Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jul 24 2024 20:15:52 FOUS30 KWBC 242015 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 415 PM EDT Wed Jul 24 2024 Day 1 Valid 16Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR THE UPPER TEXAS COAST AND SOUTHWEST LOUISIANA... ....16Z Update... No major changes were made to the Moderate and Slight risk areas. For the Moderate, the steadiest and heaviest rainfall has just about ended across the area for the day. A weak disturbance/mesolow over Texas locally enhanced the forcing resulting in heavy rain from the southeast tip of Texas near Port Arthur north along the Sabine River to about the Toledo Bend Reservoir. The heaviest rain has now shifted into central Louisiana...leaving behind much lighter rain along much of the Texas Coast. The storms are largely being=20 driven by anomalously high moisture with PWATs to 2.4 inches and=20 instability over the Gulf, which continue inland enhanced by the=20 aforementioned mesolow. With daytime heating, expect the mesolow to largely dissipate and more widespread instability to result in=20 decreasing organization to the storms. The Moderate Risk now is=20 largely to account for an expected redevelopment of shower and=20 thunderstorm activity tonight over much of the same area along the=20 TX\LA border near the Gulf Coast. For the rest of the Southeast, the Slight largely remains the same as the stalled out front continues to draw plentiful Gulf moisture northeastward, resulting in training convection over many areas hard-hit in recent days with heavy rainfall. NASA Sport imagery shows that for nearly all of the area drawn in the Slight from Texas to Virginia, soils are at or near saturation with the exception of central Georgia and southern South Carolina. Thus, additional expected rainfall due to training storms will result in widely scattered flash flooding. For the Marginal out west, the area was greatly expanded with this update to include much of eastern and southeastern California, as well as eastern Oregon, central Idaho, and western Montana. In recent days even isolated storms have been able to produce flooding due to the sensitivity of the soils in those areas due to terrain, and a repeat day is expected once again this afternoon. Thus, isolated flash flooding is likely...though exactly where that happens is very difficult to say. Wegman ....Previous Discussion... ....Texas and Louisiana Coast... After coordination with the local Lake Charles and Houston WFO's, a targeted Moderate Risk was issued across the Upper Texas coast into Southwestern LA, including Galveston Island. At the surface, a coastal trough is positioned just off the Upper TX coastal plain with a deep moist axis aimed orthogonal to the coast between Galveston up through the Lower Sabine Valley. The entire area within the coastal plain is positioned within a very anomalous axis of elevated moisture with PWAT deviations running between +3-4 from CRP up through all of Southeastern TX into LA. 00z soundings out of KLCH depict a very tropical airmass with a deep moisture presence through the column, basically running from the surface to the tropopause. Warm cloud layer depth is running around 15.5k feet, a classic signature of heavy rain potential with a greater propensity to exude very efficient rainfall rates from warm rain processes. Even within any deep layer convection, this would spell trouble for local rates breaching 2"/hr with the sampled environment generally capable for upwards of 4"/hr within the stronger cell cores. The latest HREF hourly rate probabilities are indicative of just that with a corridor of elevated probability signatures between 25-40% for rates exceeding 3" within the confines of the coast stretching from Galveston up into places like Port Arthur/Beaumont as the frictional convergence regime begins towards dawn this morning and maintains prominence through the early afternoon before dissipating. The probability fields for total rainfall were the biggest signal for the risk upgrades with the HREF EAS outputs considerably bullish within the 2" (45-60%) and 3" (15-30%) markers. Considering the conservative nature of the EAS due to the process which it's calculated, this is a significant output that is typically reserved for those higher end potential events. Assuming the factors involved and the presence of deep, tropical moisture advection off the Western Gulf, there was enough of a signature for a MDT risk upgrade in the location with the highest probabilities and mean QPF footprint indicating a solid 3-6" with locally as high as 9" possible, indicated with the LPMM HREF. ....Southeast to Southern Mid Atlantic... The same quasi-stationary front that has been stuck in the Southern Mid Atlantic down through the Southeastern U.S will continue to be a boon for the the region as another round of heavy thunderstorms will develop within an axis of elevated theta-E's located along the boundary from LA to Southeastern VA. Several small impulses also within the stagnant upper pattern will aid in the necessary upper forcing to promote small cell clusters of storms capable of a larger areal extent of heavy rainfall that would promote flash flood concerns within more urbanized corridors across the South, along with any slower moving convection that gets anchored to lingering cold pools that are running rampant from the previous days of convection. A deeper moisture flux will be entering into the Deep South with an embedded stronger mid-level vorticity maxima pivoting northeastward after it exits the Gulf. This will spur a more organized convective cluster downstream over places like MS/AL/GA. This is causing a well defined bullseye of higher precip within that corridor and is represented within the probability fields with the >3" neighborhood output hovering between 40-70% extending from Southern MS across into the SC Low Country. The