Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 230419

Area Forecast Discussion
National Weather Service San Diego CA
919 PM PDT Mon May 22 2017

Another hot day is anticipated Tuesday as a ridge of high
pressure along the West Coast stregthens slightly. The Coachella
Valley should see it`s warmest day of the year with afternoon
highs in the 109-112 degree range. A sharp cooling trend will
arrive during the second half of the workweek as a trough of low
pressure traverses the region. Inland areas can expect afternoon
highs 10-20 degrees below seasonal averages by Friday. Quiet
weather and a weak cooling trend are forecast for the upcoming



At 900 PM, weak ridging was in place over SoCal, with a shortwave
trough moving through the Intermountain West. The trough was
bringing a quick burst of high clouds region. West of the
mountains the 00Z NKX sounding indicated a marine layer with a
depth at or just below 1,000 ft msl. The marine layer stratus deck
was returning rapidly along the coast.

Tonight and Tuesday morning, the main concern will be areas of fog
within 10 miles of the coast. Compared to last night, the marine
layer is about 500 ft shallower. Given this, areas of dense fog
are possible over the coastal mesas, and along portions of
Interstates 5, 8, 405, and 805 during the morning commute. At this
time confidence in widespread dense fog is to low for the
issuance of a Dense Fog Advisory, will allow the overnight shift
to monitor/reevaluate with the morning forecast package. Any fog
that develops is expected to lift by 9 AM.

For Tuesday afternoon, the focus shifts to another day of
unseasonably hot conditions, as the ridge along the West Coast
stregthens. Inland areas will receive the brunt of the heat, with
afternoon highs forecast to be 8-16 degrees above seasonal
averages. The coast will be shielded by a shallow marine layer.
This will be the hottest day of the year thus far in the Coachella
Valley, where afternoon highs will be in the the 109-112 range.
An Excessive Heat Warning was considered, but not issued due to
the short duration of this event.

A cooling trend will arrive for the second half of the workweek as
a weak shortwave trough undercuts the ridge of high pressure
along the West Coast. By Friday, inland temperatures will fall to
10-18 degrees below seasonal averages, a 10-25 degree drop from
Tuesday`s highs. Outside of the welcome decline in temperatures,
there are two other features to keep and eye on during the second
half of the week. The first feature is a weak impulse that will
rotate around the base of the trough Wednesday afternoon. Model
guidance suggests very modest CAPE of 100-500 j/kg, mainly for
the San Gabriel Mtns and points north. This could bring a few
afternoon cumulus buildups for these areas and potentially the San
Bernardino Mtns if the track shifts slightly southeast. The
second feature will be the main shortwave as it moves inland,
producing a rapidly deepening of the marine layer Thursday night
into Friday morning. At this time sub 850 moisture and marine
layer depth look sufficient for a few light sprinkles/drizzle
during this period. Gusty winds will also develop Thursday and
Friday for the deserts slopes and deserts, with Wind Advisory
strength winds possible.

Next weekend, weak ridging is forecast to return over California
during this period. For our area this will bring a warming trend
and shallower/less persistent marine layer. By Sunday afternoon
temperatures should be near seasonal averages under fair skies.


230325Z...Coast/Valleys...Stratus with bases 600-800 ft MSL filling
in along the coast through 06Z then spreading 10-15 SM inland
overnight. Bases may lower to around 300-500 ft MSL by morning,
producing widespread vis of 5SM or less along the coast and areas of
vis 1/2 SM or less over higher coastal terrain and on coastal mesas,
including at KCRQ. Stratus clearing to the immediate coast 17-19Z,
with low clouds lingering along the beaches through the afternoon.
Stratus may return to KSAN and KCRQ at times 21-02Z, though
confidence is low. Stratus spreading back inland after 24/02Z with
bases near 500-800 ft MSL.

Mountains/Deserts...SCT clouds AOA 20000 ft MSL and unrestricted
vis through Tue evening.


A shallower marine layer may bring locally dense fog with visibility
near 1 NM to the coastal waters tonight into Tuesday morning, and
possibly again Tuesday evening.




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