11/315 AM.
Onshore flow will return to the region today as a ridge of high pressure continues to weaken and shift eastward. A shallow marine layer will bring dense fog at times to the coast, possibly reaching the valleys by Wednesday morning. A trough of low pressure will approach the West Coast on Wednesday night and bring rain, heavy at times through Thursday night or Friday. Drier but cool weather is expected over the weekend.
(tdy-Thu), 11/319 AM.
The ridge of high pressure aloft has finally started weakening and shifting towards the east. As the ridge moves out of the area, SW flow aloft will return as well as pressure gradients switching to onshore. Wednesday night through the week, an upper level trough of low pressure will move over the region bringing rain and a slight chance of thunderstorms to much of the area.
High clouds will be prevalent across the region today and Wednesday out ahead of the aforementioned trough. Despite these high clouds, the onshore flow and high 500mb heights has allowed the marine layer clouds and dense fog to spread across the coasts, and a dense fog advisory is in effect until 9am for these areas (as well as the L.A. County Inland Coast). As the morning continues, the dense fog and low clouds will retreat to the beaches, but may remain at the coasts thanks to the increasing onshore flow. As the onshore flow strengthens and the 500mb heights lower today and Wednesday, marine layer stratus will deepen and likely penetrate further into the coasts and valleys overnight tonight (with the valleys possibly seeing dense fog, and some improved visibilities closer to the coasts). Additionally, temperatures will cool both today and Wednesday compared to the previous day, by about 7 to 12 degrees. This will drop high temperatures into the 70s and 80s away from the coast today, and in the 70s by Wednesday.
A moderate to strong storm with PW's around 1.5" is expected to move through the region during the latter half of the week. Could see some light precip as early as Wednesday night along the Central Coast and a few post- frontal showers Friday, but 90% of the rainfall should take place on Thursday. Most areas will experience around 6-8 hours of precip with the front, 1-3 hours of which could be heavy with up to 0.75" per hour and a 20% chance of as much as 1.00" per hour. The heaviest precip will be in the usual favored upslope areas, including the Santa Lucia's, the Santa Barbara mountains (particularly the Santa Ynez Range) into the western Ventura County mountains, the Santa Monica mountains, and the eastern San Gabriel mountains. All of those areas will be at risk for heavy rain and flooding. If models stay consistent it's likely that a Flood Watch will be issued for the most vulnerable areas, including the most recent burn scars. Projected total rain amounts remain about the same with most coastal/valley areas falling into the 1-2" range with foothills and south facing mountains in the 2-4" range. But would not be surprised to see a couple 5" totals when all is said and done. Additionally, there is a slight chance of thunderstorms along San Luis Obispo and Santa Barbara counties on Thursday, and most of the region by Friday morning. Any thunderstorm that may form may be accompanied by lightning, locally gusty winds, heavy downpours, and/or small hail.
Not expecting any snow below 8000 feet with this system until the very tail end of it, when there could be a few inches between 6000-8000 feet.
(Fri-Mon), 11/253 AM.
There is still some uncertainty with regard to how long showers will continue following the main cold front on Thursday. The latest ensembles place the trough right through LA County Friday morning with northwest flow developing during the afternoon and evening. There still could be some brief moderate to locally heavy showers before the trough passage Friday morning but most of the ensembles are indicating rain rates well below a half inch per hour after 4am Friday. Some of the ensemble members of both the GFS and EC hint at rain continuing over the weekend, (which may just be why the NBM is carrying slight chances of rain over the entire weekend), however the majority of ensemble members suggest the rain is over before the day changes to Saturday.
Therefore, dry but cool weather is expected over the weekend. There remains a small chance of a weak storm next Monday but recent model runs have backed off that system.
11/1115z.
At 0805Z at KLAX, there was a surface inversion. The top of the inversion was near 1500 feet with a temperature of 25 degrees Celsius.
High confidence in VFR conditions for KPRB, KBUR, KVNY, KWJF and KPMD. 15% chance for VLIFR conds at KPRB through 17Z.
Moderate confidence in all coastal sites. Timing of clearing may be off +/- 3 hours, and there is a 30% chance immediate coastal sites do not clear. Moderate confidence in return of cigs after 12/00Z, but low confidence in timing (+/- 4 hours) and minimum flight cat (off +/- one cat). Expecting slightly higher cigs and vsbys at most locations later tonight.
KLAX, Low to moderate confidence in TAF. Clearing may be off by +/- 3 hours, but there is a 30% chance for no clearing. Arrival of low clouds tonight may be off +/- 4 hours. Low confidence in minimum flight cat, but moderate confidence in slightly improved conditions. No significant east wind component is expected.
KBUR, High confidence in TAF.
11/150 AM.
Dense fog with visibilities less than one nautical mile will continue to be an issue for all the coastal waters through late this morning.
High confidence in relatively calm winds and seas through Wednesday, except for localized Small Craft Advisory (SCA) level wind gusts to 25 knots near Point Conception and south to San Nicolas Islands early this morning, and again this afternoon and evening. Southwest winds 20-30 knots will develop Wednesday afternoon across the outer waters as well as nearshore along the Central Coast. Southerly winds across the inner waters south of Point Conception will be in the 15-20 knot range, but SCA level gusts to 25 knots cannot be ruled out.
A cold front from a storm system will pass through the region Thursday and gusty northwesterly winds of 20-30 knots will develop across the entirety of the coastal waters. There is a 30% chance for low end Gale Force winds Thursday afternoon through late Friday, including across the inner waters south of Point Conception. Seas across the outer waters will increase to 12-14 feet, and 5-7 feet inside the Southern California Bight.
From Wednesday night through Thursday, the aforementioned storm system will bring a slight chance of thunderstorms across the coastal waters from the Santa Barbara Channel northward. Any thunderstorms that develop will be capable of producing frequent lightning, gusty and erratic winds, locally rough seas and even waterspouts.
Moderate confidence in conditions improving some over the weekend, but SCA winds are still possible, especially across the outer waters.
Ca, Dense Fog Advisory remains in effect until 9 AM PST this morning for zones 340-341-346-347-350-354-355. (See LAXNPWLOX). Dense Fog Advisory in effect until 9 AM PST this morning for zones 349-362-366-368. (See LAXNPWLOX). PZ, NONE.
