Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 03 2024 20:01:16 FOUS30 KWBC 032000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 400 PM EDT Tue Sep 3 2024 Day 1 Valid 16Z Tue Sep 03 2024 - 12Z Wed Sep 04 2024 .....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL AND SOUTH TEXAS... ....13Z Outlook Update... ....Texas... Slight risk areas were combined and expanded to include more of south Texas (near the Del Rio area). This morning, slow-moving convection has materialized and produced areas of 1-2.5 inch/hr rain rates just east of the Rio Grande. These storms are in a sufficiently moist/unstable environment with weak flow aloft for continued heavy rainfall. Low FFG thresholds (near zero in spots) were also noted. The flash flood threat should persist for most of the D1 forecast period under this regime. Reference MPD #0968 for additional mesoscale details. ....Southeastern Mississippi eastward to northern Florida... Latest CAMs indicate a robust axis of convection materializing amid weak flow aloft from the Pine Belt of Mississippi eastward to the Jacksonville Metro area. HREF probabilities of 24-hr, 3 inch+ precipitation totals maximize near Jacksonville, where models depict a persistent fetch of deep convection originating along a frontal boundary over the Gulf Stream. Multiple rounds of precipitation are expected today through the early overnight hours especially across northeastern Florida. Localized flash flood potential is expected where rainfall can occur over sensitive/urbanized locales. The Marginal risk has been expanded northwestward and eastward to include areas of isolated flash flood potential. The remainder of the forecast is unchanged. Refer to the previous discussion below for more details. Cook ....Previous Discussion... ....Texas... The mid-level vort is expected to exit the region although precipitable water values will remain high. Flooding issues will be possible in the area with a widespread 1 to 1.5 inches forecast with locally higher amounts. A Slight Risk remains in effect for central portions of the state although it was expanded y 1 tier of counties to the south to reflect the latest trends in the guidance. A slow moving tropical low will continue to bring periods of heavy rain in the form of slow- moving showers and thunderstorms from the Gulf into the adjacent Texas Coast. Most areas will not see steady rain the entire time, but very localized areas of training storms resulting in flash flooding. The heaviest rains in the state are expected to be along the Gulf Coast from Galveston south and west through Corpus Christi. Locally heavy rains to 2 inches per hour will be possible. ....Mississippi and Alabama Gulf Coast... The Marginal Risk was maintained for the potential of slow moving and training storms to the east of the low along a stalling front into coastal Mississippi and Alabama. The abundance of atmospheric moisture and instability, as well as, the slow-moving nature of the synoptic systems increases confidence that an isolated flash flood or two could develop along this stretch of coast. ....Idaho/Montana... There is potential for isolated to scattered heavy rainfall as an advancing shortwave encounters an anomalously moist airmass. PW values rising to near 1 inch will support storms capable of heavy rain that may produce isolated flash flooding should the storms impact particularly flood sensitive or more urban areas. Campbell Day 2 Valid 12Z Wed Sep 04 2024 - 12Z Thu Sep 05 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE MISSISSIPPI DELTA REGION AND ADJACENT GULF COAST... ....20Z Update... As 12Z guidance rolled in today, the precipitation shield was not as progressive as previous forecast cycles. A blend of CAMs/global guidance has shifted the focus for heavier rainfall to be along the Upper Texas coast where they will be located closest to an organizing and elongated area of low pressure in the western Gulf of Mexico. NAEFS continues to depict a swath of PWs that are topping the 99th climatological percentile from as far south as Brownsville to as far east as Pensacola. The area most at risk will likely be found from the Upper Texas coast to southern Louisiana where the combination of surface-850mb theta-e advection directly into=20 or quasi-parallel to the stationary front will trigger more=20 widespread thunderstorm activity. Southeast TX also still sports=20 >80% soil moisture percentiles within the 0-40cm layer according to NASA SPoRT-LIS, suggesting soils there would be more sensitive to=20 2-3"/hr rainfall rates. The other adjustment to the forecast was to expand the Marginal Risk east over the Florida Panhandle where an influx of anomalous PWs and modest instability along the stalled frontal boundary may trigger additional thunderstorms capable of producing torrential downpours. Areas most at risk for possible flash flooding will be areas whose soils are more saturated following today's thunderstorm activity and as far east as the Jacksonville metro area.