Area Forecast Discussion
Issued by NWS Los Angeles, CA

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000 FXUS66 KLOX 220626 AFDLOX AREA FORECAST DISCUSSION NATIONAL WEATHER SERVICE LOS ANGELES/OXNARD CA 1026 PM PST Sat Jan 21 2017 ...Aviation discussion updated... .SYNOPSIS... A strong storm system will move into southwestern California late tonight through Monday. The storm will peak Sunday into Sunday evening with periods of heavy rain and strong southerly winds. Lingering rain and mountain snow showers will prevail over the region on Monday, along with a slight chance of thunderstorms. High pressure will bring dry and milder conditions to the area Wednesday through Saturday. && .SHORT TERM...(SAT-TUE) Mid and high clouds have pushed across the entire region this evening, and some lower clouds have developed across Los Angeles County and the Central Coast. Radar showed some light echoes moving into the region at this time, and some very light rain has fallen in a few northern locations. Models all show warm frontal development later this evening and overnight, so expect at least some light rain to develop in most areas overnight. The 00Z GFS and WRF models remain in remarkably good agreement with each other and are consistent with their previous runs. Therefore, there been very little change in thinking regarding the details of what will be a very significant storm system for Southwestern California. The heaviest rains associated with the main cold front will likely affect SLO County between 5 AM and 1 PM, SBA County between 7 AM and 3 PM, Ventura County between 9 AM and 5 PM and Los Angeles County between 11 AM and 8 PM. Just ahead of the front on Sunday there will be a period of gusty se to s winds across much of the region. Will likely expand wind advisories to include most if not all coastal and valley areas south of Point Conception. Behind the front, gusty west winds will likely affect coastal sections of L.A. and VTU Counties and linger into Sunday night. While the models do not show a great deal of instability on Sunday, this system has a good amount of jet energy and impressive mid level dynamics. Some lightning strikes have been noted well off the coast of Central CA. May have to consider slight chance of thunderstorms for Sunday, but tend to agree with previous thinking that there will be a better chance of thunderstorms on Monday. ***from previous discussion*** Big storm on the way for Sunday. Models have been pretty consistent the last few days with just minor variations in speed and amounts. Very few changes in the forecast as a result and rain amounts still on track for 2-4 coast/valleys (highest near the foothills), 3-6 foothills/mtns. Hourly rainfall rates expected to peak in the 1-1.5" range during the peak of the event (mainly south of Pt Conception due to the better orographics) will likely trigger some flooding and not necessarily exclusive to the burn areas. So a flash flood watch has been posted for all of LA/Ventura Counties, the SB mtns and south coast. The three main contributors to this being a standout event include the higher pwats (1.2-1.4") due to it`s subtropical origins, slower speed through the CWA, and 50-60kt southerly winds with and ahead of the front. A high wind warning has been posted for the mountains and a wind advisory for all areas north of Pt Conception. With this being a much warmer system snow levels initially will be quite high, over 8000`. And they`ll stay that way for a large chunk of the precip which will fall Sunday into Sunday evening. Pwats drop off significantly after that but still enough moisture will remain to interact with the colder air behind the front Sunday night into Monday to generate decent snowfall amounts across the mountains. So a winter storm warning will take over after the high wind warning expires Sunday evening and snow levels drop. Snow levels should fall to between 4000 and 5000 ft by late Sunday night and hover in that range through Monday. This could pose some problems for travelers over the Grapevine on Interstate 5, though accumulations at that level should be fairly light. Monday we`re basically dead center of the trof axis with plenty of cold air aloft to trigger showers and possibly some thunderstorms. Most places should at least get a shower or two Monday but rainfall/snowfall amounts will vary widely. I suspect we`ll also see some sunshine as well at times. But a very cool day with highs mostly in the 50s at lower elevations and 20s/30s in the mountains. The trof will start to move east Monday night but there`s still enough cold air aloft and moisture to maintain at least at threat of showers just about anywhere through Tuesday afternoon. But much more hit and miss Tuesday than Monday. Depending on the amount of activity the winter storm warning may need to be extended beyond Monday evening. .LONG TERM...