previous SLGT risk was expanded further west to account for the convective signatures all the way into LA. ....Northeast... A tongue of elevated theta-E's will be advected north on the lee side of the upper trough pivoting through the Great Lakes. To the southwest, a blossoming jet streak will develop over the Southern Ohio Valley and begin nosing into the interior Northeast by later this afternoon. At the surface, a cold front will be sweeping eastward out of Ontario through Western NY and PA leading to increasing low-level convergence within the axis of greater instability. All this to say that scattered thunderstorms with rainfall rates between 1-2"/hr will be plausible across the interior with the highest chance for flash flood concerns within Central and Upstate NY where the greatest merger of upper forcing and repeated convective impacts will occur later this afternoon and evening before the front kicks the threat downstream. HREF neighborhood probabilities are generally high for at least 2" across the Adirondacks back towards the eastern Finger Lakes. The probabilities for at least 3" are still fairly modest within the neighborhood output with a bullseye of 40-50% located over the Adirondacks. The signals is scattered for the highest totals within the deterministic outputs, so the setup is right on the higher end of the MRGL and on the cusp of a small SLGT risk area. For now, maintained continuity but will note the threat of a targeted upgrade if the trends continue upwards in the next update. ....Midwest and Ohio Valley... Surface low located over the Great Lakes will begin to occlude and track northeast through Ontario with a trailing cold front swinging through the Midwest and Ohio Valley later this morning and beyond. A general consensus within the CAMs for a round of convection to develop in-of IA and IL later this morning, spreading southeast along the confines of the front. Flow will be modestly convergent along the leading edge of the boundary with some favorable large scale ascent caused by the northern half of the area sitting within the left exit region (LER) of a blossoming jet max situated over KY. Observing the latest forecast soundings off some of the CAMs indicate a signature for locally heavy rainfall with some potential training as the flow becomes a more parallel to the front as we move into the evening hrs. The threat will not persistent however, as the front will be on the move through the entirety of the forecast. This has limited the overall potential of the setup to mainly 1-2" maxes with perhaps as high as 3" as per the latest HREF neighborhood probabilities of >3" settling between 15-25% across portions of Central IL with lighter 5-10% markers further east in IN/OH. This was enough for a continuation of the inherited MRGL risk with only some minor adjustments necessary to reflect the latest QPF and probability fields. ....Southwest... Scattered Monsoonal convection will continue for another period across the Southwestern U.S with the primary heavy rain footprint aligned within the Mogollon Rim up through the Great Basin. Areal SBCAPE values will range from 500-1500 J/kg through a large extent of the west with PWAT anomalies generally +1 deviations across AZ and NM with +2 and +3 deviations aligned from Northern CA across into NV. Highest moisture anomalies will advect eastward through the period as a shortwave trough across the PAC NW begins to flatten the northern extent of the ridge sending the more prominent moisture anomalies into Northern NV, Southern ID, and Western UT. Current QPF signature within guidance is relatively scattered when it comes to the higher values, but some 1-2+" totals are likely within the current setup with the best chance focused across the Mogollon Rim and the terrain west of I-19 in Southern AZ based on the latest ensemble QPF output and probability fields. The MRGL risk from previous forecast was maintained with only a few small modifications to reflect the instability gradient and forecast QPF footprint. Kleebauer Day 2 Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE MIDDLE TEXAS COAST THROUGH CENTRAL LOUISIANA... ....2030Z Update... ....Texas & Louisiana... In coordination with HGX/Houston, TX and LCH/Lake Charles, LA forecast offices, a Moderate Risk upgrade was introduced with this update. A couple lows, one in the upper levels south of Brownsville and a second one off the coast of Brownsville are causing multiple areas of convection, most of which off the coast. However, the lows appear likely to combine forces tonight as they track close to due north. This will bring widespread convection, likely enhanced by the strengthening nocturnal LLJ, into the Moderate Risk area late tonight. As is common with these events, the rainfall will largely be split between days 1 and 2, but the primary flooding impacts are expected into the day 2/Thursday period. Expect the heaviest rain along the Middle and Upper Texas Coast, generally between Matagorda and Galveston Bays. While this area has not been as hard-hit today, the likelihood of training storms capable of up to 3 inch per hour rates is high enough in this area that numerous instances of flash flooding will be likely starting late tonight but continuing well into Thursday morning. Meanwhile up into Louisiana and far east Texas, that same convection is likely to behave very similarly to the convection this morning...namely to move well inland from the coast across central Louisiana before sufficiently weakening so as not to be as great a flash flooding threat by the time the moisture reaches northern Louisiana. Thus, the portions of central Louisiana that are included in the Moderate Risk are more heavily relying on the antecedent conditions initiated this morning when rainfall amounts of up to 7 inches were reported in the last 24 hours near Leesville, LA. Multiple flash flood warnings were issued into western and central Louisiana this morning. Additional rainfall expected Thursday, even if lighter in intensity than areas further south and west, are still likely to result in additional flash flooding for the area given the favorable antecedent conditions.