Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Sep 07 2024 19:52:55 FOUS30 KWBC 071952 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 352 PM EDT Sat Sep 7 2024 Day 1 Valid 16Z Sat Sep 07 2024 - 12Z Sun Sep 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND WESTERN U.S... ....16z Update Summary... The SLGT risk was removed from the previous update in coordination with the impacted offices. A MRGL risk remains for parts of the Southeastern U.S, with a focus along the coastal plain of Southeast LA and the Emerald coast of FL. MRGL risks remain steadfast across portions of the Western U.S. Kleebauer ....Gulf Coast and Southeast... 16Z Update: The pattern from this morning have devolved into something less robust with continued rainfall, but paltry rates to suffice much of a flash flood concern. Recent mesoanalysis has only furthered the argument for less of a risk of heavy rainfall as MUCAPE indices remain relatively benign with the coastal plain of Southeastern LA over into the FL Panhandle only subject to ~500 J/kg of MUCAPE, at best. Analysis of radar trends indicate a limit to the overall convective output this morning, correlating well with the <1" totals over the past 6 hrs for areas it has been raining through that short window. Some diurnal destabilization will be had for portions of FL and Southern GA, but the coastal plain will be bordering on low-end probability for any instability regime to really overcome the prevailing cloudiness and the convergence axis shifting off the coast as drier air advects in from the north. Despite continued rainfall this afternoon with scattered thunderstorm concerns for the immediate coast, the prospects for anything more than a rogue over-performer will limit the threat to a mere MRGL risk, thus necessitating the removal of the previous SLGT risk in place. The northern extent of the MRGL was also trimmed away to tighten the gradient of the zone for best convective development. The setup may yield more flare further east as a better diurnal destabilization regime could yield convective pattern with stronger cores capable of localized heavy rainfall >1"/hr as signaled within the 12z HREF probs and associated CAMs. This setup is best aligned across the FL Big Bend into parts of Southern GA. This is still a lower end probability for flash flood concerns due to the higher FFG indices in place over the proposed area of interest. This was enough to maintain the MRGL risk across the northern half of FL. Kleebauer ....Portions of the Southern Rockies... 16Z Update: Little to no change occurred from the previous MRGL risk forecast across portions of the Western U.S. Scattered convective signals in-of the Southern Rockies, Mogollon Rim of AZ, and the Peninsular Range of Southern CA will all benefit from a persistent mid-level ridge axis with a slightly higher than normal PWAT accompaniment as it migrates around the western and northern flank of the ridge. Modest instability coupled with the better moisture presence will provide just enough to create an isolated flash flood threat within the terrain, and over remnant burn scars located within both MRGL risk areas. Kleebauer ...Previous Discussion.. The Marginal risks in these areas remain largely the same, other than trimming out areas of southern CA and northwestern NM where less topographic influence should generally preclude any flash flooding. The Marginals thus concentrate from the Peninsular Ranges of southern California over to the western Mogollon Rim of western Arizona, as well as for the Sangre de Cristo Mountains of southern Colorado and northern New Mexico. Any burn scars will locally enhance the flash flooding potential due to a much greater percentage of any rainfall converting to runoff. Wegman Day 2 Valid 12Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE WESTERN U.S, DEEP SOUTH TEXAS, AND THE SOUTHEAST... ....20Z Update Summary... Expanded the eastern periphery of the MRGL risk across the Western U.S to include more of the Four Corners into the Southern Rockies. Only minor adjustments made to the MRGL risk areas over South Texas and the Southeastern U.S. Kleebauer ....Western U.S... 20Z Update: Anomalous mid-level moisture flux will continue to=20 pivot around the western flank of the ridge positioned over the=20 Inter- Mountain West creating a "friendlier" environment for convective development across the Great Basin over into the Southern Rockies. 12z HREF mean QPF indicates a broad coverage of light to moderate totals within the interior portions of the west with the best QPF footprint co-located within the terrain west of the Divide, agreed upon within the neighborhood probability field of the hi-res ensemble. Probability for >1" off the HREF runs around 20-40% in areal average with a max of 60-70% across the=20 western Mogollon Rim, a sufficient signal for isolated to widely scattered flash flood chances historically. The main areas of focus will pertain to small urban zones, complex terrain, slot canyons, and remnant burn scars littered across the interior West. Considering the prevalent signatures above, there was enough consensus to expand the MRGL risk further east to account for the threat over the Four Corners and the Southern Rockies.=20 Kleebauer ....Deep South Texas... 20Z Update: Consensus is growing for the eventual advection of tropical air into Deep South TX on the northwestern periphery of a tropical wave traversing through the Western Gulf. A non-GFS consensus allows the second half of the period to become more active with pivoting bands of heavy rainfall off the Gulf to protrude inland with the highest prospects for impact located between McAllen and points east. A sharp gradient within the ensemble mean maintains precedence that this will be a focused=20 event, in the early stages of a setup that is anticipated to bleed=20 into the D3 time frame (More on this in the Day 3 ERO update). 12z=20 HREF >3" probability also reflects the tight western gradient that=20 will develop with the setup as 15-25% probs lie west of McAllen=20 with upwards of 50-60% located along the coast to Brownsville. The=20 tropical airmass will allow for sufficient warm rain processes=20 that are textbook for precluding heavy rain and flash flooding=20 events, so the threat will only see to ramp up during the back-end=20 of the forecast cycle. As of now, timing directs a MRGL risk with a steady transition to a higher risk as we enter into the next=20 period. For this, we maintained general continuity with the growing consensus in both anticipated timing and impacts.