Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed May 15 2024 08:29:45 FOUS30 KWBC 150829 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 429 AM EDT Wed May 15 2024 Day 1 Valid 12Z Wed May 15 2024 - 12Z Thu May 16 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE RED RIVER VALLEY OF THE NORTH REGION, PORTIONS OF THE CENTRAL PLAINS, THE WESTERN APPALACHIANS, AND THE CENTRAL AND NORTHERN FLORIDA PENINSULA... ....Red River Valley of the North... A deepening shortwave trough in the polar jet will dip south into the Dakotas and Minnesota later today into tonight. This will support some modest cyclogenesis of a small surface low. The low will focus an area of persistent rain in the comma-head region of north and west of the center. During the day, diurnal heating will support some modest instability, allowing convection to develop in the area. Once the sun goes down and the convection largely dissipates, steady stratiform rain will continue into northwestern Minnesota. Soils in this area are about average for this time of year, but the persistent rains some of which may be briefly heavy will support an isolated flash flooding threat. A Marginal Risk area was introduced with this update. ....Central Plains... A developing front between 2 opposing air masses across the Central Plains will be the focus for convection today and especially into tonight across northeastern Oklahoma. To the north, drier air traveling southbound down the Plains will collide with a weak LLJ of warmer and moister air. Showers and thunderstorms will develop along this boundary, tracking east with time. Soils are at or above normal in this area for this time of year, with the wettest soils into northeastern Oklahoma. The inherited Marginal was adjusted, largely to remove most of western Kansas, which will both be on the dry side of the boundary and also have very dry soils for this time of year. Easterly flow into the mountains of Colorado will upslope along the Front Range, so areas south of Denver, including Colorado Springs remain in the Marginal Risk area. ....Western Appalachians... The inherited Marginal across West Virginia was expanded both north into eastern OH and western PA, and also south into southwest VA and NC. Almost all of the resultant convection will be diurnally driven and largely disorganized. However, much of the Marginal Risk area has low FFGs so any stronger storms will be slow moving and capable of isolated flash flooding. The storms will be most focused in the Marginal Risk area along a weak deformation zone in the upper atmosphere, with largely chaotic or no movement.=20 ....Central and Northern Florida Peninsula... Ongoing convection in the Marginal Risk area will continue growing upscale into a larger line of storms that will track east across the Peninsula through the morning. Training storms will be possible in the fast westerly flow across the Peninsula as they tap additional moisture ahead of a trough associated with an upper low over the Ohio Valley. Abundant moisture available for the storms to feed on will support higher rainfall rates, but accelerating storm motion, dry antecedent soil conditions, and weakening of the line by midday should keep any flash flooding isolated, particularly in urban and poor-drainage areas. Wegman Day 2 Valid 12Z Thu May 16 2024 - 12Z Fri May 17 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FROM THE PINEY WOODS OF TEXAS TO SOUTHWESTERN MISSISSIPPI... ....Southern Plains and Lower Mississippi Valley... Little has changed in the convective evolution and overall setup across eastern Texas over to southwestern Mississippi Thursday. The nose of a 100 kt jet will push a strong shortwave trough eastward across the region, increasing divergence aloft ahead of the shortwave. At the surface, a strong and very moisture-laden air mass originating in the Gulf will surge northward into a front that will quickly become a warm front as the LLJ pushes northeastward. The warm front will support continuous convection as the moisture- laden air mass rides up the frontal interface. A cold front following behind it associated with the progressive shortwave trough will enhance lift on the back/west side of the warm sector. This will support development of an MCS, which will it will have a decently quick forward speed...will support backbuilding and training strong thunderstorms capable of very heavy rain, particularly across Texas during the day, then shifting into Louisiana and Mississippi overnight Thursday night. Soils across the Piney Woods and into western Louisiana are very saturated from multiple weeks of heavy rain. The Piney Woods have the bullseye, picking up as much as 20 inches of rain in the last=20 2 weeks alone. Multiple rounds of strong and training convection capable of rainfall rates as high as 3 inches per hour over especially this hard-hit area...but really much of the South...spells a nightmare scenario for the area. Significant and considerable flash flooding is likely as a result, as multiple inches of rain fall in a matter of a few hours. The area is considered in a higher-end Moderate risk, where the probability of flash flooding is between 55 and 70 percent. The one piece of good news is that guidance is little changed on the magnitude of the=20 event, generally expecting between 3 and 5 inches of storm total=20 rainfall, albeit in a short time.