14/717 AM.
An unusually strong storm system will bring widespread rain to the area through Sunday. The period of most concern is tonight through Saturday Night when a large area of heavy rain and isolated but potentially severe thunderstorms will bring a moderate to high risk of significant flash flooding, debris flows, and damaging winds. After a very brief break, another storm will bring light to moderate rain to the area Monday into Tuesday. Yet another storm is possible for Thursday into Friday.
(tdy-Sun), 14/833 AM.
, MODERATE TO HIGH RISK OF DANGEROUS FLASH FLOODING, DEBRIS FLOWS, AND DAMAGING WINDS TONIGHT THROUGH SATURDAY NIGHT,
***UPDATE***
POTENTIAL IMPACTS: The period of most concern is tonight through Saturday Night. During this time, folks should be prepared for ample traffic incidents, delays, and a few road closures. This includes a few flooded roads, freeway lanes, and onramps and offramps. Canyon roads, especially the most vulnerable ones like Topanga Canyon, will likely see mudslide and rockslides. In and near recent burn scars, at least shallow debris flows will occur which would impact roads, with a moderate risk of significant flows blocking or damaging roads and impacting structures. This includes the Palisades, Eaton, and Bridge burn areas which have the highest threat. In creeks and rivers, the flows will be heavy with anyone in or near those channels at risk of being swept away. There will be an area of heaviest rain, but we do not know for sure exactly where that will be. Wherever it sets up, expect all these impacts to be amplified, including a risk for a neighborhood or two experiencing shallow water spilling into businesses and homes. In addition to the water risks, there is a significant threat for severe thunderstorms capable of producing very localized but damaging winds and or a brief tornado.
WHAT YOU CAN DO: Avoid the roads, stay indoors as much as possible, and stay aware of your environment. If you have to drive, allow extra time as traffic will be increased due to slippery roads, low visibility, and localized flooding. If you are near a burn scar, heed the advice of local officials as they know your area best. Reschedule and avoid outdoor activities. If you are outside and hear thunder, see lightning, experience sudden wind shifts, or a sudden increase in rain intensity, head indoors immediately and stay away from windows. Stay out of, and far away from, any streams, rivers, and canyons - especially campers. Avoid parking near tall trees. Be ready for sudden power outages. Boaters, please stay in safe harbor. For everyone, stay tuned to your local news outlet and weather.gov for any updates.
UPDATED DETAILS: While there remains some uncertainty in terms of timing and placement of the heaviest rain, all available guidances remain locked in for an increase in rain coverage and intensity tonight through Saturday Night. Peak rain rates during this period of most concern will likely be in the 0.5 to 1.0 inch per hour range over a large portion of our area, with 0.25 to 0.50 inches per hour elsewhere. When considering the rain we have already seen and will continue to see today, which should precondition the ground and lower the bar of what sort of additional heavy rain is needed to trigger flash flooding, the risk for significant and potentially damaging flooding continues to be moderate to high with the expected rain rates. The current Flood Watches look good and there are no immediate plans for changes. In terms of the severe weather threat, we continue to see a number of parameters favoring supercell thunderstorms capable of producing localized damaging winds and or a brief tornado. This include updraft helicity, which the HREF suite of models all point to Los Angeles County having the highest risk with values that are about as high as we have seen for this area. This does not guarantee a tornado, and 99% of the population will NOT see these conditions, but the risk exists for a very localized high impact situation.
***From Previous Discussion***
, AT LEAST 3-6 INCHES OF RAIN ACROSS THE AREA WITH MAJOR IMPACTS MAINLY FRIDAY AND SATURDAY, INCLUDING POTENTIAL FOR DANGEROUS FLOODING AND DEBRIS FLOWS,
Part one of the storm system is now over the area. Currently the heaviest rain is focused over SBA county. The front will then slowly move to the east with heavier rain moving over VTA county towards dawn and over LA county mid morning. The rain will then taper off in the afternoon as the front pushes out of the area. Currently about 1 to 1.5 inches of rain has fallen over coastal SLO county and western SBA county. VTA county has only seen about a quarter inch of rain with the exception of slopes north of Ojai where up to an inch has fallen. As expected the SBA south coast has received the most rain anywhere from an inch to in excess of 4 inches.
Part two of the storm will start this evening. An upper low currently to the west of Pt Conception will move to the SW through the day. This evening it will swing up to the ENE. This will put an area of southerly flow and max PVA over the area for at least 18 hours. The max intensity of the this system will be over LA and VTA county. The PVA max will generate a threat of TSTMs through the period with the best chc occuring Saturday morning. Forecast soundings continue to indicate favorable profiles for TSTMs and severe weather, including a small chance of weak tornados, strong and gusty winds, very heavy rain and hail. During this 24 hour period (6pm tonight to 6pm Saturday) 2 to 3 inches of rain will likely fall across the flat portions of the csts/vlys south of Pt Conception with 2 to 4 inches likely across the coastal slopes. Areas north of Pt Conception and the far interior will likely receive 1 to 1.5 inches. Rainfall rates will mostly be near a half inch per hour but the heavier bursts will be near 1 inch per hour and these rates are capable of producing flash flooding and a flood watch is in effect until Saturday evening everywhere south of SLO County except for the Cuyama Valley. The watch does include Catalina Island and the Channel Islands.
Snow will not be much of an issue with snow levels over 7500 ft.
The rain will slowly taper off Saturday night and by mid morning Sunday the area should be fairly dry. Another system riding into the area will bring an increasing chc of rain later Sunday afternoon (Central Coast) and overnight south of Pt Conception). This system will not be nearly as wet as the current system.
Needless to say with all of the rain and clouds, max temps will be well below normal with almost all max temps across the csts/vly reaching only the upper 50s to mid 60s.
(Mon-Thu), 14/309 AM.
Both the GFS and the EC forecast another upper low to rotate down into the through the area on Monday and Tuesday. This is a colder system with lower PWATs (.5-.75 inches). So while there will be periods of rain through the period the two day rainfall totals will likely range from a half to three quarters of an inch (with some higher amounts in the mtns and lesser amounts across the far interior). Since this is a cooler system, snow levels will be lower likely around 6000 ft.
Wednesday looks dry for now, but the storm pattern will persist and another upper low will bring a chc of rain to the area Thu and Fri.
Max temps will mostly be in the lower to mid 60s across the csts/vlys which is 6 to 12 degrees blo normals. Wednesday will be the exception with its drier and a little sunnier conditions allowing highs to warm into the mid and upper 60s with a few lower 70s.
14/1700z.
At 1650Z at KLAX, there was no marine layer. However, there was a deep moist layer up to 10000 feet.
Overall, low confidence in 18Z TAF package. High confidence in periods of rain for all sites through the period, but low confidence in timing of flight category changes (and the flight categories themselves). CIGs and VSBYs will vary from VFR levels down to IFR through the period.
There is a 20-30% chance of thunderstorms for TAF sites south of Point Conception after 22Z. Any thunderstorms will be capable of producing heavy rain, frequent lightning and gusty/erratic outflow winds.
KLAX, Low confidence in 18Z TAF. High confidence in varying intensities of rainfall through the period. However, low confidence in timing of flight category changes as well as the flight categories themselves. MVFR conditions most likely through this evening with IFR conditions becoming more likely overnight. Through 11Z, there is a 50-60% chance of easterly winds above 8 knots, then a 60-80% chance of easterly winds above 10 knots 11Z-17Z.
KBUR, Low confidence in 18Z TAF. High confidence in varying intensities of rainfall through the period. However, low confidence in timing of flight category changes as well as the flight categories themselves. MVFR conditions most likely through this evening with IFR conditions becoming more likely overnight.
14/728 AM.
There is a 20-30% chance of thunderstorms across the coastal waters south of Point Conception from late this afternoon through Saturday night. Any thunderstorms that develop will be capable of producing brief heavy rain, dangerous cloud to surface lightning, gusty and erratic winds, locally rough seas and possible waterspouts.
For the Outer Waters, moderate to high confidence in current forecast. Today through Saturday morning, high confidence in combination of Small Craft Advisory (SCA) level winds and seas for all of the Outer Waters. From Saturday afternoon through Sunday, high confidence in wind and seas remaining below SCA levels. For Monday through Tuesday, there is a 70-90% chance of a combination of SCA level winds and seas.
For the Inner Waters north of Point Sal, moderate to high confidence in current forecast. Today through tonight, high confidence in SCA level seas, but winds should remain below SCA levels. For Saturday and Sunday, high confidence in winds and seas remaining below SCA levels. For Monday and Tuesday, there is a 60-80% chance of a combination of SCA level winds and seas.
For the Inner Waters south of Point Conception, moderate to high confidence in current forecast, though lower confidence in winds on Saturday. For today, there is a 60-80% chance of SCA level winds. For tonight, high confidence in winds and seas remaining below SCA levels. On Saturday, there is a 40-60% chance of SCA level southeasterly winds, with a 30% chance of brief Gale Force winds. However, the strength and direction of winds may vary on Saturday as models are not handling the low pressure system placement well. On Sunday, high confidence in winds and seas remaining below SCA levels. For Monday through Tuesday, there is a 60-80% chance of SCA level winds.
14/728 AM.
A moderately long period west-northwest swell will bring hazardous surf conditions to portions of Southwest California today through at least Saturday morning. In addition to the west-northwest swell, a storm system will bring gusty southerly winds which will add a southerly wind swell component to the surf. HIGH SURF ADVISORIES remain in effect for the Central Coast and the Ventura county beaches. Please refer to the LAXCFWLOX and LAXSRFLOX for details.
There is a 20-30% chance that HIGH SURF ADVISORIES may be needed for the LA county beaches today and tonight.
Additionally, there is a 20-30% chance of thunderstorms across all the beaches at times from this afternoon through Saturday night. Any thunderstorms that form will be capable of producing locally strong winds, small hail, dangerous lightning, heavy rainfall, and possibly even a weak/brief tornado.
Ca, Flood Watch remains in effect from late tonight through Saturday evening for zones 87-88-346>348-353>358-362-366>375-377>383-548>550. (See LAXFFALOX). High Surf Advisory in effect until 9 AM PST Saturday for zones 340-346-354. (See LAXCFWLOX). Flood Watch remains in effect through Saturday evening for zones 349>352-376. (See LAXFFALOX). PZ, Small Craft Advisory in effect until 3 AM PST Saturday for zones 645-670-673-676. (See LAXMWWLOX). Small Craft Advisory in effect until 3 PM PST this afternoon for zones 650-655. (See LAXMWWLOX).
