Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sun Feb 11 2024 00:50:12 FOUS30 KWBC 110050 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 749 PM EST Sat Feb 10 2024 Day 1 Valid 01Z Sun Feb 11 2024 - 12Z Sun Feb 11 2024 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GULF COAST STATES TO THE SOUTHEAST U.S... An area of rain with embedded heavier convective cells continues to stream northeastward across portions of LA/MS/AL this evening. A lack of deep instability is keeping convection fairly shallow in nature and rainfall rates generally 1"/hr or less. This lack of more intense rates is likely keeping the flash flood risk in check across the region. Recent HRRR forecasts suggest rainfall rates could locally exceed 1"/hr and approach 2" in 3 hours overnight...but even values of this magnitude will generally stay below FFG. With that said still looking at 1-3" of rain through 12z as activity will briefly train along the low level boundary as it only slowly drifts eastward. Thus while this is likely a lower end Slight risk, we still could see some runoff issues and localized flooding where heavier cells briefly train...especially if this happens over a more sensitive basin or urban area. Thus while we did shrink the Slight risk to match current observational and model trends, we will maintain the Slight risk into the overnight hours from northeast LA into central/northern MS/AL and southeast TN into far northwest GA. A better chance for deep and intense convection exists later tonight over portions of central TX, where better instability and forcing will likely result in strong convective development by 06z. Overall this activity will probably be more of a severe threat than flood risk given its expected quick forward movement off to the east. Nonetheless, heavy short term rainfall rates could cause some isolated excess runoff, and the Marginal risk covers this threat. Chenard Day 2 Valid 12Z Sun Feb 11 2024 - 12Z Mon Feb 12 2024 ....THERE IS A MARGINAL RISK AREA OVER PORTIONS OF THE SOUTHEAST U.S. ON SUNDAY... 21Z update... The western bounds of the Marginal Risk area was expanded westward across northern Louisiana and into far eastern Texas where additional rain will keep threat for local flooding concerns elevated. Consensus continues to focus the highest amounts for this period over the northern half of Georgia and western portions of South Carolina. Areal averages remain in the 1 to 2 inch range although locally higher accumulations may be possible. Campbell Guidance continue to depict an areal footprint of QPF across the Gulf states and into the Southeast U.S. during this period as one shortwave scoots eastward early in the period and cyclogenesis occurs vigorous shortwave trough continues to head towards the north/northeast followed by deep layered cyclogenesis over the southern Plains and southern Mississippi Valley. This reinvigorates the on-shore flow of Gulf moisture and results in the flow aloft becoming increasingly difluent. While there is some overlap with a northwest to southeast stripe in Alabama where 14-day rainfall amounts of a couple inches was observed...the axis of heaviest rain should generally be over areas that have had little or no rainfall recently. With WPC deterministic QPF generally below 2 inches...felt a Marginal risk area still adequately covers the risk of excessive rainfall. Bann Day 3 Valid 12Z Mon Feb 12 2024 - 12Z Tue Feb 13 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE SOUTHEAST U.S.... 21Z update... The area of precipitation advance to the Southeast/Mid-Atlantic region as the frontal system progresses eastward. The latest guidance continue to depict the highest amounts of 2 to 3+ inches to align near the coastal areas of North and South Carolina while locations to the west/north will be closer to 1 to 2 inches. The Marginal Risk area encompassed the part of the country that will have an elevated threat for excessive rainfall so no adjustments were needed at this time. Campbell There is a continuing threat of excessive rainfall on Monday...mainly over the Carolinas...as the large scale system moves over parts of the Tennessee Valley. At the surface...one area of low pressure forms and heads northeast over the far eastern TN/OH Valleys while a second area of low pressure forms over parts of the Carolinas...supported by the coupling of upper level jets and the associated vertical velocities supported by the upper level divergence. It is this component which results in the continued risk of excessive rainfall over the Southeast from Monday into the early-morning hours on Tuesday. Precipitable water values increase to values at or slightly above 1.75 inches ahead of a cold front sweeping across the region. Deterministic QPF from the GFS and UKMET seem better than the ECMWF and Canadian and tend to be aligned with the WPC deterministic amounts. Given high FFG numbers...especially where spaghetti plots of model QPF are 2 or 3 inches overlap...will keep the Marginal carried by the WPC Medium Range Desk yesterday. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-nfyAkLugWwmGz1PKtGSUpLJ7HYJntPYBKgXHLoZcMxG= BCVuuP9BnkX8vl04Bc860sZhFrUKAQ_pALKODQqkFvnGmgQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-nfyAkLugWwmGz1PKtGSUpLJ7HYJntPYBKgXHLoZcMxG= BCVuuP9BnkX8vl04Bc860sZhFrUKAQ_pALKODQqkhSBjl8k$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-nfyAkLugWwmGz1PKtGSUpLJ7HYJntPYBKgXHLoZcMxG= BCVuuP9BnkX8vl04Bc860sZhFrUKAQ_pALKODQqkor6o6oE$=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 3.14-Linux * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (1:2320/105) .