Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jul 25 2024 08:18:51 FOUS30 KWBC 250818 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 418 AM EDT Thu Jul 25 2024 Day 1 Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR THE MIDDLE AND UPPER TEXAS COAST THROUGH SOUTHWEST LOUISIANA... ....Texas and Louisiana... Current upper air and surface analysis across South TX indicates the development of a weak surface reflection with a closed 85H (850mb) low present as of the latest observations. This will play=20 a significant role in the upstream convective potential through the period, especially along the immediate TX coast from CRP up=20 through the Southwest coast of LA. There's a near unanimous=20 consensus on the latest 00z CAMs of a targeted area of significant=20 rainfall within the region spanning from Matagorda Bay up to just=20 south of Galveston during the first half of the forecast period.=20 850mb low across South TX will slowly migrate to the northeast=20 with a focused LLJ on the eastern flank of the circulation. The=20 enhanced jet and increasing low level convergence under the=20 surface and 850mb low(s) will generate a skinny axis of heavy=20 rainfall with embedded convection given marginally sufficient=20 MUCAPE located near the coast. 00z sounding out of KCRP was=20 indicative of deep tropical moisture entrenched within the column=20 as PWATs nestled just under 2.4", a solid 3 deviations above normal and within the 99th percentile climatologically, just falling=20 short of the daily maximum of 2.42". Regardless of a record or not, the environment is indicative of a moist, tropical airmass that=20 will exude efficient warm rain processes through any convective=20 development and the evolving pattern this evening through the=20 morning is one that will very likely promote intense rainfall=20 locally within the confines of the surface and closed 850mb=20 reflections. This setup will slowly migrate to the northeast during the morning, but the setup for later this afternoon and the=20 following evening still has the "jury out" on exactly how the 850mb low behaves.=20 As of now, the mean QPF output via the HREF and associated ensembles are depicting a narrow axis of significant rainfall totals extending from just northeast of Port Aransas up through Galveston Bay and eventually hitting the Upper Texas coast near the Sabine River, the same place that was just impacted recently from significant rainfall yesterday. The probability fields for >3" of rain are pronounced within that corridor, but the upper quartile outcomes and associated neighborhood probabilities for at least 5" (50-80%) and 8" (20-40%) are much higher further south along the Middle TX coast with the bullseye situated around Port Lavaca up through Matagorda with the former being the focal point for the heaviest rainfall as of the latest model output. Further away from the coast remains a question on the inland extent of the heavy rain as the QPF from many of the CAMs and even some global deterministic are all shying away from a larger protrusion inland with the focus still remaining primarily at the coast. So much of this is contingent on the behavior of the 850mb low as a longer=20 maintenance of the low across the coast would likely enhance a=20 secondary push of convection inland by nightfall thanks to a=20 redevelopment of the LLJ aimed orthogonal to the coast after 00z=20 this evening. Models that do have this outcome are more bullish in=20 a larger impact footprint to the north as the low drifts that=20 direction and allows for a greater inland focus to arise as we step into the back end of the forecast period. These solutions also=20 give credence to higher potential impacts for greater Houston, so=20 that will be something to monitor in time where there could be=20 adjustments to the current MDT risk forecast.=20 For now, the main threat is tied closer to the coastal areas with=20 the previous MDT risk generally maintained by expanded to the south to the northeast of CRP, mainly from Matagorda Bay on northeast.=20 The northeastern extent of the MDT was trimmed back across LA with=20 guidance becoming less enthusiastic with the push of deeper=20 moisture inland near Lake Charles and points north and maintaining=20 a more confined coastal heavy rain threat, similar to where the=20 impacts were felt this past forecast period.=20 ....Southeast into the Southern Mid Atlantic... An intensifying upper jet across the Ohio Valley will extend into the Northeastern U.S with a strengthening core upwards of 130kts reflecting some fairly significant large scale ascent within the right entrance region (RER) of the jet. At the surface, a cold front will translate east and southeastward with progression through the Mid Atlantic and the Southern Ohio Valley through the period leading to some modest surface convergence along the front with scattered to widespread convection out ahead of the boundary thanks to the present large scale ascent from the aforementioned jet. Sufficient low-level buoyancy located across Southern VA through the Carolinas into the Deep South will promote an opportunity of heavy thunderstorms with the highest probability located over the Carolinas into Eastern Ga due to the presence of a quasi-stationary front still bisecting the area. This will become=20 a focal point for low-level convergence and will provide an anchor=20 for storms to develop and slowly drift within the confines of the=20 boundary. Rainfall rates of 1-2"/hr will be most common in any=20 storm development, but a max of 3"/hr is still plausible given the=20 low probabilistic output of 10-20% over a large portion of the=20 Carolinas into far Eastern GA for those 3"/hr potentials. Mean QPF=20 output via the HREF was solidly between 1-2" spatially with the=20 individual CAMs output signaling some much higher local totals that could exceed 4" in some cases, a much greater threat for flash=20 flooding potential.