Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Mar 07 2024 21:04:35 FOUS30 KWBC 072020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 318 PM EST Thu Mar 07 2024 Day 1 Valid 16Z Thu Mar 07 2024 - 12Z Fri Mar 08 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF NORTH-CENTRAL TEXAS... ....16z Update... No major changes necessary for this update, only tweaking the boundaries of the Marginal Risk area based on the latest (12z) hi-res guidance. Some modest expansion of the Marginal on the west-central and southwest fringes was necessary due to the potential for convective initiation farther west near the dryline, while a reduction in the northern extent of the Marginal also seemed appropriate due to lack of instability with totals of under an inch expected. The Slight Risk was maintained for the same region of north-central TX, where the highest rates are favored (1-2"/hr) with the highest neighborhood probabilities for 3" exceedance (30-60%, per the 12z HREF). While some CAMs indicate similar localized totals farther north into central MO and surroundings, these rates should generally be capped at 1"/hr (and occur over a longer period time), limiting the potential for flash flooding. Churchill ....Previous Discussion... ....North-Central Texas... In coordination with FWD/Fort-Worth Dallas, TX forecast office, a Slight Risk was introduced for portions of North-Central Texas. Much of the CAMs guidance is in good agreement that a line of thunderstorms will move southeastward across north-central Texas this afternoon and this evening. The storms are likely to be in the area of the Dallas-Fort Worth Metroplex during the evening commute this afternoon. Thus, in addition to increasing forecasts for rainfall in the area, with a widespread 1-2 inches of rain over the flood-sensitive urban area expected, any resultant flash flooding will have outsized impacts due to the time of day of the heaviest rain. The storms will form at the tail end of a dry line attached to a developing low over Oklahoma. The low will track northeastward into the evening, roughly parallel to the dry line. This track will work against a fast forward movement of the line to the southeast. The line will be moving though, as it keeps up with the intensifying feed of Gulf moisture moving into the mid-Mississippi Valley. However, without the added support of the movement of the parent low, it's still expected that any one area of the Slight Risk will see 1-3 hours of rainfall, with many areas seeing an hour of heavy rain associated with the initial line of storms. ....Central Plains to Mid-Mississippi Valley... This area will be closer to the track of the parent low as it moves from eastern Oklahoma to the MO/IL/IA junction point by Friday morning. Heavy rain will be favored as the initial line of showers and storms supported by the warm conveyor belt moves across Missouri, but as the low passes, the comma-head region north and west of the low is expected to have enough forcing and moisture-availability to extend the rainfall into the overnight hours. Eastern Kansas into western Missouri looks to be the area most likely to be impacted by the extended rainfall. For now, it appears this will remain largely south of the Kansas City Metro, but may impact Topeka. Wegman Day 2 Valid 12Z Fri Mar 08 2024 - 12Z Sat Mar 09 2024 ....THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF CENTRAL ALABAMA AND GEORGIA... ....2030z Update... Not much change in the forecast and expected impacts with this update, which resulted in only a modest expansion of the inherited Moderate Risk area (on the western and southeastern fringes). The new Moderate Risk area generally encompasses portions of central Alabama and Georgia, including the Birmingham and Atlanta metro areas. The 12z HREF indicates neighborhood probabilities for 3" and 5" exceedance thresholds of 50-80% and 30-50%, respectively. The bulk of this precipitation may occur over a period of 6 hours or less (mainly in the evening and overnight hours), suggesting the potential for numerous instances of flash flooding (with localized instances of significant flash flooding, given the potential for 6"+ amounts). A broader Slight Risk area encompasses much of the Southeast, though the chances for these high-end amounts should generally be confined to a training west-east axis from MS/AL/GA along the advancing warm front. Churchill ....Previous Discussion... ....Southeast... In coordination with BMX/Birmingham, AL and FFC/Peachtree City, GA forecast offices, a Moderate Risk upgrade was introduced with this forecast update. With ample upper level support from a 150 kt southwesterly jet, a surface low is expected to develop Friday afternoon across northeast Texas and track into the Ohio Valley by Saturday morning. Ahead of this low, a robust and impressive low level jet that will already have been established will intensify Friday night. PWATs with the LLJ will increase to above 1.75 inches in some areas of the Southeast. Meanwhile the 40-50 kts of south to southwesterly flow associated with the LLJ will provide a very effective and prolific supply of Gulf moisture into the southeast. The warm front associated with the low is expected to be draped through the Slight and Moderate Risk areas, roughly aligned southwest to northeast. The key will be mid-level support in the form of a front that will be roughly collocated. This will provide the forcing to lift all the moisture rapidly advecting into the area, resulting in a nearly stationary boundary along which numerous showers and thunderstorms will form and track northeastward along. The training storms, ample moisture and relatively stationary boundary will favor the development of a rather narrow area where rainfall totals could easily exceed 4 inches (and will may approach as high as 8 inches in localized locations), depending on the microscale nature of the storms. Individual storms will likely move along quickly, but the boundary and moisture inflow will support backbuilding convection, especially as the LLJ gradually realigns to more of a southwest to northeast orientation. This will speed along the individual storms, but being more parallel to the frontal boundary, will support increased backbuilding and training. The storms will initially form with peak heating Friday afternoon, but the heaviest rainfall will occur overnight Friday night. The timing of the storms will