Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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222 FXUS66 KLOX 011547 AFDLOX Area Forecast Discussion...UPDATED National Weather Service Los Angeles/Oxnard CA 847 AM PDT Wed May 1 2024 .SYNOPSIS...01/319 AM. Ridging aloft building into eastern Pacific Ocean will continue to bring a general warming trend through Thursday. The warming trend may be muted across the southern coastal areas as onshore flow strengthens slightly. Stronger troughing over the West Coast over the weekend will bring some cooling along with a chance of rain. Otherwise, night through morning low clouds and fog will continue at times, remaining mostly intact for the southern portion of the area. && .SHORT TERM (TDY-FRI)...01/846 AM. ***UPDATE*** Satellite imagery and surface observations indicate lower- elevation fog over Santa Barbara County dissipating, where the Dense Fog Advisory will expire at 9AM PDT today. Also, wind gusts are decreasing from the southern VTA County mountains to the Antelope Valley and vicinity, where the Wind Advisory will expire at 9AM PDT today. The forecast has been updated to account for the expiration of these advisories. Otherwise, the forecast is well on-track, and no other changes have been made to the ongoing forecast. ***From Previous Discussion*** The latest water vapor imagery shows a shortwave trough digging south in zonal flow this morning as ridging aloft is building across the eastern Pacific Ocean. Onshore flow is strengthening across the southern portion of the area due to an eddy circulation in place near San Nicolas Island, but some weakening is starting to take place north of Point Conception. A low cloud field is stretching from Santa Barbara to the southeast into the San Pedro Channel and pushing into the Los Angeles and Ventura County valleys this morning, but the cloud field is quite disorganized with breaks in the clouds across the area. A quite complex pattern is setting up over the next several days in the zonal flow. The latest forecast ensembles offer up a range of solutions with a wider spread as one heads south and farther inland. This will make for a difficult forecast over the coming days as offshore flow will wage a battle against the onshore regime. The most likely outcome will be the ridge aloft to the west asserting some influence for areas north of Point Conception. Onshore flow could end up being replaced by offshore flow on Thursday and bring some warming to San Luis Obispo and Santa Barbara Counties. To the south, onshore flow will likely stay wedged in. Although it will be less classical of a synoptic pattern for May, it is very plausible that onshore flow could strengthen. Any offshore push will not be strong enough to dislodge the eddy circulation off the coast. With the ridge aloft adding some heating to the interior areas outside the marine influence, onshore pressure gradients will likely increase through the period. With 500 mb heights increasing into Thursday and onshore pressure gradients strengthening, clouds could end hugging the beaches over the coming days. The latest tabular forecast guidance and EPS ensemble members buy into this idea and keep Los Angeles and Ventura coastal terminals near persistence over the coming days. The forecast keeps a mention of low clouds across the South Coast of California into Friday. By Friday morning, the marine layer induced low cloud field could end up becoming more entrenched as troughing along 160W ejects over the top of the ridge and drops down into the region. The marine layer depth should become more organized and deepen as onshore flow strengthens. The forecast ensembles agree that a cooling trend should take shape, establishing on Friday. .LONG TERM (SAT-TUE)...01/435 AM. The latest forecast ensemble agree a cooling trend will continue into Sunday as the semi-permanent Aleutian low pressure system kicks out an upper-level trough that will dig into the West Coast. EPS, GEFS, and CMC ensemble members are leaning more toward an inside slider type trough of low pressure to affect the area over the weekend. The latest PoPs break away from NBM values to emphasize the highest rain chances for the northern areas and along the northern slopes Saturday night through Sunday morning. Temperatures also break away from NBM values on Sunday to buy into better cooling, but EPS and GEFS ensemble means also lean toward Monday being equally as cool. Rainfall amounts will be light, on the order of a quarter of an inch or less, but there is a 15-20 percent