=20 Kleebauer ....Southeast... 20Z Update: No real changes to the previous forecast. Did trim back the northern periphery of the MRGL risk given the latest ensemble mean QPF and a growing consensus for the best instability to remain focused along the Gulf coast and interior of the FL Panhandle.=20 Kleebauer ...Previous Discussion.. The eastward extent of the same front as impacting Deep South Texas will remain across northern Florida (both Panhandle and Peninsula) as well as southern Georgia. The same pattern will remain on Sunday as on today, in that instability will be greatly lacking with any distance from the coast, so the greatest threat for flooding will be along the immediate Gulf Coast of Florida. Wegman Day 3 Valid 12Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP SOUTH TEXAS... ....20Z Update Summary... Limited changes were necessary for the SLGT risk located across Deep South TX, along with the MRGL risk over portions of the Southeastern U.S. A MRGL risk was introduced over the Interior Western U.S as convective potential lingers along and west of the Continental Divide.=20 Kleebauer ....Coastal Deep South Texas... 20Z Update: There is now a fairly good consensus within the global deterministic and ensemble/blend output to continue the SLGT risk across Deep South TX as the tropical wave over the Western Gulf will slowly migrate northward during the period. The best chance for heavy rainfall remains focused within the coastal areas of South TX, mainly south of KCRP with the bullseye pegged closer to KBRO up through the King Ranch area. This is a continuation from the D2 time frame with multi-day totals exceeding 4" becoming increasingly likely within that coastal portion of South TX, an area a little more prone to flash flooding due to the urbanization aspects of the area. As of now, the signal still remains more beneficial to have a SLGT risk and not a higher grade, however it will be a period to monitor with short term trends likely to dictate any further upgrades as the system evolves. For now, general continuity with a mention of a higher-end SLGT confined to the coastal portion of Deep South TX.=20 Kleebauer ...Previous Discussion.. A tropical wave moving across the Gulf will continue to spread heavy rains along much of the Gulf Coast on Monday, continuing from Sunday. The bulk of the heaviest rain associated with the wave is likely to impact coastal Deep South Texas and adjacent coastal northeast Mexico. Thus, there is higher confidence that on Monday very heavy rainfall will occur along the coast of Deep South Texas as compared with Sunday. However, as mentioned in the Day 2 discussion, there is increasing uncertainty as to the track of the tropical wave, which greatly impacts the forecast and the potential for flash-flood-producing heavy rains. Guidance has been trending east/offshore with the heaviest rain, so as on Sunday, any further eastward shifts will require downgrades to the ERO risk areas. For now, the Slight risk upgrade is contingent on at least 2 days (today and Sunday's) rains further lowering FFGs in advance of the main rainfall event in the area on Monday. If Monday is the third consecutive day with heavy rainfall, as it's highly likely to be the wettest of the 3 days, then it stands to reason compounding influences by increasingly saturated soils will quickly increase the potential for flash flooding. ....The Rest of the Gulf Coast... 20Z Update: Only minor adjustments were necessary from the previous forecast package as ensemble output remains steadfast with little deviation in proxy and magnitude of expected rainfall. The period will still need to be monitored for targeted upgrades, especially over those locations that were hit hardest in the past few periods. Kleebauer ...Previous Discussion.. No change to the pattern is forecast for the Gulf Coast from upper Texas east into Florida. As on previous days, most of the heaviest rain should remain offshore of these areas, with a predominant light ENE flow along the front allowing some of the heavier rains to move into FL and GA from the Gulf, though quickly weakening with increasing distance from the Gulf. Once again, this area will be swamped with multiple days of light to occasionally heavy rainfall. Thus, it's quite possible a Slight risk upgrade may be needed with future forecast in the areas hardest hit with heavy rains both today and on Sunday. The inherited Marginal was greatly expanded to the north with this update to cover for all intents and purposes the entirely of the US Gulf Coast. Wegman ....Western U.S... Remnant mid-level moisture and energy associated with previous convective developments will continue to plague areas along and west of the Continental Divide as ensemble QPF signatures indicate another round of scattered shower/thunderstorm activity that will have the ability to entice flash flooding prospects. The signal is not as robust as previous days, but the compounding effects from prior periods, plus the instability axis/magnitude still culpable=20 for stronger convective cores allows for a low-end MRGL risk over the Interior, mainly within the Great Basin, Four Corners, into the Southern Rockies. Totals will be mainly <1", but a few areas could see higher totals which would allow for flash flood concerns, mainly within the complex terrain, slot canyons, and burn scar locations.=20 Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!6UJxqCTOyI1mvPLUZqHP8Mm1EKRHlW3Dxli6nfg_QXRx= kcK0K0CdMRCA5qW0YU0P5K5cYeIYp0hykywmCW9qxpoWbJM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!6UJxqCTOyI1mvPLUZqHP8Mm1EKRHlW3Dxli6nfg_QXRx= kcK0K0CdMRCA5qW0YU0P5K5cYeIYp0hykywmCW9qCA_MrOA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!6UJxqCTOyI1mvPLUZqHP8Mm1EKRHlW3Dxli6nfg_QXRx= kcK0K0CdMRCA5qW0YU0P5K5cYeIYp0hykywmCW9qXqUZuS8$=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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