Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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661 FXUS66 KLOX 171804 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1104 AM PDT Mon Jun 17 2024 updated aviation discussion .SYNOPSIS...17/345 AM. Temperatures will gradually trend downwards in most areas this upcoming week, following the warm to hot conditions this weekend. Strong northwest to north winds will impact the Central Coast through interior sections of Santa Barbara County to the Antelope Valley through Monday night, with fire weather concerns. && .SHORT TERM (TDY-WED)...17/1006 AM. ***UPDATE*** The Red Flag Warning for gusty northwest to north winds and low relative humidity for the Interstate-5 corridor in northwest Los Angeles County and the Ventura County mountains is now in effect until 6 PM PDT Tuesday, and includes an expected wind shift to the northeasterly direction between midnight and 6 AM PDT Tuesday. In addition, the Red Flag Warning has been extended in areal coverage to include the western San Gabriel Mountains and Highway 14 corridor from Midnight tonight to 6 PM PDT Tuesday, where gusty northeast winds and relative humidity falling to the single digit readings are expected. Please reference the Fire Weather section below for additional information. A Wind Advisory has also been issued in conjunction with the Red Flag Warning for the western San Gabriel Mountains and Highway 14 corridor. Smoke from the Post Fire complex will affect a large portion of LA County and Ventura County and will bring air- quality impacts to many areas. The forecast has been updated to account for the addition of smoke. Please reference the Air Quality Alert Message for additional information. For additional details regarding the forecast, please reference the discussion sections below. ***From Previous Discussion*** Good eddy circulation in progress across the inner waters. The marine layer has deepened to about 2000 ft, and clouds have surged northward from coastal L.A. County into the valleys and into VTU County as well. By daybreak, expect low clouds in all coastal and most valley areas of L.A./VTU Counties with the possible exception of the Santa Clarita Valley and some of the interior valley locations of VTU County. There is even a chance that low clouds will push into southeastern coastal sections of SBA County, but that remains in doubt given the strong offshore N-S gradients across SBA County. N of Pt. Conception, skies were clear. Some patchy low clouds and fog may form in southern portions of the Central Coast and the Santa Ynez Valley by morning. Any low clouds N of Pt Conception should dissipate quickly this morning. Across VTU/L.A. Counties, clouds will clear more slowly today, generally by mid to late morning in the valleys and by early afternoon across the coastal plain. High res models suggest that clouds could linger at the beaches today, which is entirely possible given the strength of the onshore gradient between KLAX and KDAG both currently, and during this afternoon. Early this morning, strong N-S gradients continued to produce strong NW-N winds across much of southwestern SBA County, the mountains of interior SBA County eastward thru the mtns of L.A. County, including the I-5 Corridor, in the Antelope Valley foothills, and in the Santa Clarita Valley. There were still some local High Wind Warning levels gusts in the mtns and thru the I-5 Corridor, but winds were mostly at advisory levels in these areas. Winds had dropped below advisory levels on the Central Coast and in much of the Antelope Valley. An unseasonably strong and cold upper low was moving thru eastern Oregon early this morning and it will push into Idaho and western Montana later today. A sharpening trough axis extending S from this low will push across the area today. Behind this trough, NW flow aloft will increase and there will be increasing subsidence across the area. In addition, there will be some weak cold advection. This should translate to an increase in northwesterly winds across the forecast area this afternoon/evening. Given this expected bump up in winds again this afternoon/evening, will keep the warnings and advisories as is. With lowering heights and thicknesses, increasing onshore gradients between KLAX and KDAG, and lots of low clouds this morning, expect several degrees of cooling in most areas today. The exception will be across southern SBA County, where gusty north winds may bring a few degrees of warming late this afternoon and early this evening. Low level flow will turn more northeasterly across L.A./VTU Counties late tonight, with gradients between KLAX and KDAG actually turning weakly offshore. N-S gradients across SBA County will begin to weaken tonight. This should cause the NW winds to drop below advisory/warning levels in most areas by late tonight. NE flow at 850 mb will actually briefly increase to 40 to 45 kt across portions of L.A. County/eastern VTU County Tue morning, with a corresponding increase in surface winds, similar to a Santa Ana wind event. Expect a period of strong to possibly advisory level NE winds in the mtns of L.A. County and possibly eastern Ventura County Tue morning. In coastal and valley areas, the forecast is more challenging, and it all depends on the marine layer tonight. At this point, it is expected that low clouds will push into coastal and at least the lower valley areas of L.A./VTU Counties. Where there are low clouds, the winds would have to erode the stubborn marine