Area Forecast Discussion
Issued by NWS Los Angeles, CA
Issued by NWS Los Angeles, CA
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039 FXUS66 KLOX 191158 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 458 AM PDT Thu Sep 19 2024 .SYNOPSIS...19/336 AM. An unsettled weather pattern will develop across the region through tonight as an early season low pressure system moves over the region through Friday. A cool air mass will remain in place through Friday, but turn more showery through tonight. While not every location will see showers and thunderstorms, any shower activity will be capable of producing heavy rainfall, especially across interior portions of Ventura, Santa Barbara, and San Luis Obispo Counties through this evening. Some showers or thunderstorms could linger into Friday before the low pressure area and the associated air mass exits the region. A warming and drying trend should develop over the weekend and into early next week. && .SHORT TERM (TDY-SAT)...19/455 AM. The latest satellite imagery shows a vigorous early season trough off the central California coast early this morning. This trough, located near 35N and 122W, or about 80 miles west of Point Conception will wobble around the Point Conception area through this afternoon while digging slowly southeast. Shower activity is already developing across interior San Luis Obispo County this morning, while a weaker band is currently located just offshore closer to the trough. Isolated thunderstorms cannot be ruled out for northern San Luis Obispo County this morning as the most favorable lifted index values will push along the San Luis Obispo-Monterey County border this morning. A few lightning strikes have been observed to the north in Monterey County. Shower and thunderstorm activity should become more expansive as the day progresses and daytime heating steepen lapses rates and destabilizes the atmosphere. PoPs mostly remain untouched through this afternoon with only a few minor tweaks, but PoPs were increased and a slight chance of thunderstorms were added to the forecast tonight through Friday morning as the 500 mb cold pocket will move across the southern California bight tonight through Friday morning. High-resolution multi-model ensemble members keep the highest probability of rain across the interior portions across San Luis Obispo, Santa Barbara, and Ventura Counties this afternoon and tonight. A flood watch remains in effect for these areas where the highest probability of the heaviest rainfall exists and includes Hurricane and Apache burn scars. To south, low clouds and fog are filling in across the Southland and into southern California bight and Santa Barbara Channel, but low confidence exists in the cloud forecast as middle and high level clouds streaming over the region are disrupting the marine layer stratus formation. Where there are clouds, the instability with the trough will likely produce patchy drizzle this morning as the instability interacts with the deep marine layer depth in place. The latest AMDAR soundings from KLAX indicate a 3200 foot deep marine layer depth in place. Clouds should continue to fill in across the Southland as the trough position should promote low-level moisture to increase. A cooler air mass and unsettled air mass will remain in place for the next couple of days as the trough center slowly drifts southeast across the area, coming ashore in Orange County on Friday morning. Shower chances linger in the forecast into Friday. It should be noted that the trough position is favorable for wrap-around shower activity across the interior areas into Friday. PoPs have been extended into Friday for this reason. A breezy afternoon looks on tap for Friday as the surface pattern tightens some. It appears to be marginal for any wind advisories, but a wind advisory cannot ruled out across southern Santa Barbara County on Friday evening. A drying and warming trend should begin on Saturday as high pressure aloft starts to build into the region. Some low clouds and fog could remain across the region, but less low cloud coverage should be expected into the weekend. .LONG TERM (SUN-WED)...19/456 AM. High pressure aloft will build into the region for early next week and bring a warming trend. The latest temperature forecast breaks away from NBM values for Sunday and Monday when high pressure and onshore flow weaken. Potentially, weak offshore flow could develop across the region and make temperatures several degrees warmer than indicated. EPS cloud cover means indicate minimal clouds for early next week during the night through morning period, thus marine layer induced low clouds and fog coverage could be less in coverage for Sunday and Monday than what the current forecast alludes to. Some cooling should develop for the latter half of next week as another trough will attempt to dig south. This trough is not expected to be as strong as the current one and will only increase onshore flow and make the low cloud field a bit more expansive. There is quite a spread in the forecast ensembles for late week as there are plenty of solutions offering warmer solutions. The forecast trends the way of the majority of the cooler solutions with more low clouds and fog. && .AVIATION...19/0747Z. At 06Z at KLAX, there was a 5000 ft deep moist layer and no inversion. Good confidence in TAFs for KPMD and KWJF. Low to moderate confidence in the remainder of the TAFs. Cigs heights will vary frequently due to presence of upper level low moving over the region. For KPRB, KSMX, KSBP, and KSBA -DZ is possible through the morning. Then there is a 20% chance of TSTMs with brief heavy rain and gusty and erratic winds 17Z through the period. Timing of flight cat changes and rain timing could be off by +/- 3 hours. For Ventura and Los Angeles County TAFs (except desert TAFs), Cigs will vary between MVFR and VFR through 16Z, then lift to VFR. -DZ is possible at all sites through the morning due to the marine layer rapidly deepening. There is a 10% chance of isolated TSTMs at KOXR and KCMA after 00Z Fri, and a 20% chance of VCSH at all sites after 04Z. There is a 20% chance of MVFR cigs at L.A. sites after 04Z Fri. KLAX...Low to moderate confidence in TAF. Cigs may vary between BKN015-035 frequently through 17Z as upper level clouds move overhead. -DZ is possible through 17Z Thurs. There is a 20% chance of BKN025-030 cigs after 04Z Fri. VCSH could occur as early as 06Z. No significant east wind component is expected. KBUR...Low to moderate confidence in TAF. There is a 20% chance of BKN022-030 conds through 15Z Thurs and after 04Z Fri. && .MARINE...19/348 AM. Low confidence in the current forecast for the period. Higher confidence exists in the forecast for seas. Lower confidence in the current forecast for winds and thunderstorms. Isolated to scattered thunderstorms will be possible across all of the coastal waters this afternoon through at least this evening as an upper-level low pressure system drops from Northern California to near Pt. Conception by the afternoon. The trough will move southeastward across the southern California bight tonight and onshore over southern California through Friday. A Marine Weather Statement is in effect to communicate the hazards from thunderstorms to the marine community. Winds and seas will likely remain below Small Craft Advisory (SCA) levels through the majority of the period. However, for the waters from Point Conception south to San Nicolas Island, in the vicinity of the Channel Islands, and across the western portion of the Santa Barbara Channel, there is a 30 percent chance of SCA level wind gusts this afternoon/evening, and a 40 percent chance Friday afternoon/evening. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Flood Watch in effect through this evening for zones 38-344-345-377. (See LAXFFALOX). PZ...NONE. && $$ PUBLIC...Hall AVIATION...Lund MARINE...Lund/Hall SYNOPSIS...Hall weather.gov/losangeles Experimental Graphical Hazardous Weather Outlook at: https://www.weather.gov/erh/ghwo?wfo=lox