A subtropical fetch of moisture ahead of an approaching trough of low pressure will likely bring periods of moderate to heavy rain to the area late Tuesday through Thursday. Cooler than normal temperatures will prevail through Thursday due to the unsettled weather. This will be followed by a trough of low pressure expected to linger over the area into this weekend, with dry and slightly milder conditions.
(Mon-Thu), 19/931 PM.
***LARGEST STORM OF THE SEASON WITH MAJOR CONCERN FOR RECENT BURN AREAS***
A broad trough of low pressure over the eastern Pacific Ocean is current sitting off the California Coast near 34N and 140W. A plume of subtropical moisture can be seen flowing out of the tropics towards the California Coast. Middle and high level clouds will thicken overnight tonight and into Tuesday morning. A mostly cloudy day looks on tap for a majority of the area on Tuesday. Shower activity or sprinkles could spread in ahead of the main rain band on Tuesday. Steady moderate to locally heavy rainfall should spread into Western San Luis Obispo County on Tuesday afternoon, spreading south into the Santa Barbara Central Coast and the San Luis Obispo County valleys as the afternoon progresses. PoPs have been nudged higher for Tuesday along the Central Coast.
With the deep subtropical moisture plume and a warm air advective pattern with southerly winds, this storm continues to hold the potential to be a major rain producer for Southern California. A Flash Flood Watch remains in effect for the recent burn areas of Southwest California. High confidence exists in rain for the region, but model solutions continue to lack run-to-run consistency and timing as to where and when the rain band for this atmospheric river event will set up. Models advertise a persistent band of moderate to heavy rainfall setting up somewhere between Monterey and Los Angeles Counties (and anywhere between) for which bulk of the rainfall will occur. The placement of the atmospheric river will determine which area is hit hardest. Its orientation determines the amount of orthogonal component to the topographic features in Southern California, which will greatly affect the rainfall amounts and rates.
*** FROM PREVIOUS DISCUSSION ***
Currently, we are expecting 2-5 inches of rain in the coasts and valleys and 5-10 inches of rain in the foothills and mountains. The higher amounts will be for the areas of eastern SBA County and western VTU County, with the least amounts in the LA area. Peak rainfall intensities of 0.5 to 0.75 inches per hour are expected with potentially higher rates in the SBA/VTU Co foothills.
Peak rainfall will move from north to south beginning in SLO Co on late Tuesday night into Wednesday, SBA and VTU Co on late Wednesday into Thursday and LA Co on Thursday.
Due to the prolonged period of rain it is possible there will be small stream and urban flooding in addition to debris flows in burn areas. We expect to issue flash flood watch products by later this afternoon.
Snow should not be a big issue with this storm due to high snow levels because of the tropical nature of this moisture. Probably no snow accumulation below 9000 feet.
(Fri-Mon), 19/221 PM.
Expecting the rain to stop by Friday mid-morning in LA County and drier northwest flow will settle over the area. By late Saturday there will be mild offshore flow and things should begin to dry out. In the wake of the storm it will turn cooler again with most cst/vly max temps only in the mid 60s.
Sunday and Monday will be dry, expecting a slight warming trend to near normal temps with mostly clear skies.
At 23Z, there was no marine inversion at KLAX.
MVFR conditions will spread in after 18Z Tuesday with ceilings at or below 5000 feet likely thereafter. There is a chance of IFR conditions north of KSBA after 20Z.
KLAX, VFR conditions will persist through at least 20Z Tuesday, then there is a 30 percent chance of MVFR conditions. Ceilings at or below 5000 feet will become likely after 20Z. East winds greater than 7 knots are possible on Tuesday evening.
KBUR, VFR conditions will persist through at least 20Z Tuesday, then there is a 30 percent chance of MVFR conditions. Ceilings at or below 5000 feet will become likely after 20Z.
For the Northern and Outer Waters, high confidence in the current forecast. Winds and seas will remaining below Small Craft Advisory levels through tonight, then slowly increase through Tuesday night. There is a 80-90 percent chance of advisory level southeast to south winds through Wednesday afternoon, decreasing to 60-70 percent between Wednesday evening through Thursday. There is a 30 percent chance of gales being reached or exceeded between Wednesday night and Thursday. On Thursday, there is a 60 percent chance of at least advisory level west to northwest winds.
For the Inner Waters south of Point Conception, moderate-to-high confidence in current forecast. there is a 50 percent chance of advisory level southerly winds Wednesday and westerly winds Thursday.
Ca, Flash Flood Watch in effect from Tuesday afternoon through Thursday afternoon for zones 34-36-39-40-44-51>53. (See LAXFFALOX). Flash Flood Watch in effect from Wednesday evening through late Thursday night for zones 41-46-54-88-547. (See LAXFFALOX). PZ, Small Craft Advisory in effect from 3 PM Tuesday to 3 PM PDT Wednesday for zone 645. (See LAXMWWLOX). Small Craft Advisory in effect from 3 AM Tuesday to 3 PM PDT Wednesday for zone 670. (See LAXMWWLOX). Small Craft Advisory in effect from 9 AM Tuesday to 3 PM PDT Wednesday for zone 673. (See LAXMWWLOX). Small Craft Advisory in effect from 9 PM Tuesday to 3 PM PDT Wednesday for zone 676. (See LAXMWWLOX).