Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Mon Sep 09 2024 15:45:17 FOUS30 KWBC 091545 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1145 AM 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.=20 Kleebauer=20 ....Coastal Deep South Texas... 16Z Update: The northern periphery of the tropical feed from now=20 Tropical Storm Francine has reached the far southern tip of Texas=20 with the primary centers of impact remaining South Padre Island=20 over to Brownsville. Rates currently are sub-par for any flood=20 concerns with an areal average of 0.25-0.5", but totals are=20 starting to breach the 1" threshold over the course of the morning=20 given the persistence. This will continue for a few more hours=20 before the precip field breaks with more scattered instances of=20 rainfall through the evening. As Francine noses north-northwest, a=20 deeper tropical moisture fetch will begin impacting the southern=20 reaches of TX once again, this time with a more robust convective=20 footprint leading to enhanced hourly rates and rapidly accumulating rainfall. The period of greatest potential remains between 06-12z=20 Tuesday, the very end of the period leading to a small window for=20 enhanced impact. 12z HREF neighborhood probability for >5" is=20 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=20 accumulations greater than 3" leading to less of an opportunity for flash flood concerns as you move west along the Lower Rio Grande=20 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 ALL OF THE GULF COAST WEST OF PENSACOLA.. ....Gulf Coast... Potential Tropical Cyclone 6 (PTC6) will continue organizing over the western Gulf based on the latest NHC forecast as it tracks north and east over the northwestern Gulf. Heavy rains associated therewith will impact all of the Gulf Coast, especially into Louisiana and Texas. The latest guidance has been shifting the axis of heaviest rainfall east, in response to very dry air north of a strong front that will be guiding PTC6 northeastward. Thus, the westward extent of the associated rainfall will be greatly limited. Thus, the inherited Slight Risk was trimmed from the west, especially over Texas. With that said, any heavy rain associated with PTC6 will have well above average amounts of atmospheric moisture to work with, making it more common for prodigious rainfall rates to occur. Much of southern Louisiana has been very wet in recent weeks, so saturated soils will only quicken the onset of flooding once the heavy rain gets going. Into Florida, the same stalled out front will shear some of the moisture from PTC6's circulation, adding fuel for diurnally driven showers and storms once again on Tuesday afternoon. Isolated flash flooding will be possible once again. ....Utah/Colorado... No changes were made to the inherited Marginal Risk area, where numerous showers and storms may develop Tuesday afternoon. The most concentrated and heaviest showers and storms will be in the Marginal Risk area from eastern Utah into western Colorado. Isolated flash flooding will be possible in flood prone areas such as slot canyons. Wegman 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... Potential Tropical Cyclone Six (PTC6) is forecast to continue strengthening up until landfall in southwest Louisiana on Wednesday. Abundant associated moisture will advect north ahead of and alongside the low center, resulting in multiple inches of rain over almost all of Louisiana and adjacent southern Mississippi. For Louisiana, much of the southern half of the state has seen abundant rainfall over the past couple weeks, resulting in highly saturated soils. A hurricane making landfall here will cause most of the rain to convert to runoff. Thus, in coordination with LCH/Lake Charles, LA; LIX/Slidell, LA; JAN/Jackson, MS, and SHV/Shreveport, LA forecast offices, a Moderate Risk upgrade was introduced with this update. The greatest risk of significant and potentially life- threatening flash flooding will be in the hardest hit areas of the I-10 corridor from New Orleans west through Baton Rouge to Lafayette. A higher-end Moderate is considered in effect for this portion of the I-10 corridor. Considerable uncertainty persists on the north and west side of the ERO risk areas. To the west, a notable and persistent drying trend continues, partially due to eastward shifts in the track and partially due to disagreement as to whether PTC6 will begin extratropical transition around landfall. With the center expected to track along the spine of Louisiana, locations to the west will be on the dry side of the storm, which will quickly cut down on rainfall totals. Given these trends, the Slight was trimmed on the western side. To the north, the concern will be more about rapidly deteriorating antecedent conditions, as northern Louisiana, Mississippi, and Arkansas have all been much drier than points further south towards the Gulf Coast. This in turn will make the watersheds much more able to absorb heavy rainfall before significant flooding commences. Thus, the Slight and Marginal Risk areas are quite conservative, as the first few inches of rain should be largely beneficial in these areas. The aforementioned eastward shift in track should mean wetter conditions into Alabama and the Florida Panhandle, and the Slight was expanded east for that reason, though Alabama has been similarly dry away from the Gulf Coast as its neighbors to the west. With PTC6 really still trying to form, expect continued adjustments in the track and expected rainfall associated therewith. Uncertainty with locations and amounts have also precluded some areas well away from the current forecast track of the center from being upgraded, but may be with future updates, especially east of the track. ....Intermountain West... 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!4g5tsK0PNvikuIEXWTWqDXxf9GGwnQNnCkmq72zxXFE2= m3gWy__wIqoqjTYv-i5XciThRCjTKQdBBgDvZFhuYwXB_D0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4g5tsK0PNvikuIEXWTWqDXxf9GGwnQNnCkmq72zxXFE2= m3gWy__wIqoqjTYv-i5XciThRCjTKQdBBgDvZFhu-FNbby8$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4g5tsK0PNvikuIEXWTWqDXxf9GGwnQNnCkmq72zxXFE2= m3gWy__wIqoqjTYv-i5XciThRCjTKQdBBgDvZFhu6v4wlfk$=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) .