Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 262047
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
124 PM PDT MON SEP 26 2016

.SYNOPSIS...
High pressure over the interior west will be weakening through
Wednesday, which will allow easterly winds to decrease. At the
same time, a low pressure area will be drifting north over Southern
California bringing more clouds, higher humidity, cooling, and a
chance for scattered showers and isolated thunderstorms, especially
over the mountains. Cooler for the latter half of the week as onshore
flow and the marine layer slowly return.

&&

.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
COUNTIES...

Mountain wave clouds developed this morning and increased into
the early afternoon, creating a nearly solid cloud deck over
much of central San Diego County and portions of Riverside County.
More clouds covered southern AZ and should spread westward tonight
into Tuesday. Despite the clouds, subsidence driven heating has been
maximized along the coastal areas today where early afternoon
temperatures were running 95 to 105 degrees. It was a bit cooler
inland across the valleys to the foothills but still quite warm.
Despite the withering heat, previous heat waves this time of year
make it difficult to break records, so expect few. a Heat Advisory
remains in effect through 8 PM PDT.

It has been breezy, even along and off the coast due to the east
winds, and yet, they have not been quite as strong as expected, so
the Wind Advisory has been cancelled. Surface pressure gradients were
running around 10 MBS offshore from NV to KSAN but still about flat
to the lower deserts.

High pressure over the interior West will be weakening through
Wednesday as a cut-off low moves north from the Baja and drifts over
Southern California. The low will become absorbed into the
westerlies to the north on Thursday as another weak upper-level
trough drifts NE over the region. The net result will be more
clouds, less wind, cooler days, and a chance for some scattered
showers and isolated thunderstorms as the atmosphere destabilizes.
Easterly flow aloft on tue, and possibly into Wed could result in
some of the showers drifting west of the mountains. Precipitation
is expected to remain mostly light and scattered.

A stronger longwave trough will amplify over the West over the
coming weekend. The 12Z GFS/ECMWF are coming together on an
amplified trough solution over the West early next week, so it
looks like this heat may be a distant memory as strong onshore flow
and a deep marine layer take center stage.

&&

.AVIATION...
262030Z...Areas of easterly winds will continue through 23Z today,
especially in San Diego County, even near the coast. KSAN will
likely switch back to the west around 22Z. Most of the gusts will be
below 30 knots the rest of this afternoon. Some easterly flow, with
gusts mostly below 25 knots, will occur tonight, with some LLWS
VCNTY KSAN and KCRQ.

Increasing clouds at/above 10000 ft MSL will continue through
Tuesday with unrestricted vis. There is a slight chance of
thunderstorms over the mountains Tuesday afternoon with strong
up/downdrafts and tops to 35000 ft MSL.

&&

.MARINE...
Local gusty east winds 10 to 20 knots will occur near the
immediate coast of San Diego County with choppy seas through 23Z.
Otherwise, no hazardous marine weather is expected through Saturday.

&&

.FIRE WEATHER...
Critical fire weather conditions will decrease this afternoon and
evening so the Red Flag Warning will be allowed to expire on schedule
at 3 PM PDT. 10 to 20 MPH Easterly winds will continue along the
coastal mtn slopes and on the foothills, with some higher gusts
at times in wind-prone areas, especially Riverside and San Diego
Counties but RH will be rising as moisture from a low pressure system
to the south increases across the region.

Tonight into Tuesday...upper-level low pressure over northern Baja
will bring a gradual recovery in RH, and an increase high clouds.
This combination will result in a cooling trend. Easterly winds may
still gust over 30 MPH in spots on the coastal mountain slopes, but
a weak sea breeze should make it to the western valleys by afternoon.
There is a chance for isolated showers/sprinkles as well, but
rainfall is not expected to be significant. Critical fire weather
conditions are not expected.

By Wednesday...Cooler with more clouds and a chance for showers and
isolated thunderstorms. Much weaker easterly winds over the mountains
will turn westerly by late afternoon with minimum RH expected to
range from 35 to 45 percent. A few dry lightning strikes are
possible, but lower-level moisture looks sufficient for wetting
rains beneath any convection that develops.

&&

.SKYWARN...
Skywarn activation is not requested. However weather spotters are
encouraged to report significant weather conditions.

&&

.SGX Watches/Warnings/Advisories...
CA...Red Flag Warning until 3 PM PDT this afternoon for Orange County
     Inland Areas-Riverside County Mountains-Including The San
     Jacinto Ranger District Of The San Bernardino National
     Forest-San Bernardino County Mountains-Including The
     Mountain Top And Front Country Ranger Districts Of The San
     Bernardino National Forest-San Bernardino and Riverside
     County Valleys  -The Inland Empire-San Diego County Inland
     Valleys-San Diego County Mountains-Including The Palomar
     And Descanso Ranger Districts of the Cleveland National
     Forest-San Gorgonio Pass Near Banning-Santa Ana Mountains-
     Including The Trabuco Ranger District of the Cleveland
     National Forest.

     Heat Advisory until 8 PM PDT this evening for Orange County
     Coastal Areas-Orange County Inland Areas-San Bernardino and
     Riverside County Valleys-The Inland Empire-San Diego County
     Coastal Areas-San Diego County Valleys-Santa Ana Mountains
     and Foothills.

PZ...None.

&&

$$

PUBLIC/FIRE WEATHER...JAD
AVIATION/MARINE...Maxwell



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