Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 12 2025 15:34:28 FOUS30 KWBC 121534 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1134 AM EDT Thu Jun 12 2025 Day 1 Valid 16Z Thu Jun 12 2025 - 12Z Fri Jun 13 2025 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF SOUTHEAST TEXAS... ....Texas through Arkansas... 16Z Update: The general expectation for the MDT risk inherited has come to fruition this morning leading to only necessitating a formal "trimming away" of areas that have already been impacted and will see no additional threats this period. The MDT risk is now more confined to the middle and upper TX coasts with a minor expansion to the northeast to include the Beaumont area. MUCAPE=20 gradient is well-documented within both the latest Mesoanalysis and the derived GOES-East satellite parameter. Values between=20 1000-2000 J/kg are still situated within the confines of the MCS=20 over the middle TX coast with a 2000-3000 J/kg signature still=20 aligned upwind of the flanking line extending back inland. Cloud=20 streaks moving northward across Deep South TX depict a sufficient=20 moisture advection pattern continuing across the region and will=20 play a role in the convective maintenance through the next 2-4=20 hrs. The good news is the cold pool associated with the setup is=20 already starting to race out into the Gulf on the northern flank of the complex with the back edge beginning to see some advancement=20 of the outflow ahead of the main line which will likely degrade the convective pattern later this afternoon. As a result of the=20 evolution, a sharp delineation of the convective threat for the=20 rest of the period has materialized with a likely diffusion of the=20 heavier rain prospects between 18-21z as noted via 12z CAMs output. 4-8" with locally 10-15" of rainfall are plausible when the entire event is done for areas southwest of Houston to back along the=20 flanking line. Rainfall rates of 2-4"/hr are possible within the=20 strongest convective cores over the next few hrs before the degradation of the overall setup. Anticipating a removal of a lot of the risk area across coastal TX back into the I-35 corridor later as the threat wanes, so will be monitoring closely for that specific adjustment.=20 Across Northeast TX into the ArkLaTex, MCV propagating to the north-northeast has allowed for a narrow band of heavier rainfall within the convergent sector of the low to mid-level circulation. This will continue to press north over the next several hrs with flares of additional heavier convective cores basically maintaining precedence through the afternoon and early evening. 2-4" areal average with totals as high as 5" are possible across the ArkLaTex through Arkansas today as additional waves of convection will propagate northward behind the vacating MCV leading to a compounding effect of heavy rainfall on saturated soils. The best chance for flash flooding will be within those urban zones and areas that have received significant rainfall the past 24 hrs. Considering the favorable output already, and the expectation as depicted from CAMs, the SLGT risk with higher-end threshold is still forecast over the above zones. A small expansion on the northern periphery of the SLGT and MRGL risks was also made to account for the latest HREF mean QPF signature and small adjustment north of the better probs for greater than 2" and 3".=20 Kleebauer ...Previous Discussion.. In coordination with HGX/Houston, TX forecast office, a Moderate Risk upgrade was introduced with this update. Convection blossoming over central Texas right now is forming in response to increased upper level divergence ahead of a sharp upper level trough over west Texas. This trough is also supporting a potent LLJ advecting deep tropical moisture out of the Gulf and into east Texas. This steady supply of plentiful moisture along with a very slow-moving upper level trough are both primary ingredients for slow-moving thunderstorms capable of producing very heavy rainfall. Storm coverage will continue to increase so that by the start of the period later this morning, a well-defined MCS will have developed north and west of Houston. The storms will build southeastward towards the Gulf as they advect towards the moisture and instability source. Additional storms forming along the coast will also advect north into the MCS. Since the southerly flow associated with the LLJ will continue through the day, this will promote backbuilding storms along the cold pools of the existing storms, resulting in new activity. The result will be several hours of heavy rainfall in some areas, which even in areas with higher FFGs, are likely to be exceeded due to the prolonged duration of heavy rain in any one area. On the south side of the MCS, the backbuilding storms may form an east-west oriented line which could train over the areas that part of the line moves over. This will locally further enhance any flash flooding threat, which