21/239 AM.
A significant cooling trend is expected through Friday as onshore flow redevelops ahead of trough offshore. Light rain or drizzle is possible at times tonight through Friday. A warmer and drier weather pattern will resume early next week as offshore flow reestablishes.
(tdy-Fri), 21/827 AM.
***UPDATE***
High clouds continue to stream into the area as the low pressure system spins off the coast. Weak offshore flow continues this morning in the mountains and hills of Ventura County, but will quickly switch to onshore by this afternoon. Rain chances will start late this afternoon along the Central Coast. The very slight chance of rain for Ventura and LA Counties tonight will increase to a 30-50% chance through the day on Thursday, with lingering showers possible through Friday. Forecast remains on track with no major changes with this update. ***From Previous Discussion***
The latest satellite imagery shows a high-amplitude ridge over the state breaking down this morning. An upstream trough centered near 34N and 130W will continue to plow into the ridge and move it east through tonight. Offshore pressure gradients continue to weaken across the region this morning. Low clouds and patchy dense fog at the coast and into some of the lower valley areas allude to the idea of onshore flow developing more strongly across the area later today. A shallow marine layer depth in place this morning will deepen over the coming days as onshore flow firmly takes hold of the flow regime.
The trough will continue to move east toward the central California coast through late tonight, then likely dig south to 27N and 120W by Friday night. The main story will be the cooling trend with more cloud cover over the coming days. Light rain or night through morning drizzle is possible across portions of the area. EPS ensemble members mostly favor light rain developing across the region, while GEFS members are much drier. High-resolution multi-model ensemble members suggest the best chance of any rain for areas north of Point Conception for tonight, then the focus shifting to Los Angeles County between Thursday and Friday.
Amounts will be mostly light with generally 0.25 inch or less of rainfall expected, but isolated thunderstorms with heavier rainfall cannot be ruled out from tonight through Friday. There is a 10 percent chance that heavier showers could develop with rainfall rates topping out between 0.25 and 0.50 inch per hour. The highest rainfall totals will likely occur across south-facing slopes of the Transverse ranges. WPC estimates favor the south- facing slopes of Santa Barbara and Ventura Counties receiving the most rainfall with this storm, but high-resolution multi-model ensemble members advertise the highest probabilities of amounts of greater than 0.10 inch in the eastern San Gabriel mountains. Due to the system's cutoff nature, less certainty should be placed in the actual totals as any movement to the east or west will change precipitation amounts wetter or drier, respectively.
As the coldest air aloft remains west of the area, snow levels will likely remain high, keeping any snowfall to the highest elevations and mountain peaks through Friday. 700 mb temperatures only cool to -4 to -5 degrees Celsius by Friday night and this should keep snowfall in the higher elevations. There is a 10 percent chance that more impactful snow could fall down to the 5000 foot level.
(Sat-Tue), 21/233 AM.
Ridging aloft will replace the trough over the weekend. Weak ridging will strengthen through early next week with another blocking ridge setting up between Monday and Wednesday. A warming and drying trend should develop for early next week.
EPS ensemble members give a moderate chance of gusty northerly winds developing on Saturday night and into Sunday morning. Then, advisory level Santa Ana winds will be possible for the typical Santa Ana wind corridor across eastern Ventura County and western Los Angeles County early next week.
A dry weather pattern is favored through the end of the month.
21/1743z.
At 1638Z at KLAX, the marine layer was 800 fT deep. The top of the inversion was at 1900 ft with a temperature of 18 C.
High confidence in TAFs for desert airfields (KPMD & KWJF).
Low to moderate confidence for remaining TAFs. Timing of CIG/VSBY restrictions may be off by +/- 3 hours and categories by 1 or 2. Expecting -SHRA chances for coastal and valley sites, thus have included PROB30 groups from previous package.
KLAX, Low to moderate confidence in TAF. Generally expecting BKN/OVC 010-025 through the period, although could fall to 008 through 22/12Z. 20-40% chc of -SHRA 14-23Z. Moderate confidence that any east wind component remains under 8 kts thru the period.
KBUR, Low to moderate confidence in TAF. IFR-MVFR cigs likely to return after 06Z Thu. Forecast sounding and MOS guidance suggest that there exist a slight chance for LIFR CIGs to develop 06Z-12Z.
21/755 AM.
Winds and seas are expected to remain relatively calm through Thursday, then there is a 30 percent chance for Small Craft Advisory (SCA) winds across the Outer Waters and into the western Santa Barbara Channel Friday into Saturday morning. Another round of light Santa Ana winds is forecast to return Saturday night into Sunday, but chances are low for any impactful nearshore winds at this time.
Patchy dense fog with visibilities less than one nautical mile is expected through later this morning, likely impacting nearshore waters.
Rain showers are possible from this afternoon through Thursday night, starting north and moving south through the timeframe. This activity could linger into Friday morning across the waters adjacent to Los Angeles and Orange Counties. There is a very low chance (5 to 10 percent) for a thunderstorm or two to develop at some time, with areas south of Point Conception favored - the thunderstorm threat looks highest on Thursday.
Ca, NONE. PZ, NONE.
