044 FOUS30 KWBC 170814 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 AM EDT Thu Jul 17 2025 Day 1 Valid 12Z Thu Jul 17 2025 - 12Z Fri Jul 18 2025 ...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE LOWER ATCHAFALAYA BASIN IN SOUTH-CENTRAL LOUISIANA... ...Northern Gulf Coast... A highly disorganized tropical disturbance continues tracking west across the far northern Gulf this morning, with a center off the coast off Mississippi as of the time of this writing. This disturbance is responsible for a larger area of thunderstorms well west of the center, moving across southern Louisiana. The storms will turn southward and move out into the Gulf through the morning. More widespread, albeit widely scattered thunderstorm activity will develop today as the center moves into southeast Louisiana. Much of the strongest activity is expected to focus west of the center of circulation, but will push westward as it does so. Beyond ample atmospheric moisture is in place, with New Orleans sounding measuring PWATs of 2.27 inches from the 00Z sounding. Thus, while the storms ongoing over Louisiana are largely sticking to rainfall rates under 1.5 inches per hour...with ample instability developing with daytime heating today, any storms that form will have more than enough instability and moisture available to locally produce rainfall rates in excess of 3 inches per hour. However, since the storms will be moving westward, any one location will see rainfall rates that heavy for only a short time. With FFGs near their average at 3.5 to 4 inches per hour over much of Louisiana, it will take repeating rounds of storms to exceed FFGs enough to induce significant flash flooding. This is likely to be a high bar to overcome in most areas, as despite the ample moisture and instability, the forcing associated with this weak, slanted, and overall disorganized low is very lacking in most areas. The Moderate Risk includes the Lafayette area and the Lower Atchafalaya Basin in large part because of the V-shaped nature of the river basin, heavy rain could funnel into flood prone areas quickly if the heavy rain over the basin is persistent enough. In coordination with LCH/Lake Charles, LA forecast office, the Moderate Risk previously in the Day 2/Friday period was transferred to today, as the likelihood for more organized convection across Louisiana will be significantly greater today, resulting in a greater coverage of heavier rain today than is now expected tomorrow/Friday. The surrounding Slight Risk area extends from the far western Florida Panhandle west along the coast to the Texas border. Many of the same thunderstorms impacting southeastern Louisiana will continue west into southwestern Louisiana before dissipating, resulting in the continuation of the Slight Risk. Any heavy rain will have to contend with very high FFG values across all of the Louisiana coast. Meanwhile further east from coastal Mississippi to the far western Florida Panhandle, the combination of urban areas, and southerly onshore flow off the Gulf will support a northward convective motion, favoring training in these areas. ...Ozarks to the Ohio River Valley... A stalled out front over this region acting as a barrier between much drier air to the north over the Great Lakes and a hot and very humid air mass from the Ohio Valley south will act as a focus for eastward-moving complexes of showers and thunderstorms to develop and push quickly east as new thunderstorms from to their west. This will occur over a long line from the central Plains to the central Appalachians this afternoon and tonight. Persistent southerly flow of moisture-laden air will support numerous thunderstorms occurring over these areas today as that southerly flow runs into the stalled out front and turns eastward. While portions of the Slight Risk area have seen more prior rains than others...during the day today expect PWATs routinely over 2 inches to support thunderstorms capable of up to 3 inch per hour rainfall rates. These heavy rainfall rates will be offset by their fast movement, but given the stationary front in place directing all the storms along the same track...training is likely. The Slight Risk area was expanded a bit into southeastern Virginia and northwestern North Carolina with this update, in coordination with RNK/Blacksburg, VA forecast office. Elsewhere few changes have been made since the pattern remains steady state. ...Northeastern New Mexico and Adjacent Areas... Only a very modest eastward shift was made to the previous Slight Risk area, which is largely centered over northeastern New Mexico, a southerly LLJ running into both terrain and a thermal low will act as a focus for rather fast moving thunderstorms to track ESE out of CO/NM and eventually into the OK/TX Panhandles. Like the other higher risk areas, southerly flow of moisture will support backbuilding and additional convective development behind the initial round of storms. Training over flood-sensitive areas may result in widely scattered instances of flash flooding this afternoon into this evening. Wegman Day 2 Valid 12Z Fri Jul 18 2025 - 12Z Sat Jul 19 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CENTRAL GULF COAST, MID-ATLANTIC, AND THE UPPER MIDWEST... ...Central Gulf Coast... The center of circulation of a tropical disturbance over south- central Louisiana will stall out early Friday morning and turn north through much of the Day 2/Friday period. Much of the guidance and the official rainfall forecast have 2 distinct maxima of total rainfall Friday and Friday night: One is over western Louisiana, with a second into the Mississippi and Alabama Gulf Coasts. While these are maxima, in most cases rainfall amounts will hold between 2 and 4 inches for the day. As mentioned in the Day 1/Thursday ERO discussion, the tropical disturbance will simply be too weak and disorganized to support sustained convection for more than a few hours. Despite help from daytime heating and with ample atmospheric moisture, without a well-defined source of forcing, the