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   FOUS30 KWBC 192041   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   341 PM EST Fri Dec 19 2025   
      
   Day 1   
   Valid 16Z Fri Dec 19 2025 - 12Z Sat Dec 20 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL MAINLY OVER   
   PORTIONS OF NORTHWEST CALIFORNIA AND NEW ENGLAND...   
      
   The AR continues to sag southward, reduce residency time, as well   
   as generally weakening in flux intensity. All factors support a   
   reduction of the category to Marginal Risk for the remainder of the   
   day. Also, have removed the northern portions within Oregon   
   where even the northern edge of the AR plume has moved south   
   limiting rainfall totals to below excessive concerns.   
      
   Thinking and placement of the Marginal Risk in New England remains   
   on track; so no changes were made for this update.   
      
   Gallina   
      
      
      
   ~~~~Prior Discussions~~~~   
   ...Pacific Northwest...   
      
   Ongoing atmospheric river will drift southward while swift flow   
   aloft transports the plume of deep layer moisture into parts of   
   the northern Great Basin. IVT values at or above 700 units are   
   forecast to cover much of Oregon initially before it weakens as   
   the axis of the moisture plume and associated precipitation settles   
   into northern California. The Marginal Risk was maintained for   
   southwest Oregon and northern California, as well as the Slight   
   Risk over northwest California. Minimal adjustments were made to   
   account for the latest QPF trends and WPC forecast.   
      
   ...Southern New England...   
      
   Locally heavy rainfall will be possible for portions of the   
   Northeast during this period as moisture drawn northward encounters   
   the advancing trough over the eastern states. With time the   
   the moist flow aloft becomes increasingly diffluent and a surface   
   warm front approaches from the south early on Friday. WPC   
   Deterministic QPF in the 1.0 to 1.4 inch range is forecast along   
   the track of the low. Offsetting the potential of excessive   
   rainfall by limiting rainfall rates/amount is the meager   
   instability expected to be in place. A Marginal Risk was maintained   
   from the New York/Connecticut/Massachusetts border to downeast   
   Maine.   
      
   Campbell/Bann   
      
      
   Day 2   
   Valid 12Z Sat Dec 20 2025 - 12Z Sun Dec 21 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF   
   NORTHERN CALIFORNIA|...   
      
   20Z Update: Continuity from the previous forecast was maintained as   
   12z model suite continued to project similar results from earlier   
   runs leading to little/no variability in the forecast. Expecting   
   areas within the northern Sierra Nevada to receive the most   
   appreciable precipitation given the trajectory of the IVT pulse and   
   persistent 850-600mb moisture feed with added upslope components   
   near the terrain. Greatest flood concerns likely in the adjacent   
   foothills on the windward side of the Sierra's. Highest probs for   
   >5" are across the I-80 corridor at the CA/NV border with the   
   highest terrain still likely to be snow, limiting the threat for   
   flash flooding for the period to more of a MRGL stance. Thus, kept   
   the previous forecast with no change.   
      
   Kleebauer   
      
   ..Previous Discussion..   
      
   A relative lull in moisture surges expected during the majority of   
   this period before the next stronger atmospheric river from the   
   Pacific reaches northern California towards the end of Day 2.   
   Although there may be some overlap in heavy rainfall, the axis of   
   the strongest on-shore winds and the greatest on-shore moisture   
   transport looks to be south of the axis of the previous event.   
      
   Precipitable water values increase to between 1.2 and 1.4 inches   
   along the northern California coast north of San Francisco south of   
   the Oregon/California border by early Saturday evening. Given such   
   high snow levels...there is not a lot of concern about   
   precipitation type until the moisture laden flow reaches the   
   northern end of the Sierra Nevada range (save for a few mountain   
   tops along the trajectory inland from the coast). The urban centers   
   between Sacramento to San Francisco will be more at play this   
   period lead   
      
   Campbell/Bann   
      
      
   Day 3   
   Valid 12Z Sun Dec 21 2025 - 12Z Mon Dec 22 2025   
      
   ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR   
   NORTHERN CALIFORNIA...   
      
   20Z Update: A more substantial IVT pulse will impact the northern   
   half of California come Sunday leading to overlap of impacted   
   areas from D2. Multi-day accumulations coupled with higher rates   
   will induce greater flash flood potential over a large area with   
   the greatest risk over the northern Sierra Nevada foothills.   
   Multi-day totals of 5-10" are still forecast in that region of the   
   state, enough to warrant a continuation of a SLGT risk with   
   probabilities approaching the higher end of the risk threshold as   
   of this time. There is an opportunity for an upgrade in the   
   following updates, so please stay tuned for the latest in this   
   evolving threat.=20   
      
   Kleebauer   
      
   ..Previous Discussion..   
      
   The Atmospheric River will strengthen over the northern half of   
   California resulting in scattered to widespread areas of enhanced   
   rainfall overlapping much of the same footprint from the Day 2   
   period. The latest guidance is suggesting daily accumulations of 3   
   to 7 inches, with 48 hour totals nearing 5 to 9 inches. A Slight   
   Risk is in effect from the West Coast to much of the Sierra Nevada   
   Range. There may be a need for an upgrade for portions of the   
   Sierra Nevada should guidance continue to increase over this area.   
      
   Campbell   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!8L034ioToXxna13oielQ6S959jlwKlp3N-7c9ObD5I-y=   
   rvxMeVh7TynTYpl7O8qtk9POmu4C2HN1jWD9gPcYKFyb4nU$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!8L034ioToXxna13oielQ6S959jlwKlp3N-7c9ObD5I-y=   
   rvxMeVh7TynTYpl7O8qtk9POmu4C2HN1jWD9gPcYVrVGHhI$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!8L034ioToXxna13oielQ6S959jlwKlp3N-7c9ObD5I-y=   
   rvxMeVh7TynTYpl7O8qtk9POmu4C2HN1jWD9gPcYrso9V4U$=20   
      
   =3D =3D =3D   
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