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   FOUS30 KWBC 291940   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   240 PM EST Mon Dec 29 2025   
      
   Day 1   
   Valid 16Z Mon Dec 29 2025 - 12Z Tue Dec 30 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Cook/Wegman   
      
   Day 2   
   Valid 12Z Tue Dec 30 2025 - 12Z Wed Dec 31 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Cook/Wegman   
      
   Day 3   
   Valid 12Z Wed Dec 31 2025 - 12Z Thu Jan 01 2026   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   ...21Z Outlook Update...   
   Most 12Z guidance has increased QPF totals across portions of the   
   central and eastern Transverse Ranges compared to prior forecast   
   runs. This increased expected rainfall totals into the 3-4 inch   
   range, with most of that rainfall occurring in the 06-12Z January 1   
   timeframe. Some consideration to a Moderate Risk upgrade was given,   
   but with several model cycles left to go before the landfalling AR   
   arrives (and attendant uncertainty), decided to hold at Slight for   
   now.   
      
   See the previous discussion below for more details.   
      
   Cook   
      
   ...Previous Discussion...   
      
   A potent upper level shortwave trough will move east through the   
   day Wednesday and approach the southern California coast New Year's   
   night. As it approaches, the attendant surface low will tap into a   
   large plume of subtropical moisture that will advect north on the   
   southerly flow ahead of the trough. The trough itself will be   
   negatively tilted, supporting cyclogenesis. The surface low will   
   elongate thanks to the negatively tilted trough, allowing the   
   southerly flow to spread rainfall north along essentially the   
   entire California coast. However, with southerly flow, the east-   
   west oriented Transverse Ranges will be particularly well set up to   
   capture that moisture and wring it out in the form of upslope   
   rainfall, in much the same way as the Christmas rainfall event a   
   week ago.   
      
   The difference between the Christmas and New Year's rainfall events   
   in California will be the depth of moisture (less of it this time   
   around), and lack of instability this time. This combined with the   
   first part of the event having a southeasterly flow in the lower   
   levels should help to mitigate the total rainfall impacting the   
   L.A. Basin and the Transverse Ranges relative to the Christmas rainfall   
   event. Forecasted PWATs will be around 1.25 inches near the coast.   
   This is still 4 sigma above normal for this time of year, and   
   therefore still represents a highly unusual amount of available   
   moisture. The latest forecast has up to an inch of rain   
   overspreading much of the lower elevations of the L.A. Basin, while   
   1-3 inches of rain are forecast for the south- facing slopes of   
   the Transverse Ranges. Full reservoirs from the Christmas event,   
   some draining still ongoing due to wetter than normal soils for   
   this time of year, burn scars, and urban effects should all work to   
   worsen any flooding effects from the expected rainfall. Thus,   
   while forecasted rainfall amounts are lower, expect that there will   
   still be widely scattered instances of flash flooding, especially   
   in and around burn scars.   
      
   A Slight Risk upgrade was introduced with this update, in   
   coordination with LOX/Oxnard, CA forecast office. Fortunately,   
   unlike the Christmas rainfall event, for the aforementioned   
   reasons, do not expect to have additional upgrades in this area.   
   Short of some much larger forecast change that results in much   
   higher forecasted rainfall amounts into SoCal, a possibility with a   
   low probability of occurring, this should be the final risk   
   category upgrade. Nonetheless the forecast should continue to be   
   monitored for the latest updates.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!9_c0p62XLhsHii5xfk7FHUNvzdwvbd8pwIqRlSscxDPi=   
   gheq3ymKmqLz5xFFvmorQ-nyPYO619S_TRW2shetZCHKzhs$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!9_c0p62XLhsHii5xfk7FHUNvzdwvbd8pwIqRlSscxDPi=   
   gheq3ymKmqLz5xFFvmorQ-nyPYO619S_TRW2shetSgeRru0$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!9_c0p62XLhsHii5xfk7FHUNvzdwvbd8pwIqRlSscxDPi=   
   gheq3ymKmqLz5xFFvmorQ-nyPYO619S_TRW2shetFkatxxY$=20   
      
   =3D =3D =3D   
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