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Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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017 FXUS66 KLOX 170627 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 1127 PM PDT Tue Jul 16 2024 .SYNOPSIS...16/137 PM. Another heatwave is expected to impact Southern California for Friday through this weekend and into next week. The most dangerous heat is expected over the interior, though very warm conditions may extend toward the coast. Monsoonal moisture will increase across the region this weekend and next week, which will have the potential to bring thunderstorms over the interior mountains and valleys. Also, gusty Sundowner winds will affect portions of southern Santa Barbara County during the evening and overnight hours through Wednesday night. && .SHORT TERM (TUE-FRI)...16/824 PM. ***UPDATE*** Sunny skies dominated the day today with only some flat topped CU over the mtns in the afternoon. Max temps were mostly 2 to 4 degrees blo normal except for the Antelope Vly where they were about 4 degrees warmer than normal. Despite a 2 mb onshore trend to the east low clouds are slow to form. It is likely that the clouds will form after midnight over the Central Coast/Santa Ynez Vly and the LA south coasts. Gusty near advisory level winds have developed across the western half of the SBA south coast. The N to S gradient is a little stronger tomorrow and advisories will be likely for that area Wednesday evening. The early 00Z mdls continue to show that the forecasted warm up is on track starting tomorrow. The forecast was updated to reduce the evening clouds and tweak Wednesday`s max temps. ***From Previous Discussion*** For the rest of this week, an upper trough off the Pacific Northwest coast will be repeatedly reinforced by the progression of a series of deep lows coming from the Gulf of Alaska -- staying well to the north of the local area. This will have a significant influence on the evolution of downstream sub-tropical ridging, causing the persistent midlevel anticyclone over the western states to build and gradually elongate northward across the Intermountain West from the central Great Basin and sharpen with northward extent into adjacent western Canada. In response to these developments, heat will continue to build over interior areas for the remainder of this week, with related impacts becoming increasingly significant by the end of this week. Temperatures are expected to rise by at least a couple of degrees each day through the remainder of this week. By Friday, HeatRisk is likely to become Major for the interior mountains and valleys, as well as the Santa Ynez Range and the Santa Monica Mountains, as high temperatures reach 100-110 degrees -- highest across the Antelope Valley. There is at least an 80% chance that these conditions warrant heat headlines, starting on Friday. One element of uncertainty regarding the degree of warming, and onset timing of most significant HeatRisk, is the influx of rich monsoonal moisture on the western flank of the aforementioned midlevel ridge. Present indications are that re-orientation of the ridge aloft over greater latitudinal expanse will favor a moisture influx from a source region deeper into the sub-tropics. Thus, with the onset of more significant heat late in the week, the influx of this moisture could have a tendency of limiting the degree of surface heating -- aided by any convection or convective debris clouds that move across the region. These factors cast uncertainty on the onset of more significant heat. An increasingly shallow marine layer closer to the coast will limit the degree of warming there, with temperatures at the beaches only expected to gradually rise through the 70s from day- to-day through the rest of this week. Patchy night and morning dense fog will be a possibility with the increasingly shallow marine layer late this week. However, with marine-layer depths decreasing in response to rising heights aloft, the spatial extent of the building heat will increase from day to day, with temperatures by Friday expected to rise well into the 80s in many coastal valleys, and a few 90s near the foothills. Regarding the potential for convection, thunderstorm chances are currently below 20% through the end of this week. However, minor convection-induced perturbations embedded in the long fetch of monsoonal moisture aloft may have the potential to bring isolated showers and thunderstorms by Thursday or Friday to the San Gabriels and Antelope Valley. These chances will be re-evaluated in subsequent forecasts as confidence grows regarding the mesoscale details of any such atmospheric perturbations, which are currently accompanied by very low predictability. Through the rest of this week, winds will primarily be light to moderate across the region and diurnally driven. However, a small area of enhanced flow will be focused across Sundowner-wind- favored locations in southwest Santa Barbara County late today into tonight and again late Wednesday into Wednesday night. Areas affected include the western Santa Ynez Range and western parts of the Santa Barbara County South Coast. Despite lacking upper support, enhanced surface ridging over the Pacific waters will favor offshore Santa Barbara-Santa Maria pressure gradients around 2-3 mb through mid-week. Local gusts may reach 45 mph late today into tonight, with a 20% chance for Wind Advisory issuance. Slightly more pronounced offshore gradients late Wednesday into Wednesday night correspond to a higher probability of Wind Advisory issuance then (40% chance). These offshore pressure gradients are expected to weaken by late week. .LONG TERM (SAT-TUE)...16/137 PM. The potentially dangerous heatwave will be ongoing this weekend and continue into next week, as heights aloft build along the