Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jul 23 2024 20:14:56 FOUS30 KWBC 232014 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 414 PM EDT Tue Jul 23 2024 Day 1 Valid 16Z Tue Jul 23 2024 - 12Z Wed Jul 24 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF THE SOUTHERN PLAINS AND LOWER MISSISSIPPI VALLEY AS WELL AS THE SOUTHERN MID ATLANTIC... ....16Z Update... No major changes were needed to most of the ERO risk areas for today. The guidance remains somewhat steady state highlighting portions of south central and southeast Texas as well as the Carolinas for the most rain this afternoon through tonight. This continues to match well with the inherited Slight risk areas. One of the two changes of note was to trim back the Slight and Marginals around the Big Bend of Texas, as much of the expected rain has already happened. The front causing the rain will continue to sag south and east today, so no redevelopment of heavy rain is expected along the portion of the Rio Grande north and west of Del Rio. The other change was to expand the Slight in NC and VA westward to include more of the flood-sensitive mountains along the NC/TN border as well as portions of southwest Virginia. The front to the west is likely to direct training storms northeastward into the region later this afternoon into this evening. Given the saturated soils and the sensitivity to flooding of the terrain of this region, the upgrade appears necessary. Wegman ....Previous Discussion... ....Southern Plains and Lower Mississippi Valley... Two distinct areas of interest for the D1 period with different synoptic and mesoscale evolutions to impact each area. The first area will be across Central TX within the Edwards Plateau, Hill Country, to the I-35 corridor. A persistent shortwave embedded within the base of the trough located over the Southern Plains will enhance regional upper forcing through the first half of the forecast period, interacting with a weak quasi-stationary boundary bisecting the aforementioned area. As high pressure noses in from the north, a tightening theta-E gradient will transpire within the confines of the shortwave impulse. Enough favorable large scale forcing within a weakly capped environment will allow for the initiation of a line of convection located within the proxy of the front. Models are in agreement on the development and overall impacts expected within the line of convection promoting locally heavy rainfall within areas that were impacted over the previous 24 hrs. Area FFGs are much lower within the Edwards Plateau out through Hill Country after yesterdays storms leading to a lower threshold for impacts from flash flooding as the top layer of soil moisture now sits between 60-80% according to the latest NASA SPoRT viewer. Neighborhood probabilities for at least 3" are up around 50-80% within a large portion of the above area, extending eastward to the I-35 corridor from Killeen down through New Braunfels. >5" probabilities are also present, but more scattered in the presentation with percentages closer to 15-25% leading to a somewhat capped upper potential. This was sufficient for the current SLGT risk continuity with minimal change from the previous forecast. Further southeast into the Texas coastal plain and the adjacent Lower Mississippi Valley, the pattern will yield a much different impact scenario thanks to the evolving upper levels leading to a persistent Gulf moisture advection regime within a developing coastal trough. Small impulses will round the western periphery of a mid-level ridge extending through the Gulf, making their presence known as they enter the proximity of the middle and upper TX coasts. Bands of heavier rain with tropical origin in the moisture field will promote efficient rain makers as they work their way onshore. Just away from the coast, adequate surface buoyancy with deep moisture presence will allow for the generation of more thunderstorms just away from the coast leading to a larger QPF footprint between the two convective initiation standpoints. As of now, the prospects for flash flooding are highest along the Upper TX coast between Houston to the southwest corner of LA as all deterministic, ML, and associated ensemble means have overlapped with the best focus of heavy rainfall around that corridor in question. Areal average of 2-4" will be forecast within that span of the coastal plain with QPF maximum upwards of 7-8" possible if one particular area sees extended training. Probability fields indicating at least 5" are running between 25-40% across an area between Matagorda through Port Arthur right along the coast. The probabilities are a bit less for the >5" totals away from the immediate coast, but still generally between 10-20% for places like Houston and Beaumont. This is bordering on a higher risk category, but the protrusion of heaviest rainfall being mainly at the coast leaves this on the fence, but well within the upper bound of the SLGT risk. Will be monitoring the progression of the setup closely as a targeted MDT is possible, especially if conditions allow for the population centers within Houston/Galveston up towards Port Arthur and Beaumont. Some of the moisture entrainment within the northwest flank of the ridge in the Gulf will be pulled inland to the northeast over LA and MS leading to an expected band of heavy thunderstorms capable of rates between 2-3"/hr at peak and totals pushing 2-5" locally extending from Lake Charles up through Jackson, MS. The prospects for localized flooding within urban corridors has grown