=20 Mullinax --Previous Discussion-- ....Gulf Coast and Lower Mississippi Valley... The surface front will continue to be draped west to east across the South, albeit further south than previous forecast cycles. This frontal boundary will provide focus for showers and thunderstorms to spread across the entire Gulf Coast from south Texas all the way to the Florida Peninsula on Wednesday. Most of the guidance shows 1 to 3 inches will concentrate over this part of the region thus increasing the risk for excessive rainfall and local flooding potential. There is the potential for a few locations for local maximums to exceed 6 inches near northern Louisiana/Arkansas. Onshore flow across southeast Louisiana and southern Mississippi could yield 1 to 4 inches as well. The focus for the heaviest rains will be in the central Gulf Coast from Louisiana east through the Florida Panhandle. Locally heavy rains are likely, especially from New Orleans east through Pensacola, where a higher-end Slight is in place. With increasing amounts of rain forecast in and around New Orleans, continued increases in rainfall there may require an eventual targeted Moderate Risk should trends continue. Given the trends, the back edge of the Marginal Risk was trimmed eastward toward the Texas coast in addition to reducing the northern boundary by 1 tier of counties across Alabama and Mississippi. Campbell/Wegman Day 3 Valid 12Z Thu Sep 05 2024 - 12Z Fri Sep 06 2024 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ALONG THE CENTRAL GULF COAST... ....20Z Update... Similar to the Day 2 update, guidance has trended slower with the progression of the QPF associated with an elongated area of low pressure in the northwest Gulf of Mexico. This led to the threat areas being drawn further west this cycle with the focus for heavy rainfall being centered over southern Louisiana. However, there=20 remains a notable amount of spread in both the northern extent of=20 the QPF axis and in rainfall totals. The GEFS remains more of a=20 northern outlier compared to ECENS/GEPS members. While dispersion=20 is on the higher side on Day 3, there is a better consensus for heavier rainfall along the central Gulf Coast given the bulk of PWs=20 >99th climatological percentile residing along the Gulf Coast. The region is also closer to the surface-850mb southeasterly flow that will sustain a steadfast regime of low level theta-e advection=20 into the nearby stationary frontal boundary. Additional forecast=20 changes are likely given so much of this setup lies with the=20 development of low pressure along the Gulf Coast.=20 Elsewhere, a Marginal Risk was added over portions of the Southern Rockies due to the PWs topping 0.75" and up to 500 J/kg of MUCAPE out ahead of an approaching cold front. Given the region's=20 sensitive soils, both from saturated soils and burn scars, opted to issue a Marginal Risk for the localized flash flood potential=20 there Thursday afternoon and evening. Mullinax --Previous Discussion-- ....Gulf Coast and Lower Mississippi Valley... The area of precipitation mentioned for the Day 2 period will shift eastward across the central Gulf Coast with the heaviest QPF still focusing across portions of southeast Louisiana, southern Alabama, southern Mississippi and clipping western portions of the Florida panhandle. Again, there was a notable southern shift in placement within a majority of the guidance from previous cycles.coastlineThe inherited Marginal Risk areas was removed from southern Tennessee and northern portions of Alabama, Mississippi and northwest Georgia while trimmed eastward across central Louisiana. The Slight Risk spans along the Gulf Coast from west-central Louisiana to eastern Choctawhatchee Bay Florida. Campbell Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!7YuvPkgnOtMCDId0vR7TPKK35mK0FcWNGFp-b_vCQ_af= F6Q0yq7GfSIm_OhtFwpEU1TgfXYAjbMjmgXe2uh7t-lKxpo$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!7YuvPkgnOtMCDId0vR7TPKK35mK0FcWNGFp-b_vCQ_af= F6Q0yq7GfSIm_OhtFwpEU1TgfXYAjbMjmgXe2uh7IAbfq7A$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!7YuvPkgnOtMCDId0vR7TPKK35mK0FcWNGFp-b_vCQ_af= F6Q0yq7GfSIm_OhtFwpEU1TgfXYAjbMjmgXe2uh7l3KaTTg$=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) .