FXUS66 KSGX 192121
Area Forecast Discussion
National Weather Service San Diego CA
215 PM PDT Mon Aug 19 2019
Building high pressure over the Southwest will bring warmer days
through Wednesday. An Excessive Heat Warning is in effect for the
deserts Wednesday. Near the coast a cooler marine airmass will
keep temperatures near their seasonal averages. The weather will
turn a little cooler Thursday and Friday, and a surge of monsoon
moisture is possible this weekend.
.DISCUSSION...FOR EXTREME SOUTHWESTERN CALIFORNIA INCLUDING ORANGE...
SAN DIEGO...WESTERN RIVERSIDE AND SOUTHWESTERN SAN BERNARDINO
There isn't much change in the weather outlook this week. Heat
will build through Wednesday, then it won't be quite as hot
Thursday and Friday as the upper ridge aloft weakens.
Forecasting maximum temperatures this week is the biggest challenge.
We have a high degree of confidence that Wednesday will be the
hottest day, because that's the day when the 850 mb temperatures
peak at 46.0C over the lower deserts. Last Thursday the 850 mb
temperature peaked at 46.8C at Thermal, and the surface high that
day was 120F. The airmass will be very dry this week (just as it was
last week) with dewpoints in the 20s and low 30s. So we anticipate
highs in the Coachella Valley and the Anza Borrego Desert will range
from 112 at the low end to 118 at the high end. An Excessive Heat
Warning will be in effect for the deserts Wednesday.
Record highs are within reach at a few sites including Palm Springs
(117), Thermal (118), Indio (115) and Borrego (116). High
temperatures in upper deserts east of Apple Valley will
range from 103 to 109 on Wednesday.
Temperatures west of the mountains will be kept in check by the
cooler marine air, especially areas within 10 miles of the coast. By
Wednesday morning the marine layer will mostly be confined to areas
near the coast, and that will allow the hotter airmass to spill into
the valleys. High temperatures in the Inland Empire will likely
eclipse 100 degrees in a few areas on Wednesday. In San Diego
County, the eastern valleys and foothills will see highs from 95-
The heat is the result of upper level ridging over Texas that will
expand westward Tuesday and Wednesday. This will be a brief heat
episode, and temperatures will start trending downward Thursday and
Friday as an anomalous upper level trough moves in from the
northwest. The onshore flow will turn stronger, and marine layer
will deepen, and low clouds will return to the far western valleys
Long Range Outlook...Invest 95E is forecast to strengthen and
move northwest off the coast of western Baja this weekend. At
this early stage of the forecast process, the GFS and ECMWF ensemble
runs show a ridge of high pressure over the southwest blocking the
northward progress of this system. With this scenario, the bulk
of the moisture would remain south of the border. Limited
moisture on its northern periphery does make its way into SoCal
for possible showers or thunderstorms early next week.
192000Z...Coast/Valleys...Clear with unrestricted vis through this
evening. Low clouds moving ashore after 05Z Tuesday with bases 900-
1200 ft MSL and tops to 2000 ft MSL, spreading 15-20 miles inland
overnight. Low confidence in cigs reaching KONT overnight. Scatter
out expected after 17Z Tuesday.
Mountains/Deserts...Mostly clear with unrestricted vis through
No hazardous marine conditions expected through Friday.
Skywarn activation will not be needed today.
CA...Excessive Heat Warning from 11 AM to 8 PM PDT Wednesday for
Apple and Lucerne Valleys-Coachella Valley-San Diego County
Deserts-San Gorgonio Pass Near Banning.
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