from recent updates allowing for an extension of the SLGT risk over TX to be pulled more northeast to account for the threat. ....Southern Mid Atlantic... Another day of scattered to widespread convective impact anticipated across the Southern Mid Atlantic with the heavy rain threat continuing within Central and Northeast NC up through the VA Tidewater. Heavy rain is likely within any cell development thanks to a persistent +2 deviation PWAT anomaly situated south of the quasi-stationary front bisecting the Central Mid Atlantic. Multiple smaller impulses will advect from the southwest with the mean flow aloft along with entering into the RER of an upper jet streak forming to the north. This will correlate to cell initiation within more favorable upper dynamics and modestly buoyant environment in place over the Southeast VA through the Carolinas creating another threat of heavy rainfall and repeated impact of cells along the corridor from the SC Piedmont and points northeast. HREF EAS probabilities between 25-40% for at least 1" covers a large expanse of the Southern Mid Atlantic, a traditional symbol for agreement within the CAMs on a widespread convective heavy rain threat and within the threshold for a SLGT risk upgrade. Based on the setup, repeated threat continuing of heavy rain and flash flood concerns, and the probability fields insinuating more 3+" totals locally in Southern VA and through NC, a SLGT risk was issued over the aforementioned area(s). ....Southwest and Southern Rockies... Scattered convection will occur once again across much of the Southwestern U.S with isolated threats of flash flooding within the flashy complex terrain, remnant burn scars, and slot canyons located within the Great Basin. The ridge axis will shift a bit further to the west aligning more of the convective potential through eastern CA and much of the southern half of NV leading to more coverage of the MRGL risk across those areas. QPF means are generally light, but some of the higher end outputs within the deterministic suite are upwards of 1-2", especially across the Mogollon Rim. This is a classic Monsoonal setup with the favored terrain being the primary focus for the period. The MRGL risk inherited was maintained with full continuity as ensemble mean QPF did not change much in terms of magnitude and precip placement from previous forecast. ....Midwest... Shortwave trough across the Great Lakes will continue to push southeastward with modest upper forcing under the trough axis. Modest instability will linger across the Midwest heading into today with some trailing vorticity maxima entering Central Midwest by late morning and afternoon. Convection will develop upstream over the Upper Midwest and pivot south into Central and Southern WI, IA, Northern IL, and eventually spread eastward into parts of Michigan. Isolated heavy rain signals within the CAMs backed by some modest probabilities for 1-2" and locally up to 3" promote a general continuation of the MRGL risk in place. Only some minor adjustments were made to the previous forecast given the relatively stable ensemble QPF footprint. Kleebauer Day 2 Valid 12Z Wed Jul 24 2024 - 12Z Thu Jul 25 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF SOUTHEAST TEXAS AND SOUTHWEST LOUISIANA, AS WELL AS PORTIONS OF THE SOUTHEAST... ....2030Z Update... ....Middle Texas Coast to Southwestern Louisiana... A slow-moving disturbance in the western Gulf at the southern end of a stationary front draped across the Southeast will be the focus for heavy rain along much of the western Gulf Coast on Wednesday. The front will make slow but steady progress east across interior Texas, but a persistent low-level jet (LLJ) of abundant Gulf moisture and instability will continue to pump that moisture into the front across east Texas and western Louisiana. There has been a small eastward nudge to the forecast axis of heaviest rain. Much of the heaviest rain will remain off the coast of Texas, but the southerly flow will threaten much of the immediate coast especially through Wednesday morning. Due to less instability inland because it's the coolest time of the day, it's expected that much of the heaviest rain will fail to make it very far inland before weakening and becoming much less of a heavy rain threat. That said, much of the area has seen multiple days of heavy rain, so rivers and streams...already running high in some areas from previous months...remain unusually susceptible to additional flash flooding should heavy rain occur over an extended period of time. Given the favorable pattern for training convection, this is certainly possible. On the flip side of the flooding threat coin is the susceptibility of the CAMs to have a north bias...meaning frequently heavy rain occurs south of there the guidance suggests it will be. Any southward deviation of the heaviest rain from where it's depicted now and the entirety of the heavy rain remains off the coast. Thus, there is a high bust potential. It's important to note with about 90 percent of the forecast heaviest 24-hour rainfall already off the coast, that the guidance appears to (rightly) suggest that the Gulf is the primary fuel source for the rainfall. Thus, it's reasonable to expect that if moving inland results in a rapidly decreasing supply of "fuel" for the storms, then it's a very reasonable expectation that the storms don't even try to move inland. It appears most likely that the storms that do impact coastal communities will move into southwestern Louisiana, rather than Texas. This is due to the predominant