layer to allow them to surface, which is no small task. Therefore, it will be difficult for the northeast winds to surface in coastal and lower valley areas of L.A./VTU Counties. This will be most likely to occur in the valleys of Ventura County and northern/western L.A. County for a few hours Tue morning. That is far from a sure thing, given how unusual this would be for mid June. But there is still a chance that it will happen, and it will have to be watched closely. Given the offshore gradients, do expect much faster clearing of any low clouds Tue. Max temps should jump up several degrees in most areas Tue, especially in the valleys of L.A./VTU Counties. If winds do actually surface, it will likely be warmer than currently forecast on Tue in coastal and valley areas of L.A./VTU Counties. Expect little change in max temps across SLO and SBA Counties. A broad trough will sag southward thru the West Coast Tue night and Wed. N-S gradients should still be strong enough for some gusty winds during the late afternoon thru late evening hours across SBA County Tue/Tue night, but much there should be much less in the way of wind across L.A./VTU Counties. Expect more widespread night thru morning low clouds and fog in coastal and valley areas Tue night/Wed morning, with low clouds likely returning to areas N of Pt. Conception as well. Max temps should be down several degrees in most coastal and valley areas south of Pt Conception Wed, with somewhat less cooling elsewhere. .LONG TERM (THU-SUN)...17/507 AM. The broad upper level trough will linger across the region into Thu. Expect widespread night thru morning low clouds and fog in most coastal/valley areas Wed night/Thu. There may be slight warming Thu, mainly across the interior, with max temps within a few degrees of normal in most areas. Heights will gradually rise Fri and Sat as a large upper high over the south central U.S. expands westward into the forecast area. The marine layer will shrink in depth, with night thru morning low clouds and fog likely squeezed out of most valley areas by Fri, or at the latest Sat. Max temps should rise a few degrees both Fri and Sat, especially away from the immediate coast, and should be several degrees above normal in most areas by Sat. Temps could approach 105 degrees in the Antelope Valley, and rise well into the 90s to near 100 degrees in the valleys west of the mtns. As the marine layer becomes more shallow late this week into the weekend, the marine inversion will become stronger, which may make for slower clearing near the coast, minimizing any warming there. Expect mostly minor changes in the marine layer depth Sat night/Sun as the upper level pattern remains rather stagnant. An increase in onshore flow may bring slight cooling to coastal areas Sun, but otherwise, expect little change. && .AVIATION...17/1803Z. At 17Z at KLAX, the marine layer was 2500 feet deep. The top of the inversion was at 5400 ft with a temperature of 18 deg C. Overall, moderate to high confidence in the 18Z TAF package. For most sites, VFR conds are anticipated through the period. However for KSMO, KLAX, and KLGB, cig restrictions to MVFR will be possible until around 20Z today for KSMO and KLGB, then sometime between 05Z-18Z tonight and Tue for all three airfields. However, there is also a 20% chance of MVFR cigs developing tonight at KBUR. The timing of the onset and any dissipation of the low clouds may be off +/- an hour or two. Gusty W to NW winds are expected this afternoon into this evening for KSBP, KSMX, KPMD and KWJF, strongest in the deserts where gusts over 40 knots are possible along with a 20% chance of restricted vsbys due to blowing dust. KLAX...Moderate to hi confidence in the 18Z TAF. VFR conds will prevail into this evening. For tonight, moderate to hi confidence in return of MVFR cigs between about 06Z and 20Z. The timing of the onset and dissipation of the low clouds may be off +/- an hour or two. An east wind component tonight up to 6 knots is expected from about 06Z-17Z. KBUR...Moderate to high confidence in the 18Z TAF. VFR conds are expected thru the TAF period, except for a 20% chance of MVFR cigs developing tonight aft about 07Z and linger into early Tue morning. && .MARINE...17/824 AM. For the Outer Waters, high confidence in current forecast. Winds are expected to remain at Gale force levels into late tonight. Seas will peak today around 10 to 13 feet. For Tuesday through Friday night, winds will subside, but remain at Small Craft Advisory (SCA) levels. Seas drop below 10 feet Tuesday night. For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. SCA level winds will persist through tonight, and seas will peak today at around 9 to 12 feet. Then Tuesday and Wednesday, there is a 60-80% chance of SCA level winds, in the afternoons and evenings. For Thursday through Friday night, winds and seas should remain below SCA levels. For the Inner Waters south of Point Conception, moderate to high confidence in current forecast. Noon today through this evening, Gale Force winds are likely for the western Santa Barbara Channel. Tuesday and Wednesday, there will be a 60-80% chance of SCA level winds across the western half of the Santa Barbara Channel, during the afternoons and evenings. South of the SBA Channel, there is a 50-70% chance of SCA level winds for the western and northern portions of the inner waters, this afternoon into evening. Otherwise and elsewhere, SCA conditions are not expected. && .BEACHES...17/824 AM. Short-period wind waves, due to