Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Sep 08 2024 20:17:15 FOUS30 KWBC 082016 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 416 PM EDT Sun Sep 8 2024 Day 1 Valid 16Z Sun Sep 08 2024 - 12Z Mon Sep 09 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHWEST, DEEP SOUTH TEXAS, AND SOUTHEAST... ....16Z Update... Very little changes were made to the overall D1 outlook with the consensus remaining steady for low to medium end MRGL impacts expected across portions of the West, Southeast, and Deep South Texas. A small shift was made across South TX where the MRGL risk was tightened even further to the coast to confine mainly Brownsville and South Padre Island as the two main areas of focus with the northern periphery of any banding associated with the tropical wave to the south. The latest 12z HREF mean QPF output depicts a sharp western gradient for precip through the period with the best potential aligning with the overnight time frame as the wave scoots more to the northwest, bringing some of the deeper tropical moisture north into the tip of South TX. There's some discrepancy on the timing of any banding with a 50/50 split amongst CAMs and some global deterministic. There's enough merit to warrant the continued MRGL to maintain continuity, especially with a tropical disturbance positioned to the south. There's a chance this materializes after the period, but as a precaution for the low-end threat, decided against removal. Over the Southeast, the main focus will reside along the coasts of FL and perhaps over the coast of SC as convection will align near the coastline during peak diurnal destabilization. The best opportunity for heavy rain will likely occur with the sea breeze across Northeast FL between Jacksonville to Melbourne with the HREF neighborhood probabilities and EAS both fairly aggressive in the latest update for >3" of rainfall in the vicinity. The airmass in place is sufficient for these higher hourly and intra-hour rates to produce some localized flash flood concerns, especially within the urbanized areas along the coast. This was enough maintain the MRGL risk with no real changes to the risk positioning from previous forecast. Out West, the only change occurred over the northern extent of the Peninsular range where 12z CAMs indicate a few cells developing just north of where the risk area was forecast. HREF mean SBCAPE values lie within the 500-1000 J/kg with a few CAMs a bit more robust in the depiction. Considering a marginally more favorable airmass overhead and the complex terrain additions over the area, thought the expansion was favorable and was agreed upon in coordination with the San Diego WFO. Otherwise, no other changes were necessary from the previous issuance across the west. Kleebauer ...Previous Discussions.. ....Southwest... A bit more widespread convection is expected with the monsoon this afternoon across the Southwest. Much of the convection will once again be focused along the various mountain ranges from the Peninsular Ranges of southern California through the Sangre de Cristos of Colorado. As usual in the desert Southwest, somewhat limited moisture will be the limiting factor, even though areas could be 2 sigma above normal for atmospheric moisture for this time of year. Thus, the primary flood threat is for urban areas, slot canyons, and arroyos where slow moving convection in chaotic steering flow may result in long-duration localized heavy rain from the most persistent convection. Any resultant flash flooding will be isolated. ....Deep South Texas... The Marginal Risk area was trimmed closer to the coast with this update, now primarily for the urban area of Brownsville over to the South Padre Island beaches. A stalled out front will begin to focus an area of heavy rainfall associated with a slowly developing tropical low over the western Gulf. Guidance has shifted east a bit, resulting in lower forecasted rainfall totals for all of Deep South Texas, with little if any rain expected west of Brownsville through tonight. However...given the abundance of atmospheric moisture available any stronger showers and storms will be capable of very heavy rainfall rates which could cause isolated flash flooding. ....Southeast... The stalled out front that has been haunting the Southeast for well over a week now remains in place from far southern South Carolina through the northern Florida Peninsula. Besides the front itself being strong...indicative of a tight gradient between the extremely humid air mass over the Gulf and an outright autumnal air mass to its north...instability remains very lacking since this front hasn't moved in days, so diurnal heating is limited under the extensive low cloud cover. Thus, almost all of the rain will be light across this region. This is countered by the aforementioned abundant amounts of atmospheric moisture available for any convection that can form. Thus, a low-end Marginal remains in place, with the greatest threat for flooding over the Florida Peninsula well south of the front where diurnal heating can actually make a difference. Wegman Day 2 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... Only minor changes were necessitated during the D2 time frame with the SLGT risk maintained across Deep South TX, but brought in closer to the coast. The MRGL risks across the Southeast and Western U.S were generally maintained outside small adjustments to the northern periphery of the risk areas.