FOUS30 KWBC 200043 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 843 PM EDT Fri Apr 19 2024 Day 1 Valid 01Z Sat Apr 20 2024 - 12Z Sat Apr 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Chenard Day 2 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... 2030Z Excessive Rainfall Discussion... Except for extending the Slight and Marginal Risk areas a bit eastward from the earlier issuance...no changes were needed to the Excessive Rainfall Outlook issued earlier today. Numerical guidance continues to advertise the approach of a large...but weakening...shortwave trough that will support a growing area of convection capable of producing excessive rainfall on Saturday. Precipitable water values in the 1.5 to 1.8 inch range were already in place...and timing of the shortwave trough remained similar in the 12Z cycle compared with earlier runs. Bann 0830Z Excessive Rainfall Discussion.. A large but weakening shortwave trough will eject into the southern Plains from the Rockies on Saturday. Meanwhile, south to southeasterly flow from the Gulf will advect increasing amounts of Gulf moisture into south Texas ahead of the shortwave. At the surface, building High pressure will move into the central Plains, bringing a much cooler air mass into the Plains and Midwest. The contrast of these clashing air masses along with the upper level support from the shortwave will support shower and thunderstorm activity Saturday and Saturday night across much of eastern Texas, extending east into the Arklamiss. Some light rain on Friday may help saturate soils a bit across northern Texas, but for the most part as regards FFGs/antecedent conditions, the storms will be on their own. However, the building Gulf moisture will bring PWATs to 1.75 inches, which in some parts of Texas will be 3 sigma above normal for this time of year. That will be plenty of moisture to support strong, moisture-laden thunderstorms across the Slight Risk area. Instability will be a factor working against flash flooding for northern Texas...but as the storms push southward, instability will increase enough to support the storms. Portions of south Texas haven't had a soaking rain in a while, so FFGs are quite high across this area. Meanwhile the Slight Risk area was expanded east into the Arklamiss due to relatively more recent heavy rainfall. By Saturday evening, the upper level shortwave support will be somewhat diffuse, which will result in multiple rounds of showers and storms, further increasing the flash flooding threat due to increased probability of overlapping storm tracks. Portions of east-central Texas are in a high-end Slight category to account for the increased likelihood of multiple rounds of storms moving across this region. This area is little changed from previous forecasts, as well as the broader Marginal and Slight Risk areas. For now it appears the somewhat fast storm motions will preclude any areas picking up enough rainfall to result in a further upgrade to a Moderate, but the area will continue to be monitored. Wegman Day 3 Valid 12Z Sun Apr 21 2024 - 12Z Mon Apr 22 2024 ...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST... ...2030Z Excessive Rainfall Discussion... A Marginal Risk of excessive rainfall remains in place along portions of the central Gulf coast of Sunday as low pressure and an associated cold front approaches from the north. With good run to run continuity in the model mass fields and deterministic QPF from the 12Z suite of numerical guidance and WPC...saw little reason to make more than a couple of minor adjustments to the northern/western boundary of the risk area. Bann ...0830Z Excessive Rainfall Discussion... Convection ongoing Sunday morning across far east Texas and the Arklamiss will move eastward into the rest of Louisiana. While the storms will be weakening in the most climatologically-hostile time of day for the storms, expect the abundant moisture from the eastward shifting LLJ will still support storms capable of locally heavy rain. Recent rainfall events have made portions of eastern Louisiana more sensitive to flooding, along with urban concerns around New Orleans. The New Orleans metro is the highest concern within the broader Marginal. The LLJ will move eastward more quickly through Sunday afternoon, which with increasing westerly flow will allow the storms to also move more quickly eastward with time. Thus, the flash flooding threat Sunday night will be greatly diminished into Alabama, the Florida Panhandle and Georgia. Thus, the Marginal risk for southern Georgia and adjacent Florida and Alabama was removed with this update. Any heavy rainfall will likely be over or moving much too quickly across Georgia by Sunday night to pose a flooding threat, so the Marginal is largely for the daylight hours on Sunday. Wegman Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt