Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 14 2025 15:52:57 FOUS30 KWBC 141552 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1152 AM EDT Fri Mar 14 2025 Day 1 Valid 16Z Fri Mar 14 2025 - 12Z Sat Mar 15 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE LOWER TO MID MISSISSIPPI VALLEY INTO PARTS OF THE OHIO AND TENNESSEE VALLEYS... A deepening mid-level upper low will move northeastward today from the southern Plains and over the Middle Mississippi Valley. At the same time, strengthening southerly flow will help to advect a=20 moist, unstable airmass increasingly northward and ahead of an=20 eastward moving cold front. Storms are expected to rapidly develop=20 along the cold front by late afternoon possibly as far west as the=20 Middle/Lower Missouri Valley before increasing in coverage as the=20 front reaches the Middle Mississippi Valley this evening, with=20 eventual development also extending southward into the Tennessee=20 Valley. Both the front and accompanying storm motions are expected=20 to remain rather quick moving given the strong dynamics in place.=20 Therefore, this convection will almost certainly pose more of a=20 severe risk than a flash flood threat. However, given the strong=20 dynamics, along with sufficient instability for deep convection,=20 expect peak rainfall rates will be briefly intense enough=20 (~1-2"/hr) to produce localized amounts that exceed current model=20 forecasts (2-3 inches over relatively short period) which could=20 lead to some isolated flash flooding particularly in urban areas.=20 There is also an increasing signal in the guidance that some=20 additional storms may develop ahead of the front as an 850 mb jet=20 ramps up from the Lower Mississippi Valley into the Tennessee=20 Valley in the evening/overnight hours. These storms would also pose a threat for some locally heavy rain rates and the potential for a quick 2-3 inches of rainfall.=20 Putnam Day 2 Valid 12Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE TENNESSEE AND OHIO VALLEYS... ....Ohio and Tennessee Valleys... Even as the previously noted low lifts to the north through the Great Lakes into Ontario, the upper pattern will remain amplified across the central U.S. as an upstream shortwave dives southeast into the southern Plains on Saturday morning and spreads eastward over the Lower Mississippi Valley into Saturday afternoon/evening. A surface wave developing along the leading system's trailing cold front will help focus deeper moisture tapped by strengthening southerly winds and with increasing upper-level dynamical support as the southern shortwave approaches. This will help lead to the continued development and maintenance of an intense line of thunderstorms pushing eastward along the front. The general consensus of the most recent 12Z guidance indicates that widespread accumulations of 2-4 inches centered over parts of the Tennessee and lower Ohio Valleys are likely (with locally higher amounts). A large Slight Risk was once again maintained across the region, reflecting the higher probabilities (50-70%) for 2 inches or more shown by the GEFS/ECENS (and available CAMs) where this favorable combination of deeper moisture, interacting with strong forcing, along a relatively slow moving front, is expected to raise the potential for heavy amounts late Saturday into early Sunday from training storms. This remains a higher-end Slight Risk for parts of the area, and a more focused corridor with the potential for an upgrade to a Moderate Risk with the addition of more CAM data extending fully into the period later today. The latest 00z guidance indicates this potential remains highest from northeast Mississippi/northwest Alabama northeast through central Tennessee into south-central Kentucky where locally higher totals of 5-6" could be realized (as the FV3 in particular highlights). However, have opted to remain at a higher-end Slight Risk for now as it is difficult to pin down the targeted area that may need a Moderate risk upgrade. The threat will be evaluated once again with the 12z CAM suite with the potential for a targeted Moderate risk upgrade. ....Pacific Northwest... Introduced a Marginal risk to portions of the Pacific Northwest (mostly coastal/western Oregon and a bit of far northwestern CA) with a notable uptick in QPF with this forecast cycle, suggesting the potential for 2-5" localized rainfall totals along the coast (with the highest amounts indicated in the OR/CA coastal border area). This is due to a faster/stronger AR reaching into the Day 2 period (as an inherited Marginal risk has been maintained for this update for Day 3) with IVT of 400-600 kg/ms indicated perpendicular or nearly perpendicular to the coast from 21z Sat well into Day 3. Churchill/Putnam/Pereira Day 3 Valid 12Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE EAST COAST, FROM THE SOUTHEAST TO NEW ENGLAND, AND FOR PORTIONS OF THE PACIFIC NORTHWEST... ....East Coast... As the aforementioned low lifts northward into Canada, what's left is a weakening cold front as the upper-level pattern becomes a bit less amplified (shortwave over the Southeast lifting a bit and cutting off from northern/polar stream). The result looks to be scattered localized totals of 1-2" over a large region from the Southeast (central FL, southeast GA, and Carolinas) into the Mid- Atlantic and portions of the Northeast/New England. The 00z GEFS and ECENS both show areas with the potential for 2-3" totals, but they are not in agreement on the regions (GEFS showing potential for the Mid-Atlantic to Northeast, while ECENS has lower probs overall and centered from the Mid-Atlantic to the Southeast). ....Pacific Northwest... The aforementioned AR looks to bring in stronger moisture transport into Day 3 with the potential for IVT in the 600-800 kg/ms range near the OR/CA border region for a period. Model consensus suggests an additional 2-4" QPF through Day 3, and that could necessitate a targeted upgrade to Slight risk in future updates (depending on forecast consistency and trends in the next cycle or two). Churchill Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!8gg33NHNwzOBDUsK750tsPjHoFykGh0k69btDEWH0q9s= gx7_19SOxbXxbudb2GKYoIvaySYzGjaolk-_v7XNa3sa6xY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8gg33NHNwzOBDUsK750tsPjHoFykGh0k69btDEWH0q9s= gx7_19SOxbXxbudb2GKYoIvaySYzGjaolk-_v7XNkhgy7as$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8gg33NHNwzOBDUsK750tsPjHoFykGh0k69btDEWH0q9s= gx7_19SOxbXxbudb2GKYoIvaySYzGjaolk-_v7XNaJ-4tIc$=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) .