An approaching low pressure system will spread marine layer clouds and below normal temperatures farther inland tonight through Saturday. There is a chance of drizzle tonight into early Thursday, and again Thursday night into Friday. A ridge of high pressure will build over the area this weekend, and a warming trend will bring temperatures back to near or above normal by early next week. In addition, the marine layer clouds will be confined to near-coastal areas.
A well-developed marine layer stratus deck is taking shape this evening as a trough of low pressure approaches the area. The trough of low pressure about 300 miles offshore of the Central Coast will slowly drift into the Southern California Bight through Friday morning. Strong onshore flow and a deepening marine layer will bring a cooling trend through Friday. If NAM BUFR time height sections are correct, the marine layer should deepen to near 4000 feet deep by Thursday morning and to 5000 feet deep by Friday morning. KLAX-KDAG and KSMX-KBFL surface pressure gradients are strongly onshore this evening and should strengthen additionally on Thursday afternoon.
With the trough nudging closer to the area, patchy drizzle or light showers cannot be ruled out as the marine layer deepens and interacts with the upper-level dynamics associated with the weak weather system over the next couple of days. PoPs have been nudged up across the area and mentions of drizzle have been added to the Central Coast tonight through Thursday morning.
With strong onshore pressure gradients, gusty southwest winds will likely develop across the Interior Valleys the next couple of days. The wind advisory for the Antelope Valley has been extended into Thursday evening, but it reasonably could be extended into Friday or Friday evening. Future shifts may also need to consider a wind advisory for the San Luis Obispo County Valleys for Thursday afternoon and evening. Model guidance for KPRB is suggesting winds strengthening above advisory level criteria, but there has been discontinuity across the model guidance. For now, any decisions on expanding a wind advisory for the San Luis Obispo County Valleys will be tabled for more data and better agreement.
*** FROM PREVIOUS DISCUSSION ***
With the trough axis moving through Friday night there should be much earlier and complete clearing of the marine lyr Saturday. Highs expected to be a few degrees warmer though still below normal.
A high amplitude upper ridge will develop along the west coast Sunday and Monday. At the same time onshore flow will be weakening and daytime temps are expected to climb to several degrees above normal by Monday. Another weak upper low is expected to undercut the ridge Tue/Wed forcing a deepening of the marine lyr and cooler temps.
At 00Z, the marine layer depth at KLAX was near 1700 feet. The top of the inversion was around 3200 feet with a temperature of 21 degrees Celsius.
Moderate confidence in the current forecast. IFR to MVFR conditions will likely spread into most coastal and valley terminals through 08Z. IFR conditions are likely in any drizzle, mainly between 07Z and 16Z. MVFR conditions will likely linger through much of the day at coastal and valley terminals.
KLAX, There is a 80 percent chance of MVFR conditions throughout the period. Between 03Z and 15Z, there is a 60 percent chance of IFR conditions in drizzle. After 20Z, there is a 30 percent chance that VFR conditions could develop; however, ceilings at or below 5000 feet will likely persist. South winds around 10 knots are possible between 13Z and 17Z.
KBUR, IFR to MVFR conditions will likely spread into KBUR between 04Z and 08Z. There is a 60 percent chance of IFR conditions in drizzle between 07Z and 15Z. After 20Z, there is a 40 percent chance that VFR conditions could develop; however, ceilings at or below 5000 feet will likely persist.
Mainly light winds are expected across the coastal waters through Friday. Northwest winds will begin to increase Saturday afternoon, with a 50% chance of SCA gusts by Saturday evening over the outer waters. Over the nearshore waters north of Point Sal, SCA winds will be possible Sunday and Monday. For the nearshore waters south of Point Conception, conditions are likely to remain below SCA criteria through the period.
Patchy fog and drizzle should affect all the waters from midnight through around noon on Thursday.
Ca, Wind Advisory in effect until 9 PM PDT Thursday for zone 59. (See LAXNPWLOX). PZ, NONE.