Swell Matrix

Area Forecast Discussion

National Weather Service San Diego CA

934 am PDT Fri Sep 24 2021

Synopsis

An upper level low pressure system will remain across the area through Saturday, then move slowly eastward and out of the area on Sunday and Monday. Some showers and thunderstorms over the mountains and deserts this afternoon may move westward to the coast. Decreasing chances for afternoon showers and thunderstorms for the mountains, deserts, and inland valleys will continue into Sunday. A slow cooling trend will continue through Sunday with Monday a little warmer. Another low pressure system moving inland to the north will bring more cooling for Tuesday and Wednesday followed by warming on Thursday.

Discussion

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

.Morning Update, Our upper level low has arrived and is already bringing some showers and storms to the San Diego County deserts at this hour. Some of these storms over the eastern mountains and deserts may also produce heavy rain that could lead to flash flooding, hence our issuance of the Flash Flood Watch through this evening. Href/wrf/hrrr guidance continues to keep this area active through the evening with the possibility of some storms pushing westward toward the western valleys and even the coastal areas. We will continue to look at the forecast today for this potential. Storm chances shrink a bit tomorrow into Sunday as the system pushes off to the east by early next week.

.Previous Forecast as of 442 AM today,

Short Term

(today through Sunday), Upper level low pressure centered over northern Baja will move slowly westward today drawing moisture into southwestern California in easterly flow aloft. Higher resolution models focus development of showers and thunderstorms this afternoon in the mountains in Riverside and San Diego Counties, some that could move west of the mountains toward the coast during the afternoon. HREF LPMM shows the potential for isolated heavier rainfall to around 1.5 inches, mainly in the mountains. That easterly flow will also push smoke out of the area during the late afternoon and evening.

On Saturday, with the upper low centered off the northern Baja coast, steering level flow becomes southeasterly and weakens. Development of afternoon showers and thunderstorms is focused mainly over the mountains from the San Bernardino Mountains southward with coverage and rainfall potential less than today.

For Sunday, the center of the upper low will move slowly eastward into northwest Mexico and southwest Arizona. Moisture will be decreasing, but a few thunderstorms are possible in the mountains on Sunday afternoon.

The marine layer has deepened to around 1500 feet with satellite imagery showing widespread coastal low clouds extending inland into the westward San Diego County valleys and spreading northward across the Orange County coastal plain. Night and morning coastal low clouds and fog are expected to spread farther into the valleys over the weekend.

Long Term

(Monday through Thursday), Weak high pressure will move across southern California as the weekend upper low moves eastward and a large low pressure system centered off the British Columbia coast moves toward the West Coast.

Monday may be a little warmer for inland areas followed by slow cooling for Tuesday and Wednesday as that low pressure system moves inland to the north strengthening the onshore flow across southern California.

Aviation

241613z, Coast and offshore, OVC008-012 with tops near 1500 FT MSL becoming SCT010-015 by 18Z. 18Z-03Z SCT-BKN100 with LCL VIS 5SM in ISOLD SHRA/-TSRA and CB bases near 6000 feet MSL. After 03Z, Areas of BKN010 redeveloping through 06Z.

Valleys/Mountains/Deserts, SCT-BKN100 with SCT SHRA/TSRA forming over the mountains, some of which may drift west over the valleys with bases 060-100 and tops to FL400. The stronger thunderstorms will briefly lower to BKN-SCT around 6000 feet MSL in +RW that will severely restrict visibility, along with gusty and erratic winds.

Marine

There is a high probability of no significant weather over the coastal waters Friday through Monday. However, a small chance does exist for a few showers and thunderstorms, in association with a developing upper-level low pressure system off the Baja Ca Coast from late Friday through Sunday morning.

Watches, Warnings, Advisories

Ca, Flash Flood Watch until 7 PM PDT this evening for Coachella Valley-Riverside County Mountains-San Diego County Deserts- San Diego County Mountains.

PZ, NONE.

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