Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Mon May 19 2025 08:20:00 FOUS30 KWBC 190820 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 AM EDT Mon May 19 2025 Day 1 Valid 12Z Mon May 19 2025 - 12Z Tue May 20 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTHERN ARKANSAS AND SOUTHERN MISSOURI... A digging longwave trough will rapidly become negatively tilted as a vigorous upper level shortwave rounds the base of the trough over the Texas and Oklahoma Panhandles. This will greatly increase the forcing downstream of this feature across Arkansas and Missouri. The forcing will run into a very unstable and moisture-rich air mass, due to a vigorous low level jet extending from deep South Texas north into Missouri. A well-defined dry line will demarcate the westernmost extent of the upper level jet, and as the dry line pushes east, it will further enhance low level forcing for ascent. The result of all of these ingredients coming together will be the explosive development of several rounds of storms, initiating around peak heating this afternoon, then tracking east along the AR/MO state line. The storms will subsequently weaken as they outrace the forcing. This should limit the eastward extent of the heaviest convection to no further than the Mississippi River. CAMs guidance as come into better agreement, albeit far from unanimous, about how the convection will evolve this afternoon and tonight. Due to the extra forcing from the upper level shortwave, expect multiple rounds of storms to track across northern Arkansas this afternoon and tonight, becoming the southern end of an MCS that is largely over Missouri. The southern end of the MCS will have the best feed of moisture from the LLJ, and will therefore have the best opportunity to convert that moisture into heavy rain via the various thunderstorms. As is very typical with convective scenarios, the small details become very important, as its likely the initial storms' cold pools feed the development of the next round of storms. This is especially true in this environment with the LLJ very effectively replacing the moisture lost to rainfall. Nonetheless, with the 2 upper level shortwaves, both the one at the base of the trough and a second rounding an upper level low further north, likely driving and enhancing the convection in a way that favors training and subsequent flash flooding, where they track will be critical. It is in this critical detail that the high- resolution CAMs differ some, which greatly impacts where the heaviest rainfall ultimately occurs. Much of the guidance suggests this will be across the Moderate Risk area in northern Arkansas and southern Missouri, and given the sensitivity of the soils there, this was enough to push the confidence level into the low-end Moderate category. Elsewhere, as mentioned above there will be a secondary maximum of heavy rainfall, likely near the MO/IA border, which will be forced by a shortwave rounding the upper level low embedded within the broader trough over the central Plains. While various guidance suggest this area may see even more rain than areas further south, that solution was discounted as contrary to the conceptual model that if there are two simultaneous rounds of storms occurring, the southern-more of the two will be the dominant due to unobstructed inflow of Gulf moisture from the LLJ, whereas the northernmore set of convection gets the "leftover" convection, and is therefore very frequently weaker. Should the heavier rainfall totals verify in this area, then Moderate Risk level impacts will also be possible here as well. For now however, the area is in a higher-end Slight, along with the rest of the state of Missouri. No significant changes were made to the risk areas elsewhere as the overall pattern remains well-resolved. Wegman Day 2 Valid 12Z Tue May 20 2025 - 12Z Wed May 21 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MIDWEST, OHIO, AND TENNESSEE VALLEYS... The deep trough across the middle of the country and its 2 separate shortwaves forcing the heavy rainfall from the Day 1/Monday period will phase the shortwaves by the start of the Day 2/Tuesday period, and also quickly weaken as the flow quickly becomes zonal. The result will be a widespread shearing front and surface low which will propagate east-southeastward from southern Iowa to northern West Virginia through the period. The low tracking somewhat parallel to its leading front will result in an elongated area of rain stretching from the northern Plains to the Mid-Atlantic. The greatest forcing will be near the surface low and the phased and weakening shortwave energy in the upper levels rapidly flattening to zonal flow. The further north and west along the line you go, the more "used up", the moisture and instability will be, favoring more stratiform rain, especially across Minnesota and Wisconsin. Due to the lack of instability which would be critical for heavy rain rates in this area, the Slight Risk was trimmed out of those states with this update. Elsewhere the Slight remains largely unchanged, especially for portions of Kentucky and West Virginia, which have been hard hit with recent storms, thus have sensitive soils to additional rainfall. The long duration of the rainfall may play as big a role as individual storms' rainfall rates in many areas. The surrounding Marginal Risk area was expanded in the Mid- Atlantic, particularly across Virginia, where recent heavy rains may have made soils in the most sensitive areas prone to isolated instances of flash flooding, even as far east as the coast. Elsewhere, only minor changes were made. Wegman Day 3 Valid 12Z Wed May 21 2025 - 12Z Thu May 22 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE MID-ATLANTIC AND DEEP SOUTH TEXAS... The same storm system that brought heavy rains from the Plains to the Midwest today and Tuesday will shift east into the Mid-Atlantic and portions of the Northeast on Wednesday. However, lack of instability will greatly cut down on heavy rainfall rates in most areas by Wednesday. This should mean most of the rainfall is stratiform north of the Mason-Dixon line. Across Virginia however, marginal amounts of instability may help fuel a few stronger thunderstorms capable of isolated instances of flash flooding. Once again though, previous days' rain will likely be a major, if not dominant factor in the amount of flash flooding, as rates are not expected to get very high. Urban and small stream flooding along the I-95 corridor from DC to NYC will be a concern on Wednesday. For Deep South Texas, as with previous days, expect the strongest storms and the heaviest rainfall to remain over the mountains of northern Mexico on Wednesday. By Wednesday afternoon however, some of those storms may try to drift off the mountains and across the Rio Grande, leading to isolated instances of flash flooding as ample instability will allow the storms to become quite strong and capable of heavy rain. Considerable uncertainty remains as to how far east into Deep South Texas the storms will get, so the inherited Marginal was left as is with no changes for this forecast update. Wegman $$ --- SBBSecho 3.20-Linux * Origin: ILink: CCO - capitolcityonline.net (454:3/105) .