Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 09 2024 20:26:41 FOUS30 KWBC 092026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 426 PM EDT Mon Sep 9 2024 Day 1 Valid 16Z Mon Sep 09 2024 - 12Z Tue Sep 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR COASTAL DEEP SOUTH TEXAS... ....16Z Update Summary... Limited changes were necessary for the 16z update as model consensus remains steadfast on the convective impact areas anticipated during the period. Kleebauer ....Coastal Deep South Texas... 16Z Update: The northern periphery of the tropical feed from now Tropical Storm Francine has reached the far southern tip of Texas with the primary centers of impact remaining South Padre Island over to Brownsville. Rates currently are sub-par for any flood concerns with an areal average of 0.25-0.5", but totals are starting to breach the 1" threshold over the course of the morning given the persistence. This will continue for a few more hours before the precip field breaks with more scattered instances of rainfall through the evening. As Francine noses north-northwest, a deeper tropical moisture fetch will begin impacting the southern reaches of TX once again, this time with a more robust convective footprint leading to enhanced hourly rates and rapidly accumulating rainfall. The period of greatest potential remains between 06-12z Tuesday, the very end of the period leading to a small window for enhanced impact. 12z HREF neighborhood probability for >5" is sufficiently high (70-90%) in-of the Brownsville area with slightly less probs further west to Harlingen. Further west to McAllen, the probabilities drop off significantly for any appreciable accumulations greater than 3" leading to less of an opportunity for flash flood concerns as you move west along the Lower Rio Grande Valley. The signal was quite consistent with the latest CAMs with a relatively high correlation in the potential given the higher EAS prob fields for all 1/2/3" thresholds across coastal South TX with much less agreement for anything appreciable further west. The SLGT risk was adjusted to match the latest EAS prob fields with a tightening of the SLGT and MRGL to account for the probabilistic output. Kleebauer ...Previous Discussion.. The Slight Risk for the coastal sections of Cameron, Willacy, and southeast Kenedy Counties remains largely unchanged, though trimmed a hair on the west/inland side with this morning's forecast update. The northern edge of the convection associated with Potential Tropical Cyclone Six (hereafter, PTC6), will try to move into the coast today into tonight. A strong front remains draped over South Texas, along the coast, but extending west/inland over Deep South Texas. This front is keeping dry air nearby, greatly limiting any westward extent to the rainfall, but is also a source of forcing for the very deep tropical moisture currently in place over most of the Gulf. Thus, as PTC6 organizes today into tonight, waves of rain may move into the coast over South Padre Island and Brownsville. Widely scattered instances of flash flooding are probable, especially where the storms are the most persistent today. Wegman ....Louisiana through Florida Gulf Coast... 16Z Update: Prevailing easterly flow along and north of the quasi- stationary front over the Gulf will lead to instances of scattered heavy convection pivoting off the Central Gulf coast into Southeastern LA. The main premise for the MRGL is to encompass the more urbanized setting around New Orleans and zones in-of Lake Pontchartrain. 12z HREF neighborhood probs are generally indicative of a 3-5" maxima somewhere near New Orleans proper with the best chance located over the Parishes just to the Southeast. The signal was high enough to warrant the MRGL risk forecast, thus maintained continuity with little variance from the previous forecast. Kleebauer ...Previous Forecast.. Few changes were made to the Marginal Risk area...which will be largely in place for afternoon sea breeze convection as deep tropical moisture associated with PTC6 move east across the Peninsula and along the Gulf Coast. There has been a nominal southward shift in the guidance, so the Marginal was trimmed to just the immediate Gulf Coast except over the Florida Peninsula where sea breezes and outflow boundaries will be much more of a factor in forcing storms capable of producing heavy rain and resultant flash flooding. Wegman ....Four Corners... 16Z Update: No major changes necessary across the Four Corners and Interior Mountain West. The signal remains low-end with the MRGL risk threshold, however the combination of scattered convection with several slot canyons, arroyos, and burn scar remnants over the anticipated zone of impact was enough to warrant the MRGL risk continuation. Kleebauer ...Previous Discussion.. No changes were made to the Marginal Risk area, where isolated flash flooding is possible especially in slot canyons and arroyos. Guidance has backed off a bit on the areal coverage of convection in this region, so this area is a low-end Marginal. Wegman Day 2 Valid 12Z Tue Sep 10 2024 - 12Z Wed Sep 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR MUCH OF THE=20 GULF COAST WEST OF PENSACOLA.. ....20Z Update Summary... Risk areas across the Gulf coast of Texas were tightened a bit closer to the coastal plain with the removal of the SLGT over=20 portions of the TX coast near Corpus Christi. The MRGL risk was=20 expanded to the southeast over the Southern Rockies.=20 Kleebauer ....Gulf Coast... 20Z Update: Now Tropical Storm Francine will continue making=20 headway to the north with a strengthening core, eventually forecast to become a hurricane within the next 24-48 hours. Guidance=20 continues to show a general contraction of the storm size as it=20 matures over the Western Gulf leading to less of a tropical rain=20 threat over portions of coastal TX as it moves latitudinally. This will inhibit the western periphery of the heavy rain potential as a majority of the convective cores around the circulation will be adjacent to the coast, but just off-shore. There are some indications that rogue bands could sway further away from the outer periphery of the circulation leading to isolated instances of heavier rainfall, especially along the concave portion of the TX coast, including the Corpus Christi/Port Aransas/Rockport areas up towards Port Lavaca. Ensemble mean QPF and relevant neighborhood probabilities for rainfall >3" have deviated away from those areas being a beneficiary of some of the heavier tropical footprint that would necessitate a higher risk, including the previous SLGT forecast. With the consensus growing on the track being slightly more east of the previous NHC forecast cyclone track, the SLGT risk located between central Kenedy County up through Port Lavaca was removed with a maintenance of the MRGL risk to account for the rogue heavier cells that may protrude the immediate coast in those areas.