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   Mesoscale Precipitation Discussion 0004   
   NWS Weather Prediction Center College Park MD   
   1251 PM EST Sat Jan 03 2026   
      
   Areas affected...Southern CA   
      
   Concerning...Heavy rainfall...Flash flooding likely   
      
   Valid 031750Z - 040200Z   
      
   SUMMARY...Excessive Rainfall rates around 1"/hr will likely lead   
   to additional flash flooding along the Coastal Range and Topatopa   
   Mountains today where soils are sensitive due to recent heavy   
   rainfall. Burn scars in the area are also prone to flash flooding   
   and debris flows. Localized urban-induced flash flooding may occur   
   in the suburbs north and west of Los Angeles   
      
   DISCUSSION...Doppler Radar shows a narrow band of   
   moderate-to-heavy rain oriented SW-to-NE over the Santa Ynez and   
   San Rafael Mountains. A narrow squall line ahead of the cold front   
   is also analyzed just west of Santa Barbara where a couple rain   
   gauges southeast of Solvang have reported ~1"/hr rainfall totals.   
   Aside from the squall, most 1-hour rainfall observations show   
   >0.5" totals as the band of heavy rain marches east towards Santa   
   Barbara. Some 3-hr totals have reached 1.75", which also coincides   
   with 1-hr averaged rainfall rates just over 0.5". This ribbon of   
   rainfall is embedded just ahead of an approaching cold front with   
   the warm sector approaching the Santa Barbara Channel. As the warm   
   front collides with the western Transverse Ranges, a sliver of   
   100-250 J/kg of MUCAPE will become available within a highly   
   saturated atmosphere. Latest RAP forecast soundings near Oxnard   
   show PWATs approaching 1.25" (above the 97.5 climatological   
   percentile) and a fully saturated warm-cloud layer that is 9,000ft   
   deep. Latest 12Z HREF probabilistic guidance does show moderate   
   chances (40-60%) for 6-hour rainfall totals >3" over the Topatopa   
   Mountains in Ventura County with similar probabilities along the   
   Santa Ynez just north of Santa Barbara.   
      
   As the frontal system moves east, so will the shield of heavy   
   rainfall as it heads for the L.A. metro area. However, PWATs   
   approaching 1.25" will retreat over off the coast and 850mb winds   
   south of L.A. will tend to be weaker than the >30kt winds farther   
   west. Therefore, rainfall rates should struggle to get much higher   
   than 0.5"/hr except for the mountain ranges to the north and west   
   where upslope enhancement will play a big role. In terms of   
   impacts, the Santa Ynez, San Rafael, and Topatopa are of greatest   
   concern given their lingering high soil moisture content (>95th   
   soil moisture percentile in sfc-100 cm layer) and burn scars in   
   these ranges. Debris flows and rock slides cannot be ruled out,   
   particularly within burn scars. As for the urbanized communities,   
   recent heavy rainfall has made soils sensitive and the greater   
   concentration in hydrophobic surfaces naturally provides some   
   flash flood concerns. Given the decreasing rates this afternoon,   
   however, any flash flooding would generally be ponding on roads   
   that could pose a hydroplane threat for motorists. In summary, the   
   mountains north and west of L.A. are likely to witness additional   
   flash flooding today, although localized flash flooding in more   
   urbanized settings north and west of L.A. cannot be fully ruled   
   out.   
      
   Mullinax   
      
   ...Please see https://urldefense.com/v3/__http://www.wpc.ncep.noaa.gov__;!!=   
   DZ3fjg!-7HaO-2_1JAsI4qeaTRyryPz2LAxCyYFSoB0xzT6DK0BMumMO9tgiqjxmTRaGq3lfeD3=   
   Kt9uO69SZ7BkF6l3r30Lzy4$  for graphic product...   
      
   ATTN...WFO...LOX...   
      
   ATTN...RFC...RSA...NWC...   
      
   LAT...LON   35201980 34861950 34741920 34761878 34731841=20   
               34471830 34191839 33971867 34151933 34281964=20   
               34331992 34402043 34712047 35142022=20   
      
   =3D =3D =3D   
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