Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Dec 18 2024 16:00:49 FOUS30 KWBC 181600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1100 AM EST Wed Dec 18 2024 Day 1 Valid 16Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF THE OHIO AND TENNESSEE VALLEYS ALONG WITH THE SOUTHEAST FLORIDA COAST... ....16Z update... ....Southeast FL... Weak 0-6 km AGL and LFC-EL mean layer winds (both ~5 kt on the 12Z MFL sounding, but slightly stronger flow to north) will continue to support the potential of slow moving showers/thunderstorms during the day today. While the earlier cell near FLL has weakened since this morning, a low-end threat for mainly urban flooding will linger into peak heating. A convectively enhanced mid-level trough just east of the FL Peninsula is expected to continue weakening=20 with an eastward translation today. Low level flow has become more=20 southerly and while the bulk of convection today is expected to=20 remain offshore, an isolated chance for a slow moving cell later this afternoon will be possible with daytime heating within the anomalous moisture over southeastern FL. Will keep the Marginal Risk, though the threat appears to be diminished compared to earlier today. ....Parts of the Mid-South to Lower Ohio Valley... Opted to remove the Marginal Risk for this portion of the country with this update. A SW to NE axis of locally heavy rain extended=20 from southeastern KY through middle TN and northwestern AL at=20 1545Z, moving off toward the east. There has been brief training=20 within this line with MRMS- estimated rainfall rates near 1 in/hr, but overall, the line has been progressive with some possible=20 minor flooding earlier this morning along the western KY/TN border. As a surface low over KY tracks northeastward today, the front and associated precipitation axis will advance steadily off toward the east with some weakening due in part to a lack of=20 moisture/instability downstream. Additional spotty rainfall of 1=20 to 1.5 inches will be possible on a localized basis. Otto ....previous discussion follows... ....Parts of the Mid-South to Lower Ohio Valley... Maintained the Marginal Risk area across parts of western and central Tennessee into southern Kentucky as models still advertise an upper-level trough amplifying to the west which allows for a more curved upper level jet streak on the lee side of the trough. That configuration may result in some enhanced rainfall rates...an idea supported by the 18/00Z HREF neighborhood probabilities showing potential for rainfall rates exceeding an inch per hour between 12-18Z today (mainly in Tennessee). With rainfall blossoming to the west of the outlook area in the pre-dawn hours this morning helping to prime the soils a bit more and a general slowing of the eastward progression across the Tennessee Valley compared with earlier forecasts...opted to extend the Marginal risk area a bit farther westward than previously indicated. The HREF probability of exceeding flash flood guidance remained low but did reflect somewhat higher probabilities west of the same guidance from the 17/12Z and 17/18Z runs. The decision to expand westward slightly was also driven by the observed rainfall during the past day or two over parts of northwest Tennessee and southwest Kentucky which has resulted in the latest 1-hour and 3- hour flash flood guidance coming down and the 0-40km soil moisture percentiles per NASA SPoRT have climbed to 60-80% for most areas. ....Southeast FL... Post frontal easterly low level flow off the Atlantic will linger allowing for a possible repeat of convective clusters over and near the southeast Florida coastline today. The 00Z runs of the high-res CAMs continued to show onshore low-level moisture flux/convergence and sufficient deep-layer instability to support localized downpours. The 18/00Z HREF supports rates of 2+ inch per hour peaking around 10 percent during the afternoon with and a 50-70 percent 40km neighborhood probabilities of at least 5 inches of additional rain and 30-50 percent probabilities of 8+ inches. Given that these numbers are comparable with the 17/12Z and 17/18Z runs of the HREF...saw little reason to make too many changes other than nudges to the boundary of the previously issued Marginal risk area. Bann Day 2 Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 3 Valid 12Z Fri Dec 20 2024 - 12Z Sat Dec 21 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4IMnT7g2skWYqSIJM-hDdlj5SqgMjFuz2E-dkoJCMSc6= 13K6zwjdEjf0FL6iPuWAs3cuQEXd8q1ZnmOo1xowkjieblc$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4IMnT7g2skWYqSIJM-hDdlj5SqgMjFuz2E-dkoJCMSc6= 13K6zwjdEjf0FL6iPuWAs3cuQEXd8q1ZnmOo1xowrCiJrIk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4IMnT7g2skWYqSIJM-hDdlj5SqgMjFuz2E-dkoJCMSc6= 13K6zwjdEjf0FL6iPuWAs3cuQEXd8q1ZnmOo1xowZkp4a3s$=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. 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