Area Forecast Discussion...UPDATED
National Weather Service Los Angeles/Oxnard CA
535 AM PDT Tue Apr 7 2020
SYNOPSIS...06/810 PM.
Scattered rain and snow showers will continue through Thursday with some snow on the major passes early Tuesday and Wednesday morning. There is a slight chance of thunderstorms tonight through Wednesday including across the coastal waters. Conditions will be dry and warmer on Friday and through the weekend.
SHORT TERM (TDY-THU)...07/134 AM.
An upper low will bring interesting and changeable weather to the area over the next three days. Currently the upper low is 180 miles WSW of Pt Conception. By noon it will be 125 miles SW of Pt Conception. It will then proceed to spot just south of San Miguel island where it will sit and spin through mid morning Wednesday. it will then head east and pick up speed ending up over central Riverside County by Wednesday evening. On Thursday it will track to the NE and into NV.
Currently the forecast area is under a large area of NVA and most of the area if shower free and under partly cloudy skies. The only exception is the far eastern portion of LA county where there are persistent bands of showers rotating into and through the area. The shower activity will remain at a minimum through late morning but will then change a large lobe of PVA moves up and into LA/VTA/SBA counties and eventually SLO county a an hour or two later. This will produce widespread shower activity which will persist overnight. The PVA and the difluent flow aloft should be enough to bring a slight chc of TSTMs as well despite the lack of really great instability. During this time most of LA/VTA/SBA counties will see a quarter to a half inch of additional rain with inch amounts possible over some mtn locations and lesser amounts over SLO county.
Snow levels will start out near 4000 feet will quickly rise to 5500 feet and then fall to 4500-5000 ft this evening. A winter weather advisory covers the mtns now since the snow shower activity is so much less than it was ydy. The advisories will turn into Winter Storm Warning over the LA and VTA mtns early this afternoon as the widespread showers develop. It is likely that the higher elevations of the San Gabriels will see about 10 inches of new snow from this afternoon through this evening. Mtn areas between 5500 feet and 7000 feet will see 5 to 9 inches of new snow with lesser amount over the SBA mtns and from 4000 ft to 5500 feet. The heavier snow Will arrive after the snow level have risen so the major mtn passes will see rainy conditions but not snowy ones.
Well below Max temperatures will continue today with most cst and vly temps topping out in the lower to mid 60s.
All mdls agree that from later Wednesday morning through Thursday the upper level flow will switch from the SW to the NE or N as the upper low tracks to the east. This will shift the focus of the precipitation from the coasts and vlys to the mtns and the interior. The csts/vlys will likely only see scattered showers and. 10-.25 additional rainfall (xcp up to a half inch across the San Gabriel Vly). The interior from the Cuyama Vly the to Antelope Vly including the northern sections of the SBA/VTA/LA mtns will likely recieve a .50-1.00 inches of additional rain. A slight chc of TSTMs will persist as well again more dynamically driven than instability driven. The mtns will continue to see impressive snowfall amounts. S now levels will remain around 5500 feet and another 5 to 10 inches of new snow is possible during this period with the greatest amounts falling above 7000 ft over the eastern San Gabriels
Max temps will fall a couple of degrees each day as the lows path brings lower hgts to the area in addition to the cloudy skies and showers. By Thursday the coasts and vlys will only see max temps a degree or two either side of 60 which is 8 to 12 degrees blo normal for most sites.
Both the NAM and the GFS agree that the upper low move to the east Thursday evening leaving only a slight chc of showers over eastern LA county interior and mtns. But the latest EC retrogrades the upper low back to the west and directly over VTA county which keeps all of the forecast activity going over night for all of the area. Given the oddness of this upper low cannot rule out that is the right soln so will keep some chc of rain in the fcst.
AVIATION...07/1233Z.
At 12Z, there was no marine inversion at KLAX.
High confidence in VFR conditions for all sites through at least mid morning, then low to moderate confidence. Showers are likely to spread over the region by this afternoon from east to west. The best chance for showers will be for LA County sites. Low confidence on timing of showers and potential flight category changes. MVFR conditions are most likely during periods of rain, but IFR conditions are possible in heavier showers. There will be a slight chance of thunderstorms for all sites starting this afternoon.
MARINE...07/1241 AM.
High confidence that winds and seas will remain below SCA level through the forecast period.
Isolated TSTMs will be possible across all of the coastal waters this afternoon and evening, then mainly confined to the southern waters on Wednesday. Any thunderstorms will be capable of producing brief heavy rain, small hail, rough seas, and possibly waterspouts.