Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 406 AM EDT Sat Apr 20 2024 Day 1 Valid 12Z Sat Apr 20 2024 - 12Z Sun Apr 21 2024 ...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR CENTRAL AND EAST TEXAS THROUGH PORTIONS OF THE ARKLAMISS... The primary change to the ERO from the previous forecast was to expand the Slight a bit to cover all of central Mississippi all the way to the Alabama border. The latest guidance and CAMs suggest that convection that forms at the nose of a robust LLJ will track along to the east a bit more quickly than previous forecasts. Training convection will be likely in this scenario as the storms track along a nearly stationary boundary. Unfortunately in the Arklamiss, the convection will track near or over an area that was quite hard hit a few days ago, covering the AR/LA/MS tripoint along the Mississippi River and extending across central Mississippi. Thus, despite the fact the forecast remains wetter across portions of east Texas, the antecedent conditions and saturated soils in the Arklamiss should make up for the lesser amounts of rainfall expected to still result in widely scattered instances of flash flooding. Depending on how well the convection holds together in this area, localized considerable flash flooding is possible, especially if the stronger storms track in the same areas hard hit a few days ago. Across Texas, the Slight risk area is largely unchanged. As a strong shortwave ejects out into Texas today, increasing Gulf moisture moving north along a strong LLJ will raise PWATs above 1.5 inches in many areas, which is about 2-2.5 sigma above normal. This will support a large area of rain over much of north Texas, which will quickly expand east with time. The shortwave and resultant cyclogenesis will shift the LLJ east with time. Meanwhile, strong Canadian high pressure will dive south down the Plains, increasing the thermal gradient...along which the convection will form and move east. The shortwave and strongest moisture flux will support the strongest convection across east Texas this afternoon at peak heating, where a higher-end Slight is considered in effect. As the rainfall pushes east into the Arklamiss this evening, the thunderstorms should gradually weaken, resulting in slightly less rainfall, but as mentioned above the antecedent conditions are more favorable for flash flooding development. While changes to the Slight and surrounding Marginal Risk areas were minimal, they were nudged southward with this update in keeping with both guidance trends and typical climatology of these kinds of events, where stronger storms and heavier rains tend to form a bit south of guidance where both instability and moisture availability are both greater. Wegman Day 2 Valid 12Z Sun Apr 21 2024 - 12Z Mon Apr 22 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. The inherited Marginal Risk area for the central Gulf Coast was removed with this update, in coordination with LIX/Slidell, LA and MOB/Mobile, AL forecast offices. A bit faster evolution of the rainfall and dry air encroachment from the central Plains will allow remaining rainfall and potentially some embedded thunderstorms to move east quickly by Sunday morning. As a result, forecast rainfall in this area continues to rapidly decrease, as has been the trend in the guidance for the past few days. Wegman Day 3 Valid 12Z Mon Apr 22 2024 - 12Z Tue Apr 23 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Wegman Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt