Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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477 FXUS66 KLOX 201704 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 1004 AM PDT Fri Sep 20 2024 .SYNOPSIS...20/946 AM. Shower and thunderstorm chances have shifted to LA County and eastern Ventura County today as a cold upper continues to move through the area. Some showers or thunderstorms could have heavy downpours, and gusty winds. High pressure will build in over the weekend and bring a significant warming and drying trend to the region over the weekend into early next week. && .SHORT TERM (TDY-SUN)...20/1002 AM. ***UPDATE*** The upper low has moved southeast and is currently between Palos Verdes and Catalina. A line of showers and thunderstorm stretches from Mojave to eastern Ventura County, though most of the storms are north of Antelope Valley. A the day goes along and the low continues to shift to the southeast, that line will pivot to a north/south orientation and focus on the eastern portion of LA County, including the Bridge fire area. Showers and thunderstorms chances will continue in that area through early evening with periods of heavy rain possible. Steering flow is expected to be 15-20 kt from the north, but some training of storms is possible. Moisture profiles today are not as favorable for significant flooding today but can`t rule it out entirely. Latest hi res ensembles show the 90th percentile for hourly rain rates peaks at around .60 inches, which is about a quarter inch shy of the thresholds for burn area debris flows, so for now will not be issuing a flash flood watch. Precip expected to taper off this evening with dry and warmer weather over the weekend. ***From Previous Discussion*** A mostly north to south flow on the back side of the low pressure trough will steer any shower activity from the mountains into the valleys and coastal areas later today and into this evening. The trough`s slower movement has contributed to PoPs being held on for longer the system has slowed down. The highest PoPs are indicated over the mountains and into the foothills of the mountains, but showers and thunderstorms pushing into Southland valley area and into the Ventura County coastal plain seems elevated in confidence at this point. Otherwise, a cooler air mass will continue across the area through tonight as a deep marine layer depth will remain in place. Clouds are pushing well into the coastal slopes of the mountains this morning, even with the disturbed and weaken marine inversion over the region at this time. A marginally tight northerly surface gradient could bring some sub-advisory northwest to north winds across southern Santa Barbara County mountains this evening and through the Interstate 5 Corridor tonight. Gusts to between 30 and 40 mph will likely be observed with local gusts up to 45 mph in a few spots. A drying and warming trend will take place over the weekend as the upper-level trough will move off to the east. High pressure aloft will nose into the region and bring much temperatures, especially Sunday and Monday. Night through morning low clouds and fog should remain in the forecast for tonight, then much less marine coverage .LONG TERM (MON-THU)...20/357 AM. The latest forecast ensembles agree that Monday will likely be the warmest day of the week. Low cloud coverage could end up being confined to the coast by Monday morning as onshore flow weakens. There is still a large spread of solutions for the remainder of next week, the forecast ensembles lean trend toward a cooling trend for Tuesday and Wednesday. The forecast temperatures break away from NBM values for Monday, but otherwise, keeps the NBM values for now. Forecast ensemble leans towards a cooler air mass with more marine influence for Tuesday, Wednesday, and Thursday. && .AVIATION...20/1301Z. At 1216Z at KLAX, there was a deep moist layer up to 5000 feet. The top of the inversion was at 5500 feet with a temp of 11 C. Moderate confidence in TAFs for KPMD and KWJF. There is a 20% chc of TSTMs thru 21Z. Low confidence in remaining TAFs. Cig heights will vary frequently due to an upper level low moving over the region. Timing of flight cat and VCSH/VCTS changes could be off by +/- 4 hours. For KPRB, there is a 30% chc of IFR conds after 06Z Sat. For KSMX and KSBP, low confidence in cig height and timing. LIFR conds possible (20% chc) thru 17Z, with VLIFR conds possible (20% chc) after 06Z Sat. For KSBA, and Ventura and Los Angeles County TAFs, there is a 20-30% chance of -DZ or -SHRA at all sites thru 00Z, and a 10-20% chance of thunderstorms between thru 00Z. The highest chances will be for KSBA, KOXR, KBUR, and KVNY. Any thunderstorms that form may produce brief +RA and gusty, erratic winds. KLAX...Low confidence in TAF. Cigs may vary between BKN015-035 frequently thru at least 18Z, and there is a 30% chc of -SHRA and a 15% chance of TSTMs thru 15Z Fri-00Z Sat. Low confidence in hgt of CIGs after 06Z sat, which could be between Bkn008-018. No significant east wind component expected. KBUR...Low confidence in TAF. Cigs may vary between BKN015-035 frequently thru at least 18Z. There is a 20% chance of TSTMs 15Z Fri-00Z Sat. Low confidence in hgt of CIGs after 06Z sat, which could be between Bkn008-015. && .MARINE...20/838 AM. Isolated thunderstorms will be possible across the eastern portion of the Santa Barbara Channel through the southern inner waters and out to Santa Cruz Island into the afternoon. Any thunderstorm that forms has the potential for frequent cloud to surface lightning, gusty and erratic winds with rough seas, and small hail. For the Outer Waters (Central coast south to San Nicolas Islands), moderate to high confidence in the forecast. High confidence in winds and seas remaining below Small Craft Advisory (SCA) level thru early next week in the northern zone (off the Central Coast) except for a 20-30% chance of SCA level winds this afternoon thru evening. For the southern zones (Pt Sal to San Nicolas Island), there is a 60-70% chance of SCA level winds through late tonight across the waters from Point Conception to San Nicolas Island, with the furthest southern zone already hitting SCA levels this morning. From Saturday thru Wednesday, high confidence in winds and seas remaining below SCA level, and then there is a 20-30% chance of winds picking up Wednesday night. For the inner waters along the Central coast, high confidence in conds remaining below advisory levels through Wednesday. For the Santa Barbara Channel, moderate confidence in the forecast. There is a 50-70% chance of SCA level wind gusts across the western portion of the channel this afternoon through evening with a 20% chance of winds extending into the eastern portion, therefore a SCA was issued. Then, moderate to high confidence in conditions remaining below SCA levels through Wednesday. For the inner waters off the coast of LA and Orange Counties, moderate to high confidence in conditions remaining below advisory levels through Wednesday. However, there is a 20-30% chance of SCA level wind gusts thru the San Pedro Channel Sun afternoon. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...Small Craft Advisory in effect from 3 PM this afternoon to 3 AM PDT Saturday for zones 650-673. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Saturday for zone 676. (See LAXMWWLOX). && $$ PUBLIC...MW/Hall AVIATION...Lund MARINE...Lewis/Lund SYNOPSIS...MW weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox