Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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534 FXUS66 KLOX 180636 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1136 PM PDT Mon Jun 17 2024 .SYNOPSIS...17/240 PM. Strong northwest to north winds will impact the Central Coast through interior sections of Santa Barbara County to the Antelope Valley through this evening. Later tonight, a surge of northeasterly winds will spread across the Los Angeles County mountains and adjacent eastern Ventura County, and potentially through the Santa Monica Mountains. Critical fire-weather conditions are expected across the Interstate-5 corridor of northwest Los Angeles County and the Ventura County mountains, as well as the western San Gabriels, through tomorrow. Temperatures will be generally within a few degrees of normal readings this week, before temperatures across the interior warms considerably this upcoming weekend. && .SHORT TERM (MON-THU)...17/831 PM. ***UPDATE*** Satellite continued to indicate very limited fire activity near the Post Fire area in northwest Los Angeles County into far northeast Ventura County. This was likely due to elevated humidity through the day compared to yesterday. Mountain wave clouds have formed in the lee of the southern Sierras into portions of southwest CA and are indicative on the stable descending air on the backside of an unseasonably strong trough across the region. The well advertised uptick in winds is already underway with Sandberg already gusting to near 60 mph last hour and and handful of sites focused near the I-5 corridor in the mountains into the western Antelope Valley and near the Santa Ynez Range gusting in the 30s and 40s to near 50 mph. The latest guidance continues to support winds these areas peaking through around midnight before dropping off through early Tuesday morning. The current wind products look well placed and timed. Very challenging low cloud forecast tonight as a strong eddy challenges the increasing northerly flow. Best bet for low clouds is across the LA Basin where some clouds have already banked up against the foothills. Much lower confidence exists the further north you go. ***From Previous Discussion*** Satellite loops indicate a deep cyclone crossing the northern Rockies, with unseasonably high-amplitude troughing extending southward into Southern California. A shortwave impulse advancing through the base of the trough will glance the region tonight, reinforcing ample upper support for strong winds. The passage of the impulse will also facilitate a wind shift from the northwest and north to the northeast overnight tonight over the higher terrain of LA and Ventura Counties, before a drier air mass overspreads the interior on Tuesday. Wind gusts are expected to increase to around 60 mph once again across High Wind Warning areas (Ventura County mountains eastward to the western Antelope Valley Foothills including the Interstate-5 corridor of northwest Los Angeles County, as well as the southwest Santa Barbara County mountains and coast). Surrounding Wind Advisories are in effect for many areas from the Antelope Valley westward to the Central Coast, where gusts of 35-55 mph are expected. Winds will gradually taper off during the day Tuesday. Of note, the southward mass flux in the low levels, in response to the strengthening upper support, is expected to continue fostering the formation of a strong coastal/barrier northerly jet just offshore. As has been evidenced in longer-range visible satellite imagery, the interaction of cross-stream speed shear across the jet, with coastline curvature in the Southern California Bight, will continue supporting the development of a strong Catalina Eddy. The circulation around this eddy is expected to drive a Santa Ana wind-type response, which is also a major factor supporting the wind-shift to the northeast overnight tonight into early Tuesday morning. And with nocturnal drainage flow enhancements, the northeasterly wind surge will remain quite strong late tonight into Tuesday, with gusts upwards of 45-55 mph through Tuesday morning, tapering off during the day Tuesday. Moreover, the re-orienting surface pressure gradient will support the eastward spread of strong northeast wind gusts across the western San Gabriel Mountains, where a Wind Advisory has been issued for the northeast winds late tonight into Tuesday. The southern extent of the northeasterly wind surge remains in question, owing to the deepening marine layer being transported northward within the eastern semicircle of the Catalina Eddy. There is a 40% chance for wind headlines to be extended farther toward the coast across the Los Angeles County mountains and Santa Monicas including adjacent parts of Ventura County in later forecasts -- if the reinforcing marine layer were to be less widespread and/or deep. However, greater than 50-50 odds are present for the marine layer to extend well inland from the coast, causing the southern extent of the northeast wind surge to remain confined to the western San Gabriels. Low relative humidity will be combining with the strong winds to continue prolonging critical fire-weather conditions. A Red Flag Warning is in effect for the Interstate-5 corridor in northwest Los Angeles County and the Ventura County mountains until 6 PM PDT Tuesday. In addition, a Red Flag Warning is in effect for the western San Gabriel Mountains and Highway 14 corridor from Midnight tonight to 6 PM PDT Tuesday. Additional information is available in the Fire Weather section below, as well as the Fire Weather Planning Forecast. Also, smoke from the Post Fire complex will affect a large portion of LA and Ventura Counties and will bring air-quality impacts to many areas. Please reference the Air Quality Alert Message for additional information. By Tuesday night and into Wednesday, the midlevel height gradient is expected to weaken while broad upper troughing persists across the region. Correspondingly, low level winds across the region are forecast to begin a weakening trend, and present indications are that wind headlines will be unlikely after Tuesday in most areas. However, Sundowner winds over southern Santa Barbara County could necessitate additional Wind Advisories (40-60% chance) With the reinforced upper troughing persisting over the region through mid-week, temperatures are not expected to be as warm across the region as they were this past weekend. In addition, surface pressure gradients at the larger scale will be gradually turning more onshore through early to mid week. This will favor deepening of the marine layer with increasing coverage of marine stratus and fog, and perhaps night/morning drizzle, aided by persistence of eddies just off shore. This will result in additional cooling for the coasts and coastal valleys. Temperatures through Thursday are generally expected to be within a few degrees of normal, mostly in the 70s except upper 60s at the beaches and in the 80s and 90s over interior valleys and foothills. .LONG TERM (FRI-MON)...17/240 PM. The axis of broad mid and upper troughing will shift slightly offshore for Friday, as small-scale impulses pivoting through the surrounding cyclonic flow reposition its primary axis. This will favor continued deepening of the marine layer, with abundant night and morning marine stratus and fog over the coasts and coastal valleys. As a subtropical ridge builds westward from the south- central CONUS for late in the week into next weekend, larger-scale onshore pressure gradients will substantially strengthen. This will reinforce low clouds and fog at the coast while supporting stronger heating over the interior. With the midlevel ridging, high temperatures are forecast to reach the 90s to around 100 degrees over many interior valleys, and upwards of 105 degrees in the Antelope Valley for the upcoming weekend. In addition, as midlevel heights rise during the upcoming weekend, more substantial diurnal clearing may occur over inland areas away from the coast owing to increasingly shallow marine- layer depths. This could expand the areal coverage of very warm to hot temperatures closer toward the coast next weekend. There appears to be a 10-30% chance for heat-related headlines to be issued for the upcoming weekend into early next week. && .AVIATION...18/0636Z. At 0515Z at KLAX, the marine layer was 1600 feet deep. The top of the inversion was at 2000 ft with a temperature of 16 deg C. Good confidence in TAFs for KSBP, KSBP, KPMD, KWJF and KPRB. At KPMD and KWJF, brief restrictions due to BLDU or FU is possible. Moderate confidence in TAFs for KSMO, KLAX and KLGB. There is a 30 percent chc of rapid clear before 13Z. Moderate Confidence in TAFs for KCMA and KSBA with a 25 percent chc of IFR cigs 12Z-17Z. Low confidence in TAFs for KBUR, KVNY and KOXR with a 40 percent chc of IFR cigs through 16Z. VFR or MVFR FU or HZ is possible from KSBA and south. Weak to moderate LLWS is possible through 18Z at KBUR and KSBA. KLAX...Moderate confidence in TAF. There is a 30 percent chc of clearing before 13Z. Low cloud returns Tuesday evening may be as late as 08Z. There is a 10 percent chance of an east wind component of 8 kts between 10-16Z. KBUR...Low confidence in TAF. There is a 40 percent chc of BKN008 conds 10Z-16Z. Lgt LLWS is possible through 18Z. There is a 20 percent chc of 01015KT winds from 13Z to 18Z. && .MARINE...17/957 PM. For the Outer Waters, high confidence in current forecast. Winds are expected to remain at or near Gale force levels into late tonight. For Tuesday through Friday, winds will subside, but remain at Small Craft Advisory (SCA) levels. Seas have already fallen below 10 feet for most of the waters, but will continue to trend down Tuesday. Conds are then expected to below SCA levels Friday night through Saturday night. For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. SCA level winds will persist through Thursday night, strongest afternoons and evenings. For Friday through Saturday night, winds and seas should remain below SCA levels. For the Inner Waters south of Point Conception, moderate confidence in current forecast. Expecting SCA level winds over western portions of the Santa Barbara Channel tonight, before dropping off overnight. For Tuesday through Thursday, there will be a 60% 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 30% chance for SCA winds Thursday evening. Otherwise, winds and seas will generally be below SCA levels. && .BEACHES...17/121 PM. High surf is no longer expected for the Ventura County beaches. However, elevated surf conditions and dangerous rip currents will still continue for some west and northwest facing beaches of southwest California through this evening. && .FIRE WEATHER...17/240 PM. 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 continue gusting 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. With some increase in midlevel moisture, minimum afternoon relative humidity values today will be slightly elevated compared to yesterday -- generally ranging 20-35 percent, today, and locally as low as 15 percent in downslope-flow favored areas. Only poor to moderate overnight recovery to around 25 to 45 percent is expected tonight. Late tonight into Tuesday, relative humidity will quickly fall as very dry air overspreads the region behind a passing midlevel impulse, reaching the single digit readings during the day 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. 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 20-45 percent tonight falling to 5 to 10 percent Tuesday. While live fuel moisture readings are still quite high, ample fuel loading of dead fuels has likely contributed to extreme fire behavior with the Post Fire complex, and increasingly favorable meteorological conditions may 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 the impulse, 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...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). Wind Advisory remains in effect until 6 PM PDT Tuesday for zone 379. (See LAXNPWLOX). Red Flag Warning in effect until 6 PM PDT Tuesday for zones 376>379. (See LAXRFWLOX). PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 2 AM PDT Tuesday for zone 650. (See LAXMWWLOX). Gale Warning in effect until 3 AM PDT Tuesday for zones 670-673-676. (See LAXMWWLOX). && $$ PUBLIC...Munroe/Cohen AVIATION...Rorke MARINE...Phillips/Sirard BEACHES...Sirard FIRE...Cohen/Gomberg SYNOPSIS...Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox