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   FOUS30 KWBC 180826   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   326 AM EST Tue Nov 18 2025   
      
   Day 1   
   Valid 12Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS WESTERN=20   
   ARIZONA, FAR SOUTHEASTERN CALIFORNIA, AND FAR SOUTHEASTERN=20   
   NEVADA...   
      
   Upper difluence will become quite pronounced over the Lower   
   Colorado River Valley today downstream of a strong mid-level trough   
   approaching from California. Meanwhile, pockets of surface heating,   
   near 50s F surface dewpoints, and cold air aloft will all combined   
   to support one or two focused bands of showers and thunderstorms   
   especially across western Arizona. These storms will migrate   
   quickly to the north given seasonably strong wind fields aloft for   
   steering. However, pronounced backbuilding should develop toward   
   the source region of nearly 1000 J/kg SBCAPE (across   
   southwestern/west-central Arizona) that should result in potential   
   for several hours of heavy rainfall. While local rates may not   
   exceed 0.5 inch/hr on a widespread basis, multiple hours of   
   convective training potential will support at least a few instance   
   of flash flooding as rainfall totals exceed 2 inches in several   
   areas from Mohave to Yavapai Counties. A Slight Risk has been   
   maintained for this outlook due to the aforementioned trends.   
      
   A few areas of heavier rainfall may also occur across southeastern   
   California during the afternoon and evening as the large/cold upper   
   trough moves into that area. Buoyancy should be a bit weaker   
   compared to farther east, although recent rainfall and relatively   
   wet soils suggests at least modest/isolated excessive runoff=20   
   potential. Marginal Risk remains in place for this scenario.   
      
   Farther east across the Ohio Valley, models are all indicative of   
   scattered, west-to-east oriented convection that should gradually   
   propagate southeastward through much of the forecast period.   
   Substantial differences exists, however, with respect to peak   
   precip maxima - Nam solutions tend to favor higher precipitation   
   totals across southern Ohio/northern West Virginia on the nose of   
   confluent 850mb flow in that area. Other solutions and higher-res   
   guidance/CAMs point to more backbuilding and training across   
   Kentucky (and perhaps as far west as the MS/OH River Confluence).   
   The spread in solutions has necessitated broad expansions of the   
   inherited Marginal Risk across the region, although the likelihood   
   of widespread training and or greater-than-isolated flash flood   
   potential seems a bit on the low side. Modest PW values (around   
   1-1.25 inch) and instability (especially with eastward extent) also   
   support the notion of relatively isolated excessive runoff issues.   
      
   Cook   
      
   Day 2   
   Valid 12Z Wed Nov 19 2025 - 12Z Thu Nov 20 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   SOUTH CENTRAL TEXAS TO A PORTION OF THE ARKANSAS RIVER VALLEY AND=20   
   OVER THE SOUTHWEST...   
      
   Maintained continuity across both Marginal Risk areas but with   
   minor adjustments based on latest model runs.=20   
      
   The upper level trough moving across the Southwest US continued to   
   trend somewhat slower than in earlier model runs...allowing for   
   more coverage of scattered showers and embedded thunderstorms are=20   
   likely to persist into Wednesday across these areas. Overall it=20   
   looks like less of a focus than the Day 1 period but nonetheless=20   
   still enough of a convective signal to suggest localized flash=20   
   flooding could continue to be a concern.   
      
   Farther east...low level moisture will start to be drawn northward   
   with dewpoints in the 60s by Wednesday afternoon across portions of   
   Texas into eastern Oklahoma and western Arkansas. That should help   
   feed the development of showers and a few thunderstorms during the   
   latter half of the period...moreso from late Wednesday night into early   
   Thursday morning. Model guidance continued to show most-unstable=20   
   CAPE values pushing 1000 J per kg and precipitable water values in   
   excess of 1.5 inches becomes established along and south of a front   
   from parts of south-central Texas into eastern Oklahoma and=20   
   western Arkansas. Based on the GEFS...the precipitable water axis=20   
   is roughly between 2 and 2.5 standardized anomalies above=20   
   climatology for this time of year. Based on that=20   
   combination...suspect locally heavy rainfall rates are possible=20   
   leading to localized flash flooding concerns. Deterministic QPF=20   
   values remain fairly modest...but with the event spanning into=20   
   Thursday with broader areal coverage and an increasing risk of=20   
   excessive rainfall...kept the Marginal risk on the chance that the=20   
   event begins a little earlier than shown by the current model runs.   
      
   Bann   
      
      
   Day 3   
   Valid 12Z Thu Nov 20 2025 - 12Z Fri Nov 21 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF   
   THE SOUTHERN PLAINS...   
      
   ...SOUTHERN PLAINS...   
      
   The flow aloft backs over portions of Texas and Oklahoma and   
   becomes increasingly difluent Thursday morning as an upper trough   
   makes its way from the southern Great Basin into and across the   
   southern Rockies. This configuration aloft with increasingly moist   
   flow in the low levels ahead of a surface cold front will foster=20   
   an environment producing excessive rainfall by means of intense=20   
   rainfall rates by individual storms and by training of cells/repeat   
   convection. The axis of highest precipitable water   
   values...generally in the 1.5 to 1.75 inch range...sets up over=20   
   the eastern half of Texas into southeast Oklahoma and western=20   
   Arkansas...is roughly 2.5 standardized anomalies above climatology=20   
   for this time of year feeding into storms that form during the=20   
   afternoon and evening. Localized rainfall rates in excess of an=20   
   inch per hour are expected with such a set up that results in=20   
   flooding or problems from run off in regions of poor drainage.=20   
   Model deterministic QPF is greatest over portions of Oklahoma into=20   
   portions of western Arkansas...although there is some potential for   
   higher rainfall amounts developing over parts of central Texas=20   
   based on hints from the models for cell training Thursday night.   
      
   ...CALIFORNIA...   
   Maintained a Marginal Risk area introduced by the WPC Medium Range   
   desk over portions of coastal Southern California as the next area   
   of low pressure produces another round of on-shore flow of moisture   
   and associated rainfall Thursday into early Friday. Rainfall   
   amounts at this point look to be pretty modest but will be falling   
   on areas where antecedent conditions have been wet for the better   
   part of 5 days already. That may result in additional localized   
   flooding on Thursday or Thursday night.   
      
   Bann   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!6yhYa_0FIB_4m8-cZpkNhJPBswdRyVgIWaqvWlG6ifAv=   
   bDthFcORxQ46OU9ZpQOUmZzkkCpWUvub1cm57pndglhOrx0$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!6yhYa_0FIB_4m8-cZpkNhJPBswdRyVgIWaqvWlG6ifAv=   
   bDthFcORxQ46OU9ZpQOUmZzkkCpWUvub1cm57pndNUGxeNE$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!6yhYa_0FIB_4m8-cZpkNhJPBswdRyVgIWaqvWlG6ifAv=   
   bDthFcORxQ46OU9ZpQOUmZzkkCpWUvub1cm57pndpWbKUqo$=20   
      
   =3D =3D =3D   
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