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   QPFERD   
      
   Excessive Rainfall Discussion   
   NWS Weather Prediction Center College Park MD   
   149 PM EST Tue Dec 2 2025   
      
   Day 1   
   Valid 16Z Tue Dec 02 2025 - 12Z Wed Dec 03 2025   
      
   The probability of rainfall exceeding flash flood guidance is less   
   than 5 percent.   
      
   Otto   
      
   Day 2   
   Valid 12Z Wed Dec 03 2025 - 12Z Thu Dec 04 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF   
   SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA...   
      
   Mid-level flow will amplify dramatically across much of Texas ahead   
   of an approaching mid-level wave near the Four Corners. Meanwhile   
   at the surface, 850mb flow will increase dramatically into a small   
   part of southeast Texas and adjacent areas of the northwestern Gulf   
   and southwestern Louisiana. Ascent on the nose of this low-level   
   flow will combined with modest increases in low- and mid-level   
   instability to force mostly elevated deep convection initially   
   across southeastern Texas around 03-06Z Thursday. These cells will   
   will form in an environment favoring appreciable storm motions,=20   
   although the development of convection relative to an east-west=20   
   oriented warm frontal zone across the region will likely support=20   
   areas of training. CAMs are depicting local 3+ inch rainfall totals   
   in areas southwest of Houston that could be realized especially if   
   storms can become rooted in the boundary layer and/or training=20   
   becomes focused along a local corridor in the region. A Marginal=20   
   Risk/5% area was added to the outlook to account for these model=20   
   trends. This risk is likely to begin after 03Z Thursday and persist   
   through 12Z and into the D3 forecast period.   
      
   Cook   
      
   Day 3   
   Valid 12Z Thu Dec 04 2025 - 12Z Fri Dec 05 2025   
      
   ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE CENTRAL=20   
   GULF COAST...   
      
   ...21Z Outlook Update...   
   The previous discussion and forecast philosophy is still on track,   
   with an extensive overrunning event expected to produce widespread   
   areas of 2-4 inch rainfall totals especially across southern   
   Louisiana. Urban/flood prone areas will be most susceptible to   
   flash flooding in this regime. The notion that most convection   
   will be elevated atop a cool stable layer precludes any   
   introduction of Slight Risk probabilities for this outlook.   
   Marginal Risk areas have been trimmed/confined southward to   
   locations expected to experience the highest rainfall totals.   
      
   Cook   
      
   ...Previous Discussion...   
      
   A strong warm advection regime of deep Gulf moisture will have   
   developed along the central Gulf Coast on Thursday morning. A polar   
   high over the Midwest will suppress the northward extent of the   
   precipitation. A unidirectional WSW flow regime will set up over   
   all of the Southeastern U.S. with a 170 kt jet streak aligned   
   roughly along the Ohio Valley, with a longwave positively tilted   
   trough over the Southwest. Weak impulses of shortwave energy will   
   eject out of the trough and along the jet streak, locally enhancing   
   any convection. The primary forcing for precipitation however will   
   be overrunning, as warm Gulf air tries to lift over the much colder   
   air mass over much of the eastern U.S. The combination of   
   unidirectional flow at all levels out of the WSW, overrunning of   
   anomalously moist air, and some, albeit far from impressive   
   instability will support training thunderstorms capable of 2 inch   
   per hour rates. These storms will align in lines parallel to one   
   another. Those lines will be slow moving, supporting multiple   
   rounds of training storms over much of southern/southeastern   
   Louisiana and into coastal Mississippi. NASA Sport imagery shows   
   soils in much of southern Louisiana to be bone dry, with moisture   
   levels at single digit percentages as compared with climatology.   
   Thus, much of the rainfall, especially in areas where it remains   
   light, will be beneficial for these areas. The Drought Monitor also   
   shows this area to be in D2/Moderate Drought. Thus, it will take   
   some time for enough rain to fall to support flash flooding.   
   However, guidance suggests the New Orleans metro to be in the   
   bullseye with the greatest chances for prolonged heavy rain. Given   
   all of the above, the area remains in a Marginal Risk, but the area   
   in and around New Orleans to the urbanized Mississippi Gulf Coast   
   will be evaluated for the need for a Slight Risk upgrade with   
   future updates.   
      
   Wegman   
      
      
   Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/94epoints.txt__;!!DZ3fjg!9JX-EnnIA5U8Si1ylASq88VPPYfLKI-eB776FOQcGeax=   
   3iGEmvgRhCgG5Kwp05VO7dmHZrVq88rRljmlCkn85S-ZtQI$=20   
   Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/98epoints.txt__;!!DZ3fjg!9JX-EnnIA5U8Si1ylASq88VPPYfLKI-eB776FOQcGeax=   
   3iGEmvgRhCgG5Kwp05VO7dmHZrVq88rRljmlCkn8Jsv847k$=20   
   Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=   
   v/qpf/99epoints.txt__;!!DZ3fjg!9JX-EnnIA5U8Si1ylASq88VPPYfLKI-eB776FOQcGeax=   
   3iGEmvgRhCgG5Kwp05VO7dmHZrVq88rRljmlCkn8KCmCqF0$=20   
      
   =3D =3D =3D   
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