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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1034 AM EST Wed Dec 31 2025   
      
   Day 1   
   Valid 16Z Wed Dec 31 2025 - 12Z Thu Jan 01 2026   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   ...16Z Outlook Update...   
   The ongoing forecast is largely on track, with areas of moderate to   
   heavy rainfall expected across the Transverse Ranges especially   
   after 06Z tonight.=20=20   
      
   The only minor change to the ongoing forecast for this update was   
   to broaden Marginal across portions of San Luis Obispo and Santa   
   Barbara counties, where greater rainfall totals (around/over 1   
   inch) could occur atop sensitive areas/burn scars during the   
   forecast period). Isolated areas of excessive runoff cannot be   
   ruled out in these areas today and tonight.   
      
   See the previous discussion below for more forecast information.   
      
   Cook   
      
   ...Previous Discussion...   
      
   A slow-moving upper level low centered well west of the California   
   coast will move towards California through the Day 1/New Year's   
   period. The upper low will remain negatively tilted as it   
   approaches the coast. This will allow a surface low, already formed   
   at the start of the period, to move north along the coast.   
   Southerly winds ahead of the low will drive plentiful Pacific   
   moisture into southern California. Upslope into the Transverse   
   Ranges will wring out much of that moisture, resulting in multiple   
   inches of rainfall, especially New Year's night, into the L.A.   
   Basin and adjacent Transverse Ranges. Once the center of the   
   surface low moves north of the Bay Area, its trailing cold front   
   will move into southern California. This will turn the flow to out   
   of the southwest, which will further enhance the upslope flow into   
   the Transverse Ranges, causing heavier rainfall Wednesday night.   
      
   The inherited Slight Risk for much of the L.A. Basin and higher-end   
   Slight for the Transverse Ranges themselves was left largely   
   unchanged with this update. The forecast weather pattern remains   
   largely unchanged, with more parallel southeasterly flow into the   
   Ranges supporting mostly light rain through much of New Year's   
   Eve and into the evening, then the rain turning heavier around the   
   stroke of midnight for the L.A. Basin. While soils across Southern   
   California have dried out a fair bit since the Christmas rainfall   
   event, both soils and rivers are still running above normal even   
   several days later. Thus, the renewal of heavy rainfall Wednesday   
   night should quickly refill any rivers, with much of the lighter   
   rain during the day Wednesday saturating the soils in time for the   
   heavier rains Wednesday night to largely convert to runoff. Thus,   
   the Slight Risk looks to be in good shape, with no major changes to   
   rainfall amounts expected requiring any further major changes to   
   the excessive rainfall forecast.   
      
   Wegman   
      
   Day 2   
   Valid 12Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   Heavy rain will be ongoing at the start of the period Thursday   
   morning across much of southern California. As the upper level   
   shortwave moves into the coast, the winds will turn out of the   
   west-southwest, or exactly orthogonal to the Transverse Ranges.   
   This will maximize the upslope enhancement of the rainfall just as   
   the maximum forcing is moving ashore. Through 18Z, this period New   
   Year's morning will be when the heaviest rain of the event is   
   expected into southern California. PWATs between 1 and 1.25 inches   
   will be between 3 and 4 sigma above normal for this time of year, a   
   sign of how much moisture will be available to convert into   
   rainfall and runoff from the Transverse and to a lesser extent,   
   Peninsular Ranges. Once the shortwave moves ashore and inland by   
   Thursday evening, the combination of weakening of both upper level   
   and surface lows, as well as rapid weakening of the onshore west-   
   southwesterly flow behind the low, will lead to rapid diminishing   
   of the rainfall across Southern California, such that by Thursday   
   evening, little if any rainfall will be left.   
      
   The Slight Risk inherited is largely unchanged both from the   
   previous forecast and from the Day 1/Wednesday Slight Risk area.   
   Both days' Slight Risks will be splitting the two halves of the   
   same event, so little should be different. There is a higher-end   
   Slight in effect for most of the Transverse Ranges from about   
   Malibu east through San Bernardino. This will largely be for the   
   morning hours, as once the rain weakens, the flooding threat   
   associated with additional rainfall should also rapidly diminish,   
   likely resulting in an early downgrade of the Slight Risk Thursday   
   afternoon.   
      
   Further north, along the track of the aforementioned upper level   
   shortwave, rainfall will persist for much longer into the northern   
   Coastal Ranges and much of the Sierra Nevada foothills, especially   
   those along the Sacramento Valley. However, this area has more   
   thoroughly recovered from the Christmas rainfall event and also is   
   better equipped naturally to handle long duration rainfall. As   
   such, only an isolated flash flooding threat, consistent with the   
   unchanged Marginal, is expected.   
      
   Wegman   
      
   Day 3   
   Valid 12Z Fri Jan 02 2026 - 12Z Sat Jan 03 2026   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   CALIFORNIA...   
      
   Light rain over most of northern California from about 06Z Friday   
   through 18Z Friday will end as another large upper level low begins   
   to make its way towards northern California and Oregon. This upper   
   low, while bigger, will be less amplified/elongated as compared   
   with the last upper low. That should focus most of the rainfall   
   into northern California. Further, the surface low will be a   
   vertically stacked one, whose occluded front has wrapped clear   
   around the low center. It is this occluded front making its way   
   ashore that will restart any heavier rain into the southward facing   
   slopes of the coastal ranges, Klamath, and the northern Sierra   
   Nevada foothills. Rainfall totals will generally fall in the 2-3   
   inch range in most of the mountains, with around an inch into the   
   Sacramento Valley. This amount of rain should be enough to raise   
   river levels with perhaps an isolated instance or two of flash   
   flooding in some of the flashier creeks and streams, but overall   
   impacts should still be minimal, despite some precipitation   
   occurring prior to this on Day 2. Much of northern California is   
   well-suited to handle rainfall amounts such as these, so widespread   
   impacts are not expected. The area's forecast will be monitored   
   for any potential Slight Risk upgrades needed.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!7o8Eg9AOCNMgaJC05jFAIC1fkLbfj77oMUDCFRAo6PPM=   
   j1sM1Yy5BqelH3I-e4JsONR7Lq7-msUc_9z02JkDybKSlnk$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!7o8Eg9AOCNMgaJC05jFAIC1fkLbfj77oMUDCFRAo6PPM=   
   j1sM1Yy5BqelH3I-e4JsONR7Lq7-msUc_9z02JkDvBeINxA$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!7o8Eg9AOCNMgaJC05jFAIC1fkLbfj77oMUDCFRAo6PPM=   
   j1sM1Yy5BqelH3I-e4JsONR7Lq7-msUc_9z02JkDwSaCW4g$=20   
      
   =3D =3D =3D   
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