Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 12 2025 20:03:30 FOUS30 KWBC 122002 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 402 PM 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 gradient is well-documented within both the latest Mesoanalysis and the derived GOES-East satellite parameter. Values between 1000-2000 J/kg are still situated within the confines of the MCS over the middle TX coast with a 2000-3000 J/kg signature still aligned upwind of the flanking line extending back inland. Cloud streaks moving northward across Deep South TX depict a sufficient moisture advection pattern continuing across the region and will play a role in the convective maintenance through the next 2-4 hrs. The good news is the cold pool associated with the setup is already starting to race out into the Gulf on the northern flank of the complex with the back edge beginning to see some advancement of the outflow ahead of the main line which will likely degrade the convective pattern later this afternoon. As a result of the evolution, a sharp delineation of the convective threat for the rest of the period has materialized with a likely diffusion of the 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 flanking line. Rainfall rates of 2-4"/hr are possible within the 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. 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". 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. 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 motions will enhance locally heavy rainfall capabilities with rates between 2-3"/hr plausible in a few of the stronger cores. Urbanized zones within the region referenced above will be the most susceptible for flash flooding prospects with some of the latest CAMs going as high as 4-5" in some of the greater outputs anticipated. A MRGL risk is in place for the areas above with a minor expansion on the southern and western edge of the inherited risk. 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... ....Arkansas/Mid-South... Slowly-moving upper low will be centered over the Ozarks Friday morning and drifting northeastward amid a very moisture-laden environment. Instability will remain modest as the cool core passes through, which will support another round of afternoon storms (following rainfall today) with rainfall rates of 1-2.5"/hr.=20 Increase in LLJ overnight tomorrow night may again enhance some=20 rainfall over the area, and have maintained the Slight Risk for=20 excessive rainfall over the region with a small extension=20 eastward/northeastward into the stronger IVT zone. Soils are wet=20 and FFG values will lower today/tonight, leaving the area=20 (especially in the hillier terrain) more susceptible to=20 localized/scattered instances of flooding. With the wettest soils=20 in Arkansas from about Little Rock west, this area was maintained=20 in an internally-denoted higher-end Slight Risk.=20 ....Mid-Atlantic/Carolinas... Expanded the Marginal Risk northward to near the PA border and westward into eastern West Virginia given an increase in QPF from the 12Z guidance. Frontal boundary sinking southward will stall over the region, acting as a focus for heavier rain and rates (1-2"/hr) where it has been rather wet coming out of May. Over VA into the Carolinas, PW values will remain elevated (1.75-2.00") and some localized flash flooding is possible. CSU Machine Learning ERO guidance shows a small Slight Risk area from near the DC Metro area southward, so this area will need to be watched further. ....Northern WI/Michigan U.P.... Area of low pressure moving eastward through MN may help drive a small risk for heavier rainfall over northern WI into the U.P. Friday morning, but instability is limited.=20 ....Northern Rockies/Western High Plains... The Marginal from Montana to northeast Colorado was maintained=20 with minimal changes along the western interface of the deeper=20 moisture over much of the Plains.=20 Fracasso/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. The Slight=20 Risk area was maintained for Arkansas and Mississippi to capture the early morning rainfall potential and due to previous expected rainfall (and lower FFG values). Plentiful moisture over 1.5=20 inches of PWAT will still support any storms that do form to be=20 capable of heavy rainfall.=20 ....Mid-Atlantic... Maintained the Marginal Risk area much of the Midwest to Mid- Atlantic that remains in a moisture-laden environment near and south of a frontal boundary. Targeted upgrades to Slight Risks may be warranted pending rainfall distribution over the next two days (especially over areas that have above normal soil moisture). CSU Machine Learning ERO guidance does show a Slight Risk over northern VA so this will be monitored in future shifts. Fracasso/Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-op0PBiZVctE8T_Ck9Xj-3ORHNd0BGGihOIxvzphA7ha= scX_xl4JswLCNi_pvh7jVz1ph2svG6huroHK35CMVTaQFYo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-op0PBiZVctE8T_Ck9Xj-3ORHNd0BGGihOIxvzphA7ha= scX_xl4JswLCNi_pvh7jVz1ph2svG6huroHK35CMsI34UJA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-op0PBiZVctE8T_Ck9Xj-3ORHNd0BGGihOIxvzphA7ha= scX_xl4JswLCNi_pvh7jVz1ph2svG6huroHK35CMh_6ZXKI$=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) .