north- south-oriented expansive upper ridge over the western states. Heights at 500 mb are forecast to reach around 596 dam from central California to central Nevada this weekend and into early next week. This pattern will prolong the heatwave into next week, with daily high temperatures forecast to reach or slightly exceed 110 degrees across the Antelope Valley starting this weekend, while reaching 100-105 in most interior mountains, valleys, and foothills. These conditions will likely result in Major to Extreme Heat Risk, and there is at least an 80% chance for heat headlines to be issued continuing into next week. A continuing element of uncertainty will be the potential for rich monsoonal moisture over the area to limit the degree of heating. Such uncertainty mainly applies to the maximum degree of warming and just how extreme this heatwave becomes. Regardless, this will likely be a significant heatwave, and those susceptible to heat impacts should consider preparing for dangerous heat. In addition, closer to the coast, patchy dense fog will be a possibility with the increasingly shallow marine layer this weekend into next week. Similar to late this week, the areal extent of significant heat will continue increasing this weekend into next week. High temperatures each day this weekend into next week are expected to be in the 80s in many coastal valleys, with temperatures rising well into the 90s in some of these areas -- especially closer to the foothills. Present indications are that the gradual influx of rich deep-layer moisture will continue through the weekend and next week. As moisture continues to stream into the region from the deeper sub- tropics, precipitable water values may be exceeding 1 inch in most areas by early next week -- perhaps upwards of 1.25-1.50 inches in Los Angeles and Ventura Counties. While the overall upper pattern may not be particularly conducive for significant coverage of thunderstorms, the combination of the substantially increased moisture and ample heating resulting in terrain-driven ascent will have the potential to conditionally favor strong thunderstorms and locally heavy rains. Confidence is very low in monsoonal thunderstorms developing, and forecast chances for such activity are presently less than 20%. Regardless, the low- predictability potential for increased thunderstorm impacts does exist from this weekend into next week over the higher terrain and interior valleys of Los Angeles and Ventura Counties -- perhaps even extending into Santa Barbara County by early next week. The conditional potential for strong, erratic outflow winds causing rapid spread rates of fires, lightning-induced fire ignitions, and flash flooding from localized intense rainfall rates will exist -- if thunderstorms were to develop (less than 20% chance). && .AVIATION...17/0626Z. At 0531Z at KLAX, the marine layer was 600 feet deep. The top of the inversion was at 4000 feet with a temperature of 27 deg C. High confidence in TAFs for KSBA, KBUR, KVNY, KPMD, and KWJF. Low confidence in coastal TAFs through 18Z, then high confidence. Flight cat changes may be off by 1-3 hours. There is a a 20% chance that KCMA and a 30% chance KSMO, KLAX, and KOXR become IFR 12Z-17Z. There is a 40% chance that KLGB remains VFR through the period. There is a 10% chance for brief IFR cigs at KPRB between 12Z-16Z. There is a 30% chance that KSBP remains VFR through the period, and a 10% chance KSMX and KSBP become VLIFR due to dense fog. KLAX...Low confidence in TAF through 18Z, then high confidence through 06Z, then low confidence afterward. Low confidence in cig arrival time, which may be as late as 12Z. 30% chance of flight cat becoming BKN006-008 after 08Z, lifting to BKN010-015 MVFR between 13Z-17Z. Good confidence in clearing by 18Z. There is a 30% chance of BKN008-010 cigs after 09Z Thurs. No significant east wind component expected. KBUR...High confidence in TAF. && .MARINE...16/824 PM. For the outer waters, high confidence in current Small Craft Advisory (SCA) for the waters south of Point Sal tonight. Expecting SCA level winds to spread throughout the outer waters Wednesday, with somewhat lower confidence for waters north of Point Sal. High confidence for SCA conditions continuing through at least early next week, peaking Friday and Saturday. There is a 30 percent chance of SCA conditions for the nearshore Central Coast waters each afternoon and evening through Thursday, with chances increase Friday and Saturday for winds and steep seas. For the Santa Barbara Channel, current SCA looks on track tonight due to steep, choppy seas. Winds are somewhat below SCA criteria. Wednesday afternoon may need a short-lived SCA with continued choppy seas, and winds may be borderline SCA levels again. By Thursday, winds appear to be more northerly and SCA level winds will struggle to push into eastern portions of the Channel. There is still a chance (20-40%) for SCA conditions each afternoon/evening Friday through the weekend. For the nearshore waters off of Los Angeles and Orange Counties, the chance for SCA conditions is less than 20 percent through most of the week, although local gusts to 20-25 knots are possible in the San Pedro Channel Wednesday afternoon to evening. Choppy seas will be building each day as well. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Beach Hazards Statement in effect through Thursday evening for zones 354-362-366. (See LAXCFWLOX). PZ...Small Craft Advisory in effect until 3 AM PDT Wednesday for zone 650. (See LAXMWWLOX). Small Craft Advisory in effect from 9 AM Wednesday to 6 AM PDT Thursday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect until 6 AM PDT Thursday for zones 673-676. (See LAXMWWLOX). && $$ PUBLIC...Cohen/Rorke AVIATION...Lund MARINE...Phillips/Kittell SYNOPSIS...Cohen weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox