Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000 FXUS66 KLOX 210440 AFDLOX AREA FORECAST DISCUSSION...UPDATED NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 940 PM PDT Wed Sep 20 2017 .SYNOPSIS...
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An upper level trough will persist over the forecast area through much of the weekend. A weak disturbance in the flow will move across the region bringing a chance for some light rain or drizzle across much of Los Angeles and Ventura Counties Thursday. High temperatures will lower significantly Thursday with gusty winds behind the the front. There will be a weak offshore flow developing by Saturday and persisting into next week. Expect a significant warming trend by late this weekend into next week.
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&& .SHORT TERM...
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(WED-SAT) The latest infrared imagery indicates an upper-level trough of low pressure along the Oregon coast. A cold front can be seen pushing south across Northern California. This front will move over the region Thursday and bring primarily a breezy to windy pattern with some shower possibilities. A marginal wind advisory was added for the Antelope Valley and for the Santa Barbara south coast and mountains tonight as winds should reach criteria. Winds will increase on Thursday and Thursday night, bringing high confidence in advisory level winds to these areas. A wind advisory may also need to be considered for the Central Coast and the Santa Ynez and San Luis Obispo County Valleys for Thursday afternoon and evening, if forecast guidance continues to fall inline with advisory criteria being met. There is a chance that advisory level wind criteria may also be met through the Interstate 5 Corridor on Thursday night and into Friday morning. Clouds will increase throughout tonight and into Thursday morning as the front advances on the region. Thursday is shaping up to be a mostly cloudy day for the Southland, clearing out as cold air advection pushes into the South Coast Basin. Pops have been nudged higher for the mountains and desert as jet dynamics and the trough positioning should interact to bring the best rain chances in these areas. A local mix of snow and rain cannot be ruled out on the higher mountain elevations as the first `fall` system rolls in. High temperatures on Thursday and overnight low temperatures on Thursday night have been cooled. The first freezing temperatures of the fall-winter season are forecast for the mountains on Thursday night. Further cooling will likely need to be introduced for Friday night, as well. *** FROM PREVIOUS DISCUSSION *** Over the weekend we`ll be transitioning, slowly, to offshore flow and warmer temps. There`s still a weak eddy circulation in place Saturday morning and with warmer air aloft the inversion should strengthen enough to allow low clouds to reform at least across LA County and possibly further north/west. Clouds may linger at the beaches well into the afternoon and inland highs will still be well below normal, but warmer than the previous couple days. .LONG TERM...(SUN-WED) Offshore gradients are expected to continue all of next week, mostly on the light side, generally under 3mb, but enough to push temps at least into the 80s for coastal areas and 90s for the valleys each day. There is potential for higher temps and stronger winds if gradients and upper support are stronger than what the models are showing now but as it stands now neither factor is really that impressive for most of the week. Tuesday appears to have the best upper support as a second shortwave drops into the mean trough over the Great Basin and brings a reinforcing shot of cold air and northeast winds aloft across southern CA. After that a ridge builds in from the west and the air mass warms up quite a bit with thicknesses climbing each day through Friday, up to 580dm and 950 temps near 30c. So we might see an initial spike in high temps Tuesday west/south of the mountains, then either little change or slight cooling Wednesday before warming again at the end of the week as the ridge builds.
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&& .AVIATION...
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21/0300Z. At 02Z, there was a double inversion at KLAX. The marine layer depth was around 1600 feet deep at KLAX, with deeper moist layer up to around 3800 feet. The top of the highest inversion was around 6100 feet with a temperature of 16 degrees Celsius. Low to moderate confidence in the current forecast. IFR to MVFR conditions will become predominant through 13Z. There is a chance of LIFR conditions at terminals north of Point Conception. There is a chance that conditions could improve sooner than forecast at terminals north of Point Conception. MVFR to VFR conditions at or below 5000 feet will likely linger into the afternoon hours on Thursday south of Point Conception. KLAX and KBUR...MVFR conditions will become predominant through 13Z. MVFR to VFR conditions at or below 5000 feet will likely linger into the afternoon hours on Thursday south of Point Conception.
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&& .MARINE...
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20/900 PM. Moderate to high confidence in the current forecast. Gale Warnings were converted to Small Craft Advisory for all remaining Outer Waters through late tonight before becoming warnings again early Thursday morning. At least Small Craft Advisory conditions will be prevalent by late Thursday afternoon. There is a 80 percent chance that gales for the nearshore waters north of Point Conception and through Santa Barbara Channel, with a 40 percent chance of gales for the Inner Waters south of Point Mugu to Dana Point. Steep short-period seas are likely on Thursday afternoon and evening across much of the waters.
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&& .LOX WATCHES/WARNINGS/ADVISORIES...
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CA...Wind Advisory in effect until 4 AM PDT Friday for zones 39-52. (See LAXNPWLOX). Wind Advisory in effect until 10 PM PDT Thursday for zone 59. (See LAXNPWLOX). PZ...Small Craft Advisory in effect until 3 PM PDT Thursday for zones 645-650. (See LAXMWWLOX). Gale Watch in effect from Thursday afternoon through Thursday evening for zones 645-650. (See LAXMWWLOX). Small Craft Advisory in effect from 9 AM Thursday to 3 AM PDT Friday for zone 655. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Thursday for zones 670-673-676. (See LAXMWWLOX). Gale Warning in effect from 3 AM Thursday to 3 AM PDT Friday for zones 670-673-676. (See LAXMWWLOX).
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&& .HAZARD POTENTIAL OUTLOOK...
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(SAT-WED) No significant hazards expected.
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&& $$ PUBLIC...Hall/MW AVIATION...Hall MARINE...Hall SYNOPSIS...Kaplan weather.gov/losangeles

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