Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Nov 06 2024 00:52:08 FOUS30 KWBC 060051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 751 PM EST Tue Nov 5 2024 Day 1 Valid 01Z Wed Nov 06 2024 - 12Z Wed Nov 06 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TO MID MISSISSIPPI AND LOWER OHIO VALLEYS... ....01z Update... Little change needed to the inherited Slight Risk for overnight, as the expectation for localized 2-3" totals remains on track. The edges of the Slight and Marginal Risk areas were adjusted a bit, based on observational and model trends.=20 Churchill ....1600Z Update... Overall, modest changes made for the late-morning update to account for the latest 12Z HREF guidance and recent HRRR trends. There has been some overall tightening of the QPF gradient from the lower MS Valley up into the lower OH Valley for the late-day and overnight hours, but still a threat for a ribbon of locally heavy rainfall totals. This will include areas from northwest MS through western TN and western KY where the latest guidance still supports a 40 to 45 kt southerly low-level jet ahead of the cold front approaching the area, and thus a channel of rather strong moisture transport. Instability though is overall expected to rather modest and this will likely tend to cut down on the rainfall rates of the areal shower and thunderstorm activity. Nevertheless, there may be some localized training of the convection this evening, especially in the 00Z to 06Z time period, and a few pockets of 2 to 3 inch rainfall totals will be possible. Based on this, the Slight Risk area has been maintained although adjusted just slightly off to the east and trimmed in areal extent to account for the 12Z HREF consensus. Given the eastward advance of the overall rainfall shield currently over the lower/middle MS Valley and southeast TX, the western portion of the Marginal Risk area has been trimmed away. Orrison ....Previous Discussion... A cold front tracking east across the middle of the country is expected to stall out generally along the Mississippi and Ohio Rivers late tonight. Southerly flow off the Gulf will continue pumping plentiful moisture into the front. Meanwhile, in the upper levels, a large longwave trough over the west will establish itself as a positively tilted trough by tonight as it digs southwestward over the Southwest. Before it does so, the last potent shortwave of the "old" trough will race northeastward into the Great Lakes as the leading edge of the trough becomes a roughly straight line southwesterly from the Southwest right through into eastern Canada. The potent shortwave and associated surface low will both race northeastward through the period. While they're over the upper Mississippi Valley, they will move the front currently stalled from Texas through Michigan eastward a bit to align with the Slight and Marginal Risk areas along the Mississippi and Ohio Rivers. This will give the hard hit northwestern Arkansas region a break as the heaviest rain shifts east. However, by tonight, both features will be well into the Great Lakes and Canada, so the forcing pushing them east will quickly diminish, resulting in another stall-out of the front. With continuous Gulf moisture, another round of training thunderstorms is expected up the Mississippi and lower Ohio Rivers tonight. Given both the prior history of this front stalling as well as urban concerns in the Memphis area especially, a Slight Risk area was introduced with this update. Despite the stalled out front keeping the rainfall largely confined to a narrow corridor, the upper level forcing quickly weakening tonight should hold rates and coverage in check much better tonight as compared to the ongoing storms over Texas, Arkansas and Missouri right now. Thus, forecast rainfall amounts are considerably lower today and tonight than yesterday. The Slight is considered a "lower-end Slight" with any flash flooding impacts expected to be largely confined to low-lying flood prone areas and urban areas. Like areas further west, this area has also been very dry prior to this rainfall, so it will still take a prolonged period of heavy rainfall to result in flash flooding. Wegman Day 2 Valid 12Z Wed Nov 06 2024 - 12Z Thu Nov 07 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF GEORGIA AND SOUTH CAROLINA... 20z Update: Main change to the inherited risk areas was to expand the Slight risk to the GA/SC coast. Overall guidance is split on where convergence will be maximized, with the GFS/ECMWF and a couple HREF members inland over GA and SC, the HRRR and Gem Reg more towards the coast, and the ECMWF AIFS in between. Confidence in which model camp is right remains low...thus the Slight risk was expanded to include both possible outcomes. Overall still think this event has some higher end potential along a narrow axis. The dry antecedent conditions will initially limit the flood risk...however the ingredients are there to overcome this wherever convergence can be maximized for the longest time. Could eventually need a targeted MDT risk, but given continued uncertainty with the axis of max rainfall, think a Slight should suffice for now. Chenard ....Previous Discussion... A large plume of deep tropical moisture will overspread all of Florida and move into Georgia and South Carolina