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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   709 PM EST Tue Dec 30 2025   
      
   Day 1   
   Valid 01Z Wed Dec 31 2025 - 12Z Wed Dec 31 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Bann   
      
   Day 2   
   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...   
   The ongoing forecast is on track and no changes are needed. Areas   
   of 1-3 inches of rainfall are expected in the Transverse Ranges   
   during the 06-12Z Thursday period that should result in instances   
   of excessive runoff, mud/rock slides, and debris flows. USGS   
   analyses depict enhanced streamflows suggesting that the region   
   still hasn't recovered from prior rainfall and further enhancing   
   the flood/flash flood risk.   
      
   Additional details are in the previous discussion below.   
      
   Cook   
      
   ...Previous Discussion...   
      
   A large upper level Pacific low will track east and approach the   
   coast of Southern California on New Year's Eve. The low will   
   gradually weaken with time, and remain under a ridge in the   
   polar jet, thus keeping the low cut off from the main jet stream.   
   The low will draw an unusually moist, tropical air mass northward   
   up the Mexican coast and into Southern California. The predominant   
   850 flow will be out of the southeast for much of the day   
   Wednesday, but will turn more out of the south-southwest by New   
   Year's Night. This pattern will keep rainfall into the Transverse   
   Ranges, and much of southern California for that matter, rather   
   light through the day. However, once the flow turns to become more   
   orthogonal to the coast Wednesday night, rainfall rates will   
   increase into the Transverse Ranges. The orthogonal turn of the   
   flow will correlate with the closest approach of both the upper   
   level trough and corresponding surface low. These will also enhance   
   rainfall rates into the mountains.   
      
   Hydrologically, while Southern California has had some time to   
   recover from the Christmas rainfall event, it's still too soon   
   after the event for full drainage to have occurred. Forecast   
   rainfall has increased in the past 24 hours, due in part to a   
   closer approach of the surface low to the coast relative to the   
   forecast last night. As such, a higher end Slight was issued for   
   much of the Transverse Ranges, where 2-4 inches of rain are   
   forecast through the period. Much of that rain will fall between   
   06Z and 12Z Thursday morning. Meanwhile, in the adjacent L.A.   
   Basin, 1-2 inches of rain are expected. Between burn scars and   
   urban concerns, this amount of rain should lead to scattered   
   instances of flash flooding. Thus, the Slight Risk inherited   
   remains in place. Further increases to the rainfall forecast may   
   require an additional upgrade to a Moderate Risk with future   
   updates.   
      
   Wegman   
      
   Day 3   
   Valid 12Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   ...21Z Outlook Update...   
   The ongoing forecast philosophy is on track. Areas of 2-4 inch   
   rainfall amounts are expected during the first half of the forecast   
   period (12Z Thu through 00Z Fri) across the eastern Transverse   
   Ranges with slightly lesser amounts across the Los Angeles Basin   
   and Peninsular Ranges. Several instances of excessive runoff,   
   rock/mud slides, and flash flooding are expected.   
      
   See the prior discussion below for additional information.   
      
   Cook   
      
   ...Previous Discussion...   
      
   A Slight Risk upgrade was introduced with this update, for many of   
   the same areas as the Day 2 Slight Risk. At the start of the period   
   New Year's morning, the brunt of the event will be well underway   
   across Southern California. Heavy rain will continue into the   
   morning hours as the surface low and upper level trough finally   
   move ashore during the day Thursday. Thus, the heaviest rains are   
   expected Thursday morning. Behind the low Thursday afternoon and   
   into the overnight, the rain should largely end for most, tapering   
   to a few showers here and there. Thus, this Slight Risk is really   
   for the first half of the day, and should be able to be dropped   
   after that. Nonetheless, an additional 1-2 inches of rain on top of   
   the rains from the overnight will continue to cause flooding   
   concerns across Southern California. So while amounts in total will   
   be less Thursday than on Wednesday, the continuation of Wednesday's   
   rain on into Thursday will keep the commensurate impacts ongoing as   
   well.   
      
   Further north, a Marginal Risk remains in place for portions of the   
   northern California coastal ranges and extending south along the   
   Sierra Nevada foothills. The rain in these regions Thursday and   
   Thursday night will be much more evenly spread in time. However,   
   these areas are much more equipped to handle the expected rain,   
   especially since full recovery from the Christmas rainfall event   
   has already occurred. An isolated instance or two of flash flooding   
   can't be ruled out, hence the Marginal, but overall impacts into   
   central and northern California should be minimal.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!4mZPy_huc3qi418XhBa3trUqZ72Z_OUUOuWF08SRQFEG=   
   QE-UmlnqQsZkyBziNVgjjb83owLACla3aUK2txnCxoDh_LA$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!4mZPy_huc3qi418XhBa3trUqZ72Z_OUUOuWF08SRQFEG=   
   QE-UmlnqQsZkyBziNVgjjb83owLACla3aUK2txnCIZdVMQc$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!4mZPy_huc3qi418XhBa3trUqZ72Z_OUUOuWF08SRQFEG=   
   QE-UmlnqQsZkyBziNVgjjb83owLACla3aUK2txnCACwZNtM$=20   
      
   =3D =3D =3D   
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