Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 192053

Area Forecast Discussion
National Weather Service San Diego CA
150 PM PDT Sun Aug 19 2018

High pressure aloft over Southern California will shift southeast to
the Rio Grande Valley by Tuesday. This will allow some monsoonal
moisture to return, possibly sparking more thunderstorm activity
over the mountains and deserts.  At the same time, a trough will
develop over the Pacific Northwest and slowly broaden over the West
Coast through the week, bringing a drying trend, along with slightly
cooler weather later in the week. The night and morning low clouds
along the coast are expected to decrease a bit into Wednesday, then
become more widespread during the latter half of the week.



Some patchy marine clouds near the Coast and cumulus over the
mountain crests were being detected in satellite imagery early this
afternoon, otherwise skies were clear. Winds were generally light
with temps close to values observed yesterday.

High pressure aloft over SoCal today is headed SE and will be
centered over West TX by Tue. This will briefly throw parts of
southeastern CA into a monsoonal flow. Based on blended TPW
imagery...Moisture is already beginning to increase over eastern
portions of the CWA and will likley continue into Tue morning as an
easterly wave rides north into the Lower Colorado River Valley. The
latest WRFEMS shows some convective showers firing late tomorrow
over the lower deserts, with much more activity creeping north along
the mountains and desert areas on Tuesday. We are already forecast
to be on the edge of this activity, so any shift to the east would
leave the entire area dry. For now, low POPS are in place for mts/
lower deserts.

Once the easterly wave passes, there is a shift in the upper-level
flow to predominantly westerly through the week. This becomes even
more pronounced early next week as both the GFS/ECMWF models show a
marked shift in the pattern over the Western Hemisphere. If the
models are correct...This shift will allow the strong subtropical
high to set up over the Lower Mississippi Valley, while a longwave
trough develops over the West to end August. Besides driving any
Monsoonal activity completely out of the region, it would bring
widespread and substantial cooling. Lets see if a few more model
runs keep this trend.

In the meantime...Don`t look for any meaningful changes in the
weather across SoCal this week. Humidity will likely remain elevated
with onshore flow and a warmer than average sea surface. Daytime
temps will be within a few degrees of average, while overnight lows
continue 3 to 7 degrees above average.

Despite the low POPS, PW values of near two inches will be available
if thunderstorms develop on our desert slopes on Tuesday. Even on
Monday, the threat for any thunderstorms looks to be in the lower
deserts. The passage of a wave on Tuesday increases forcing in this
potentially rich CAPE environment. The only wild card that could
enhance updrafts even more would be enough afternoon sun (which
could be in short supply). The main threat area for severe
convection and Flash Flooding at this time appears to be the Borrego
Desert areas, the Coachella Valley, and adjacent mountains.


192030Z...Coasts/Valleys...Coastal low clouds will gather tonight
after 06Z with bases around 1200 feet MSL and moving into western
valleys overnight. Scatter out by 16-17Z Monday.

Mountains/Deserts...Cumulus clouds based at around 10000 feet MSL
through 01Z today. Cumulus clouds over mountains and moving into
deserts after 19Z Monday, growing into isolated thunderstorms. Bases
around 9000 feet growing to 35000 feet.


No hazardous marine weather is expected through Friday.


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




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