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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   332 AM EST Thu Dec 18 2025   
      
   Day 1   
   Valid 12Z Thu Dec 18 2025 - 12Z Fri Dec 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER=20   
   WESTERN OREGON EXTENDING INTO A SMALL PORTION OF WASHINGTON AND THE   
   NORTHWEST CORNER OF CALIFORNIA...   
      
   A well define plume of deep moisture will continue to approach the   
   coast and then progress inland bringing widespread coverage of=20   
   moderate or greater rainfall to portions of western Oregon and a=20   
   small part of adjacent Washington and the northwest corner of=20   
   California. After several runs where the guidance has shown a=20   
   subtle northward shift of the moisture plume and associated=20   
   rainfall...there was subtle shift southward in the latest suite of=20   
   guidance necessitating a small shift in the north/south position of   
   the Slight Risk along/near the Oregon Cascades. Overall...the risk   
   within the Slight Risk area remained along the west aspect of the=20   
   Cascades where the moisture plume encounters the Cascades and is=20   
   forced to lift orographically. Guidance showed widespread 3 inch to   
   6 inch precipitation amounts within 24hrs with isolated higher=20   
   totals depicted by the convective allowing models in and near the=20   
   terrain. Both the HREF and RRFS continued to show high=20   
   probabilities of 0.50"+/hr rainfall rates especially after 21Z=20   
   today which persists through 12Z Friday. Therefore, the potential=20   
   for small stream and/or river flooding continues to increase with=20   
   this event (per collaboration with the National Water Center).=20   
   However the potential for flash flooding (more short- fused=20   
   inundation), especially over burn scars, is also increasing.=20   
      
   Bann   
      
   Day 2   
   Valid 12Z Fri Dec 19 2025 - 12Z Sat Dec 20 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL MAINLY OVER=20   
   PORTIONS OF NORTHWEST CALIFORNIA...   
      
   ...Pacific Northwest...   
      
   The atmospheric river that came on-shore in the Day 1 period will=20   
   be on-going across Oregon...although the event should be winding=20   
   down in terms of rainfall rates and additional amounts fairly early   
   in the outlook period. A well-defined plume of deep layer moisture   
   will continue to be drawn from the eastern North Pacific Ocean=20   
   eastward into parts of the northern Great Basin by fast westerly=20   
   flow aloft. IVT values at or above 700 units are forecast to cover=20   
   much of Oregon initially...then weaken as the axis of the moisture=20   
   plume and associated precipitation settles into northern=20   
   California. Maintained a large amount of continuity in terms of=20   
   placement of the Slight Risk area clipping northwest California and   
   a portion of adjacent southwest Oregon.=20   
      
   Saw little reason to make any significant changes to the expanded   
   coverage of the Slight Risk area made yesterday to cover more into   
   the Trinity Mountains given latest QPF.=20   
      
   ...Southern New England...   
   A trough crossing the eastern United States will draw increasing   
   amounts of moisture northward into parts of New England...helping   
   set the stage for locally heavy rainfall when flow aloft becomes   
   increasingly difluent 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=20   
   potential of excessive rainfall by limiting rainfall rates/amount is   
   the meager instability expected to be in place.=20   
      
   Bann   
      
   Day 3   
   Valid 12Z Sat Dec 20 2025 - 12Z Sun Dec 21 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN PORTIONS OF=20   
   NORTHERN CALIFORNIA|...   
      
   Renewed rainfall is expected across northern California as yet=20   
   another push of moisture arrives on the heels of the AR which=20   
   departed during the day on Friday, The axis of the strongest on-=20   
   shore winds and the greatest on-shore moisture transport looks to=20   
   be south of the axis of the previous event, but there is likely to=20   
   be at least some overlap between the two rainfall footprints.=20   
   Precipitable water values increase to between 1.2 and 1.4 inches=20   
   along the northern California coast north of San Francisco south=20   
   of the Oregon/California border by early Saturday evening. Given=20   
   such high snow levels...there is not a lot of concern about=20   
   precipitation type until the moisture laden flow reaches the=20   
   northern end of the Sierra Nevada range (save for a few mountain=20   
   tops along the trajectory inland from the coast). With the areal=20   
   average amount of the rainfall forecast being in the 1 to 2 inch=20   
   range...will keep the Marginal Risk inherited from yesterdays Day=20   
   4 ERO and plan to adjust as needed when placement and amounts=20   
   become more locked in,   
      
   Bann   
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!4EmAmY6NdM37RBWwn8UUmFrSqPU6s8VtD61HYYrDwPHJ=   
   duHql51FXByo2be4Kk94uKCAJN7cqSzmnsDLQ80yBpC3Yhc$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!4EmAmY6NdM37RBWwn8UUmFrSqPU6s8VtD61HYYrDwPHJ=   
   duHql51FXByo2be4Kk94uKCAJN7cqSzmnsDLQ80y1hIU7OI$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!4EmAmY6NdM37RBWwn8UUmFrSqPU6s8VtD61HYYrDwPHJ=   
   duHql51FXByo2be4Kk94uKCAJN7cqSzmnsDLQ80yfhh5wHs$=20   
      
   =3D =3D =3D   
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