=20 Kleebauer ....Coastal Deep South Texas... 20Z Update: The pattern remains conducive for locally heavy rainfall in-of the South TX coastal plain with the heaviest precip focus situated east of McAllen towards Brownsville and South Padre Island. The tropical disturbance to the south will be slowly progressing northward through the period with the deeper tropical moisture feed aligned right along the coast with some protrusion inland as the disturbance jogs northwesterly the second half of the forecast. Recent ensemble QPF output pins the 2-4" potential right within Brownsville proper with a sharp western cutoff the QPF as drier air located to the northwest of the weak circulation maintains a foothold on the area. Latest HREF neighborhood probabilities have a pretty solid signal for >3" (80-90+%) and >5" (50-80%) confined right in that far Southern fringe of the Lone Star State with a precipitous drop off in the prob fields as you extend westward. This correlates well with the blended mean QPF and the recent ML output dictated within the ECMWF AIFS and GFS Graphcast outputs. There was enough consensus for tightening the SLGT risk closer to the coastal plain and dropping the northern fringe of the SLGT a bit further south to match the probability fields. This is an evolving situation with the future of the disturbance still in question, so until guidance has a more defined structure of the tropical system, it will be hard to discern finer details on the threat. For now, there was enough agreement on the SLGT risk maintenance with some minor adjustment.=20 Kleebauer ...Previous Discussion.. Weak easterly flow north of a developing tropical low over the southwestern Gulf will advect increasingly deep tropical moisture into coastal Deep South Texas on Monday. This is highly likely to result in much more rainfall for South Padre Island and over to Brownsville as compared with Day 1/Sunday. Given PWATs that could exceed 2.5 inches, or 3 sigma above normal for this time of year, any convection that moves west off the Gulf into the coast will be capable of very heavy rain rates. Thus, the real question regarding flood coverage will be how frequent storms producing those rates move over urban areas. Think right along the coast widely scattered instances of flash flooding can be reasonably expected. Any training could quickly result in significant flash flooding. That said, the system will only really just be getting organized, so overall rains will likely be more intermittent in most areas. Urban areas such as Brownsville will be at a higher likelihood for flash flooding. Wegman ....Four Corners Region... 20Z Update: One change will be noted this forecast cycle with an extension of the MRGL risk further north into Northeastern UT where the core of the heaviest QPF is being depicted on both hi-res and global deterministic. The signal is in conjunction yo a trailing shortwave that will cross through the Inter- Mountain=20 west Monday afternoon while situated on the fringe of the left exit region of an upper jet cruising into the Southwestern U.S. This allowed for the primary convective axis to be positioned over the Wasatch down through Central UT for the period. The rest of the forecast wavered very little from run to run leading to general continuity for the remainder of the forecast. Kleebauer ...Previous Discussion.. Convective coverage should shift northeastward a bit on Monday. Thus, the flooding threat shifts into the Four Corners region and away from the West Coast. Flooding concerns will be greatly focused in area slot canyons and arroyos of southern Utah and southwest Colorado Monday afternoon and evening with peak heating. Wegman ....Rest of the Gulf Coast... 20Z Update: Little to no changes were made to the MRGL risk area located across the Southeast. Heavy rain cores will be plausible in-of much of the FL Peninsula with the heaviest focus over the interior. A strong signal within the probability fields for >3" (80-90+%) exists with a sharp decline in the probability for anything over 5". This limits the upper end of the potential, thus maintaining the previous MRGL risk signal inherited.