guidance and the forecast generally agree that the convection will be highly disorganized in clusters that are most likely to track over those two aforementioned maximum areas, but could just as easily form in other areas, rain heavily, then collapse and weaken. New storms may form in similar areas, but once again they will remain widely scattered. PWATs of 2.25 inches and higher will still support any storms that can form to be capable of short bursts of very heavy rainfall of 3+ inches per hour. The disorganized nature of the storms may still support multiple separate clusters moving over the same unfortunate area, resulting in highly localized double digit rainfall totals. However, it is impossible at this point to pinpoint any one area that is likely enough to see 6+ inches of rain in a large enough area to continue to paint it in a Moderate Risk area. Thus, while it remains likely localized areas will see enough heavy rain to cause localized flash flooding, some of which may be significant, the threat for numerous flash flooding has continued decreasing due to the disorganized nature of the disturbance as a whole. Thus, given the paltry signals for heavy rain that are highly changeable, the inherited Moderate Risk for western Louisiana was downgraded with this update. Meanwhile, further east into coastal Mississippi and Alabama, these areas east of the center of circulation will be under a predominant southerly flow of deep tropical moisture. With the center of circulation drifting northward through the period, expect nearly stationary "bands" of very heavy showers and thunderstorms to develop as early as the predawn hours Friday. Any "bands" will have embedded thunderstorms with much heavier rain in between lighter showers. Given any heavy rain in these areas from the Day 1/Thursday period will likely have saturated the soils in these largely urbanized areas, the higher likelihood for additional heavy rainfall into Friday will only worsen the localized flash flooding threat from Gulfport east through Pensacola, including Mobile. A higher end Slight is in effect for this area. ...Mid-Atlantic... A new Slight Risk area was introduced with this update from central Virginia southeast through the Tidewater and into northeastern North Carolina with this update. Disturbances from prior thunderstorms over the Appalachians will track ESE across this region on Friday. Ample moisture and instability remain in place from multiple days of showers and thunderstorms over the past few weeks. Thus, much of this region has very saturated soils that cannot handle much additional rainfall. Given the front stalled over this region will act as a focus, the storms will likely form over western VA and WV, then track southeastward across the Slight Risk area and into the Carolinas. Additional thunderstorms are likely to impact eastern areas prior to that, resulting in multiple rounds of storms impacting the same area in some cases. While the storms will be fast-moving, which may limit the time of the heavy rain over any one area, the likelihood of training and backbuilding thunderstorms could still result in multiple hours of heavy rain over some areas, resulting in locally significant flash flooding, especially were this to occur over any urban area. ...Upper Midwest... Building southerly flow into a stalled out front will result in scattered convective development over far southern Minnesota and northwestern Iowa Friday afternoon. While the storms are in their formative stage...slow movement and cell mergers could result in widely scattered instances of flash flooding. The storms are likely to congeal into a line in eastern Iowa, which will increase in forward speed as the line races southeast into Illinois by Friday night. Once the line has formed, the flash flooding risk should decrease since the line will track orthogonal (southeastward) to its orientation (southwest to northeast). Soils are at or above average for this time of year in the area, so the heavy rainfall likely in this area will support any flash flooding due to excessive runoff. Wegman Day 3 Valid 12Z Sat Jul 19 2025 - 12Z Sun Jul 20 2025 ...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST INTO THE MID-ATLANTIC... Multiple rounds of thunderstorms are likely to impact areas from Iowa southeast to the VA/NC coast on Saturday. Thunderstorms ongoing from the Friday night period may impact the Ohio Valley and central Appalachians Saturday morning. With daytime heating additional storms will form over much of Virginia, as lines and other congealing storms move into the Appalachians. Meanwhile an upper level shortwave will support repeating thunderstorms from Iowa southeast through Indiana from Saturday afternoon through Saturday night. All of these are likely to occur over the same areas hit with strong thunderstorms capable of heavy rainfall on previous days. Further, the remnants of the tropical disturbance over the northern Gulf will further add to the moisture available for storms to use to produce heavy rain over the Midwest. Thus, in some instances where repeating storms occur, it may not much matter how saturated the soils are, as the prolonged heavy rain will result in flash flooding by overcoming the areas where soils are dry. The Slight Risk area was expanded into southeastern VA and northern NC with this update. While confidence isn't quite as high on D3/Saturday as compared with D2/Friday, the signal for heavy rain, if reduced from Friday, will still support flash flooding in these hard hit areas. ...Lower Mississippi Valley... The inherited Slight Risk area over northern Louisiana was downgraded to a Marginal with this update. There is good agreement in the guidance that any remnants of the northern Gulf tropical disturbance will be dissipated by Saturday, with only showers and isolated thunderstorms remaining over Louisiana, posing only a Marginal flash flooding risk at most. The downgrade was coordinated with SHV/Shreveport, LA forecast office. 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