southwesterly flow supporting the storms tracking northeast...and the Louisiana coast would be the first land northeastward moving storms would encounter. Ultimately the positioning of the front, which would locally enhance lift, will determine how much rain occurs into Texas. ....Southeast... The Slight risk area across the Southeast was expanded in multiple directions with this update. Guidance indicates increasing likelihood for heavy rain across central Alabama. The area has been hit with multiple days of scattered thunderstorms that have locally produced heavy rain and saturated soils. NASA Sport imagery shows much of central Alabama well above normal for soil moisture and above 70% saturation in the near-surface soils. Thus, expect a renewed round of heavy rain in the area to have a higher-than- normal likelihood of resulting in flash flooding. The Slight was expanded southwest roughly following where the lower FFGs may coincide with heavy rain. The Slight was also expanded west to include the mountains of the western Carolinas. Low FFGs and terrain considerations will both locally increase the likelihood of flash flooding should heavy rain impact the area. Given the well above normal atmospheric moisture expected, as PWATs approach 2 inches, the storms will be quite capable of heavy rainfall. The good news is the storms will be moving along to the northeast, so it will take some training of multiple cells to produce significant flash flooding. Finally, the Slight was also expanded to include Hampton Roads of southeast Virginia and the portion of northeastern North Carolina north of Albemarle Sound. This was largely due to both ongoing and expected convection that will impact this area on top of the multiple days of storms that have already impacted this region. ....Southwest... Monsoonal moisture will result in increased coverage of showers and storms, primarily across Arizona and the Mogollon Rim again Wednesday afternoon as compared to today. However, limited organization should keep any resultant flash flooding isolated, so the Marginal remains in place with few changes. Wegman ....Previous Discussion... ....Texas and Louisiana... A continuation of the pattern from the previous period will lead to more shortwave impulses exiting out of the western Gulf as they navigate northward between a broad upper trough to the northwest and the persistent ridge off the southeast in the Gulf and Western Caribbean. A coastal trough will strengthen at the surface right along the mid and upper TX coast leading to a more defined axis of surface convergence in-of the coastal plain. Moisture anomalies will be poking closer to +3 deviations with area PWATs generally running between 2.2-2.5 along the TX coast all the way into Southwest LA. There is still some modest uncertainty as to where the best axis of heavy rainfall will preside for the D2 period, but the consensus is slowly merging towards the Upper TX coast with a grazing of the Mid TX coast for the heaviest rain potential. Recent ensemble QPF has an axis of 2-4" located right along the immediate coast beginning near Matagorda, expanding to the northeast to as far north as Lake Charles. Ensemble probabilities within both the GEFS and ECENS are targeting the Middle TX coastal areas to the southwest of Houston with the best probabilities for upwards of 2" while the NBM has a split maxima of one overlapping the area the GEFS and ECENS are targeting along with one to the north over Port Arthur into Lake Charles. The split comes for a discrepancy in the handling of a more robust shortwave that ejects to the north later in the period. Due to that discrepancy, a potential for an upgrade was passed on to take more time for guidance to come into agreement on where the focal point for the highest totals will occur. There is enough merit given the environment that if any area is favored for at least 4" within the mean, and probabilities favor the threat of over 5-6+", there could be a targeted MDT risk in future updates. For now, a broad higher end SLGT has been maintained with emphasis on the Middle and Upper TX coast, including near Houston proper. ....Southeast... A stronger mid-level impulse will eject northeast out of LA into through the Deep South within a corridor of moderate instability and deep moist access. The favorable upper forcing and accompanying thermodynamic presence has allowed for guidance to generate a widespread area of higher QPF along the confines of the impulses path. This is consistent within the ensemble bias corrected QPF footprint with 1.5-2.5" totals in the ensemble positioned from east-central AL through Central GA/SC into extreme southern NC. Considering the prospects for 2-3+"/hr rates within the convective zone Wednesday afternoon, a SLGT risk addition was warranted to cover for the potential. ....Southwest and Southern Rockies... Scattered convection will occur once again across much of the Southwestern U.S with isolated threats of flash flooding within the flashy complex terrain, remnant burn scars, and slot canyons located within the Great Basin. The ridge axis will shift a bit further to the west aligning more of the convective potential through eastern CA and much of NV leading to more coverage of the MRGL risk across those areas. QPF means are generally light, but some of the higher end outputs within the deterministic suite are upwards of 1-2", especially across the Mogollon Rim. This is a classic Monsoonal setup with the favored terrain being the primary focus for the period. The MRGL risk inherited was