=20 By Thursday night, the MCS will pick up some forward speed as it=20 moves across Louisiana and eventually Mississippi. While this will=20 modestly decrease expected storm total rainfall in this area...the=20 abundance of moisture in the LLJ will continue to support convection capable of very heavy rainfall, and while most of northern Louisiana and southwest Mississippi hasn't seen quite as much rain in recent weeks as east Texas, there has still been 300 to 400 percent of normal rainfall in the area the past few weeks, so soils are similarly fully saturated, so all rainfall will convert to runoff. Thus, a lower-end Moderate continues for most of northeast Louisiana and with model guidance now suggesting the rain extends into Mississippi by Friday morning. The Moderate Risk was expanded eastward to account for this change, in coordination with LIX/Slidell, LA and JAN/Jackson, MS forecast offices. There remains some uncertainty in the guidance as to where exactly this axis of heaviest rainfall along the warm front will set up. The American models remain big outliers, but have been very slowly "catching up" to the rest of the model suite. As such, once again they have been largely tossed from the forecast. The 06Z HRRR suggests the axis of heaviest rainfall may be a bit south and west of the current Moderate Risk area, so additional changes to the forecast are likely in coming days. Unfortunately even the 06Z HRRR scenario does not spare the Piney Woods from multiple inches of=20 rain.=20 Further north into Oklahoma, the boundary that will serve as a focus for convection on Day 1/today will still be draped across the area and largely stationary on Day 2/Thursday. Guidance remains highly variable in how much convection develops again here, particularly Thursday night, but given expected rainfall in the area on Day 1/today, it's likely that soils will be more saturated than now by this second round Thursday night. Thus, the area was=20 upgraded to a Slight with this update. Wegman Day 3 Valid 12Z Fri May 17 2024 - 12Z Sat May 18 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHERN MISSISSIPPI AND SOUTHWESTERN ALABAMA... ....Southern Mississippi and Southwestern Alabama... By the start of the period 12Z Friday morning, a strong LLJ originating out of the Gulf and advecting air with PWATs above 2 inches into the Gulf Coast will be oriented southwest to northeast. The strong southwesterly flow will not abate much, keeping a steady supply of fuel into the ongoing thunderstorms moving across Mississippi and into Alabama. The MCS will weaken with daytime heating, as typically occurs, so expected rainfall will drop off into southeast Alabama and most of the Florida Panhandle. The dissipation of the MCS with the first round of storms will mean the LLJ will stall out and stop progressing east.=20 On Friday night, a trailing upper level shortwave, which will be deeper than the first, stronger, and slower-moving will drag a cold front east, which should allow for renewed eastward movement of the moisture-plume for a time, but will be the animus for a second round of strong showers and thunderstorms across southern Mississippi and Alabama as the moisture plume is initially drawn back to the north and west as the cold front and associated storms approach. As the second round will quite probably produce more rain than the first, any flash flooding started with the first round of storms will be greatly worsened by the second.=20 Soils across this area are very wet from storms as recently as yesterday, so all of the rain that falls from both rounds of storms will convert to runoff. Flash flooding is likely and significant flash flooding is possible. One positive is that we are in the heart of growing season and the soils are generally flood- resistant. However, this will be offset by the 2-3 inch per hour rainfall rates that will be quite possible with the strongest storms. Urban and poor drainage areas are at greatest risk for flash flooding. In coordination with MOB/Mobile, AL and JAN/Jackson, MS forecast=20 offices, a Moderate Risk upgrade was introduced with this update.=20 It's quite possible heavy rain may spread more into central=20 Alabama, but since these kinds of convective events tend to trend=20 southward with time and central Alabama is on the northern fringe=20 of the heaviest rainfall, held those portions of Alabama including=20 Montgomery, in a Slight for this update. Should trends indeed=20 evolve southward with time, then the hardest hit areas from today,=20 including Gulfport, Mobile, and Pensacola could all be in line for=20 a second round of flash flooding. Fortunately, most of the Moderate Risk area is in relatively low population areas, which should=20 limit overall impacts somewhat. ....Mississippi and Ohio Rivers Confluence Region... In coordination with SGF/Springfield, MO, PAH/Paducah, KY and=20 LSX/St. Louis, MO forecast offices, a Slight Risk upgrade was introduced with this update for the Mississippi and Ohio Rivers=20 Confluence region. Guidance has been somewhat consistent for a few days now that a secondary area of showers and storms associated=20 with a shortwave trough tracking across the area will be capable of widely scattered instances of flash flooding. The area has seen wetter soils from rainfall both yesterday and expected over the coming days, such that by Day 3/Friday, flash flooding will be more likely as the storms track along with an advancing cold front across the region.=20 Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4f6O1iuD0WkhsJOi0kEO2Vaypkk1MPyW0FhkcpZVM2ea= E-upATMUw6jiriglwivmS0wD2UZnvQ3k2_1LoWLcnc9qKEE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4f6O1iuD0WkhsJOi0kEO2Vaypkk1MPyW0FhkcpZVM2ea= E-upATMUw6jiriglwivmS0wD2UZnvQ3k2_1LoWLcpvn7fPg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4f6O1iuD0WkhsJOi0kEO2Vaypkk1MPyW0FhkcpZVM2ea= E-upATMUw6jiriglwivmS0wD2UZnvQ3k2_1LoWLc4xKgnEU$=20 =3D =3D =3D To unsubscribe from WX-STORM and you already have a login, go to https://lists.illinois.edu and use the "Unsubscribe" link. 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