=20 In addition to introducing the Moderate Risk, the surrounding Slight risk was adjusted eastward based on both trends in the guidance and the favorable antecedent conditions no present into=20 portions of northern Louisiana. ....Southeast... There has been a small eastward shift in the axis of heaviest rainfall across the Carolinas, resulting in a trimming of the Slight out of portions of central North Carolina and Georgia. There remains concerns into portions of the western NC Appalachians for widely scattered instances of flash flooding as well as across all of SC, so the Slight remains in place for those regions. ....Southwest... The Marginal Risk area across the Southwest remains largely unchanged. Persistent anomalous moisture remains across the area, but should begin to shift east on Thursday. Thus, the likelihood for convection across California has decreased enough that a Marginal Risk was not drawn in with this update. Wegman ....Previous Discussion... ....Southeast to Southern Mid Atlantic... A cold front will migrate to the southeast pressing through the southern half of the Mid Atlantic while the at the upper levels a 130kt jet streak will be passing over New England leading to better large scale forcing within the RER of the jet across the aforementioned area. Ample pre-frontal instability within anomalous deep layer moisture will also contribute to the favorable environment for heavy rain potential, especially as the front approaches and enhances low-level convergence within the confines of the boundary. After destabilization will aid in priming the environment prior to the fronts approach with the initial area of interest lying over Southeastern VA, eventually shifting focus into the Carolina's as the front progresses south. This is the area of greatest significance given the best surface to upper level forcing presence working in tandem to create a period of widespread heavy rainfall. Ensemble QPF between 2-3" is common within the confines of the front from the Hampton Roads area down through Eastern and Central NC/SC with much of the rain occurring within a short time frame as the front approaches. This is a signature for heavy hourly rates that will enhance the flood risk over the region. For that, the SLGT risk was maintained from the previous forecast. Across the Southeast, deep moisture presence with several mid-level perturbations entering the area through the period will assist in maintaining a scattered thunderstorm signature with heavy rain threats continuing thanks to the persistent elevated moisture anomalies and ample instability. The threat is not as pronounced as what will occur further to the north, but the setup is sufficient for any cell producing flash flooding, especially over GA/SC where the best instability is located. ....Southwest and Southern Rockies... 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. The area of interest remains across the Mogollon Rim down into Central AZ where Locally 1-2" with upwards of 3" is possible currently in that zone due to a stronger axis of instability under which could end of being near a 600dm ridge on Thursday afternoon. Across the north, a more prominent moisture advection pattern will continue through the Interior Mountain West as a shortwave trough over the PAC NW continues to squash the northern extent of the ridge and funnel the moisture further into the interior after it began navigating out of the CA/NV. More widespread convective coverage is forecast across UT/Southern ID/Western WY as a result leading to an expansion of flash flood concerns within those areas. The MRGL risk was generally maintained from previous forecast but did expand the risk slightly on the northern periphery to account for the moisture anomalies and accompanying forecast instability across those northern zones. Kleebauer Day 3 Valid 12Z Fri Jul 26 2024 - 12Z Sat Jul 27 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF THE EASTERN CAROLINAS... ....2030Z Update... No major changes were needed to the Slight and Marginal Risk areas across the country for the Day 3/Friday time frame. The focus will shift to the Carolinas alone on Friday as activity in Texas greatly diminishes as compared with Thursday. With some uncertainty on the northern and southern ends of the Slight, it was expanded north into the Hampton Roads of VA as well as south to Hilton Head Island, SC with this update. Elsewhere the slow eastward drift of the moisture across the West continues on Friday as well as diminishing amounts, so the Marginal remains largely the same as inherited, but a bit smaller than previous days. Wegman ....Previous Discussion... ....Carolinas... The cold front from the previous periods 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" over the aforementioned area of interest, but can already see the writing on the wall for locally heavier precip that will cause some flash flood concerns within more urbanized areas, especially along and east of I-95. Thus, maintained general continuity with some adjustments on the northern and southern edges of the risk area. ....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. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-RcCuhPu4ebcPqCJ5B8D12mrudg8RrCBgtYDTGpPMJnB= 1qEK_2AfNSBIy0tXobGHKfowUFmgjxPjkX_zb6R3tCHieVU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-RcCuhPu4ebcPqCJ5B8D12mrudg8RrCBgtYDTGpPMJnB= 1qEK_2AfNSBIy0tXobGHKfowUFmgjxPjkX_zb6R3rjwPn1o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-RcCuhPu4ebcPqCJ5B8D12mrudg8RrCBgtYDTGpPMJnB= 1qEK_2AfNSBIy0tXobGHKfowUFmgjxPjkX_zb6R3yOnCTfg$=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) .