Area Forecast Discussion
Issued by NWS San Diego, CA

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138
FXUS66 KSGX 240451
AFDSGX

Area Forecast Discussion
National Weather Service San Diego CA
945 PM PDT Sun Jul 23 2017

.SYNOPSIS...
Increasing mid level moisture tonight and Monday will bring higher
humidity and a chance for showers and thunderstorms through Monday,
with the chances greatest near the mountains. Moisture will decrease
Tuesday, but a few thunderstorms are still possible over the
mountains. Dry and seasonally hot Wednesday through Friday with
temperatures a little above average.

&&

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

Midlevel clouds have increased over the last few hours as seen on
the GOES-16 IR satellite loop. Convection fired over AZ today,
sending outflow boundaries across the Lower Colorado River Valley
and into eastern SBD/RIV and Imperial Counties. Current GPS plots
show 2+ inches of PW at Brawley and La Quinta, a sign that moisture
is advecting into the lower deserts, and will drift westward across
the rest of the Southern California overnight.

Radar this evening is detecting weak echoes near Desert Center
moving west toward the Coachella Valley. The current forecast of a
slight chance of thunderstorms in the mountains and deserts tonight
looks good. On Monday the clouds may limit the heating potential,
but an easterly wave and fairly deep moisture will continue the
thunderstorm threat Monday. A few showers or perhaps a thunderstorm
may drift west west of the mountains, but weakening easterly flow
should keep the main thunderstorm threat near the mountains. Given
the high values of PW expected (2+ inches over the lower deserts and
adjacent mtn slopes) stronger, slow-moving storms could produce
local flash flooding there.

Convection will diminish Monday night into Tue, but fire again over
the mountains Tue afternoon/evening due to residual moisture and
instability. If any storms do develop, they should drift into the
high deserts (based on forecast steering flow). The main threats
will be lightning and heavy rain leading to possible flash flooding.

*From previous forecast*

By Wednesday the subtropical ridge will be rebuilding westward,
further crimping any thunderstorm threat here, and possibly
threatening desert areas again with extreme heat late in the week.
Onshore flow is projected to continue west of the mts, maintaining a
shallow marine layer and more moderate Summer conditions. Both the
GFS/ECMWF 12Z operational runs show the high reaching near 600 DM at
500 MB over NV early next week. This position is far enough north to
allow easterly flow aloft over SoCal, which may eventually lead to
more monsoonal activity.

&&

.AVIATION...
240430Z...Coast/Valleys...Areas of BKN-OVC stratus with periodic FEW-
SCT conditions will occur within 20 miles of the coast overnight.
Bases will be 1300-1700 ft MSL with tops to 2000 ft MSL. Most vis
will remain above 5 miles. Clearing will occur in most areas 15Z-17Z
Mon. Otherwise, variable clouds will occur above 10000 ft MSL
through Monday. The valleys will have a slight chance of
thunderstorms Monday with bases 9000 ft MSL and tops to 30000 ft MSL.

Mountains/Deserts...There is a slight chance of thunderstorms late
tonight and Monday morning, with a 30-40% chance of thunderstorms
Monday afternoon. Bases will likely be 9000 ft MSL with tops to
35000 ft MSL. Strong up/downdrafts and local gusty surface winds
will be possible. Otherwise, occasional clouds above 10000 ft MSL
will occur except local clouds to 3500 ft MSL in the Coachella
Valley through 06Z tonight. Vis will mostly be unrestricted except
for a slight possibility of vis below 5 miles in blowing dust
tonight and Monday in the deserts.

&&

.MARINE...
An isolated shower is possible through Monday, though thunderstorms
are not expected at this time. No hazardous marine weather is
expected through Thursday night. From Friday through next Sunday, a
south swell from what is now Tropical Storm Hilary may arrive in the
Southern California Bight and produce 5-8 ft combined seas.

&&

.BEACHES...
A High tide near 7 ft this evening could result in minor tidal
overflow in low-lying beach areas, therefore a Beach Hazard
Statement remains in effect through midnight. Surf heights will
mostly be 1-3 ft, and this should somewhat limit the potential for
tidal overflow.

A few showers are possible Monday near the beaches, but current
guidance suggests that any thunderstorms that form are most likely
to stay inland.

&&

.SKYWARN...
Skywarn activation may be needed on Monday. Weather spotters are
encouraged to report significant weather conditions.

&&

.SGX WATCHES/WARNINGS/ADVISORIES...
CA...Beach Hazards Statement through this evening for Orange County
     Coastal Areas-San Diego County Coastal Areas.

PZ...NONE.

&&

$$

PUBLIC...Moede/JAD
AVIATION/MARINE/BEACHES...Moede



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