would be enhanced further should it occur over Houston or another urban area. Antecedent soil conditions have already been saturated in many areas of southeast Texas, so this additional rainfall could cause flash flooding quickly. The MCS will gradually develop into a southeastward moving line through the morning hours, which should gradually reduce the flash flooding threat as the storms approach the Louisiana state line. Urban concerns in and around Houston will also increase the flash flooding threat. The good news is by midday or so, the line will be diminishing and will be moving largely orthogonal to the orientation of the line, which will greatly decrease the time any one area sees heavy rain. Further, instability and competing sources of forcing should disrupt the MCS in typical diurnal fashion. This too should work to reduce the flash flooding threat. Thus, the Moderate Risk is really just for the morning and early afternoon, and will likely be able to be downgraded this afternoon. Further north into Arkansas, the northern end of the MCS will cause additional storms to form across the state this morning. Most areas will see a break through the day, but additional convection forming this afternoon through tonight behind a developing mesolow will add to the precipitation totals. Further topographic influences with the Ozarks may add upsloping to the forcing mechanisms. Thus, expect a secondary "bullseye" of heavy rainfall across much of central to southwestern Arkansas. The Slight Risk area was adjusted eastward with this update as the guidance better resolves the ongoing shower and thunderstorm activity. The risk areas were generally reduced from the DFW Metroplex west, and increased in central Louisiana and central Arkansas. A higher-end Slight is in effect for the area from the Moderate Risk north and east into central Arkansas. Wegman ....Northern Plains... 16Z Update: Only minor adjustments necessary for the inherited SLGT and MRGL risk areas across the northern tier of the CONUS. The forecast remains steadfast overall with little deviation in the expected heavy rain axis to be located within the northern periphery of a strong 850mb FGEN band situated west to east across the Northern Plains into the Upper Midwest. 12z HREF EAS prob fields for at least 2" is pretty high (40-60%), which is a testament to the agreement within the CAMs suite on the placement of the heavier precip threat. Rainfall rates between 1-2"/hr will be common in the strongest convective cores, and with training potential elevated due to parallel alignment of the mean wind to the surface/low-level based forcing mechanism(s), this will lead to a narrow axis of 2-4" with locally up to 5-6" plausible over the course of the late-afternoon and evening time frames when the event takes shape. Isolated flash flood concerns will extend as far northwest as MT with remnant shortwaves and modest instability maintaining a persistent convective presence during the course of the period with the best chances likely developing after 19z.=20 Kleebauer ...Previous Discussion.. At the northernmost head of the LLJ of deep tropical moisture out of the Gulf across Minnesota, the plume of moisture will run into a warm front that will add lift to the atmosphere as the moisture runs up the frontal interface. Divergence aloft in the right entrance region of a 120 kt jet will also support shower and thunderstorm development in the fast upper level westerly flow associated with the jet. Shower and thunderstorm activity have already broken out along the Minnesota/Iowa border this morning. As the warm front lifts north through the day, the coverage of rainfall will diminish a bit with daytime heating disrupting the synoptic scale lift in favor of much more chaotic local forcings. By tonight however, as the typical diurnal LLJ reestablishes itself, showers and thunderstorms will once again redevelop, but this time further north into central Minnesota, and likely near the Twin Cities. The strengthening supply of moisture associated with the LLJ will support backbuilding and training storms, as the cold pools work to lock where the storms form in place, despite the continued northeastward movement of the synoptic-scale warm front. This will support a rather narrow (50-80 mile wide) corridor where heavy rain on the order of 1-3 inches with locally higher amounts are expected tonight. This should cause widely scattered instances of flash flooding. The Slight Risk area was nudged northward with this update in keeping with the latest suite of guidance, which has been struggling with where to place the warm-front related convection on the map for the last few days. Wegman ....Southeast U.S... A combination of elevated moisture across the Southeast and strong differential heating away from the coastal plain will lead to a corridor of enhanced SBCAPE on the order of 2500-3500 J/kg across GA up through the Continental Divide in the Southern Mid Atlantic. Scattered to numerous thunderstorms with relatively slow storm=20 motions will enhance locally heavy rainfall capabilities with rates between 2-3"/hr plausible in a few of the stronger cores.