Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Dec 17 2024 20:01:02 FOUS30 KWBC 172000 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 300 PM EST Tue Dec 17 2024 Day 1 Valid 16Z Tue Dec 17 2024 - 12Z Wed Dec 18 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PORTIONS OF THE EAST CENTRAL TO SOUTHEAST COAST OF FLORIDA... ....16Z update... 12Z soundings from MFL and KEY showed PW values of 1.5 and 1.7 inches, respectively, or near the 90th percentile for mid- December. Low level easterly flow is expected to support showers/thunderstorms moving inland during the day today with an increasing threat for slow moving/stalled cells late today and overnight along an expected coastal/near-coastal convergence axis. 925-850 mb winds of 15 to 20+ kt are forecast by the latest guidance with a shift toward more of a southeasterly direction later tonight. The 12Z HREF indicated neighborhood probabilities for 5 inches in 24 hours (ending 12Z Wednesday) of 50-60 percent and while these probabilities have come down and are largely=20 driven by the more bullish 12Z NAM_nest and ARW2, a low end threat=20 for localized flash flooding will remain through 12Z Wednesday for=20 the eastern and southeastern FL Peninsula. Otto ....previous discussion follows... Model consensus is for increasingly active showers late Tuesday afternoon into Tuesday night/early Wednesday along the east central to southeast coast of Florida. Low level easterly to southeasterly flow in an axis of PW values 1.5 to 2+ standard deviations above the mean expected to persist into the east central to southeast Florida coast, with models showing an axis of defined boundary layer convergence in this low level inflow axis parallel to the east central to southeast coast. There will be potential for very slow moving cells along the coastal regions, supporting locally very heavy rainfall amounts and an urban flash flood threat. HREF neighborhood probabilities are high (50-90%) for 3"+ amounts from Cape Canaveral, south to Miami, while the 5"+ neighborhood probabilities are high (60-90%) from near West Palm Beach, south to Miami. The best hi res overlap, evident in the greatest EAS probabilities for 2"+ amounts, is from near West Palm, south to just north of Miami. Oravec Day 2 Valid 12Z Wed Dec 18 2024 - 12Z Thu Dec 19 2024 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG PARTS OF THE MID-SOUTH AND LOWER OHIO VALLEY ALONG WITH THE SOUTHEAST=20 FLORIDA COAST... ....TN into southern KY... Have included a targeted Marginal Risk in the updated Day 2 ERO across parts of western-central TN (especially northern portions) into southern KY, which would include the Nashville metro area. Amplifying upper trough to the west will allow for a more curved upper level jet streak on the lee side of the trough -- which will enhance the upper divergence and low-level moisture transport/flux into the outlook area. The latest (12Z) HREF show more elevated probabilities of 1"/hr rainfall rates between 12-18Z. In addition, especially when considering recent heavy rainfall over the past=20 24+ hours (1-2 inches over parts of nw TN and sw KY), the latest 1 and 3 FFG values have come down while 0-40km soil moisture percentiles per NASA SPoRT have climbed to 60-80% for most areas. As such, the HREF neighborhood probabilities of 3 hourly QPF=20 exceeding 3 hour FFG peaks between 25-40% between 12-15Z Wed, when in that short 3-hour window most areas should see additional=20 rainfall of 1-1.5" while some (isolated) locations receive closer=20 to 2+ inches. ....Southeast FL... Basically carrying over the Marginal Risk area from Day 1 into the Day 2 period, as much of the 12Z high-res CAMs show lingering convective clusters along the se FL coast (mainly south of Vero Beach). Continued favorable (onshore) low-level moisture flux/convergence and sufficient deep-layer instability on Wed will maintain elevated short-term rainfall rates, likely 1-2+ in/hr at times during the day 2 period per the HREF probabilities. The 12Z=20 HREF meanwhile also indicates 50-70% 40km neighborhood=20 probabilities of at least 5 inches of additional rain during Day 2 (12Z Wed-12Z Thu), with 30-50% probs of 8+ inches. The NAM-CONUS=20 Nest, ARW1, and 12Z HRRR in particular are leading to these high=20 HREF probabilities.=20 Hurley Day 3 Valid 12Z Thu Dec 19 2024 - 12Z Fri Dec 20 2024 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Hurley Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!99ChtqScNzFHFt1Sx9hi6HGttWsZb6Y3RkAhHODBf4BK= pdTtu8mLOa6H8YwUEV07SWB1lJstCx9wuldJSg7GODxh8XI$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!99ChtqScNzFHFt1Sx9hi6HGttWsZb6Y3RkAhHODBf4BK= pdTtu8mLOa6H8YwUEV07SWB1lJstCx9wuldJSg7G19Fvcv0$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!99ChtqScNzFHFt1Sx9hi6HGttWsZb6Y3RkAhHODBf4BK= pdTtu8mLOa6H8YwUEV07SWB1lJstCx9wuldJSg7GIIpOF0M$=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) .