=20 Considering the incessant nature of the pattern the past week with copious heavy rainfall episodes contributing to flash flooding across the Carolinas, a SLGT risk from previous forecast was maintained with a small shift on the western extent of the risk area where the probabilities dropped off significantly for the threat of flooding rainfall, carrying the signal closer to the=20 SC/GA line than previous forecasts.=20 ....West... Monsoonal convection will encompass much of the Southwest U.S with primary coverage in the Great Basin as the mid-level ridge pattern shifts focus to the west of the Four Corners. Modest moisture anomalies and relatively formidable instability across much of the region will allow for scattered thunderstorms with isolated heavy rain cores that could spell issues if they fall along complex terrain, burn scars, urban footprints, and slot canyons. More=20 widespread convective coverage is forecast across UT/Southern=20 ID/Western WY as a result leading to an expansion of flash flood=20 concerns within those areas. Neighborhood probabilities for the higher convective totals >1" are still most prominent across AZ with the focus areas being over the Mogollon Rim down through the Southern terrain near the border where the probs are running between 60-80% for the threat of at least 1". Secondary maxima are popping up across the Inner Mountains west within Utah and Western Colorado, but the threat is a little more subdued due to the lower instability outputs that favor further south under the 500mb ridge. Still, the moisture anomalies are forecasted to be best to the north, so there could be some sneaky prospects for locally higher impacts from convection, especially within the terrain and adjacent valleys. The MRGL risk was generally maintained from=20 previous forecast but did expand the risk slightly on the northern=20 periphery to account for the moisture anomalies and accompanying=20 forecast instability across those northern zones. Kleebauer Day 2 Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE CAROLINAS... ....Carolinas... The cold front from the previous period will finally make progress off the eastern seaboard across the northern half of NC while=20 making slow progress to the south through the rest of NC/SC region. Scattered thunderstorms within the persistent deep moist and=20 unstable environment will allow for a period of heavy rain=20 potential, especially along and ahead of the cold front as it=20 migrates southward. The heaviest rain will align over Eastern NC=20 into the South Carolina Low Country, areas where FFGs are a bit=20 higher and potentially more difficult to flood. Previous rainfall=20 has lowered the FFG indices, especially as you work from Columbia,=20 SC to points northeast which plays a role in the current SLGT risk=20 maintenance. Ensemble QPF output is currently between 1-2" across Southeast NC, including the Wilmington area extending down through the Low Country and coastal sections of SC. Individual deterministic output is as expected with some local maxima around 4" within any particular global member leading to ensemble bias corrected means closer to 2-2.25" over the hardest hit areas. Did trim back on the northern extent of the previous SLGT with maintenance further south and southeast across central and eastern SC up through Southeast NC where the front will slow in forward progression and become a focal point for Friday afternoon convection. Best threat for flash flooding will likely occur over more urbanized zones due to the sandier soils present in the confines of the current risk area which lead to the continuity,=20 and not expecting any upgrades in the current setup.=20 ....Southwest through the Inner Mountain West... Scattered thunderstorms within a favorable moist axis will linger through the Southwestern U.S with the northern half of the convective threat shifting further inland into the Interior before hitting the proverbial wall at the Central Rockies. Any storm within the period could exude heavy rainfall with rates generally topping at 1"/hr, an intensity that could easily cause issues within the complex terrain out west. The increased moisture advection back over NM will also open the door for more convective threats within the remnant burn scars in place over the Sangre de Cristos, so the MRGL risk was promptly positioned to cover for the threat. General rainfall maxima is expected to be between 1-2" but a small chance for locally higher remains, especially from any cells that get anchored to the terrain. The previous