084 FXUS66 KLOX 211022 AFDLOX Area Forecast Discussion National Weather Service Los Angeles/Oxnard CA 322 AM PDT Wed Mar 21 2018 .SYNOPSIS...20/1012 PM. A strong storm moving into the region will bring waves of significant precipitation to the region through early Friday. This could result in debris flows at all recent burn areas as well as periods of urban and small stream flooding. The heaviest rainfall and a chance of isolated thunderstorms will come Wednesday afternoon through Thursday. Notable snowfall will remain above 9000 feet. && .SHORT TERM (TDY-FRI)...21/304 AM. Forecast hit a pretty big bump in the road last evening and early this morning. The atmospheric river (AR) event did not really start despite the best forecasting efforts of weather forecast computers across the globe. This is not totally unexpected as the forecast of systems coming up from the SW is the most difficult to forecast since they originate over the unpopulated ocean and there is really no good data going into the forecast. Too late for this storm but the situation should improve once the latest and greatest NOAA weather satellite gets set up over the pacific later this year. But it looks like the forecast is going to get back on track over the course of the next few hours. Satellite imagery shows a much better organized band of clouds approaching the coast. Consider the last 12 hours just the prelude to the main system and its only purpose was to get the atmosphere saturated. (This was very evident last evening as there were all sorts of radar echos and little to no rainfall on the ground as all of the rain evaporated in the lower atmosphere before it could reach the ground) The main event will start around dawn this morning across the Central Coast. By mid morning the rain will reach across most of VTA county and moderate to heavy rain will be over SBA county. Mostly moderate rain should then continue over SLO/VTA/SBA county through the afternoon. The system will tilt into more of a SW to NE orientation this evening and the rain will briefly let up south of Pt Conception but will really intensify over Western SLO and SBA counties. Right now it looks like the storm will enter its most threatening period Thursday morning as the AR moves southward. Due to its SW to NE orientation its flow will be nearly perpendicular to the transverse ranges of southern SBA county and western VTA county. It looks like there will be about a 9 hour period of mdt/hvy rain Thu morning and afternoon. Its during this time when LA county will begin to see rainfall in earnest. The event will wrap Thursday evening and overnight into Friday morning. The rain will end starting over the Central Coast and ending in LA county. TSTMs are the latest wrinkle in the forecast. Quite a bit of lightning has been observed in the plume. The satellite soundings have picked up on this area of instability as well. The area of instability will move over the area this morning esp over the higher terrain. There will be a slight chc of TSTMs across SLO county and western SBA county today and this afternoon. Starting around midnight on Thursday the trof associated with parent low will move into Srn Ca and will add a little more instability and a lot more dynamics to the area North of LA county. Added a slight chc of TSTMs to SLO/SBA/VTA counties from Midnight to late afternoon Thursday. TSTMs will be very problematic as any convective lift in this moist an atmosphere could easily produce rainfall rates in excess of an inch per hour. The current rainfall estimates still look good. The forecast calls for amounts of 2-5 inches of rain across the coasts and valleys and 5-10 inches of rain in the foothills and mountains. The higher amounts should be over SBA county and western VTA County. The LA area may end up with a little less rain as it will not have the longer duration of rainfall that SLO/SBA/VTA will. Peak rainfall intensities of 0.5 to 0.75 inches per hour are expected with potentially higher rates in the SBA/VTU Co foothills due to orographic enhancement. As mentioned above any convective cells could bring rainfall rates in excess of an inch per hour (perhaps well in excess) No snow expected as snow levels will be above 8000' for the duration of this event. The sub tropical airmass will combine with the persistent clouds and rain and make for two days with only about an 8 degree swing between the below normal highs and above normal lows. There will be some lingering showers on Friday but otherwise look for clearing skies with max temps 4 to 8 degrees blo normal. .LONG TERM (SAT-TUE)...21/320 AM. Long range forecast has changed as both GFS and EC swing a decent trof through the area Saturday afternoon and evening. This will bring some more clouds to the area...prevent much of any warming and will bring a slight chc of light showers to SLO and SBA counties. Sunday will be dry...cool and breezy behind the trof. On Monday and Tuesday a ridge will push into the Pac NW while a large upper low spins over Las Vegas. This will set up an offshore event but not as strong as it was looking yesterday. Hgts remain below normal so despite the offshore flow max temps will remain blo normal. && .AVIATION...21/0608Z. At 0530Z, there was no marine inversion present at KLAX. Moderate confidence overall in the 06Z TAF package. Rain will gradually spread south and east across TAF sites tonight with MVFR conditions likely with steady rain aft about 08Z for KPRB, KSBP, KSMX and KSBA, then lower to IFR for these airfields early Wed morning with moderate to heavy rain thru Wed evening. For KOXR and KCMA, VFR cigs are expected tonight then lower to MVFR Wed morning thru the afternoon with steady rain before increasing to VFR cigs early Wed evening, but there is low confidence in this. VFR cigs should prevail at remainder of airfields tonight through Wed evening, altho there will still be a 30-40 percent chance of MVFR cigs at times Wed into Wed evening. Overall confidence in timing of lowering flight cats is low, and could be off +/- 2 hours or more. KLAX...Moderate confidence overall in the 06Z TAF. VFR cigs are expected thru the TAF period with an increasing chance of rain on Wed and Wed night which should lead to MVFR vsbys later Wed night. There will still be a 30-40 percent chance of MVFR cigs at times Wed into Wed night. Overall confidence in timing of any lowering flight cats is low, and could be off +/- 2 hours or more. KBUR...Moderate confidence overall in the 06Z TAF. VFR cigs are expected thru the TAF period with an increasing chance of rain on Wed and Wed night. There will still be a 30-40 percent chance of MVFR cigs at times Wed into Wed evening. Overall confidence in timing of any lowering flight cats is low, and could be off +/- 2 hours or more. && .MARINE...21/256 AM. SCA gusty south to southeast winds will continue into Thursday for the Outer Waters and Inner Waters north of Point Sal. There is a 20 percent chance of reaching Gale force winds for the Outer Waters sometime this evening into Thursday morning. Isolated thunderstorms are possible at times across the waters through Thursday evening. Brief gusty and erratic winds with an isolated water spout is possible near any thunderstorm. SCA winds are likely again late Friday into this weekend across the Outer Waters with a 30 percent chance of reaching the inner waters at times. There is also a 20 percent chance of Gale force winds across the Outer Waters by Sunday. && .LOX WATCHES/WARNINGS/ADVISORIES... CA...Flash Flood Watch in effect through Thursday afternoon for zones 34-36-39-40-44-51>53. (See LAXFFALOX). Flash Flood Watch in effect from this evening through late Thursday night for zones 41-46-54-88-547. (See LAXFFALOX). PZ...Small Craft Advisory in effect until 3 AM PDT Thursday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect until 9 AM PDT Thursday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect until 3 AM PDT Friday for zone 673. (See LAXMWWLOX). Small Craft Advisory in effect until 3 PM PDT Thursday for zone 676. (See LAXMWWLOX). && .HAZARD POTENTIAL OUTLOOK (FRI-TUE). No significant hazards expected. && $$ PUBLIC...Rorke AVIATION...Sirard MARINE...Munroe SYNOPSIS...Kj weather.gov/losangeles