Therefore anticipate low clouds to fill in
once again tonight across the coastal waters with minimal inland
intrusion similar to last night. Patchy dense fog is also expected
again early Monday morning.

Latest observations show temperatures are very similar to
yesterday at this time across the northern half of the region
while the southern half is running anywhere from 3 to 5 degrees
warmer with some locations such as Watsonville and Big Sur
running upwards of 10 degrees warmer. The big difference from
yesterday is significantly less clouds cover. Temperatures are
expected to finish off in the mid 60 to mid 70s along the coast,
upper 70s and 80s around the San Francisco Bay, and 90s inland
with a few locations hitting 100 degrees in the warmest inland
areas.

Warm temperatures will continue into Labor day as a broad upper
level ridge centered over the Four Corners region remains the
dominant weather feature through the remainder of this Holiday
weekend. An approaching upper level trough currently positioned
off the Washington coast will introduce a slightly cooler airmass
to the region beginning on Labor Day as the marine layer
gradually deepens over the California coast. This will
lead to 3 to 5 degrees of cooling over the area on Labor Day.
The coast will remain mild with only a few degrees of cooling
while inland temperatures cool on average 5 degrees or so.
Model remain in fairly good agreement that an upper trough will
develop off the Northern/Central California coast this coming week
and deepen as the week progresses. This will lead to a gradual
cooling trend that will continue through the rest of the work
week. The trough is forecast to move inland next weekend possibly
leading to a more significant cooling.

From previous discussion...As the ridge moves east and the
western trough deepens, there is a chance that the southwesterly
midlevel flow could bring monsoonal moisture northward with the
accompanying dynamics to produce some precipitation. Considering
models are still disagreeing, it will be left out of the forecast
and the likelihood is minimal. Secondly, it appears that a new
tropical system is forming in the Eastern Pacific, southwest of
Mexico. Longer range models show it eventually weakening and
moving north, funneling tropical moisture over Central California
next weekend. Once again though, the models disagree in the amount
of moisture and placement, with more models showing moister
mostly to the east of our region. Again, next weekend is still too
far out in time to have great confidence in this forecast, but
the potential for tropical moisture advecting to the Central Coast
is worth monitoring.

&&

.AVIATION...As of 10:38 AM PDT Sunday...For 18Z TAFs. Visible
satellite imagery shows stratus along the San Mateo coast and
northward through Marin County with clear skies elsewhere. VFR
conditions to prevail while a shallow marine layer remains in
place. Patchy stratus and fog is expected again tonight and
tomorrow morning. Low confidence as to timing and coverage of
redevelopment. Light winds this morning increasing and turning W
to NW this afternoon 10-15 kt with locally higher winds at KSFO.

Vicinity of KSFO...VFR conditions to prevail through the period
with patchy stratus expected to form again overnight. W to NW
winds 10-15 kt increasing to around 20 kt this afternoon with
gusts to 25 kt possible.

SFO Bridge Approach...Similar to KSFO.

Monterey Bay Terminals...VFR conditions this afternoon and evening
before IFR/LIFR conditions are forecast to return tonight due to a
combination of low clouds and fog. Generally light winds turning
onshore this afternoon 10-15 kt.

&&

.MARINE...as of 8:11 AM PDT Sunday...Light to moderate northwest
winds will continue over the coastal waters through the long
weekend and into the start of next week as high pressure sits over
the eastern Pacific and a thermal trough resides over California.
Expect breezy winds across the waters this afternoon and into the
early evening, especially along the inner coastal waters. Mixed
seas will persist with shorter period northwest waves at around 5
to 7 seconds in addition to a longer period southerly swell. A
long period northwest swell then arrive mid- week.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tngt...SCA...Mry Bay until 9 PM
             SCA...Pt Arena to Pt Reyes 0-10 nm until 9 PM
             SCA...Pt Reyes to Pigeon Pt 0-10 nm until 9 PM
             SCA...Pigeon Pt to Pt Pinos 0-10 nm until 9 PM
             SCA...Pt Arena to Pigeon Pt 10-60 nm until 9 PM
             SCA...Pigeon Pt to Pt Piedras Blancas 10-60 nm until 9 PM
             SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm

&&

PUBLIC FORECAST: CW
AVIATION: AS
MARINE: AS

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