Area Forecast Discussion
Issued by NWS San Diego, CA

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FXUS66 KSGX 221651

Area Forecast Discussion
National Weather Service San Diego CA
930 AM PDT Mon Jul 22 2019

High pressure aloft will expand over the Southwest early this week,
bringing warm days and less extensive night and morning coastal
low clouds. Increasing moisture from the southeast will mean more
high cloudiness and an increasing chance of showers and
thunderstorms, mainly in the afternoons and early evenings and
mainly over the mountains and deserts. The best chances of strong
thunderstorms with locally heavy rain is Tuesday through Thursday.
Dry weather will likely return by next weekend.



The monsoonal moisture has arrived from the southeast with clouds in
the mid and high levels dominating. Precipitable water increased to
1.55" in the Miramar sounding from 5 AM. There was mostly virga,
but an isolated thunderstorm formed just northwest of Palm Springs
which produced a couple of lightning strikes. Areas of low clouds
were decreasing along the coast, and due to the monsoonal moisture
the next few days, they will be just patchy. It appears that some of
the clouds will decrease the next few hours, which will increase
instability and produce more thunderstorms this afternoon. Due to
somewhat vigorous flow from the southeast due to the weak upper-
level wave to our south, most precipitation should be short-lived,
so for today the flood risk is low.

The upper high is now over the 4 Corners and will stay in that area
through about Thursday and bring us moist southeast flow aloft.
Winds in the mid levels will decrease by about half, which will help
the taller pulse-type thunderstorms to better form, though near
saturation at times in the 500-300 MB layers may produce enough
clouds at times to inhibit the convection. Some of the higher level
moisture will be dependent on convection upstream, such as over
Sonora, the day before. Where thunderstorms form, there could be
brief heavy rain, with a possibility of isolated flash flooding,
though HREF solutions show a very low chance of greater than one
inch of precipitation anywhere Tuesday, at least for now.
Thunderstorms will mostly be over the mountains and deserts, but
some convergence along the Elsinore Convergence Zone in the Inland
Empire could produce a few thunderstorms there Wednesday afternoon,
especially with surface temperatures around 100 degrees providing
some instability. The moisture decreases some Thursday, though the
height level of the moisture, more around 700 MB, less above, may be
more favorable for thunderstorms partly due to fewer high clouds.

GEFS is consistent with its ensemble solutions with the placement of
the upper high over the 4 Corners through Friday, then the solutions
gradually divergence over the weekend with more than half the
solutions having the upper high shift south which would place us
under drier southwest flow, mostly due to troughing in the northern
part of the West, but other solutions only have a slight shift
southwest, which could maintain isolated thunderstorms. For now,
will lean towards the drier solution but keep our eyes open.
Temperatures will be a little above normal this week, except near
or just slightly below normal in the deserts due to the moisture,
though the humidity will make it feel warmer there. With the drying
late in the week, temperatures in the deserts could rebound above
110 degrees again.


221540Z...Coast/Valleys...Low clouds with bases 1000-1500 ft MSL and
tops to 1800 ft MSL expected to become mostly SCT by 17Z. SCT-BKN
clouds AOA 10000 ft MSL will continue into the early afternoon. Low
clouds will be less in coverage tonight, but could impact KSAN, KCRQ
and KSNA at times between 08Z and 16Z Tuesday.

Mountains/Deserts...SCT-BKN clouds AOA 10000 ft MSL through early
afternoon. A few TSRA is possible over the mountains this
afternoon/evening, but are not expected to impact KPSP nor KTRM.


No hazardous marine weather expected through Friday.


There is a small chance that Skywarn activation will be needed later
today. Skywarn could be activated any afternoon and early evening
through Friday due to thunderstorm activity.




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