make any resultant flash flooding more dangerous as it occurs at night. Rainfall totals of 2-3 inches occurred during the day Wednesday in the Atlanta area, which resulted in significant flooding. This next event is likely to impact the Atlanta area once again with at least similar amounts of rain, and potentially more. Thus, the impacts and widespread nature of the expected flooding prompted the Moderate Risk upgrade. Further south and west into Alabama, the storms will impact these areas first, which may include either Birmingham or Montgomery. The heaviest totals of the entire event may be over this area, as has been generally agreed upon in guidance trends. Thus, the potential for Moderate Risk impacts led to the Risk contour to include those metro areas. Further south and west, the Slight Risk includes portions of the central Gulf coast largely in deference to the EC solution, suggesting multiple inches of rain may fall along the immediate coast from eastern Louisiana into the western Florida Panhandle. While this is an outlier solution, the past several runs of the EC have all shown this solution with little variability. In deference to the reliance of the guidance, the Slight remains in effect for these areas. Should these coastal storms occur, this will greatly reduce the potential for heavy rain further north into central Mississippi, though it is in good agreement that heavy rain is likely in the Moderate Risk area. Given that dichotomy and generally lesser storm total rainfall amounts expected that far west, the Slight remains somewhat uncertain, and the Moderate Risk was not expected with the latest guidance. To the north and east into the Southern Appalachians, the storms that impact Alabama and western Georgia will be weakening as they track northeastward into the western Carolinas. While the storms will be weakening, they will have the benefit of some upslope flow...though the primary flow will be parallel to the mountains. Nonetheless, the moisture-laden nature of the atmosphere should result in 2 to as much as 4 inches of rain Friday night into the western Carolinas. The Slight remains in effect for these areas with little change. ....Mid-Mississippi, Ohio Valleys to Chicagoland... The Marginal Risk remains in effect for these areas, with the Chicagoland area added with this update. Significant uncertainty remains in the forecast with the structure and nature of any showers and thunderstorms that develop in this part of the country. Despite having better forcing in the form of a potent cold front and a low (the same as from day 1) tracking into the upper Great Lakes, the lack of moisture will serve as a significantly limiting factor. As such, the guidance remains highly variable in the various solutions...with little confidence on where any heavier rain will occur. Much of the Marginal Risk area in this part of the country had heavy rain in some isolated areas 2 nights ago, which may support the development of isolated flash flooding should heavy rain coincide. Wegman Day 3 Valid 12Z Sat Mar 09 2024 - 12Z Sun Mar 10 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE CAROLINAS... ....2030z Update... The inherited Slight Risk area was expanded a bit for this update, based on the new ensemble guidance which suggests a slightly broader area with the potential for 2-3"+ localized totals. In addition, combined the two separate Marginal Risk areas to encompass the full Mid-Atlantic region (given that ensemble guidance suggests some localized totals of 1"+ between the Carolina maxima and the Northeast maxima). Overall meteorological thinking remains unchanged from the prior outlook. Churchill ....Previous Discussion... ....Carolinas... In coordination with CAE/Columbia, SC and ILM/Wilmington, NC forecast offices, a Slight Risk upgrade with introduced with this forecast update. Storms ongoing Saturday morning at the nose of a southwesterly LLJ emanating from the central Gulf will be impacting portions of the Carolinas by Saturday. The low over the Ohio Valley will keep a warm front associated with the nose of the jet moving eastward into the Carolinas, as it gets hung up along the mountains, which will eventually separate the front from the parent low. Rain is likely to be ongoing for at least the western portions of the Slight risk across northern South Carolina on Saturday. The support of the strong LLJ will keep convection going strong as it moves with increasing speed along the Carolina border and into the Atlantic. The heaviest convection associated therewith is expected to occur Saturday morning. By Saturday afternoon most of the convection will be off the coast or impacting coastal southern North Carolina. The LLJ will shift off the coast as a strong cold front sweeps across the Southeast and pushes all of the significant rainfall well off the coast Saturday night. ....Great Lakes into the Northeast... A strong low tracking northeast from the Ohio Valley Saturday morning to New England Sunday morning will drag a powerful cold front behind the front. This will provide ample forcing to lift residual moisture and convection associated with the warm conveyor belt in the form of locally heavy rain across the Marginal Risk area. The lion's share of the available atmospheric moisture will stay over the Southeast, but the ample forcing and some moisture will support some heavier rain. The key limiting factor to flash flooding will be the fast movement of the low, which in turn will bring a dry slot in from the southwest behind the low, ending any significant rainfall. Nonetheless, rain ahead of the low in the warm sector and rain associated with the cold conveyor could drop a quick inch of accumulation, which could cause isolated flash flooding from the Great Lakes into the Northeast, with a slightly higher threat into the Northeast due to very saturated soils in that area into New England due to ongoing and recent heavy rains. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-yALjHr7ICBV-yos8IZorTlBbSrnbQqOJTc0xEM4M0LA= w7xkLuxlgGmzgISEEk2iDT6JQa2e5S1TOz_pbJ_51JEU8fE$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-yALjHr7ICBV-yos8IZorTlBbSrnbQqOJTc0xEM4M0LA= w7xkLuxlgGmzgISEEk2iDT6JQa2e5S1TOz_pbJ_5P5teGzk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-yALjHr7ICBV-yos8IZorTlBbSrnbQqOJTc0xEM4M0LA= w7xkLuxlgGmzgISEEk2iDT6JQa2e5S1TOz_pbJ_5JSz-TTU$=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) .