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   FOUS30 KWBC 041938   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   238 PM EST Sun Jan 4 2026   
      
   Day 1   
   Valid 16Z Sun Jan 04 2026 - 12Z Mon Jan 05 2026   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE   
   SIERRA AND FOOTHILLS...   
      
   16z Update: Locally heavy rainfall rates into coastal Monterey   
   County will continue to result in an isolated flash flood and   
   mudslide risk through ~00z. Hourly rainfall between 0.5"-1" will   
   continue to locally occur, with a slow southward shift expected in   
   these heavier rainfall rates through the day. Rainfall totals of   
   2-4" are expected along the coastal portion of Monterey County.   
      
   Another focus for heavy rainfall will be across the eastern valley   
   into the foothills of the central Sierra. Southerly flow up the   
   valley, combined with westerly flow behind an approaching cold   
   front, will locally enhance convergence and support some periodic   
   low topped convection capable of rainfall rates over 0.5"/hr. The   
   combination of upslope enhanced rainfall and these locally higher   
   rates will result in a continued isolated to scattered flash flood   
   risk through today.   
      
   Showers into southern CA through today may periodically drop ~0.5"   
   of rain within an hour...but the coverage and magnitude of rainfall   
   is not expected to match what occurred yesterday over Santa   
   Barbara and Ventura counties. By tonight the approaching cold   
   front should increase shower coverage, but again hourly rainfall   
   should generally peak around 0.5" or less. HREF FFG exceedance   
   probabilities stay below 15%, and while there is a risk of   
   isolated flash flooding and mudslides, the magnitude of the threat   
   is lower than yesterday and also mainly focused farther south.   
      
   Chenard   
      
   ...Previous Discussion...   
      
   Precipitation is expected to increase in coverage and intensity   
   Sunday morning across northern to central California as the next   
   batch of shortwave energy rotates through the mean west coast trof   
   and inland across central CA. PW values and 850-700 mb moisture   
   flux are expected to increase Sunday associated with this next   
   batch of shortwave energy. This will support locally heavy precip   
   totals along much of the California coast Ranges and into the   
   upslope of the Sierra. Hourly rainfall rates in the .25-.50" range   
   possible along the central CA coast and into the upslope of the   
   northern Sierra. A small slight risk area was maintained in the   
   upslope of the northern Sierra where model consensus is for   
   potential of additional 1 to 2" of rain. The HREF probabilities of   
   3 hr precip exceeding 3 hr FFG values shows an area of 15-30%   
   probabilities sinking southward along the upslope of the northern   
   Sierra Sunday where the slight risk area is drawn. Overall, no   
   changes of note made to the previous outlook for the upcoming day 1   
   period.   
      
      
   Oravec   
      
   Day 2   
   Valid 12Z Mon Jan 05 2026 - 12Z Tue Jan 06 2026   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE CENTRAL   
   TO NORTHERN CALIFORNIA COAST AND NORTHERN CALIFORNIA...   
      
   19Z Update: Our model preference is shifting towards the closer to   
   the coast low track and thus wetter scenario of the   
   ECMWF/GEM/UKMET/AIFS. Still not a high confidence forecast, but=20   
   this scenario seems to have more support than the farther offshore   
   GFS/NAM solution. The upward trend in QPF supports a solid=20   
   Marginal risk of excessive rainfall, with amounts of 1-3" and some=20   
   flood impacts likely across central/northern CA. We gave some=20   
   consideration to a Slight risk upgrade, but it remains unclear=20   
   whether rainfall rates will get high enough for that level of flash   
   flood risk. So for now we will keep things at the Marginal level=20   
   and continue to monitor. It's worth noting that most 12z HREF=20   
   members appear too far offshore with the closed low, with this poor   
   synoptic scale forecast negatively impacting their QPF forecast.=20   
   Thus we were not really able to rely on the high res hourly=20   
   rainfall forecasts this cycle...so hopefully the 00z HREF will=20   
   trend more in line with the global model consensus, which should in   
   turn result in more useful hourly rainfall rate data to diagnose=20   
   the flash flood risk.   
      
   Chenard   
      
   ...Previous Discussion...   
      
   There are large scale differences developing day 2 with the   
   amplification of a closed low in the base of the mean trof off the   
   Central to Southern CA coast. The NAM and GFS are showing a more   
   southwest solution compared to the EC, CMC and UKMET. This farther   
   southwest solution is resulting in a trend of less precip along   
   the central CA coast to the south of San Francisco and into the   
   Northern Sierra in the GFS and NAM as the low level flow has a   
   much weaker onshore component compared to the EC, CMC and UKMET.   
   Models often struggle with closed formation downstream of   
   flattening upper ridges with overall low confidence at the moment.   
   The latest WPC qpf did trend towards less precip across the central   
   CA Coast Range and over the Northern Sierra, but not as dry as the   
   GFS and NAM. Subsequently, the previous marginal risk area was   
   decreased in size. There may be additional changes needed to the   
   risk area with future model cycles, depending how the evolution of   
   the closing off mid to upper level center evolves.   
      
   Oravec   
      
      
   Day 3   
   Valid 12Z Tue Jan 06 2026 - 12Z Wed Jan 07 2026   
      
   ...The probability of rainfall exceeding flash flood guidance is   
   less than 5 percent.   
      
   Chenard   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!9mKkEMKK1V8M4zUKSgH6brhInGUvJrsfJNRgyc6yrHdB=   
   hmT5qpXhBzD7_HSqkR5_oeRRZ59xDmlLnO6pQklnG0pkbzk$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!9mKkEMKK1V8M4zUKSgH6brhInGUvJrsfJNRgyc6yrHdB=   
   hmT5qpXhBzD7_HSqkR5_oeRRZ59xDmlLnO6pQklnRFHk8RU$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!9mKkEMKK1V8M4zUKSgH6brhInGUvJrsfJNRgyc6yrHdB=   
   hmT5qpXhBzD7_HSqkR5_oeRRZ59xDmlLnO6pQkln4TO8T2E$=20   
      
   =3D =3D =3D   
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