=20 Kleebauer ...Previous Discussion.. Rinse and repeat weather pattern will continue, albeit shifted just a bit south of Sunday's focus. Thus, most of the north Gulf Coast will get a relative break from the potential for steady heavy rain, as the heaviest rains shift south a bit away from the coast. The exception will be into the Florida Peninsula, where a repeat day of sea breeze showers and thunderstorms can be expected to the south, with more widespread lighter rain expected further north. Wegman Day 3 Valid 12Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TEXAS, LOUISIANA, AND MISSISSIPPI GULF COASTS... ....20Z Update... The tropical disturbance in the Western Gulf will continue to progress northward through the course of Tuesday into Wednesday with the western fringe of the precip shield becoming aligned within the TX coastal plain. This forecast package has seen some deviation away from the precip extending further west into TX leading to an adjustment tighter to the coast for the SLGT risk forecast. The biggest question becomes the organized nature of the low and how close will it be to the coast of TX to orient enough moisture to affect to the area of interest within the SLGT. A further east track of the primary low will create a very focused corridor of heavy rain right at the coast, limiting the potential somewhat for greater urbanized zones just inland of the coastline. Increasing southwest shear on the western quadrant of any organized system will also have the ability to thwart organized convection and really hamper the ability for any heavy rain to occur inland which is noted by a few global deterministic and ensemble members of the EPS/GEFS combo. This is the period to watch as it pertains to greater flash flood impacts ramping up with the disturbance in question. As of now, the only change was that tightening of the SLGT closer to the coast, but still manage a broad area of coverage for the risk in question due to some uncertainty and the ensemble mean QPF depicting the threat still being on the table.=20 Across the west, the MRGL risk over CO was expanded back to the west into UT to account for a lingering convective pattern over the terrain with the primary focus within the Wasatch. The signal is less enthusiastic on the potential compared to the previous period, but does fall within the low-end criteria of the MRGL with pockets of higher QPF located in-of the mountains where flood concerns are highest. The remainder of the area was relatively unchanged as scattered thunderstorms are forecast across Western CO with the more prone slot canyons, arroyos, and burn scars still the zones of highest flash flood opportunity.=20 Kleebauer ...Previous Discussions.. ....Gulf Coast... In much of the guidance, a strengthening tropical low in the Gulf will push north up the far western Gulf from Mexico and into the northwestern Gulf. This will both push the stalled out front that has been in this area to the north back into the South, and much better organize its associated rain. The result will be widespread heavy rainfall along the northwestern Gulf Coast on Tuesday. The bulk of the heaviest rain will be along much of the Texas Gulf Coast Tuesday and Tuesday night. Steady heavy rain directly associated with the low will hold until well into Tuesday night for the Upper Texas Coast. Meanwhile into Louisiana and points east, the rain expected in these areas will be moisture peeling off the low in the western Gulf and tracking along the stationary front separating the moisture abundant air mass over the Gulf with the fall-like air mass over much of the eastern U.S. Thus, forcing will be lesser into Louisiana and points east than in Texas. The Slight was introduced not only for the potential for long-duration light to moderate rainfall, but also due to very favorable antecedent conditions (saturated soils) from the week plus of consecutive days of rain from the same stalled front. As mentioned in previous discussions, while there's good agreement that the low will be organizing in the western Gulf as it moves north by Tuesday, the fact that the low is only just beginning to get its act together as of the time of this writing has resulted in widely variable solutions as to when and where the heaviest rains will be along the Gulf Coast by Tuesday. Thus, expect potentially significant changes in the forecast rainfall footprint in the coming days as the guidance gets a much better handle on the track of the low and its associated rainfall. ....Inter-Mountain West... A small Marginal Risk area was added across Southwest Colorado for lingering monsoonal moisture hitting the same mountain areas as on previous days' afternoons. Rains will also spread into the Intermountain Northwest through UT/ID/WY. With better agreement in the guidance, the Marginal may need to be expanded into this region with future updates. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7981KTZzy1oU_rnjvkwApi2bwYbu0fCYABQe1jGWP3O1= AnBf8PDrOB2f4J5JL1fZ4lGsoDysOYAva8HONQOqJDZM-HU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7981KTZzy1oU_rnjvkwApi2bwYbu0fCYABQe1jGWP3O1= AnBf8PDrOB2f4J5JL1fZ4lGsoDysOYAva8HONQOqrYodcNE$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7981KTZzy1oU_rnjvkwApi2bwYbu0fCYABQe1jGWP3O1= AnBf8PDrOB2f4J5JL1fZ4lGsoDysOYAva8HONQOqoD9GYtc$=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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