=20 Urbanized zones within the region referenced above will be the most susceptible for flash flooding prospects with some of the latest=20 CAMs going as high as 4-5" in some of the greater outputs=20 anticipated. A MRGL risk is in place for the areas above with a=20 minor expansion on the southern and western edge of the inherited=20 risk.=20 Kleebauer Day 2 Valid 12Z Fri Jun 13 2025 - 12Z Sat Jun 14 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF ARKANSAS... The very slow moving upper level trough will continue to drift across Missouri and Arkansas through the period. Continued northward advection of deep tropical moisture will run into the Ozarks as a dryline also gradually pushes east out of the Plains. The combination of each of these forcings over an area with topography and likely very wet antecedent soil conditions will all support continued potential for scattered instances of flash flooding across much of Arkansas. The heaviest rain will likely hold off until after midnight Friday night, with the diurnal strengthening of the low level jet, a faster push of the dryline and associated sharp gradient in moisture, as well as the peak divergence as the upper level shortwave trough amplifies a bit. All of these support the continuation of the Slight Risk over Arkansas, which was shrunk a bit due to better confidence in the placement of the heaviest rain, allowing areas further north towards Paducah to be dropped from the Slight Risk. With the wettest soils in Arkansas from about Little Rock west, this area was maintained in a higher-end Slight with this update. Elsewhere, lingering rain from Minnesota east through the U.P. will support isolated instances of flash flooding in that region, largely on Friday morning. A new Marginal Risk was introduced from southeast Virginia through the Carolinas and into Georgia. While convection in this area will likely remain disorganized and dominated by cold pool forcings, the abundance of moisture on the order of over 2 inches of PWAT will still favor any storms that form having the capability of multiple inch per hour rainfall rates. The Marginal from Montana to northeast Colorado was maintained with minimal changes along the western interface of the deeper moisture over much of the Plains. Wegman Day 3 Valid 12Z Sat Jun 14 2025 - 12Z Sun Jun 15 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM CENTRAL ARKANSAS THROUGH NORTHERN MISSISSIPPI... ....Arkansas through Alabama... After multiple days of shower and thunderstorm activity in this area, there should be some decrease in the organization of the overall shower and thunderstorm activity expected Saturday from Arkansas east through Alabama. Despite that decrease, very wet soils through Mississippi and a lingering frontal boundary providing some forcing should still lead to enough organization to support widely scattered instances of flash flooding. For this region, a Slight Risk area was introduced for Arkansas and Mississippi with this update. The flash flooding risk has likely decreased a bit due to a lack of upper level support by Saturday, so the Slight Risk remains a lower end Slight. Plentiful moisture over 1.5 inches of PWAT will still support any storms that do form to be capable of heavy rainfall. Further, a lack of forcing and organization will increase the time any storms may impact any one area, to include much of the period, day and night. In other words, there isn't one specific period of time where the threat for heavy rain and subsequent flash flooding is significantly higher than any other point. ....Mid-Atlantic... A second signal for heavy rain has shown in the guidance across northern Virginia and into portions of the Tidewater. A slow, southward moving front over PA will interact with the divergence ahead of the slow-moving shortwave trough that caused the problems with flooding further west the past couple days. Since this signal is relatively new and highly subject to change, a Slight risk upgrade has not yet been introduced, but is being considered. Should the heavy rain signal drift north to the DMV area, then a Slight Risk will likely be needed. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9Cgrj3qKAuD-HkFzQHvmTb0O_E0Hesp6lP5VxWstDj_1= uTo7CzGG-d5VqQbD_eOLvjmmV8YP82UBxeZ1EEsP9ePuHS4$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9Cgrj3qKAuD-HkFzQHvmTb0O_E0Hesp6lP5VxWstDj_1= uTo7CzGG-d5VqQbD_eOLvjmmV8YP82UBxeZ1EEsPSbN6Az0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9Cgrj3qKAuD-HkFzQHvmTb0O_E0Hesp6lP5VxWstDj_1= uTo7CzGG-d5VqQbD_eOLvjmmV8YP82UBxeZ1EEsP3baNZqM$=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. Otherwise email Chris Novy at cnovy@cox.net and ask to be removed from WX-STORM. --- SBBSecho 3.20-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .