Moderate to heavy rain will drench the Southland from Tuesday night through Thursday as a deep atmospheric plume of moisture from north of the equator teams up with a trough of low pressure from the Pacific Northwest to produce the wettest storm system the Southland has experienced this rainy season and to threaten to unleash mud slides and debris flows down areas stripped bare by wildfires, forecasters said Monday.
Cooler than normal temperatures will prevail this week due to the unsettled weather through Thursday, followed by a trough of low pressure lingering over the area into this weekend, according to a National Weather statement.
Between late Monday evening and Tuesday morning, the atmosphere will moisten rapidly as subtropical moisture associated with an atmospheric river reaches the region, it said, adding: “The atmosphere looks to moisten from the top down through Tuesday.”
NWS meteorologist Rich Thompson said the effect will feel a great deal like a Pineapple Express — atmospheric moisture that builds up in the tropical Pacific before reaching the West Coast, packing a huge amount of rain.
The expected duration of the rainfall will make the approaching weather system “the most significant of the entire winter rainy season,” which begins in October, according to the statement.
“A good 36 hours of precipitation could add up to impressive rainfall totals through Thursday approaching 4-6 inches in the foothills and 2-4 inches for coastal areas,” the statement said, adding that the most rain is expected in the San Gabriel Mountains in Ventura and Santa Barbara counties.
“Rain will taper to showers Thursday afternoon with lingering scattered showers into Friday morning,” the statement said.
The statement warned that “the most critical time for recent burn areas will be from early Wednesday into Wednesday night. This is when steady rain rates could easily jump above a half-inch per hour.”
But the NWS said overnight that it would wait for additional computer models before issuing flash-flood warnings, even though it’s already clear that “periods of moderate to heavy rain are possible Wednesday through Thursday night. Rainfall rates during this time will likely exceed U.S. Geological Survey thresholds for debris flows near recent burn areas. Rainfall totals could bring local flooding to small streams and urban low-lying areas.”
The NWS forecast partly cloudy skies in L.A. County Monday and highs of 56 degrees on Mount Wilson; 64 in Avalon; 66 in Palmdale and Lancaster; 67 at LAX; 69 in Saugus; 70 in Long Beach, San Gabriel and Burbank; 71 in Downtown L.A.; 72 in Pasadena; and 73 in Woodland Hills. Temperatures will be marginally lower Tuesday in some communities — up to 2 degrees — but several degrees lower on Wednesday, Thursday and Friday. The high in Downtown L.A. Thursday is forecast to be 65, 6 degrees lower than Monday. A high of 65 is also forecast in Woodland Hills compared to Monday’s 73.
Partly cloudy skies are also forecast in Orange County Monday, along with highs of 62 in San Clemente; 65 in Newport Beach and Laguna Beach; 71 in Mission Viejo; 72 in Anaheim and Irvine; and 73 in Irvine. Tuesday’s temperatures will be about the same as Monday’s but go down in some communities by as much as 8 degrees on the last four days of the week.
—City News Service