on Wednesday and Wednesday night. This same plume is home to T.S. Rafael down in the Caribbean. PWATs will increase to as high as 2.25 inches, meaning the storms expected to form will have plenty of moisture to convert to heavy rainfall. The front over the Ohio Valley is unlikely to play too much of a role in the storms over Georgia and South Carolina. Rather, the guidance suggests a secondary front will from at the leading edge of the moisture plume that will likely be enhanced by the right entrance region of the upper level jet streak, as well as possibly some upslope component (especially in South Carolina). Regardless of the primary lifting mechanism, there is good agreement in the guidance for an area of 3 to 5 inches of rain (though likely much higher amounts will be embedded) in the Slight Risk area, which is a small increase in amounts from the previous forecast. As with areas further west the past several days, this area has also been incredibly dry lately, with many areas not having seen appreciable rain in a month. For the swampy and sandy areas, this will mean it will take a lot of rain before runoff converts to flooding. Further north across interior SC and GA, the clay soils are likely quite hydrophobic due to the recent dry weather, which will notably increase runoff once the heavy rain gets started. Thus, locally increased impacts are possible. An internal higher- end Slight area was introduced for the peak rainfall area of east- central Georgia and west central South Carolina for this potential. Since this plume is home to T.S. Rafael, this would be considered a predecessor rain event (PRE) if Rafael were to head towards the Big Bend and GA/SC. However, since the latest forecasts keep Rafael tracking westward in the Gulf this rainfall event is not considered a PRE. Wegman Day 3 Valid 12Z Thu Nov 07 2024 - 12Z Fri Nov 08 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AS WELL AS FOR PORTIONS OF THE SOUTHERN PLAINS... 20z Update: Only minimal changes to the inherited Marginal risk areas. Outer banding east of Rafael could be scraping the Gulf Coast of FL, although lingering uncertainty with exactly how close the system will track to the coast. Likely to have ongoing convection over portions of GA/SC and depending on what happens on day 2 could eventually need a Slight risk for the morning hours. However the peak of the heaviest rainfall should be Wednesday afternoon and night, with activity on a downward trend Thursday. So for now will stick with the Marginal and continue to monitor. Over the southern Plains models are trending a bit slower and south with the mid/upper low. The GFS (which had been an outlier) had the biggest jump, with the 12z run now more in line with the rest of the models. However there still may be room for a slower and further south solution...with both the UKMET and ECMWF AIFS showing something slower/south. Do think the ingredients in place will eventually support a Slight risk, but given the lingering uncertainty with placement opted to hold things at Marginal for now. Chenard ....Previous Discussion... ....Southeast... The cold front over the Appalachians that will drive the Day 2/Wednesday rainfall event will move southward to coastal South Carolina and southern Georgia Thursday and Thursday night. The southerly flow of deep tropical moisture ahead of the front will abate somewhat in favor of a more neutral easterly flow. This will keep periods of heavy rain going...but both less widespread, less persistent, and less heavily. The core of heaviest rain will shift to the coast of SC/GA and north FL, but since most of this area is likely to not have seen substantive amounts of rain on Wednesday, and the coastal swamps can usually handle a lot of rainfall with few problems, have opted to keep the area in a Marginal Risk for this update. However, recent model trends have been for the heaviest rain to increase with time as the CAMs move into this period of time. Should that happen a Slight Risk will likely be necessary. ....Southern Plains... A potent upper level low will begin to eject out of the Southwest and into the southern Plains on Thursday. A strong cold front will develop as a result with plentiful snow into much of northeastern New Mexico. This area was trimmed from the Marginal Risk with this update. For the rest of the Marginal Risk area, some Gulf moisture will be captured and advected northward across north Texas and Oklahoma, but amounts will be somewhat limited. With only broad forcing from an upper level jet streak but few coherent features away from the strong cold front, the rainfall footprint will be broad and disorganized. This area has seen plentiful rainfall over the past few days, so the additional rainfall, while not much, may still cause isolated instances of flash flooding. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5aLzdgNXd9cMNSJoMESFKHyE-uGIdMeKnOYleKgGNy1s= ZxjYiMegJTrWfOdrrvD0Tlr3hLFYs_zIoEcpHsdu6uXCo74$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5aLzdgNXd9cMNSJoMESFKHyE-uGIdMeKnOYleKgGNy1s= ZxjYiMegJTrWfOdrrvD0Tlr3hLFYs_zIoEcpHsduDl2dF0E$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5aLzdgNXd9cMNSJoMESFKHyE-uGIdMeKnOYleKgGNy1s= ZxjYiMegJTrWfOdrrvD0Tlr3hLFYs_zIoEcpHsdu2-NsLew$=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) .