Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 222029

Area Forecast Discussion
National Weather Service San Diego CA
128 PM PDT Sat Jul 22 2017

Moisture will increase early this coming week ahead of an easterly
wave, bringing more clouds, higher humidity, and a chance of showers
and thunderstorms. Best chance for significant rain looks to be over
the mountains and deserts on Monday. The wave will weaken on
Tuesday, and the threat of thunderstorms will follow. Along the
Coast...onshore flow and the marine layer will continue mild
conditions with periods of nocturnal clouds and fog. Dry and warmer
for the latter half of the next week.



Early this afternoon...satellite imagery showed the marine stratus
had cleared most of the coast and adjacent coastal waters, and other
than some small cu over the mts, skies were clear. Surface pressure
gradients were weak onshore and peak wind gust reports were under 25
MPH in the wind-prone areas. A quick check at dewpoint reports from
mtn/desert areas show they had increased today over yesterday in
northern portions of the CWA.

High pressure over the SW was keeping the weather hot and dry inland
today, while onshore flow held a moderately deep marine layer and
cooler weather over coastal areas. The high will weaken and shift
east through Mon, as a weak trough develops along the CA Coast, and
an easterly wave moves west over NW Mexico. This will allow an E-SE
flow to develop aloft over SoCal, along with an increase in
moisture, clouds, and a chance of showers and thunderstorms. The
best chance will be over the mts/deserts, as the wave is forecast to
weaken and be dragged rapidly N-NE on Tuesday.

Wednesday through Friday...the upper high builds back toward the
Four Corners region and persists for the remainder of the week. Flow
aloft over SoCal is southerly and drier initially, then gradually
turns SE and moistens aloft. This should bring periods of high
clouds, but stability, and the lack of mid-level moisture should
keep a lid on any thunderstorm development until next weekend at the
earliest. Onshore flow prevails as well this period, so the marine
layer should keep coastal areas moderate while inland areas are
seasonably hot.

The main forecast problem for the next few days will be convection
timing, intensity and placement. For Sunday, it appears low-level
moisture increases sufficiently, along with daytime heating to drive
isolated tstm development over the high terrain. Rain shafts could
be briefly heavy, but overall, precip should be limited due to the
number of cells that actually form.

Monday looks to be the most active day, as the weak easterly wave
approaches. The various Hires model runs have waffled back and
forth bringing precip to the coast and beyond, and the ECMWF/GFS
models are handling the track of the wave differently. Instability
parameters have been weak, but time/height X-sections show quite a
dewpoint surge in the 500-300MB layer on Mon, which could
destabilize the atmosphere enough for elevated convection. The
models have backed off on the RH in the 200-400MB layer, which
favors more heating on the mts to drive deeper diurnal convection
there in the afternoon. With all the uncertainty, low POPS are in
the forecast all the way to the coastal waters on Monday, with
greater chances near the mts. Any heavy precip would most likley be
confined to the mts/foothills where the risk of flash flooding will
be greatest.

By Tuesday...moisture is decreasing and the wave has been absorbed
into the south flow aloft. Lingering moisture and daytime heating
could still ignite afternoon storms over the mts/deserts. This is
supported by the 12Z WRFEMS, which is GFS based, and the ECMWF looks
favorable as well, so POPS continue into Tue evening.


222000Z...Coast/Valleys...Low clouds with bases around 1,200 ft MSL
had cleared the coast for all areas excepted southern San Diego
County, where a few patches were lingering along the beaches. Away
from the beaches, predominantly clear skies will continue through
03Z Sunday, with light afternoon sea breezes. Low clouds with bases
near 1,200 ft MSL will creep back into coastal areas around 03Z
Sunday, and spread 20-25 miles inland over the Orange and San Diego
County valleys by 14Z Sunday. The TAF sites should clear between 15Z
and 17Z on Sunday.

Mountains/Deserts...Mostly clear skies with unrestricted visibility
will continue through 18Z Sunday.


There is a slight chance for thunderstorms Sunday night through
Monday. Lightning will be the primary threat, with any storms
potentially obscured by the marine layer low clouds. After Monday,
No hazardous marine weather is expected through Thursday.


Evening high tides will be near 7 ft today and Sunday. This could
result in minor tidal overflow in low-lying beach areas, therefore a
Beach Hazard Statement remains in effect through Sunday to account
for this possibility. Surf heights will be limited to 2-3 ft, and
this should limit the potential for tidal overflow to some degree.

There is a slight chance of thunderstorms at the beaches Sunday
night through Monday, with the best thunderstorm chances occurring
Monday morning.


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


CA...Beach Hazards Statement through Sunday evening for Orange County
     Coastal Areas-San Diego County Coastal Areas.




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