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   FOUS30 KWBC 181550   
   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   1050 AM EST Tue Nov 18 2025   
      
   Day 1   
   Valid 16Z Tue Nov 18 2025 - 12Z Wed Nov 19 2025   
      
   ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS WESTERN   
   ARIZONA, FAR SOUTHEASTERN CALIFORNIA, AND FAR SOUTHEASTERN   
   NEVADA...   
      
   ...Southwest U.S...   
      
   16Z Update: Current radar and IR satellite composite continues to   
   indicate a broad area of diffluence within the eastern periphery of   
   a closed ULL situated just off the CA coast. Radar indications and   
   observational ground truth pinpoint some instances of locally   
   moderate to occasionally heavy rain this morning across western AZ,   
   alignment within the greatest MUCAPE and deepest moisture presence   
   in the proximity of 0.9-1.1" PWATs. Both 12z RAOB soundings out of   
   KVEF (Las Vegas, NV) and K1Y7 (Yuma, AZ) indicate relatively deep   
   saturation within the profile and PWATs well within the 90-99th=20   
   percentile overall for the time of year. These indicators are=20   
   textbook for more efficient rainfall prospects, especially with the   
   additional instability maxima expected later during prime=20   
   differential heating.=20   
      
   A more defined shortwave over Baja is currently progressing north-   
   northeastward with an indication of greater ascent being initiated   
   across southwest AZ at this hours as scattered convective pulses=20   
   begin materializing to the east of the Colorado River Basin. This=20   
   has been well-forecast within the CAMs on the hourly outputs, and=20   
   should maintain some persistence the next several hours with the=20   
   convective footprint shifting north within deep uni-directional=20   
   steering flow positioned along that eastern flank of the upper-=20   
   level circulation. This would put areas further north between=20   
   Prescott, AZ to Lake Havasu City, extending into the southern half   
   of Clark county in NV during the peak of the late-morning and=20   
   afternoon convective threat. Highest HREF neighborhood=20   
   probabilities for >2" of precip align within that general zone with   
   the highest probs >60% situated between Prescott and Lake Havasu=20   
   City. Considering the nature of the precip likely exhibiting more   
   convective schemes and the lower FFG presence in that area of the   
   Southwest CONUS, there is enough to warrant a continuation of the   
   SLGT risk from previous forecast with a broader MRGL encompassing   
   the lower deserts in AZ/NV/CA, including major metros of Phoenix   
   and Las Vegas proper.=20   
      
   Kleebauer   
      
   ..Previous Discussion..   
      
   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   
   potential. Marginal Risk remains in place for this scenario.   
      
   Cook   
      
   ...Ohio Valley...   
      
   16Z Update: Little change necessary in the previous forecast as the   
   prospects for flash flood remain within the lower threshold of a   
   MRGL risk, but still enough of a signal to warrant a general   
   continuation. CAMs are still in agreement on some convective   
   potential later today with the aid of some regional ascent stemming   
   from the passage of a relatively strong shortwave trough migrating   
   out of the Midwest through the northern Ohio Valley. Surface low   
   across the Mid-Mississippi Valley will shift eastward with a warm   
   front arcing over the Ohio River Valley with modest instability   
   focused along and south of the front. General convergence pattern   
   within proxy of the front will be the best area of interest for   
   some heavier convective cores which aligns well with the higher HREF   
   prob fields for >1" and >2" potential with modest signals for even   
   >3" located over northern KY into southern OH. The missing piece in   
   all this is a greater instability maxima, or any in that regard   
   with the signal leaning pretty mundane in that department for the   
   period. This should subdue the overall setup to warrant only   
   isolated flash flood instances with more of the rain leaning the   
   beneficial side of the proverbial impact coin. In any case, there's   
   just enough of a signal for a cell or two to potentially cause   
   problems over an area where 1 and 3-hr FFG's running between=20   
   1-1.5", matching the prospect rate in the forecast. This led to a   
   general continuity of the previous MRGL with some trimming on the   
   western edge to match trends in guidance.=20   
      
   Kleebauer   
      
   ..Previous Discussion..=20   
      
   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   
   OVER THE SOUTHWEST...   
      
   Maintained continuity across both Marginal Risk areas but with   
   minor adjustments based on latest model runs.   
      
   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   
   likely to persist into Wednesday across these areas. Overall it   
   looks like less of a focus than the Day 1 period but nonetheless   
   still enough of a convective signal to suggest localized flash   
   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   
   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   
   western Arkansas. Based on the GEFS...the precipitable water axis   
   is roughly between 2 and 2.5 standardized anomalies above   
   climatology for this time of year. Based on that   
   combination...suspect locally heavy rainfall rates are possible   
   leading to localized flash flooding concerns. Deterministic QPF   
   values remain fairly modest...but with the event spanning into   
   Thursday with broader areal coverage and an increasing risk of   
   excessive rainfall...kept the Marginal risk on the chance that the   
   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 diffluent 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   
   an environment producing excessive rainfall by means of intense   
   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   
   the eastern half of Texas into southeast Oklahoma and western   
   Arkansas...is roughly 2.5 standardized anomalies above climatology   
   for this time of year feeding into storms that form during the   
   afternoon and evening. Localized rainfall rates in excess of an   
   inch per hour are expected with such a set up that results in   
   flooding or problems from run off in regions of poor drainage.   
   Model deterministic QPF is greatest over portions of Oklahoma into   
   portions of western Arkansas...although there is some potential for   
   higher rainfall amounts developing over parts of central Texas   
   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!_0kZlTyW9FDxKLsF3EfNzoonPXbQBQoEj8v5vSoFA9IT=   
   GzESz2fD6186Ul-vJL61BOEWN0NEkn60zaf09_CZ9VKwpfw$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!_0kZlTyW9FDxKLsF3EfNzoonPXbQBQoEj8v5vSoFA9IT=   
   GzESz2fD6186Ul-vJL61BOEWN0NEkn60zaf09_CZf2fvj-k$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!_0kZlTyW9FDxKLsF3EfNzoonPXbQBQoEj8v5vSoFA9IT=   
   GzESz2fD6186Ul-vJL61BOEWN0NEkn60zaf09_CZXvr7ZEk$=20   
      
   =3D =3D =3D   
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