gale force winds over the nearby coastal waters, will generate elevated-to-high surf conditions and dangerous rip currents across the beaches through early this evening. West and northwest facing beaches will see the highest surf. && .FIRE WEATHER...17/1006 AM. The Red Flag Warning for gusty northwest to north winds and low relative humidity for the Interstate-5 corridor in northwest Los Angeles County and the Ventura County mountains is now in effect until 6 PM PDT Tuesday, and includes an expected wind shift to the northeasterly direction between midnight and 6 AM PDT Tuesday. In these areas -- after northwest to north winds reached the 45-60 mph range last night, with isolated locales to around 65 mph, northwest to north winds will gust to 35 to 55 mph for the rest of today, increasing up to 60 mph tonight. Between Midnight tonight and 6 AM PDT Tuesday, winds will shift to the north to northeast and gust 40 to 60 mph and then gradually decrease to 25 to 45 mph on Tuesday. For today, minimum afternoon relative humidity values are expected to range from 15 to 25 percent, locally as low as 10 percent in downslope-flow favored areas. Only poor to moderate overnight recovery to around 25 to 45 percent is expected tonight. Relative humidity will fall to the single digit readings through the day Tuesday, as a very dry air mass overspreads the region. In addition, the Red Flag Warning has been extended in areal coverage to include the western San Gabriel Mountains and Highway 14 corridor from Midnight tonight to 6 PM PDT Tuesday, where gusty northeast winds and relative humidity falling to the single digit readings are expected. In these areas -- after occasionally gusty northwest to north winds through this evening, winds will shift to the northeast after midnight with speeds of 20 to 30 mph and gusts to 45 mph, locally 55 mph. Relative humidity is expected to be 20 to 45 percent tonight falling to 5 to 10 percent Tuesday. While live fuel moisture readings are still quite high, ample fuel loading of dead fuels is likely contributing to the extreme fire behavior with the Post Fire complex, and increasingly favorable meteorological conditions are expected to foster further growth and intensification of this fire complex or any other fires developing in the Red Flag Warning areas. After 6 PM PDT Tuesday, decreasing winds and relative humidity recovery Tuesday night will decrease the critical fire-weather risk. Elsewhere across the region, the wind shift to the northeast tonight into early Tuesday morning will come with the passage of a midlevel trough to the north of the forecast area, and also with the continued consolidation and deepening of a Catalina Eddy. The same Catalina Eddy driving this Santa Ana wind-type pattern will also draw a thickening marine layer northward across the coasts and coastal valleys including much of the LA Basin. This marine layer will greatly temper relative humidity reductions at the lower elevations, possibly extending as far as the Santa Monica and Santa Susana Mountains and especially the nearby foothills, and the Santa Clarita Valley. As a result, there is a lot of uncertainty regarding how far the northeasterly wind surge will extend off the San Gabriels and the Ventura County Mountains. Nevertheless, the potential will exist for this wind surge and accompanying deep mixing with very low relative humidity to bring critical fire-weather conditions much farther toward the coast, in a Santa Ana wind-type pattern. Areas south of the western San Gabriels and Ventura County Mountains toward the coast will be closely monitored for possible expansions of Red Flag Warnings (30% chance of further extension), and elevated to brief critical conditions will be a definitive possibility in these areas (60-80% chance). Elsewhere across the region, the combination of gusty winds, warm temperatures, and low relative humidity will continue to bring elevated fire-weather conditions across the Santa Barbara County interior mountains today and Tuesday. Areawide on Wednesday, weakening winds will be lessening the fire-weather risk while dry conditions persist across the interior. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement in effect until 7 PM PDT this evening for zones 87-340-346-362-366. (See LAXCFWLOX). Wind Advisory remains in effect until 3 AM PDT Tuesday for zones 88-340-341-346-347-352-353-356-383. (See LAXNPWLOX). High Wind Warning remains in effect until 3 AM PDT Tuesday for zones 349-351-376>378-381. (See LAXNPWLOX). High Surf Advisory in effect until 7 PM PDT this evening for zone 354. (See LAXCFWLOX). Wind Advisory in effect from midnight tonight to 6 PM PDT Tuesday for zone 379. (See LAXNPWLOX). Red Flag Warning in effect until 6 PM PDT Tuesday for zones 376>378. (See LAXRFWLOX). Red Flag Warning in effect from midnight tonight to 6 PM PDT Tuesday for zone 379. (See LAXRFWLOX). PZ...Small Craft Advisory in effect until 3 AM PDT Tuesday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until noon PDT today for zone 650. (See LAXMWWLOX). Gale Warning in effect until 9 PM PDT this evening for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect from 3 PM this afternoon to 9 PM PDT this evening for zone 655. (See LAXMWWLOX). Gale Warning in effect until 3 AM PDT Tuesday for zones 670-673-676. (See LAXMWWLOX). && $$ PUBLIC...Cohen/DB AVIATION...Sirard MARINE...Sirard/Schoenfeld BEACHES...Sirard/Schoenfeld FIRE...Cohen/Gomberg SYNOPSIS...Cohen/RS weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox