Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 270351

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

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 a
large trough moving in from the west brings stronger onshore flow
and a return of the marine layer.



Mid and high level clouds continue to move in from the east this
evening, mainly over San Diego County. Isolated light sprinkles
are possible tonight as higher mid-level moisture moves in, but no
measurable precipitation is expected. At the surface, offshore
pressure gradients are weakening, currently at around -8 mb SAN-
TPH and -3 mb SAN-DAG. Gradients are nearly neutral to the low
desert. As a result, east winds continue to weaken this evening,
generally in the 20-30 MPH range in the San Diego County Mountains
and below the San Gorgonio Pass. As of 8 PM temperatures remain
well into the 80s west of the mountains, even at the beaches.

High pressure over the interior West will continue to weaken through
Wednesday as a cut-off low moves north from the Baja and drifts
over Southern California. Some locally gusty east winds will
continue into Tuesday due to moderate mid-level flow associated
with the upper low, though winds will be weaker than today. The
00Z NKX sounding shows precipitable water has already begun to
increase, up to 0.90 inches versus 0.65 inches this morning.
Moisture will increase further Tuesday and Wednesday, resulting in
higher humidity, more cloud cover, and a chance of showers and
thunderstorms. Any measurable precipitation will likely be limited
to the mountains and high desert on Tuesday, though some virga and
sprinkles are possible throughout the forecast area. Showers will
become more widespread on Wednesday, possibly extending into the
inland valleys. Lingering moisture will continue the shower and
thunderstorm chances over the mountains into Thursday afternoon.

Drier southwest flow develops on Friday as a longwave trough
amplifies along the West Coast. The trough slowly progresses
inland through the weekend into early next week. This will bring
greater cooling, a return of the marine layer which gradually
deepens each night, and gusty onshore winds in the mountains and
deserts. Right now no precipitation is expected with this trough
as any significant moisture remains to the north.


270300Z...Coast and Valleys...The moderate easterly winds that
impacted the region today will continue to diminish this evening. As
the surface winds weaken, 15-25 kt of LLWS between 1,000 and 2,000
ft AGL will become possible in the vicinity of KSAN and KCRQ, and
remain possible through 16z Tuesday. There is a low possibility of 5-
10 kt easterly winds developing at KSAN or KCRQ between 11z and 15z
Tuesday. Significant wind impacts are not expected at KSNA or KONT
overnight. Westerly winds will return to the coastal TAF sites
Tuesday afternoon. Other than winds, clouds will remain SCT to BKN
AOA 10,000 ft MSL through Tuesday with a slight chance of sprinkles
over SD County. Visibility will remain unrestricted through Tuesday.

Mountains and Deserts...Mountain wave activity will continue to
produce turbulence over the mountains through Tuesday morning. Sky
conditions will be SCT to BKN AOA 10,000 ft through Tuesday. There
is also a slight chance of showers through 18z Tuesday in SD County,
and a slight chance of thunderstorms over the mountains from 18z to
00z Tuesday.


Easterly winds of 5 to 10 kt are possible over the nearshore waters
Tuesday morning. The standard diurnally driven northwest winds will
return Tuesday afternoon and continue through the end of the week.
Otherwise no significant or unusual weather is expected.


While some gusty east winds remain over portions of the mountains,
humidity in those locations have increase to around 25 to 50
percent. Expect humidity to gradually increase throughout the
region Tuesday as moisture associated with an upper level low
spreads over Southern California, with the greatest increase in
the mountains and low desert.

By Wednesday, it will be much 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 along the edges of any
convection that develops, but any widespread dry thunderstorms
looks unlikely.


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


.SGX Watches/Warnings/Advisories...


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