12/1031 AM.
A cooling trend will continue through Thursday, with cooler than normal temperatures expected across the area into the weekend. There is a chance of mountain and occasional lower elevation convection through at least tonight and potentially lingering into Thursday morning. More robust marine layer cloud coverage is likely this week, however, higher clouds may disrupt the formation of the marine layer at times.
(tdy-Sat), 12/256 PM.
Main story for the forecast today is shower and thunderstorm chances. An upper level trough is in the process of building into the West Coast, and will remain over the region through at least Friday. Additionally, a small vorticity lobe about 200 miles west of the Central Coast is slowly moving northwestward and expected to move onshore this evening through tomorrow.
Monsoon moisture moving across the region, paired with the vort lobe and trough, is resulting in thunderstorm/shower chances through Saturday, with highest chances today. So far, only three small showers have popped up over the eastern San Gabriel Mountains, and clouds associated with the vort lobe last produced any lightning around 7 or 8 am this morning. While it is less active than it was this time yesterday, there is a 30-50% chance for the northern Ventura mountains and eastern San Gabriel Mountains, a 15-30% chance for San Luis Obispo and Santa Barbara Counties, and up to a 10% chance for the remainder of the areas. These thunderstorm/shower chances may linger overnight into Thursday afternoon. On Thursday the SW flow from the trough will bring enough drying to lower the convective threat to 5-10% for Thursday and Friday, and down to 5% on Saturday.
As for temperatures, increased cloud cover, paired with increased onshore flow to both the north an east, along lowering 500mb heights will allow several degrees of cooling for the majority of the region today and tomorrow. High temperatures today will tap out in the 60s and 70s near the coast and 80s to mid 90s away from the coast. Temperatures on Thursday will lower into the 80s for areas away from the coast, with similar temperatures expected for Friday. These temperatures will be below up to 5 degrees below normal for this time of year near the coasts, and 5-12 degrees below normal for areas away from the coast.
Marine layer clouds will reform for areas south of Point Conception tonight into tomorrow, but less certainty in areas north of Point Conception due to the vort lobe moving through that may disrupt the formation of the marine layer. More confidence in the marine layer reforming on Thursday night into Friday morning thanks to the onshore push, and marine layer clouds may even push as far as into the Santa Clarita Valley and south Salinas Valley Friday morning. Clearing will be slower than normal and some beaches may only see limited clearing.
The onshore push to the east will generate gusty winds across the Antelope Valley and Foothills. There will be a 20-30 percent chance of low end wind advisories each afternoon.
On Saturday, onshore flow will start to weaken with 500 mb heights increasing slightly again, resulting in the marine layer clouds becoming less expansive. A warming and drying trend will start up, with temperatures warming up by a few degrees, pushing valleys back into the 80s to low 90s.
(Sun-Wed), 12/301 PM.
Good agreement with all of the medium range models. The trough will be pushed to the west by a powerful upper high that spans most of the southern portion of the CONUS. 500mb heights will climb to 592 dam, with dry SW flow aloft. At the surface, Onshore flow will weaken, with the push to the east weakening slightly, but the push to the north weakening substantially. Surface flow might might even be offshore by Tuesday or Wednesday.
The weather will be benign through the period. There will be plenty of night through morning low clouds and fog across the coasts. Due to the rising heights, the marine layer will shrink to under a 1000 ft which should keep it out of the valleys. Skies, otherwise, will be mostly clear. The weakening onshore flow to the north will also lessen the amount of coastal low clouds early next week, especially if the offshore flow develops.
Gusty but still sub-advisory levels winds will continue across the Antelope Valley.
Temperatures will continue to warm each day, though still remain below normal through the extended.
12/1736z.
At 1639Z at KLAX, the marine layer was 1600 feet deep. The top of the inversion was near 2500 feet with a temperature of 21 C.
High confidence in TAFs for desert airfields (KPMD & KWJF).
Moderate confidence in remaining TAFs. Timing of prevailing groups may be off +/- 2 hours and conditions by one category.
KLAX, Moderate confidence in TAF. Arrival and clearing of MVFR CIGs (012-018) may be off +/- 2 hours. No significant east wind component expected.
KBUR, Moderate confidence in TAF. No wind issues expected.
12/815 AM.
High confidence that conditions will remain below Small Craft Advisory (SCA) criteria across all coastal waters through Friday morning. Low to moderate chances for SCA winds across favored areas south of Point Conception this weekend. Increasing chances across all coastal waters heading into next week.
There is a slight chance of a shower or a thunderstorm around and north of Point Conception later this afternoon thru early Thursday morning.
Ca, Coastal Flood Advisory in effect until midnight PDT tonight for zones 349-350-354-362-366. (See LAXCFWLOX). PZ, NONE.
