Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

200 pm PDT Sat jul 20 2019

Synopsis

The rest of this afternoon will be sunny with temperatures continuing to be below normal. High pressure aloft will expand over the southwest next week and bring warmer weather and less extensive night and morning coastal low clouds. Moisture from the southeast and more instability will bring chances of mostly afternoon and early evening mountain and desert thunderstorms Monday through Thursday, with best chances Tuesday and Wednesday.

Discussion

For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,

Sunny skies prevailed this afternoon with temperatures continuing 5- 10 deg F below normal in most areas. Winds were mostly light to moderate at early afternoon, except for a few locations on the desert slopes which had gusts over 20 MPH, including the White Water RAWS which had a recent gust of 37 MPH.

A large high pressure ridge extends from the Mid-Atlantic states west through the central and south-central US to northern Baja California, with a broad trough over the NE Pac and NW US. The upper high will gradually shift towards the southwestern US Sunday and expand over the west Mon/Tue. The GFS ensembles have been good with consistency between members and runs in recent days, with the center of the upper high likely over the Four Corners region Mon-The. This is a favorable pattern for monsoonal thunderstorms in the mountains/deserts of SoCal, and moisture will be abundant during this time at/above 700 MB, with a mostly dry layer below that except for occasional near-surface moisture surges from the Gulf of California into the lower deserts. Instability will be the key to thunderstorm development, with best instability likely Tue or Wed. 12z GFS is showing CAPEs of 2000-3000 J/kg Wed afternoon over many of our mountain areas with some CAPE along the Elsinore Convergence Zone in the Inland Empire where there is a small chance of a thunderstorm in the afternoon. National Blend of Models show Wed as the best day for thunderstorms as well. With total precipitable water exceeding 1.5 inches in the lower deserts Tue/Wed, there could be a risk of flash flooding then as well. West of the mountains, the stratus should be far less extensive and more patchy Sun night-mid week, partly due to the greater amount of longwave radiation reflecting off the mid-level moisture.

Late in the week, the upper high could shift a bit farther west, though not all models/ensemble members agree with this. If the high shifts west, monsoonal activity should decrease somewhat, and temperatures should increase a bit, though 850 MB temperatures mostly just increase in the deserts. Especially with lower humidity in the deserts late in the week, temperatures should rise to around 110 by then.

Aviation

201950z, Coast/Valleys, VFR conditions will continue this afternoon with low clouds returning to the coast after 04Z with bases 1000-1500 FT MSL. VIS may be reduced on higher coastal terrain in areas of low clouds overnight. Low clouds along the coast expected to scatter out 16-18Z Sunday.

Mountains/Deserts, SKC with unrestricted VIS through Sunday.

Marine

No hazardous marine weather expected through Thursday.

Watches, Warnings, Advisories

Ca, NONE. PZ, NONE.

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