A slight chance of showers and thunderstorms will continue into this morning, mainly for the mountains and deserts of San Bernardino County. A low pressure system to the north will bring stronger onshore flow and a cooling trend for Thursday through the weekend with Sunday high temperatures near the coolest for the date for a few locations. Southwest to west winds for the mountains and deserts during the afternoon and evening will strengthen for Thursday through Saturday, then weaken for early next week. The marine layer will deepen late in the week into the weekend with night and morning coastal low clouds extending well inland and onto the coastal slopes of the mountains for early Saturday. Then warmer for early next week, but with high temperatures remaining below average.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
Current satellite this afternoon shows a notable decrease in mid level cloud coverage from southwest to northeast as the shortwave trough that provided a bolster of moisture and synoptic lift has progressed into the Great Basin. With this, we expect an end to the elevated shower chances for the San Bernardino Mountains and High Desert by mid-late this afternoon. Meanwhile, strong upper level high pressure in control across the desert southwest has a ridge axis stretching westward over southern California. This and the added moisture from the monsoonal disturbance aloft is yielding mild afternoon high and overnight low temperatures, with some minor HeatRisk for the inland valleys, with moderate HeatRisk in the deserts. Portions of the lower Deserts of San Diego Valley are seeing locally major HeatRisk with the highest combination of temperature and humidity, where an Extreme Heat Warning remains in effect through 8 PM Thursday. Thursday will mark the beginning of a late week cooling tend, but with another mild night before the mid level moisture and increase humidity fully depart, so there is still moderate HeatRisk in the deserts into the afternoon.
All attention quickly turns to a highly unseasonable trough that is set to push into the Pacific Northwest on Thursday/Friday. This trough will kick out the monsoonal moisture for good, shove the ridge south and east, and bring much cooler air to the region for the weekend and into next week. Ensembles and clusters provide good confidence in the strength and progression of this large and anomalous trough, set to dive down into California and dominate across the West for the weekend. Temperatures quickly fall below normal area wide by Friday, with afternoon high temperatures likely to fall 15 to even 20 degrees below normal for Saturday and Sunday. The currently forecast temperatures are so unseasonably low that record cool high temperature forecasts could be challenged this weekend. In addition to the cool temperatures, this trough will bring a strengthening trend for the onshore winds, especially for the desert mountain slopes and deserts. Wind gusts on Friday afternoon could be near Advisory-level criteria with 35-40 mph for the deserts, and locally 45-60+ mph in mountain passes. The strongest winds look to peak Saturday afternoon, and then weakening into Sunday. Dry conditions in the High Deserts will likely bring elevated fire concerns, so see the Fire Weather Section below for more details.
Looking ahead into next week, ensembles indicate the large could could get a few reinforcing shortwaves that could extend its presence across the western US well into middle part of next week. Nbm currently favors below normal temperatures lingering across SoCal through at least Wednesday.
241730z, Coast/Valleys, Mostly clear today, except for patches of low clouds with bases 1200-1500 feet MSL persisting at the beaches. Cigs at the same level will push ashore after 03Z and inland overnight, slightly farther into valleys by 11-15Z Thursday. Vis reduced 1-5SM in valleys. Scatter out Thursday 17-19Z.
Mountains/Deserts, SCT-BKN clouds 10000-20000 feet MSL will continue today, but decrease and move east after 23Z, leaving mostly clear skies into Thursday morning.
No hazardous marine conditions are expected through Monday.
A long-period south swell will continue to bring surf of 3 to 6 feet through Thursday, highest on south facing beaches. See the Beach Hazards Statement for details.
Ca, Beach Hazards Statement through Thursday evening for Orange County Coastal Areas-San Diego County Coastal Areas.
Extreme Heat Warning until 8 PM PDT Thursday for San Diego County Deserts.
PZ, None.
