Cooler weather is in the forecast through Friday as a weak trough of low pressure moves through the region. Low clouds will spread farther inland each night and morning as the marine layer deepens, and strong and gusty winds will occur in the mountains and deserts. The trough may create patchy drizzle in the coast and valleys overnight into Thursday morning and scattered light showers over and west of the mountains late Thursday night through early Friday morning. High pressure aloft will rebuild over the West this holiday weekend for warmer weather and a shallower marine layer. Another cooling trend likely to begin next Tuesday as another trough develops near Southern California.
For extreme southwestern California including Orange, San Diego, western Riverside and southwestern San Bernardino counties,
It was another hot day today in the lower deserts with Thermal tying the record high for the date of 109F degrees (previously in 1983)! Elsewhere, temperatures cooled as the onshore wind flow had increased. Marine layer stratus has filled back in along the coast and has quickly spread inland to the inland valleys. Patchy drizzle is possible late. No significant changes were made to the forecast. See previous discussion below for additional forecast details.
, Previous Discussion (Issued at 128 PM PDT Wed May 24 2017),
Marine layer stratus has diminished for the most part this afternoon according to visible satellite, with scattered alto- cumulus clouds moving overhead from the southwest throughout Southern California. These alto-cumulus are associated with a short-wave trough, which can be seen on water vapor satellite imagery moving northeast over Central California. A low can also be seen far offshore of Central California, with a stronger trough over British Columbia and a weakening ridge over Southern California. The 12Z Miramar sounding shows an 11 deg C inversion at around 1500 feet MSL. Onshore flow is strengthening quickly, with San Diego to Las Vegas onshore pressure gradients up to 10.8 mb, whereas they were 2.2 mb at this time yesterday. Despite the strengthening onshore flow and strong inversion, stratus managed to clear at the beaches today. Today through early Thursday, the trough over British Columbia will push south and start to pull the low far offshore of Central California towards the California coast while the aforementioned short-wave trough continues to move northeast into the Great Basin. This increased troughing in California will serve to further weaken the ridge over us and continue to increase the onshore pressure gradients, resulting in strong and gusty west winds along the mountain ridges, desert slopes, through the passes and into adjacent desert areas beginning late this afternoon. Wind gusts of 55 mph and local gusts of 65 mph will be possible in and through the San Gorgonio Pass and in the San Diego county deserts, where Wind Advisories have been issued. The lowering 500 mb heights and onshore flow will also deepen the marine layer and bring a cooling trend for all areas today and Thursday, with highs dropping to several degrees below normal on Thursday. Temperatures currently are 5-15 degrees below what they were 24 hours ago over much of the region. Expect marine layer stratus to move completely fill the coast and valleys tonight into Thursday morning with patchy drizzle possible.
As the low moves into Southern California Thursday night into Friday morning, it weakens into a short-wave and will bring another burst of strong and gusty winds to the mountains/deserts and further deepening of the marine layer, with stratus Thursday night/Friday morning filling the coastal basin and moving into the coastal slopes and passes. Large scale and hi-res models show some very light precipitation being generated over and west of the mountains late Thursday night into early Friday morning, resulting in scattered 0.01-0.05 inch amounts.
The trough will move to our northeast by Friday afternoon, with the cooler air mass resulting in highs 10-15 degrees below normal Friday. For Saturday through Monday, an upper level ridge to the west will move in and strengthen, creating a warming trend and a shallower marine layer. Highs are likely to reach back to normal to slightly above normal levels on Sunday and then 5-10 degrees above normal on Monday. Expect night and morning marine layer stratus over coastal areas and western valleys during this period.
For Tuesday through late next week, the GFS and ECMWF now show a closed low (possibly of the cut-off variety) developing near or south of Southern California. This would likely bring cooling. However, models don't show any precip associated with this low at the moment.
250335z, Coast/Valleys/Coastal Mountain Slopes, Stratus with bases 1500-2000 ft MSL will continue to fill in west of the mountains tonight. Bases lifting to 2000-2500 ft by 16Z. Clearing expected to occur 16-19Z, though coastal airports may remain BKN-OVC through much of the afternoon. Clouds becoming BKN-OVC along/west of the mountains after 26/02Z with bases near 2500 ft MSL and tops 4000- 5000 ft MSL. Coastal slopes of the mtns obscured in clouds/fog at times through Thu night.
Mountains/Deserts, Southwest to west winds with sfc gusts 35-45 kt along the desert slopes/foothills and through the passes continuing through Thu night with MOD-STG UDDFS and pockets of LLWS over and NE/E of the mountains.
No hazardous marine weather is expected through Monday.
Ca, Wind Advisory until 8 AM PDT Friday for San Diego County Deserts- San Gorgonio Pass Near Banning.