=20 Across the Brownsville area, recent trends have led to a general continuity over the far southern reaches of TX with a strong signal for heavy rain likely to affect points along and east of McAllen with the higher end potential likely situated over Harlingen to Brownsville and South Padre Island. Additional 2-4" of rain with locally upwards of 6" will be plausible tomorrow morning before the primary bands lift north with the forward propagation of Francine. This will be on top of what will occur in the prior period leading to storm totals of 4-8" with locally up to 12" for some of the areas hit by repeating bands. The highest significant flash flood threat will reside in the urban corridors across Deep South TX, including Brownsville proper.=20 Further north, the prospects increase gradually for a locally significant heavy rain threat, but generally maintained along the immediate coast as the cyclone jogs north-northeast once it reaches the latitude of Galveston. The northern periphery of the cyclone will begin edging onshore of the Upper TX coast over into Southern LA with multiple bands of heavy rain likely impacting the area from the lower Sabine over into Morgans City. Depending on the forward speed of Francine, the timing could be off +/- 4 hours leading to a significant difference in impacts for the period. The SLGT from previous forecast was relatively unchanged in its location and inland placement to account for those uncertainties in the timing. Regardless, the threat for significant rainfall will be increasingly steadily over much of Southern LA, including over towards New Orleans where onshore easterly flow will advect deep tropical moisture ahead of the cyclone with streaming convection moving off the Gulf into Southeastern LA.=20 Kleebauer ....Inter-Mountain West and Southern Rockies... 20Z Update: Scattered convective footprint is anticipated across portions of the West with the main threat residing over the Southern Rockies into the Wasatch of UT. The threat is on the lower end of probabilities considering the middling signal within the latest HREF for >1" chances over the region. Regional instability is enough to maintain a relatively solid pulse convective regime that would allow for a few stronger cores that if falling over the wrong area (Slot canyons, burn scars, etc.), could cause some issues locally. The biggest adjustment from the previous forecast was an expansion of MRGL risk into Northern CO as recent CAMs and some of the ensemble mean QPF signaling a threat of isolated thunderstorms over the Sangre de Cristos, an area that has a heightened risk of flash flooding due to multiple burn scar areas.=20 Kleebauer Day 3 Valid 12Z Wed Sep 11 2024 - 12Z Thu Sep 12 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI DELTA... ....Southeast... 20Z Update: Tropical Storm Francine will likely strengthen to a hurricane prior to landfall on Wednesday morning with a well- defined axis of heavy rainfall making a steady progressing through much of Louisiana and the eastern fringes of TX near the state lines. The western periphery of Francine will likely be going through a strong vertical shear pattern that will inhibit the tropical convective pattern within the western half of the cyclone. Some of the pivoting bands on the northwestern quad will likely make progress towards the Lower Sabine which could cause some flood issues, but the story will be further east where the shear pattern will be less of a deterrent to the heavy rain footprint with an increasing consensus of 4-8" with local to 10" situated between the area of Sabine Pass over to Morgan City and points north. A secondary maxima over Southeast LA could occur with the prevailing onshore component on the eastern fringes of Francine's circulation. The New Orleans metro will be the most susceptible to flash flood concerns thanks to the urbanization factors with higher run off potential in the city limits. This area has also experienced a fair amount of rain leading into the event, so the soils will be primed relative to what is typical to the area. The MDT risk was maintained over the eastern extent of LA with only a small adjustment on the northeast fringe to include the Jackson, MS metro corridor due to the expected heavy rainfall at the back end of the period.=20 The eastern sections of the MRGL and SLGT risk areas will have the highest threat for locally significant impacts away from the storm center thanks to the sheared component, even as the storm moves further inland. It's not until D4 when the storm will begin the extra-tropical transition leading to a broader expanse of heavier precip within the western side of the remnant circulation. For D3, the heaviest rain will remained confined to the core and points east leading to the sharper drop off in the risk as you move outside of the TX/LA border.=20=20 ....Intermountain West... 20Z Update: A potent mid-level shortwave will allow for a broad axis of upper ascent leading to scattered convection over the Northern Rockies with a maintenance of the MRGL flash flood risk over Western MT into Northern ID. Some of the higher QPF may be in the form of snow, so will have to assess the setup closely for the snow level forecast to discern if a targeted SLGT risk is still relevant. The disturbance is fairly robust, so the threat of small areal upgrade is non-zero. Kleebauer ...Previous Discussion.. A Marginal Risk was introduced for this region as a strong digging shortwave trough interacts with topography and above normal moisture for this region. Forecast amounts may be high enough to potentially need a targeted Slight with future updates in this area. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9LnUi1untK0NSYcQ431v6RsFaR3ArRt0hAdilYF6hJaX= vpgGvUqzJ38utAu1YUsIE9ZsYpa8mH5U-vROMyyg9DCzCc8$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9LnUi1untK0NSYcQ431v6RsFaR3ArRt0hAdilYF6hJaX= vpgGvUqzJ38utAu1YUsIE9ZsYpa8mH5U-vROMyygt3w30gk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9LnUi1untK0NSYcQ431v6RsFaR3ArRt0hAdilYF6hJaX= vpgGvUqzJ38utAu1YUsIE9ZsYpa8mH5U-vROMyygGUJqGbQ$=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) .