Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 230425

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

Warm again on Sunday with isolated afternoon thunderstorms over the
mountains. Increasing monsoonal flow Monday will bring higher
humidity and a chance for more widespread showers and thunderstorms.
The brief monsoon surge weakens Tuesday, but thunderstorms may still
form over the mountains. Dry and seasonally hot Wednesday through



Moisture is slowly starting to work its way into Southern California
from the east this evening. Dewpoints this evening are 5-10 degrees
higher than 24 hours ago in the Lower Deserts, and 10-20 degrees
higher in the SAN/RIV/SBD Mountains. Yesterday evening the PW on the
NKX sounding was 0.90", and it was up to 1.29" this evening.

The IR satellite loop shows convection over AZ moving west toward
Imperial County, and that moisture will move into the San Diego
Forecast area Sunday. Model PW of 2.00" is forecast over the lower
deserts Sunday afternoon from a surge of moist easterly flow in the
500-700 mb layer. An upstream easterly wave is not readily apparent
in the water vapor loop this evening, so Sunday`s convection will be
primarily driven by thermodynamic instability and orographics. And
perhaps a surface convergence boundary over the mtns between the
onshore afternoon sea breeze and southeast winds over the deserts.
Any storms that form over the mtns could generate brief heavy rain,
but the flash flood threat will be minimized fast cell movement in
the 15-20 kt easterly midlevel flow.

On Monday the PW maxes out at 2" west of the mountains and 2.5" in
the deserts, and 1.5" in the mountains. Wind aloft will be lighter,
and the slower moving storms combined with greater available
moisture and instability will increasing the flash flood potential.

*From Previous Discussion*

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.

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.


230400Z...Coast/Valleys...Areas of stratus will gradually develop
along the coast through 09Z and spread up to 15 miles inland. Bases
will be 1000-1500 ft MSL with tops to 1600 ft MSL. Local vis 2-4
miles will be possible in the valleys 10Z-16Z. Most areas will clear
by 16Z Sunday, with clouds increasing above 15000 ft MSL. Stratus
will likely be only patchy Sunday night, and there is a slight
chance of thunderstorms late Sunday night.

Mountains/Deserts...Mostly clear skies with unrestricted visibility
will continue through Sunday morning. Clouds will increase above
10000 ft MSL Sunday with a slight chance of thunderstorms over the
mountain crests from L35 south to the Mexican border Sunday
afternoon with bases 9000 ft MSL and tops to 35000 ft MSL with
strong up/downdrafts and local gusty surface winds. A few
thunderstorms could occur Sunday night in the mountains and deserts.


There is a slight chance of thunderstorms late Sunday night through
Monday. Cloud-to-water lightning will be the primary threat. No
hazardous marine weather is expected Monday night through Thursday.


High tides will be near 7 ft this evening and Sunday evening. This
could result in minor tidal overflow in low-lying beach areas, and
therefore a Beach Hazard Statement remains in effect through Sunday.
Surf heights will mostly be 1-3 ft, and this should limit the
potential for tidal overflow to some degree.

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


Skywarn activation won`t be needed Sunday, but may be needed Monday.


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




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