Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000 FXUS66 KLOX 210251 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 751 PM PDT Tue Apr 20 2021 .SYNOPSIS...20/203 PM. Strong onshore flow will bring much cooler conditions through Thursday. Night through morning low clouds and fog will push well into the valleys by Wednesday. Drizzle or light rain is possible across Los Angeles and Ventura Counties tonight through Thursday morning. High pressure aloft will bring a slight warming trend for the end of the week into the weekend. Then a chance of rain with cooler temps later Sunday into Monday. && .SHORT TERM (TUE-FRI)...20/749 PM. ***UPDATE*** In the big picture this evening, a tightly wound low pressure system over northern california has been generating plenty of lightning while a jet streak was creating mountain wave activity over the southern Sierras. Here in southwest CA the marine layer inversion remains below 2000 feet this evening, but it`s just a matter of time before that jumps above 3500 feet with a surge of low clouds expected to cover the entire region west of the mountains by daybreak. Starting to get the best stratus push inland along the central coast and it`s filling in over the LA bight at 0230Z. Onshore pressure gradients were around 8.3 mb for LAX-DAG at 00Z and not quite as strong as the NAM had it, plus a good 2mb below forecast for LAX-TPH. Nevertheless, gusty WSW winds to 45 mph continue this evening through the highway 14 corridor near Lake Palmdale. Expect the winds to slowly decrease in the next few hours and that will work out well to end the advisory over the Antelope Valley on time. Wednesday is likely to be a cloudy day with many coastal areas not clearing at all or just briefly before stratus moves back in. There is a possibility of drizzle late tonight into Wednesday morning mainly for foothill communities as the marine inversion deepens. Temperatures will remain on the cool side tomorrow with locally gusty onshore winds. Pressure gradients will not be as strong so it doesn`t look like additional wind advisories will be needed for tomorrow. Wanted to point out that the low pressure system currently over northern CA shifts into southern NV tomorrow. Another chunk of cold air aloft (off the WA coast) shoots southward and slides over southwest CA late Wednesday night and Thursday morning. There will be weak instability across the eastern San Gabriel Mountains as this moves by and this could provide additional lift for actual showers over the foothills and mountains then. ***From Previous Discussion*** A couple of upper lows will come through the area over the next 48 hours continuing the cooling trend and pushing the marine layer up to 4000-5000`. Starting to feel the effects already with a strong eddy circulation centered near Catalina and much cooler temps area-wide today. A 4mb onshore trend this afternoon is generating some gusty winds in the Antelope Valley that will likely continue through the week. As the first upper low approaches from the north tonight, clouds should re-develop quickly and push inland up to the coastal slopes with some drizzle or light rain possible later tonight into Wednesday, especially in eastern and southern LA County. Probably not much clearing Wednesday for coast/valleys and even the coastal slopes. Possibly a reverse clearing trend with better clearing over the coastal waters. The second low will follow quickly Wednesday night into Thursday but with a more westerly track to it which could provide better dynamics and lift for drizzle and light rain, again mainly in LA County but possibly into Ventura County as well. The colder air coming in aloft will weaken the inversion quite a bit which will likely help to break up the cloud layer and allow some sunshine in the afternoon, especially for coastal areas. Some weak instability Thursday afternoon could trigger some light showers in the LA mountains but again mostly light amounts. Slightly improving conditions expected Friday as a weak upper ridge develops. Still a strong onshore flow in place so temps will remain on the cool side and low clouds likely to hug some of the coastal areas much of the day. .LONG TERM (SAT-TUE)...20/209 PM. Slightly warmer temps expected Saturday as weak high pressure remains overhead but still a few degrees below normal with the LAX-DAG pressure gradient still hovering around 8-9mb. Once again low clouds likely to hang near the coast with that onshore gradient. On Sunday a full latitude trough will arrive along the West Coast with a decent moisture tap to it. While most of the better dynamics remain well to the north, strong height falls ahead of it and the moisture should at least be enough to drop some light precip across the area later Sunday into Monday. Ensembles are still all over the place with this one. The EC is basically all in, even more so than earlier runs with precipitable waters close to an inch. The GFS ensembles though are almost the exact opposite. Still decent with the moisture but not as deep with the trough and with essentially zero southerly flow ahead of it at lower levels. As a result, while the northern parts of our area are still likely to receive some rain, the front washes out as it rounds Pt Conception with very little left as it moves into LA/Ventura Counties. Tending to side with the EC on this one with at least some light rain a good chance everywhere. But will be interesting to see which model blinks first. Probably a low impact event, but there are a few EC solutions around an inch. Snow levels would be around 6000` so some snow accumulations possible in the mountains. && .AVIATION...21/0003Z. At 2350Z, the marine layer depth at KLAX was 1800 feet. The top of the inversion was 3300 feet with a temperature of 16 degrees Celsius. High confidence in 00Z TAFs for KPMD and KWJF. For coastal/valley 00Z TAF sites, moderate confidence through 04Z then lower confidence as low clouds will rapidly move into terminals tonight and possibly persist most of Wednesday. IFR CIGS developing at KSMX by 01Z expanding across the rest of the central coast sites through 05Z. The south coast will see MVFR conditions by 02Z from KOXR to KLAX and by 05Z for valley locations KBUR and KVNY. Patchy IFR conditions possible in these valley areas mainly after 10Z. The Antelope Valley TAFs will have the usual gusty southwest winds and possible UDDFS and LLWS. KLAX...Moderate confidence in 00Z TAF. MVFR CIGS developing by 03Z and persisting possibly beyond 20Z. E winds will remain below 8 kts through Wednesday morning. KBUR...Moderate confidence in 00Z TAF. MVFR CIGS and fog developing by 05Z and persisting until at least 18Z. Possible IFR conditions 08-15Z. && .MARINE...20/735 PM. For the Outer Waters, high confidence in current forecast. Winds have diminished to Small Craft Advisory (SCA) levels this evening which should continue until late tonight. Another round of SCA level wind gusts can be expected Wednesday night. For Thursday through Sunday, high confidence in winds and seas remaining below SCA levels, except for a period of SCA level gusts Thursday evening for portions of the central outer waters zone (PZZ673). For the Inner Waters north of Point Sal, high confidence in current forecast. Winds and seas are expected to remain below SCA levels through Sunday. For the Inner Waters south of Point Conception, generally high confidence in current forecast. There is a 20%-30% chance of SCA level wind gusts at times this evening across the western half of the Santa Barbara Channel. Otherwise, winds and seas are expected to remain below SCA levels through Sunday. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Wind Advisory in effect until 9 PM PDT this evening for zone 59. (See LAXNPWLOX). PZ...Small Craft Advisory in effect until 3 AM PDT Wednesday for zones 670-673-676. (See LAXMWWLOX). && .HAZARD POTENTIAL OUTLOOK (FRI-TUE). Rain and mountain snow above 6000 feet is possible for early next week. && $$ PUBLIC...EB/MW AVIATION...EB MARINE...Sirard SYNOPSIS...MW weather.gov/losangeles

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