TZUTC: 0000   
   MSGID: 167836.weather@1:2320/105 2db020b5   
   PID: Synchronet 3.21a-Linux master/123f2d28a Jul 12 2025 GCC 12.2.0   
   TID: SBBSecho 3.28-Linux master/123f2d28a Jul 12 2025 GCC 12.2.0   
   BBSID: CAPCITY2   
   CHRS: ASCII 1   
   FORMAT: flowed   
   FOUS30 KWBC 212039   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   339 PM EST Sun Dec 21 2025   
      
   Day 1   
   Valid 16Z Sun Dec 21 2025 - 12Z Mon Dec 22 2025   
      
   ...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR   
   PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA...   
      
   ...16Z update...   
      
   Moderate to heavy rain was ongoing across northern California this   
   morning with the atmospheric river oriented southwest to northeast   
   into and across the coastline. Through 15Z, 24 hour localized   
   totals of 3 to 5 inches have been reported across the King Range of   
   southern Humboldt County and 3 to 6+ inches across the northern   
   Sierra Nevada. IVT values peaking near 800 kg/m/s were estimated   
   this morning at 15Z in/around Mendocino County with slow but steady   
   southward translation and weakening forecast over the next 12-24   
   hours. Additional 24 hour totals of 4 to 8 inches (perhaps local   
   maxima near 10 inches) are anticipated for the northern Sierra   
   Nevada with 2 to 4 inches (local 5+) for the Coastal Ranges from   
   Cape Mendocino to Monterey Bay. Valley locations will receive much   
   less, but higher than the 0 to 0.1 inch forecast values depicted by   
   the hires guidance.   
      
   A few landslides/debris flows have already been reported and   
   additional flood related impacts will be likely within regions   
   that receive additional high rainfall totals with peak hourly rates   
   over 0.5 inches at times, locally up to 1 inch in an hour.   
      
   Otto   
      
   ...previous discussion follows...   
      
      
   A significant surge in Pacific moisture will overspread much of   
   northern California during this period within the reinforced   
   atmospheric river and will lead to widespread heavy rainfall.   
   Within the Moderate Risk area over the portions of the Sierra   
   Nevada range there are several hours of >50% HREF probabilities of   
   at least 0.50"/hr rates between 12Z Sun and 03Z Mon, while HREF   
   probabilities of >8" within the 24 hour Day 2 period are >60% for   
   much of the area. There will likely be many areas where flash   
   flooding may arise, along with sloughing and debris flows. For   
   additional details please refer to WPC MPD #1267.   
      
   Campbell/Hurley   
      
      
   Day 2   
   Valid 12Z Mon Dec 22 2025 - 12Z Tue Dec 23 2025   
      
   ...THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR   
   PORTIONS OF THE SIERRA NEVADA IN NORTHERN CALIFORNIA...   
      
   ...20Z Update...   
      
   12Z guidance trends are not as far south with the peak rainfall   
   across the northern Sierra foothills; in fact, there is quite a bit   
   of overlap with the Day 2 Moderate Risk area with today's D1   
   Moderate. Given this, we have included an 'enhanced' or 'higher-   
   end' Moderate area across portions of the northern Sierra   
   Foothills, including the communities of Colfax, Grass Valley, and=20   
   Blue Canyon, where the neighborhood probabilities of more rapid=20   
   inundation (particularly over burn scars and/or more flood prone=20   
   areas) climbs to above 50%.=20   
      
   The atmospheric river will move southward during this period as   
   will the heavy rainfall footprint spread further down the Sierra   
   Nevada Range and along the coastline. Much of the same area of the   
   Sierra Nevada is projected to receive additional 2-3 inches over   
   the water logged area from Day 1. A Moderate Risk was maintained   
   along with expanding a touch to the south. Multi-day accumulations   
   will likely be well into the double digits by this time and leading   
   to several instances of rapid runoff, sloughing and debris flows   
   along the foothills/adjacent locations.   
      
   Hurley/Campbell   
      
      
   Day 3   
   Valid 12Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   CALIFORNIA...   
      
   ...20Z Update...   
      
   Not too many changes were needed for the Day 3 period. The   
   atmospheric river that had been impacting portions of northern   
   California will begin to shift south and realign more south-   
   southwest to north-northeast by Day 3/Tuesday. This will occur as a   
   deep longwave trough digs southward across the Pacific off the   
   coast of California. In contrast to the air mass containing the   
   atmospheric river, which will be warm and moisture-laden, this   
   contrast will support cyclogenesis along the greatest   
   gradient/boundary between the cold air mass within the trough and   
   the much warmer air mass in the ridge/atmospheric river. The low   
   that forms will deepen to as low as the upper 970s mb. Meanwhile,   
   the digging trough will cause the A.R. to turn more   
   longitudinal/parallel to the California coast. This means much of   
   the state will be impacted by heavy precipitation, and more   
   snow/falling snow levels are expected into northern California   
   within the much broader longwave trough. The onshore flow   
   associated with the low itself will continue very heavy rain and   
   mountain snow across northern California, while the trailing cold   
   front moves into southern California starting Tuesday night.   
      
   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 L.A. Basin. Meanwhile, up north the renewed round   
   of heavy rain will be in areas getting hard hit by the A.R. now and   
   will continue with the deepening low offshore into the northern   
   coastal ranges. The Slight Risk inherited was combined to include   
   all of the central California coast, and will now extend from close   
   to the Oregon border down through Los Angeles. Meanwhile along the   
   Sierras, in coordination with HNX/Hanford, CA forecast office, the   
   Slight Risk was extended south along the southern Sierra Nevada   
   range due to expected impacts from the heavy rain that could be   
   supported by increasing instability as warmer and more moist air is   
   advected into the region as the A.R. realigns.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!-1fo1JYvSYMvCVuAP3BB0MUQMryJLI-lwV36kHE9Wwzj=   
   BdK5PsGeBaz8JK1vY_OhYaZAk43H90UK3IDfc_5IDovhflg$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!-1fo1JYvSYMvCVuAP3BB0MUQMryJLI-lwV36kHE9Wwzj=   
   BdK5PsGeBaz8JK1vY_OhYaZAk43H90UK3IDfc_5IJysuymw$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!-1fo1JYvSYMvCVuAP3BB0MUQMryJLI-lwV36kHE9Wwzj=   
   BdK5PsGeBaz8JK1vY_OhYaZAk43H90UK3IDfc_5IDulRtrc$=20   
      
   =3D =3D =3D   
   To unsubscribe from WX-STORM and you already have a login, go to   
   https://lists.illinois.edu and use the "Unsubscribe" link.  Otherwise email   
   Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM.   
      
   --- SBBSecho 3.28-Linux   
    * Origin: Capitol City Online (1:2320/105)   
   SEEN-BY: 4/0 19/10 88/0 90/0 93/1 104/119 105/81 106/201 114/10 120/302   
   SEEN-BY: 128/187 129/14 153/757 7715 154/10 30 50 110 700 218/700   
   SEEN-BY: 218/840 220/10 20 30 90 221/1 6 360 226/18 30 44 50 227/114   
   SEEN-BY: 229/110 134 206 300 307 317 400 426 428 470 664 700 705 266/512   
   SEEN-BY: 291/111 301/1 320/219 322/757 335/364 341/66 200 207 342/200   
   SEEN-BY: 396/45 460/58 633/280 712/848 880/1 900/0 102 106 902/0 19   
   SEEN-BY: 902/26 904/0 13 905/0 2320/0 105 304 3634/12 5019/40 5020/400   
   SEEN-BY: 5075/35   
   PATH: 2320/105 154/10 221/6 341/66 902/26 229/426