Wildfire in California, July 1 to 17, 2018

Area Forecast Discussion
National Weather Service San Francisco Bay Area
831 AM PDT Sun Jul 1 2018

.SYNOPSIS...A gradual cooling trend will continue today as
onshore flow increases over the region. Much cooler conditions are
expected to develop by Monday and Tuesday with temperatures below
seasonal averages on Independence Day. Another warming trend is
then likely late next week.

&&

.DISCUSSION...as of 08:31 AM PDT Sunday...The marine layer is
currently sitting at around 2000 ft. A deeper marine layer and the
return of onshore flow allowed for low cloud development
overnight along the coast and inland into the Bay Area and down
the Salinas Valley. As a result, temperatures cross the region are
around 5 degrees cooler than this time yesterday. Current
satellite imagery shows stratus along entire coast and beginning
to dissipate over inland areas. A large plume of smoke blowing
southwest into the Bay Area from the County Fire can also be seen
on the visible satellite.

While the marine layer has deepened and brought cooler and more
moist conditions to most of the region, locations situated above
the marine layer remained warm and dry overnight. Warm and dry
conditions will persist into this afternoon across the hills of
Napa and eastern Sonoma Counties. This along with northerly winds
caused elevated fire weather conditions to continue overnight and
into this afternoon. Thus, the Red Flag Warning will remain in
place for elevations above 1000 ft in Napa and far eastern Sonoma
Counties through 3 PM. The area of greatest concern will be across
northern and eastern Napa County. Onshore flow near the surface
is forecast to return this afternoon easing fire weather concerns.

Northerly flow aloft will likely cause smoke to continue drift to
the southwest through at least this evening before onshore flow
returns allowing for some relief. Temperatures across the interior
will be noticeably cooler today with many inland areas seeing
afternoon highs in the 80s to mid 90s. The exception will be over
the far northeastern portion of the region where highs may still
breach 100 F before the marine layer deepens enough to move
further inland and allow for some cooling. Highs along the coast
will remain in the 60s and 70s.

No updates to the forecast at this time. For additional details
please see the previous discussion below.

&&

.PREV DISCUSSION...as of 03:29 AM PDT Sunday...The return of
onshore flow and a deepening marine layer has resulted in low
clouds from coastal Sonoma County down into the Monterey Bay and
Salinas Valley. Meanwhile, a much warmer and drier air mass
remains in place just above the marine inversion with temperatures
still in the 70s and lower 80s in the highest elevations across
the region, especially the North and East Bay Hills/Mountains.
With this, dry northerly flow persists in these elevations which
continue to produce elevated fire weather conditions. Given these
conditions across the higher elevations of Napa County and eastern
Sonoma County, will keep the Red Flag Warning in effect through 3
PM this afternoon. By late afternoon, onshore flow is forecast to
return to much of the region and help reduce the overall fire
risks in the aforementioned areas.

With northerly winds aloft, smoke from the County Fire (formally
Guinda Fire) will continue to impact portions of the North Bay down
into the greater San Francisco Bay Area through at least today. This
will result in smoke in the higher levels as well as poor air
quality through at least this evening. Winds aloft are then forecast
to shift onshore and will help clear smoke from the region heading
into early next week.

Before the marine air makes a strong push inland later today,
temperatures will still warm into the 80s to 90s for most inland
areas away from the coast. However, conditions will be noticeably
cooler today across the interior. Additional cooling is then
expected on Monday and Tuesday with a deepening marine layer and
continued onshore flow. Temperatures are then forecast to drop to
below seasonal averages by Independence Day as a weak mid/upper
level trough develops over the West Coast. Patchy morning coastal
drizzle may also develop over the next few days, however have kept
out of the official forecast at this time. Something we will monitor
in the coming days.

A warming trend then appears possible late in the week as high
pressure over the Desert Southwest builds toward the California
Coast.

&&

.AVIATION...As of 5:20 AM PDT Sunday...IFR/LIFR due to fog and
stratus is becoming more entrenched along the immediate coast.
Inland it's VFR, but onshore winds are carrying fog and stratus in
through the Golden Gate and a dry atmosphere above the marine
layer is allowing for decent nocturnal radiative cooling and
patches of IFR stratus ceilings are developing around the bays. In
general it appears the WRF model boundary layer relative humidity
is under-forecasting actual humidity this morning. 12z tafs were
composed with this in mind, however upon further inspection it
appears they may still be a bit optimistic (VFR forecast) for this
morning in some spots. Will monitor closely through the morning
and compare model forecasts against incoming observations.

An early return of stratus and fog is likely this evening for the
immediate coast and inland intrusion is also likely over the San
Francisco Bay Area. Gusty onshore winds will continue north of the
Bay Bridge to the Delta for most of the period.

Vicinity of KSFO...Amended forecast for tempo BKN cig 13z-17z, and
recent 5 min observations now just showing BKN 9 hundred feet.
Stratus has developed nearby in the approach and is filling in to
the north over San Francisco and over the bay waters. A light west
wind may shift to light northeast prior to onshore winds returning
by early afternoon. Light winds tonight. Thank you CWSU for recent
coordination call.

SFO Bridge Approach...San Mateo Bridge indicates an IFR ceiling.
Per CWSU coordination call clearing not forecast to return until
1630z in the approach.

Monterey Bay Terminals...IFR/LIFR until 17z-18z today. VFR this
afternoon, but stratus and fog are likely to return early this
evening.

&&

.FIRE WEATHER...As of 9:00 PM PDT Saturday...By Saturday evening
winds had decreased in the North Bay mountains and the wind direction
had switched from north to southwest in many locations. However,
the latest WRF model forecasts winds in the higher hills of the
North Bay to switch back to the north and increase once again
overnight, which will result in very poor humidity recoveries and
continued warm and dry conditions into Sunday morning. Given these
expected conditions, and the fact that an active fire is
currently burning in Yolo County near the Napa County line, have
decided to extend the Red Flag Warning for the North Bay Mountains
until 3 pm Sunday afternoon (the warning had been scheduled to
expire at 11 pm Saturday evening). Will indicate in the Red Flag
Warning that the area of greatest concern is northern and eastern
Napa County. Winds are forecast to switch to southwest and bring
in a cooler and more moist airmass by late Sunday afternoon, which
will ease fire weather concerns from late Sunday into the early
part of the work week.

&&

.MARINE...as of 3:29 AM PDT Sunday...Gusty northerly winds will
continue over the northern outer waters through mid-week. Gusty
southerly winds will develop over the southern inner waters this
afternoon. Ongoing gusty onshore winds can be expected north of
the Bay Bridge to the Delta through today and much of tonight,
gusty winds resuming Monday afternoon and evening. Elsewhere winds
will be mainly light southeasterly.

&&

.MTR WATCHES/WARNINGS/ADVISORIES...
     .Tday...SCA...Pt Pinos to Pt Piedras Blancas 0-10 nm from 11 AM
             SCA...Pt Arena to Pigeon Pt 10-60 nm
             SCA...SF Bay

&&

PUBLIC FORECAST: AS/RGass
AVIATION: Canepa
MARINE: Canepa
FIRE WEATHER: Dykema

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