Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Jul 26 2024 00:35:57 FOUS30 KWBC 260035 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 835 PM EDT Thu Jul 25 2024 Day 1 Valid 01Z Fri Jul 26 2024 - 12Z Fri Jul 26 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR THE MIDDLE AND UPPER TEXAS COAST... A Moderate Risk for excessive rainfall has been maintained for=20 this update for much of the Middle and Upper TX Coast, while being=20 downgraded for the portions of southwestern LA. Heavy rainfall has come to an end for the bulk of the region (and surroundings), but=20 the expectation is for refiring of convection once again late=20 tonight along the coast (where SB CAPE ranges from 1000-2500 J/kg with precipitable water values of 2.0-2.2 inches, which are near=20 the 90th percentile). A positively-tilted longwave trough remains=20 anchored just to the west (over TX), allowing for DPVA from the=20 southwest (at least partially derived from prior convection over=20 the terrain of northeastern Mexico). This locked pattern is tending to result in reinitiation of convection during the overnight=20 hours, and hi- res guidance once again depicts this potential=20 tonight (with 18z HREF 40-km neighborhood probabilities for 5"=20 exceedance of 20-30%). With 1-4" of rainfall (and locally higher)=20 over the past 24 hours alone, the area remains sensitive=20 hydrologically (with USGS gauges indicating above normal to near=20 record streamflows). Some models indicate intense convection=20 developing farther inland as well (to as far north as College=20 Station), and a Slight Risk has been maintained in these areas.=20 Otherwise, a Slight Risk was maintained for portions of the Carolinas and adjacent portions of eastern GA, where heavy rainfall is still ongoing (though will likely wane with loss of daytime heating). A broader Marginal Risk was maintained across the Southeast, where localized downpours may continue in the presence of widespread tropical moisture (with a large area of 1.8"+ PWATs). A Marginal Risk has also been maintained across much of the Southwest and Intermountain West, where monsoonal convection will gradually wane with the loss of daytime heating.=20 Churchill 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 EASTERN CAROLINAS... ....20Z Update... No major changes were needed for the Day 2 ERO. The front off the Carolinas has been resolved in the guidance a bit further east and off the coast, so the Slight has been adjusted accordingly. Of course, with a greater amount of the expected rainfall now expected to fall off the coast, uncertainty as to the potential for flash flooding along the coast has increased. 12Z HREF guidance still suggests an over 70% chance of 5 inches of rain in the Neighborhood Probabilities from Cedar Island, NC southwest to the South Carolina line. These amounts if they verify would almost certainly result in widely scattered instances of flash flooding. However, noting the tendency of the guidance to shift south and the trends already to shift the precip east, there is just as much potential for additional shifts eastward resulting in a downgrade to the flooding potential. Elsewhere no significant changes were made across the Southeast regarding the sprawling Marginal risk. Areas of largely disorganized convection are expected again Friday afternoon, and there will likely be localized areas where the convection organizes, resulting in better than isolated instances of flash flooding. However, there's little confidence elsewhere on where that will occur, but targeted Slights going forward are certainly possible. A very similar situation remains in the West with the higher atmospheric moisture area gradually shifting east with time. Wegman ....Previous Discussion... ....Carolinas... The cold front from the previous period will finally make progress off the eastern seaboard across the northern half of NC while making slow progress to the south through the rest of NC/SC region. Scattered thunderstorms within the persistent deep moist and unstable environment will allow for a period of heavy rain potential, especially along and ahead of the cold front as it migrates southward. The heaviest rain will align over Eastern NC into the South Carolina Low Country, areas where FFGs are a bit higher and potentially more difficult to flood. Previous rainfall has lowered the FFG indices, especially as you work from Columbia, SC to points northeast which plays a role in the current SLGT risk 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, and not expecting any upgrades in the current setup. ....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. 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 THE SOUTHEAST, MID MISSISSIPPI VALLEY, SOUTH AND EAST TEXAS, NORTHERN PLAINS AND UPPER MIDWEST, ALONG WITH PORTIONS OF THE SOUTHWEST AND INNER MOUNTAIN WEST... ....20Z Update... No significant changes were needed to the Day 3 Marginals as well. Convection across the Southeast and Mississippi Valley remain similarly disorganized as on Day 2 in the guidance. A cold front working down the Carolinas may help the storms organize ahead of it into southern SC and GA, but it's possible it will be moving too fast to result in a long enough duration of heavy rainfall to result in more than isolated flash flooding. The area will continue to be monitored should guidance come into better agreement with a slowing of the storms. One ingredient remains consistent...there remains plentiful atmospheric moisture from the Gulf in place across the Southeast, so there's little doubt the storms will be capable of heavy rainfall, but if they're moving too fast then it may not rain enough in any one location to result in flash flooding. A similar scenario may try to play out again along the Texas Gulf Coast...with mesolow development that may possibly help organize the storms that form over an area that has been hard-hit with heavy rain the past few days. Here too there's a "higher-end Marginal" threat for flash flooding that may eventually result in a Slight risk upgrade. Conditions across the West will continue to deteriorate as far as potential for flash flooding goes, so the Marginal Risk has been shrinking in this area day-to-day...however given the terrain and at least enough moisture in much of the West, it's going to be really tough to completely eliminate the flash flooding potential. The Northern Plains Marginal is probably the least confident of the Marginal Risk areas, as a slow moving front with limited moisture moves across ND and northern MN. Given the difficulty for this area to flash flood even with much more favorable dynamics in place, it seems this area has the best potential for the Marginal to be downgraded with future forecasts. Wegman ....Previous Discussion... ....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. ....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. ....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. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8CBAUcMTXixVRfHsfUZJnfnLvqVwIF7KW5-gHCC2gAcJ= GbWlrnNQGtJUiy6QFDoou8e_FkV2PUX_6xL-q_EWdDcH0x0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8CBAUcMTXixVRfHsfUZJnfnLvqVwIF7KW5-gHCC2gAcJ= GbWlrnNQGtJUiy6QFDoou8e_FkV2PUX_6xL-q_EWx9bvaRY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8CBAUcMTXixVRfHsfUZJnfnLvqVwIF7KW5-gHCC2gAcJ= GbWlrnNQGtJUiy6QFDoou8e_FkV2PUX_6xL-q_EWI2yY_Eo$=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) .