(WED-SAT) Several dry days are expected after Tuesday as models continue to show a ridge slowly developing that will last through the weekend at least. Temps will warm but only slightly Wed/Thu as a couple of ripples move across the top of the ridge. Warming trend should accelerate going into the weekend as the ridge strengthens and some weak offshore flow develops. && .AVIATION...22/06Z. At 05z at KLAX... there was no notable inversion. Overall... Moderate confidence in the TAFs. The reduced confidence is primarily due to the uncertainty in the timing of the storm. MVFR conditions will continue to spread south and east with at least MVFR conditions likely at all terminals by 19Z. IFR conditions with a chance of LIFR conditions will develop around the time of frontal passage. Gusty cross winds along with moderate to strong wind shear and turbulence will develop after 09Z and spread south and east through 16Z. There is a thirty percent chance of thunderstorms after 23/08z. KLAX... Moderate confidence in the TAFs. The reduced confidence is primarily due to the uncertainty in the timing of the storm. MVFR conditions will develop by 08Z. East winds will develop by 08Z and increase to greater than 7 kts by 13z. There is a seventy percent chance of IFR conditions between 20Z and 01Z. There is a forty percent chance of southerly cross winds greater than 20 knots between 22Z and 02Z. There is a thirty percent chance of thunderstorms after 23/08z. KBUR... Moderate confidence in the TAFs. The reduced confidence is primarily due to the uncertainty in the timing of the storm. MVFR conditions will develop by 08Z. There is a seventy percent chance of IFR conditions between 17Z and 03Z. There is a forty percent chance of southerly cross winds greater than 20 knots between 22Z and 02Z. && .MARINE...21/900 PM. Very dangerous sea conditions are expected through at least Sunday. High confidence for widespread gale force southerly winds and very steep seas developing through Sunday. Breaking waves should continue over Morro Bay and Ventura Harbor entrances...as well as near the shoreline and breakwalls. Waves like these have a history of capsizing boats of all sizes. While the swell will remain large likely through Tuesday, it will be highly variable after today due to the changes in winds and additional swell energy moving in. Low confidence therefore on the precise timing of the swell heights, but confidence is high in the general large swell picture. High confidence in at least Small Craft Advisory conditions to follow the gales Sunday night through Monday night. There is a chance that Gale Warnings may need to be extended across the southern waters into early Monday morning. Offshore flow is expected Tuesday or Wednesday. Moderate confidence in thunderstorms forming late Sunday night and Monday. && .BEACHES...21/900 PM. A very large long period west to northwest swell will continue to impact the region through at least Sunday. High surf warnings reamin in effect through 6 am Sunday. Advisory level surf looks likely through Tuesday night and possibly into Friday. Coastal flood advisories also remain in effect through 9 am Sunday. This is a dangerous situation for people near the surf zone. Beach goers should use the highest level of caution during this time. Much larger waves could wash over rocks, jetties and beach areas near the water`s edge, potentially sweeping you into the water. West facing harbor entrances will be dangerous with large breaking waves. Breaking waves may also occur in shallow water near the outer edge of the surf zone. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...High Surf Warning in effect until 6 AM PST Sunday for zones 34-35-40-41-87. (See LAXCFWLOX). Coastal Flood Advisory in effect until 9 AM PST Sunday for zones 34-35. (See LAXCFWLOX). Wind Advisory in effect from 2 AM to 2 PM PST Sunday for zones 34>38-51. (See LAXNPWLOX). High Surf Advisory in effect until 6 PM PST Tuesday for zone 39. (See LAXCFWLOX). Flash Flood Watch in effect from 7 AM PST Sunday through Sunday afternoon for zones 39-52. (See LAXFFALOX). Flash Flood Watch in effect from 10 AM PST Sunday through Sunday afternoon for zones 40-44-45-53. (See LAXFFALOX). Flash Flood Watch in effect from Sunday afternoon through Sunday evening for zones 41-46-54-59-88-547. (See LAXFFALOX). High Wind Warning in effect from 4 AM to 7 PM PST Sunday for zones 52>54-59. (See LAXNPWLOX). Winter Storm Warning in effect from 7 PM Sunday to 6 PM PST Monday for zones 52>54. (See LAXWSWLOX). PZ...Gale Warning in effect until noon PST Sunday for zones 645-670-673. (See LAXMWWLOX). Small Craft Advisory in effect until 6 AM PST Sunday for zones 650-655-676. (See LAXMWWLOX). Gale Warning in effect from 6 AM to 2 PM PST Sunday for zones 650-655-676. (See LAXMWWLOX). && .HAZARD POTENTIAL OUTLOOK...(TUE-SAT) No significant hazards expected. && $$ PUBLIC...MW/DB AVIATION...Kj MARINE...Hall BEACHES...Hall SYNOPSIS...Smith weather.gov/losangeles

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