maintained with full continuity as ensemble mean QPF did not change much in terms of magnitude and precip placement from previous forecast. ....Great Lakes and Ohio Valley... Surface low across MI will push northeast into Canada with a trailing cold front progressing east and southeast over the course of Wednesday. Isolated to scattered thunderstorms will develop along and ahead of the cold front with some modest cell cores plausible given the increased surface and upper forcing pattern. QPF footprint is overall on the weaker side compared to what you would want to see regarding more considerable flash flood threats, but the environment is capable for some totals of 2-3" in a very short time which if it falls within any of the urban corridors around the Lakes could spell some isolated flash flood concerns. The threat overall is low-end within the MRGL threshold, but enough to warrant a continuation from previous forecast. Kleebauer Day 3 Valid 12Z Thu Jul 25 2024 - 12Z Fri Jul 26 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHEAST TEXAS, WESTERN LOUISIANA, AS WELL AS PORTIONS OF THE SOUTHEAST... ....2030Z Update... ....Southeast Texas into Western Louisiana... Much of the rainfall expected into this region for the day Thursday will occur Thursday morning, before the usual weakening of the LLJ result in decreasing coverage and intensity of convection by midday. Nonetheless expect a renewed southerly push due to the peaking of the LLJ's intensity, which will result in the stationary front over eastern Texas briefly becoming a warm front as it retrogrades westward. The heaviest convection will be in eastern Texas, including the Houston area since this will be the area where the aforementioned front will be aligned parallel with and be=20 closest to the Gulf. Much of the Upper Texas Coast is considered in a higher-end Slight, both due to expected convection and soils remaining saturated from previous days' rainfall. ....Southeast... The Slight Risk area was expanded well to the south with this update, and now includes much of northern GA, all of SC, and all of coastal NC. This is largely due to a small eastward shift in the axis of heaviest rainfall expected to impact the area during peak heating Thursday afternoon as the front forcing the storms makes a small eastward jaunt across this area. With well above normal moisture in place, unidirectional southwesterly flow favorable for training, and well-saturated soils from prior rainfall, flash flooding will be widely scattered in this area...with the greatest risk in any urban areas from Raleigh east. Wegman ....Previous Discussion... ....Texas and Louisiana... A general persistence in the pattern will yield another active day across the Texas and Southwest LA coastal plain. There's still not enough agreement on the potential for an upgrade beyond the current SLGT risk forecast. Pending the outcome from the previous periods, a targeted upgrade is not out of the question, but will not commit until we have more consistency in the ensemble QPF output and we have some idea of impacts prior to the period in question. For now, another 2-4" with locally higher amounts is plausible along the TX coast from Corpus Christi up through Lake Charles. Multi-day totals could be exceeding 8" in spots after this time frame, so it bears monitoring in the coming days. ....Carolina's to the Virginia Tidewater... A continuation of the active pattern across the Carolina's into Southeast VA will unfortunately resume for another day as the area sees an introduction of a stronger shortwave out of the Southeast along with entering into the favorable RER of a 125-130kt jet max positioned across the Northeastern U.S. The combination of the maximized upper forcing within a very moist, unstable airmass sets the stage for another round of heavy rainfall with cell cores capable of exuding another 2-3"/hr in rain rates with slower cell propagation as noted by the forecast Corfidi Vectors in NC hovering around 7-10kts within the upshear component of any storms. Ensemble bias corrected QPF signatures of 2-3" are already showing up across the Central and Eastern Carolina's with the northern extent peaking up into the Hampton Roads area. This area will have been through a multitude of heavy rain events in the past week, so another day will lead to issues, especially with one of this expected magnitude. A SLGt risk was introduced across much of SC/NC up through Southeastern VA as a result. ....Southern Rockies and Southwest... 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. One area of interest is across the Mogollon Rim down into Central AZ. Locally 1-2" with upwards of 3" is possible currently in that zone due to stronger axis of instability under which could end of being a 600dm ridge on Thursday afternoon. A MRGL risk is in effect across much of the climatologically favored areas in the Southwestern Monsoon, but the Mogollon Rim and potentially over Northwest UT will be favored for an upgrade if the current forecast holds. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_7jNdB-SNwfGSFDOvlWNfxEQ55L0JdHcX9QayqGJwWFF= QVXn6MtDP3Xur5rQ6yWYT5mvuvY9syK0XGkRKA43e71buq0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_7jNdB-SNwfGSFDOvlWNfxEQ55L0JdHcX9QayqGJwWFF= QVXn6MtDP3Xur5rQ6yWYT5mvuvY9syK0XGkRKA43oOwu36M$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_7jNdB-SNwfGSFDOvlWNfxEQ55L0JdHcX9QayqGJwWFF= QVXn6MtDP3Xur5rQ6yWYT5mvuvY9syK0XGkRKA43ZMmiJWw$=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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