10/143 AM.
Dry conditions with cooler than normal temperatures will continue into early this week. A significant storm system will arrive Wednesday and last through Friday. It will bring rain, high elevation snow, and gusty winds to the region.
(tdy-Wed), 10/845 AM.
***UPDATE***
Forecast on track with no changes need with the morning update.
The focus of today will be refining details of the upcoming Wednesday through Friday storm, especially for Thursday when most areas will see the heaviest precipitation and potential for greatest impacts. This storm will very likely be the biggest precipitation producer so far this season for most of southwest California.
***From Previous Discussion***
Weak broad pos tilt troffing will move over CA today. Hgt will fall to 565 dam. The weak offshore push from east will switch to onshore this afternoon. The offshore flow from the north will be near 3 mb this morning and will weaken to about 2 mb this afternoon. A weak eddy is spinning and it will likely bring some low clouds to the LA coast and maybe the San Gabriel vly this morning. The switch to onshore flow to the east and lowering hgts will combine to bring about two degrees of cooling to SLO and SBA county. The LA/VTA csts/vlys will see more dramatic cooling of 4 to 8 degrees. Most max temps across the csts/vlys will be in the lower 60s today.
The troffing will increase on Tuesday and hgts will fall further to about 562 dam. The eddy will also be a little stronger and there will be more low clouds across the coasts and lower vlys. The low clouds will likely not clear til late morning due to the eddy and strengthening onshore flow. Most max temps will fall another 2 or 3 degrees (the SBA south coast may see a few degrees of warming). The increased onshore flow to the east along with some WNW upper level wind support will bring gusty near advisory level winds to the Antelope Vly and its western foothills. An impulse racing ahead of a larger approaching storm will overspread the area on Wednesday. Skies will turn mostly cloudy early. A chance of rain will develop across the Central Coast in the predawn hours but the best chance of rain will occur during the day. This system is racing through a weak ridge and is embedded in WNW flow which is not the most favorable orientation for rain south of Pt Conception. Rainfall during this period should add up to about an inch across the Central Coast and a half inch south of Point Conception with potentially double those amounts across the coastal slopes. But due to the upper level flow pattern and the potential for easterly rather than southerly low winds South of Pt Conception rainfall here could be less than fcst.
The deterministic mdls show a significant reduction in rain overnight Wednesday although the ensemble smear keeps likely pops across the Central Coast and chc pops for areas south of Pt Conception. Hopefully this discrepancy will resolve itself as the event draws nearer.
(Thu-Sun), 10/320 AM.
Not much change in the main storm forecast Thursday and Friday. There is very high confidence that widespread rain will affect the entire area for about a 36 hours period. A winter storm with a well defined warm and cold front backed by a strong 150 mph jet will approach and then work its way through the state early Thursday through Friday.
Look for steady rain to develop everywhere in the warm frontal sector of the storm early Thursday. The rain will continue through the day. Rainfall during this time will be lgt to mdt with a low risk intensities capable of producing dangerous debris flows. The more dangerous portion of the storm will occur with the cold front which will sweep into SLO county late Thursday afternoon. It will move into SBA county Thursday evening and then VTA/LA counties after midnight. This will be the worrisome portion of the storm as rainfall rates near 1 inch per hour will be possible just ahead and with the front. People in or near to recent burn areas will need to by hyper vigilant of weather conditions during this time frame. There is also a 10 percent chc of TSTMs with the front which will be well capable of producing dangerous rain rates in excess of 1 inch per hour as well a gusty winds and/or waterspouts.
Rain will turn to showers in the wake of the cold front on Friday. There will likely be numerous showers Friday morning. The shower activity should decrease Friday afternoon from the NW to the SE. Some models show enough CAPE and instability for TSTM development, but for now too low a chc to mention.
Not much change in the rainfall forecast for the Thu/Fri time period. Look for 1 to 2 inches of rain across the with 2 to 4 inches across the coastal foothills. Additionally, there is a chance that rainfall totals could be even higher.
Snow levels will likely start out near 5000 ft on Wednesday before rising to near 7000 ft with Thursday's warm front. The snow levels will then lower to first 6000 ft and then to near 5000 ft in the wake of the front. Elevations above 7,000 or 8,000 feet may see several feet of snow.
As for winds, warning level winds will likely occur with the front Thursday night. High temperatures will fall into the 50s each day, which is 5 to 10 degrees below normal for this time of year.
Ridging will move in behind the storm and will bring dry and somewhat warmer conditions for upcoming weekend.
Please stay tuned to this forecast as it will likely evolve over the next few days. This storm has the potential to bring life threatening debris flows in and below recent burn scars.
10/1321z.
At 12Z at KLAX, the marine layer was 1500 ft deep. The top of the inversion was at 2200 ft with a temperature of 11 C.
Low clouds have pushed into coastal sections of L.A. County and the San Gabriel Valley. Conds were mostly low MVFR, but will be locally IFR to LIFR in the San Gabriel Valley and in the hills. Patchy low clouds will affect the Central Coast as well, with IFR to LIFR conds. Expect skied to clear by mid to late morning, For tonight, expect clouds in coastal sections of L.A. County, possibly pushing into coastal sections of VTU County by daybreak. Clouds will likely also push into the San Gabriel Valley. Expect mostly MVFR conds, except IFR in the San Gabriel Valley and hills.
KLAX, Low confidence in TAF. There is a 30-40% chance that conds will remain VFR this morning. There is a 20% chance that cigs will arrive as early as 03Z this evening or hold off until after 09Z tonight. There is a 20% chance of an 8 kt east wind component 09Z-16Z Tue.
KBUR, Moderate to nigh confidence in TAF. There is a 20% chance of IFR cigs thru 16Z this morning, and again after 11Z tonight.
10/801 AM.
In the Outer Waters, Small Craft Advisory (SCA) level NW winds will continue much of the time thru early Wed morning. There may be periods where winds drop below SCA levels especially in zone PZZ670. There is a 40% chance of Gale force winds for the waters around Point Conception, Northern Channel Islands, and down to San Nicolas Island Tue afternoon into Tue night (PZZ673/676).
Winds are expected to decrease some late Wed morning into Wed afternoon, then turn to SE to S ahead of an approaching storm system. SCA level S winds are likely (60% chance) Thu/Thu night, followed by SCA level W to NW winds Fri/Fri night.
In the Inner Waters N of Pt Sal (PZZ645), SCA level winds are likely (80-90% chance) this afternoon/eve and Tue afternoon/eve. SCA conds are not expected Tue night thru Wed night. SCA level S winds are likely (60-70% chance) Thu/Thu night, followed by SCA level NW winds Fri.
In the Santa Barbara Channel (PZZ650), SCA level W-NW winds are likely (80% chance) during the late afternoon thru late night hours today and Tue. There is a 30-40% chance of Gale force gusts Tue eve. SCA conds are not expected late Tue night thru early Thu. SCA level SE winds are likely (60% chance) Thu afternoon into Thu night, followed by SCA level W winds Fri/Fri night.
In the southern inner waters, SCA conds are not expected thru Tue morning. SCA level W winds are likely (60-70% chance) Tue afternoon/eve. SCA conds are not expected late Tue night thru early Thu. SCA level S winds are likely (60% chance) Thu afternoon into Thu night, followed by SCA level W winds Fri/Fri night.
Ca, NONE. PZ, Small Craft Advisory in effect from 3 PM this afternoon to 9 PM PST this evening for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM this afternoon to 3 AM PST Tuesday for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PST Wednesday for zones 670-673-676. (See LAXMWWLOX).