MRGL risk was kept to general continuity, but did pull the MRGL risk a little further west into NV due to some linger convective potential from a trailing shortwave migrating through the region during peak diurnal instability, aligned with a further west QPF footprint as of the latest NBM and ensemble bias corrected outputs. ....East Texas through the Lower Mississippi Valley... Remnant mid-level energy will linger across Eastern TX into the Lower Mississippi Valley within a persistent anomalous moisture presence situated over the region. Sufficient buoyancy and attendant large scale forcing will likely generate periods of scattered convection capable of producing locally heavy rainfall over the aforementioned areas. There are some indications that a much more defined 500mb low will develop over the Southern Plains which would increase the general forcing pattern allowing for a more robust convective initiation over the eastern flank of the mid-level circulation. This was noted via some of the 00z global deterministic with the ECMWF being the most aggressive with the solution. There are some hints of the threat becoming more pronounced across East TX into parts of LA/AR given the recent GFS Graphcast and ECMWF AIFS ML output, so a targeted upgrade may occur over the above areas if the consensus grows and remains stable in the upcoming model iterations. For now, a MRGL was maintained from previous forecast.=20 Kleebauer Day 3 Valid 12Z Sat Jul 27 2024 - 12Z Sun Jul 28 2024 ....THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF=20 THE SOUTHEAST, MID MISSISSIPPI VALLEY, SOUTH AND EAST TEXAS,=20 NORTHERN PLAINS AND UPPER MIDWEST, ALONG WITH PORTIONS OF THE=20 SOUTHWEST AND INNER MOUNTAIN WEST... ....Texas through the Mid Mississippi Valley... Pending the evolution of the 500mb pattern across the Southern Plains, the same areas will be subject to a continuation of the active convective pattern across South and East TX, spreading northeast into the Mid Mississippi Valley as indicated by a few global deterministic. Ensemble means are relatively benign in terms of areal coverage of at least 1" with the highest located along the TX coast due to potential surface trough refocusing over the Western Gulf. A deep moist environment with prevalent mid-level synoptics would certainly support at least the chance of heavy rainfall and accompanying flash flooding, so the risk area from previous forecast was relatively unchanged. There could be a future upgrade if the mid-level pattern progression supports a more dynamic setup away from the coast, and/or the coastal plains of TX see an increasing heavy rain footprint that would call for an upgrade. Until then, the consensus was not clear enough to warrant an upgrade, so maintained continuity.=20 ....Southeast... Scattered thunderstorms capable of locally heavy rainfall will be possible during peak diurnal convection on Saturday. The moisture anomalies continue to favor a primed environment for a few thunderstorms creating some isolated flash flood concerns with totals generally around 1-2" with a chance for 3+ inches in the strongest cells. This setup is on the lower end of the MRGL threshold, so some changes are possible in future updates. For now, with the signature present in a deeper moisture field, the previous risk forecast was maintained. ....Upper Midwest... A digging shortwave trough will enter the northern tier of the Central U.S with increasing large scale ascent over a relatively sufficient moist axis and accompanying surface based instability. The threat for locally heavy rainfall will be highest during the late afternoon and evening on Saturday with some isolated higher totals that could enhance local flash flood concerns. The previous forecast was maintained for continuity as the ensemble QPF output remains generally unchanged from previous forecast issuance.=20 ....West... Scattered convection across the Rockies down into Southeast AZ will continue for another period as the presence of sufficient deep layer moisture extending from the International Border up near the Northern Rockies will aid in potential heavier convective cores that could cause issues within the complex terrain aligned over the outlined area. The area across NM and a few spots over the Central Rockies will be the highest threat for flash flooding due to burn scar remnants creating a higher probability for hydrophobic based run off within the confines of the area. Targeted SLGT risks will be possible in the future, but the ensemble mean and scattered nature of convection is too much of a factor to account for a higher risk area. General continuity was maintained outside some minor expansions over the northern section of the risk.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_e4qGSydyc0xkbkGUqHBnWyjGxVs45w1oJ3-GVE4WHUF= FPRzpgVCJkJtJQ0WlI7yDhP0d9C8Eir11fDlYxVHCVB1PNg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_e4qGSydyc0xkbkGUqHBnWyjGxVs45w1oJ3-GVE4WHUF= FPRzpgVCJkJtJQ0WlI7yDhP0d9C8Eir11fDlYxVHjzUeZGo$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_e4qGSydyc0xkbkGUqHBnWyjGxVs45w1oJ3-GVE4WHUF= FPRzpgVCJkJtJQ0WlI7yDhP0d9C8Eir11fDlYxVHGllPTX4$=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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