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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   315 PM EST Tue Dec 23 2025   
      
   Day 1   
   Valid 16Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   CALIFORNIA...   
      
   ...16Z Update...   
      
   Only minor tweaks were needed for the Slight and Marginal Risk   
   areas across much of California today. The atmospheric river (A.R.)   
   is making its long expected turn from westerly to south-   
   southwesterly. For now, that has resulted in the steadiest rain to   
   shift north to over far northwestern California and southwestern   
   Oregon. However, even here rates are generally below a half inch   
   per hour, so flooding concerns look to be minor for much of the   
   rest of the day.   
      
   A rapidly deepening low and its attendant cold front will approach   
   the California coast tonight. The low and its cold front will   
   greatly increase rainfall rates over much of California. To the   
   north rain will be associated with the low itself, as it tracks NNE   
   along the coast. To the south, the rain will be associated with the   
   trailing cold front. With the transverse ranges aligned generally   
   west-east, and the flow SSW to NNE, there will be a significant   
   orthogonal component to the mean flow which will further enhance   
   rainfall rates on the south facing slopes of the western Transverse   
   Ranges tonight, including the Santa Ynez and San Rafael ranges near   
   Santa Barbara. Late tonight, rain rates could approach an inch per   
   hour, which when combined with the numerous burn scars in the area,   
   will increase the flooding threat significantly. Thus, have opted   
   to highlight those ranges in a higher-end Slight Risk for this   
   update. A Moderate Risk may need to be considered in this region   
   for the evening update.   
      
   Wegman   
      
   ...Previous Discussion...   
      
   The S/SW to N/NE orientated atmospheric river is expected to   
   maintain scattered to widespread rainfall along the coastline and   
   inland to the Sierra Nevada range with rain for the lower   
   elevations and snow for the higher elevations. Cyclogenesis will   
   occur along the greatest gradient/boundary between the cold air   
   mass within the trough and the much warmer air mass in the   
   ridge/atmospheric river. As the low deepens the onshore flow will   
   align parallel to the coastline thus significantly increasing the   
   areal extent heavy rainfall and the higher terrain snow   
   accumulations.   
      
   The areas of greatest impact will be into the   
   northern Coastal Ranges and the Transverse Ranges. Both these   
   ranges are sufficiently aligned perpendicular to the south-   
   southwesterly flow to have a significant local upslope contribution   
   to rainfall rates. In the Transverse Ranges, local burn scars will   
   also worsen impacts from resultant flooding over and downstream of   
   any burn scars north and west of the Los Angeles Basin. The   
   renewed round of heavy rain will be in areas getting hard hit by   
   the current atmospheric river and will continue with the deepening   
   low offshore into the northern coastal ranges. A Slight Risk was   
   maintained for the Oregon border down through Los Angeles along   
   with a Marginal Risk encompassing much of the same area.   
      
   Campbell/Wegman   
      
      
   Day 2   
   Valid 12Z Wed Dec 24 2025 - 12Z Thu Dec 25 2025   
      
   ...THERE IS A HIGH RISK OF EXCESSIVE RAINFALL ACROSS SOUTHERN=20   
   CALIFORNIA...   
      
   ...2030Z Update...   
      
   Relatively few changes were needed for the peak of the atmospheric   
   river (A.R.) event across southern California. In coordination with   
   LOX/Oxnard, CA forecast office, the Moderate Risk was expanded   
   north up the coast to San Luis Obispo County. Expect the heavy rain   
   on top of the numerous burn scars in the area to result in numerous   
   instances of flash flooding. Elsewhere, the forecast is little   
   changed so the changes to the ERO risk areas were also minimal.   
      
   Wegman   
      
      
   ...Previous Discussion...   
      
   Highly anomalous 850-700 mb moisture flux on the order of 5+   
   standard deviations above the mean will yield extremely heavy   
   rainfall ahead of the amplifying mid/upper trough located offshore   
   of the California coast. The heavy rainfall totals for Southern CA   
   will likely occur over an 18 hour period, with widespread 4-7 inch   
   totals, with isolated max amounts of 9"+. Expect a well defined   
   south to north precip band in this anomalous moisture flux axis   
   with hourly rainfall rates of 0.50-1.5"+ likely. The potential for   
   high hourly rainfall totals will accentuate runoff over soil that   
   is relatively dry and or over recent burn scar areas. This will   
   pose a significant threat of flash flooding, landslides, rock falls   
   and mud slides. The greater than normal travel during the   
   Christmas Holiday will likely expose a potentially larger number of   
   people to these lift threatening hazards and was taken into   
   consideration with the upgrade to high risk.   
      
   Although snow levels will be lowering across the Sierra Nevada and   
   adjacent locations and much of the highest elevations will have   
   significant snow, some of the lower elevations could receive   
   additional rainfall of 1 to 4 inches, which is covered by a   
   sprawling Slight Risk area. Deep-layered moisture will also track   
   eastward across eastern California, southern Nevada and into   
   southern Utah and western Arizona where areal averages up to 1 to   
   2+ are forecast. A new Slight Risk area was raised for much of   
   southern Nevada, along the California/Nevada border and for western   
   Arizona. Rainfall rates will easily surpass 0.5-1.0 inch/hr rates,   
   particularly for California.   
      
   Campbell/Oravec   
      
      
   Day 3   
   Valid 12Z Thu Dec 25 2025 - 12Z Fri Dec 26 2025   
      
   ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHERN CALIFORNIA...   
      
   ...2030Z Update...   
      
   In coordination with LOX/Oxnard, CA forecast office, a Moderate   
   Risk upgrade was introduced with this update for much of the coast   
   from San Luis Obispo through much of the Transverse Ranges north of   
   L.A. Expect much of the heaviest rain to occur on D2/Wednesday, but   
   by Christmas Day, there will still be one more front that will   
   cross the area from west to east, which will result in another   
   round of heavy rainfall in southern California. Since this next   
   front will arrive soon after the heaviest rains from Wednesday,   
   expect there will be outsized impacts from the heavy rainfall   
   associated therewith. Showers and maybe even a few thunderstorms   
   will impact much of coastal southern California and the adjacent   
   mountains. While each one will be fast moving, the rivers and   
   streams all across southern California will likely already be   
   flooding, so any additional rainfall, especially where heavy, will   
   make ongoing flooding worse.=20   
      
   Elsewhere, few changes were needed. Onshore flow and periods of   
   heavy rain will impacts much of central and coastal northern   
   California on Christmas Day. Snow levels will come down such that   
   much of the Sierra Nevada will see feet of new snow. This should   
   keep downstream lower-elevation flooding a bit better since this   
   snow will not contribute to those higher river levels.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!5wiudYFhYDtjqjO-UyQJKZBXx271e48-s5WoEDOIgeNd=   
   kDo9iFHHyJADWjjTqOn0guHvxCgJ1NYeT5DDMY2C0-DrsMM$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!5wiudYFhYDtjqjO-UyQJKZBXx271e48-s5WoEDOIgeNd=   
   kDo9iFHHyJADWjjTqOn0guHvxCgJ1NYeT5DDMY2CWXhQEaE$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!5wiudYFhYDtjqjO-UyQJKZBXx271e48-s5WoEDOIgeNd=   
   kDo9iFHHyJADWjjTqOn0guHvxCgJ1NYeT5DDMY2Cv2js3Y8$=20   
      
   =3D =3D =3D   
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   --- SBBSecho 3.28-Linux   
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