429 FXUS66 KLOX 171715 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 915 AM PST Fri Jan 17 2025 .SYNOPSIS...17/857 AM. Cooler weather is expected today and Sat as onshore flow returns. Morning low clouds and fog are expected in many coastal and some valley areas this morning, and again late tonight and Saturday morning. The flow will turn weakly offshore Sunday, with local northeast breezes, less night through morning low clouds, and a few degrees of warming. Another Santa Ana wind event is likely beginning later Monday into Tuesday, and again Thursday. Temperatures will likely be below normal for much of next week, with frosts possible in wind sheltered areas away from the coast. && .SHORT TERM (TDY-SUN)...17/915 AM. ***UPDATE*** Onshore flow has returned and with it some morning low clouds and fog. The sounding out of NKX showed a much weaker inversion than the forecast soundings indicated yesterday which explains the very patchy nature of the clouds. With less clouds than expected temperatures last night were still quite cold across coast and valleys, but the onshore flow increase did create enough mixing across the Antelope Valley so temperatures there were several degrees warmer than previous nights. For today, high temperatures will be down several degrees for coast and valleys with the return of onshore flow. Most areas are already down 10 degrees from yesterday at 9am and some areas are down as much as 20 degrees. Ultimately looking at highs about 10 degrees cooler than yesterday, except little change or possibly slightly warmer in the mountains and other far inland areas. In addition, humidities are 40-60% higher today with the onshore flow. No big changes in the forecast. Models zoning in two peaks of offshore flow next week. The first one late Monday into Tuesday and the second on Thursday. Most of the ensemble solutions are in the moderate to locally strong category for both events but still quite a bit of time yet for changes. ***From Previous Discussion*** A rather benign period of weather is expected thru the weekend. A weak ridge will develop west of the region tonight/Sat, with W to NW flow aloft, and slowly rising heights. The upper ridge will amplify off the West coast Sat night/Sun, causing the flow aloft to become more NW to N by Sun. Expect some night thru morning low clouds and fog tonight/Sat morning, especially in coastal/valley areas of L.A. County and on the Central Coast. If skies remain mostly clear, frost and freeze conditions are possible late tonight/early Sat in the valleys of western L.A County and VTU County, and on the Central Coast. Rising heights and weakening onshore gradients should allow for a few degrees of warming in most areas on Sat. For Sat night/Sun, surface gradients will turn offshore once again, but only weakly so. There could be some local N-NE breezes in the mtns/valleys of L.A./VTU Counties late Sat night/Sun, but winds should remain well below advisory levels. There will likely be less in the way of low clouds Sat night/Sun morning, likely confined to coastal sections of L.A. County. Max temps should be up a few degs in most areas Sun, generally to near normal levels. .LONG TERM (MON-THU)...17/529 AM. There is some disagreement between the operational runs of the EC and the GFS regarding both the upper level pattern and the surface pattern next week. There is also a bit more spread in the ensembles than is normally desired to make a high confidence forecast, at least with respect to the fine details. In general, it looks as though there will be another stretch of offshore flow from Mon thru at least Thu, and possibly into Fri. An amplified upper ridge will be located off the West Coast Mon, and the upper flow across the western states will become northerly. A rather strong short wave will drop southward in this northerly flow aloft, carving out a sharpening trough over Arizona. This in turn will cause a sharpening height gradient over the region by late Mon and Mon night, with increasing northerly winds aloft. Cold air advection, strengthening winds aloft, and subsidence will provide more robust upper level support for winds in the forecast area Mon night thru at least Tue, and possibly into Wed. The EC, farther west with the short wave, shows better upper support for winds than does the GFS. At the surface, the determinist run of the EC also shows stronger offshore gradients than does the GFS, at least for Mon night/Tue, with the EC at -8.2 mb between KLAX and KDAG, compared to about -5 mb shown on the GFS. Both models show slightly relaxed offshore surface gradients Tue night thru Thu, with weakening upper level support thanks to a weak upper low approaching the coast. Based on the deterministic runs, it would appear that the strongest winds would be Mon night and Tue. However, some EC ensemble members show the strongest winds Tue night/Wed. Therefore, timing is still an issue. Overall, it appears that at least a moderate Santa Ana wind event is likely for VTU County and much of L.A. County in the Mon night thru Wed timeframe, with a 30% chance that this will peak as a strong Santa Ana wind event either Mon night/Tue or Tue night/Wed. Both models show another, slightly smaller peak in the winds Thu night/Fri. Gusty winds will be a threat at times from late Mon thru Fri. It will be mostly clear through the period. Very low relative humidity values are likely Tue and Wed, with widespread humidities of 10% or less, possibly down below 5% in the driest locations. It will be quite cool, at least Tue and Wed, with max temps below normal except near the coast where downslope flow will help bring some warming. In wind sheltered area, the nights will be quite cold, with frost or freeze conditions likely in away from the coast in SLO and SBA Counties, and in wind-sheltered valleys of VTU and possibly L.A. County during each morning Tue thru Thu. Some warming may begin across the region by Thu. && .AVIATION...17/1352Z. At 1314Z at KLAX, there was no marine layer. There was a surfaced based inversion with a top at 700 ft and a temperature of 10 C. Overall, moderate confidence in 12Z TAF Package. Best chances for flight cat restrictions due to low clouds/patchy fog for LA coastal areas and KSMX. There is a 10-20% chance of LIFR conds at KSMX from 02Z to 12Z Sat. There is a 20% chance of IFR conds at KPRB from 06Z to 12Z Sat. Similar chances for IFR conds at KOXR thru 18Z fri. KLAX...Moderate confidence in 12Z TAF. There is a 10% chance cigs fall below BKN/OVC010 to IFR conds from 12Z to 18Z Fri. Good confidence that east wind component remains below 8 kt thru the fcst pd. KBUR...Good confidence in 12Z TAF. && .MARINE...17/813 AM. In the Outer Waters, there is a 30% chance of Small Craft Advisory (SCA) level winds from Pt. Conception to San Nicolas Island this afternoon thru Sat night, and again Sun night/Mon. There is also a 30% chance of SCA wind gusts out of the NE for Mon night and Tue morning for the southern outer waters (PZZ676). Otherwise, sub-advisory conditions are expected through Tue night. For the Inner Waters North of Pt Sal, sub-advisory conds are expected thru Tue night. In the Santa Barbara Channel, there is a 20%-30% chance of SCA level W winds over western portions this afternoon and evening. There is also a 30%-40% chance of SCA easterly wind gusts over eastern portions Mon night and Tuesday. Otherwise, sub-advisory conds are expected thru Tue night. In the Southern Inner Waters, except for a 30%-40% chance of SCA level NE winds Mon night and Tue mainly nearshore from Malibu to Santa Monica, SCA conds are not expected thru Tue night. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...NONE. PZ...NONE. && $$ PUBLIC...MW/DB AVIATION...Black MARINE...Sirard SYNOPSIS...MW/DB weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox
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