chance that amounts could be higher than that in northwestern San Luis Obispo County, highest near Rocky Butte. Snow levels should be above 6000 feet with this system, but there is still a great amount of uncertainty with the colder air mass aloft. Any changes to the west, and the system could be wetter and more cold air could be transported into the region. The latest forecast ensembles give the Interstate 5 Corridor a 20 percent chance of seeing accumulating snowfall between Sunday and Sunday night. The main story could end up being the winds with this system as all of the EPS solutions suggest a gusty wind pattern developing across the mountains and desert and into some of the coastal areas, as well. Based upon the EPS wind gust means at terminals and the areal extent, the pattern seems to suggest gusty west to northwest winds on Saturday and Sunday, shifting to northwest to north into Monday. Zonal flow should redevelop on Monday and a warming trend is looking to establish for early next week. && .AVIATION...01/1236Z. At 12Z at KLAX, the marine layer was 2600 ft deep with an inversion top at 4500 ft with a temperature of 14 C. Low to moderate confidence in coastal TAF sites and L.A. Valley sites, with high confidence elsewhere. Cigs are starting to become patchy and scatter out, but could reform over the coastal/valley TAF sites through 18Z Weds. Otherwise, timing of flight cat changes could be off by +/- 2 hours. KLAX...Low to moderate confidence in TAF, however high confidence 19-05Z. Cigs may scatter out early this morning, but remain possible through 18Z. BKN010-015 conds possible as early as 06Z Thurs. There is a 30% chance of an east wind component reaching 7 kts through 18Z Wed, otherwise good confidence in any east wind component remaining less than 7 kt. KBUR...Low to moderate confidence in TAF. Timing of flight cat changes may be off by +/- 2 hours. && .MARINE...01/410 AM. For the outer waters, winds will likely drop below Gale levels this morning, but are expected to restrengthen by the afternoon, so decided to extend the Gale Warnings through late tonight. Swell will continue to be steep and choppy through Thursday night. After gales subside, Small Craft Advisory (SCA) conds are expected for the outer waters through Sunday. However, there is a 40-50% chance of gale force wind gusts again Thursday afternoon and evening, and again Sunday afternoon through evening (and possibly through much of next week). For the inner waters N of Pt. Sal, SCA level winds are likely through tonight. Then, there is a 60% chance of SCA winds during the afternoon and evening hours through Friday. Seas will start to diminish below advisory levels late tonight. For the Santa Barbara Channel, SCA level W-NW winds are expected across the extreme western portion this afternoon/evening, but not far enough into the channel to warrant issuing an advisory. Then, there is a 20-30% chance of SCA winds each afternoon/evening Thursday and Friday across the extreme western portion. Winds will likely remain below SCA levels for eastern portions of the channel through Friday night. Saturday afternoon through Sunday, there is a 40-50% chance of widespread W to NW SCA level winds across the entire inner waters, with a 20% chance of gale force wind gusts on Sunday. For the southern inner waters, there is a 20% chance of SCA level W to NW winds near Anacapa Island Wednesday evening. Otherwise, good confidence in conds remaining below SCA levels through Friday night. Saturday afternoon through Sunday, there is a 40-50% chance of widespread W to NW SCA level winds across the entire inner waters, with a 20% chance of gale force wind gusts on Sunday. && .BEACHES...01/1203 AM. Persistent strong winds and outer water swell heights of 10-15 feet with a 10 second period will lead to high surf along the Central Coast (7-11 feet) and Ventura County beaches (4-7 feet). Surf will be highest across west- and northwest-facing beaches. There is a 40% chance of high surf advisory criteria lingering through the evening. NO coastal flooding is expected. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...High Surf Advisory in effect until 2 PM PDT this afternoon for zones 340-346-354. (See LAXCFWLOX). PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for zone 645. (See LAXMWWLOX). Gale Warning in effect until 3 AM PDT Thursday for zones 670-673-676. (See LAXMWWLOX). && $$ PUBLIC...Hall/Cohen AVIATION...Lund MARINE...Lund/Phillips BEACHES...Lund SYNOPSIS...Hall weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox