Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Sat Jun 24 2023 16:00:19 FOUS30 KWBC 241600 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 1159 AM EDT Sat Jun 24 2023 Day 1 Valid 16Z Sat Jun 24 2023 - 12Z Sun Jun 25 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS MUCH OF MINNESOTA AND THE EASTERN DAKOTAS... ....Northern Plains into Northern Minnesota...' There is a lingering threat for heavy to locally excessive rainfall over portions of the eastern Dakotas into Minnesota as an anomalously deep mid/upper level low and associated surface low drift northward and eastward. With higher moisture characterized by dewpoints in the 60s and PWs above 1.25-1.5" wrapping into the system, a narrow corridor of heavy rainfall expected near the surface low where the best overlap of moisture, some instability, and deep forcing is present. The 12Z HREF still shows a strong signal for 2-3"+ across portions of eastern North Dakota and hourly totals between 1-1.5" look to be possible. For portions of Minnesota southward into Midwest, early morning convection should push through portions of Iowa and then additional convection is expected to develop in the afternoon/evening across Minnesota but also perhaps extending southward into Missouri, though there is more uncertainty in how convection develops that far south. With the best signal for isolated to scattered flash flooding across portions of Minnesota and the eastern Dakotas...was not compelled to make more than a few minor adjustments to the previously-issued outlook. ....Northeast... The slow moving, stubborn upper level trough will inch forward during the day, beginning this morning in the Ohio Valley and reaching New England by early Sunday morning. The combination of anomalous moisture (PWs up to 1.5"+), warm cloud depths, and skinny CAPE profiles, efficient rainfall producing showers and thunderstorms will be capable of producing localized heavy rainfall, especially this afternoon/evening with the peak daytime heating. With the mean flow remaining somewhat parallel to the overall storm motions, some repeating rounds will be possible, supporting isolated/localized instances of flash flooding. Did expand the southern and western boundaries just a bit in response to the 12Z HREF neighborhood probabilities but the changes did not reflect a fundamental shift in forecast reasoning. ....Southeast Florida... A deep axis of higher than normal moisture will position itself across South Florida and with weak flow and modest shear, higher coverage of afternoon/evening thunderstorms are expected to develop. The environment will support intense rain rates given the tall/skinny CAPE profiles and warm cloud depths. Expanded the southern boundary of the Marginal risk towards the Homestead area based on mid-morning radar imagery. Otherwise there are enough ingredients and support in the various models to maintain the Marginal Risk without change. ....Central Washington... Isolated thunderstorms this afternoon will have the potential to produced localized rain rates up to 0.5-1.0"/hr per the latest model guidance and 00Z/12Z HREF neighborhood probabilities. Given some of the burn scars in the area, particularly western Okanogan County burn scars - Cedar Creek, Cub Creek, and Muckamuck - and the potential for heavy rainfall, maintained the previously-issued Marginal Risk area. Taylor/Bann Day 2 Valid 12Z Sun Jun 25 2023 - 12Z Mon Jun 26 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF THE GREAT LAKES...OHIO AND TENNESSEE VALLEYS...AND NORTHEAST... ....Northeast... Deep southerly flow will remain in place across the Northeast, ahead of the approaching deeper closed low tracking into the Great Lakes, and with the anomalous moisture in place (upwards of +2.5 standard deviations) and expected daytime heating/instability, scattered to widespread thunderstorms are expected across mainly the interior Northeast. Mean flow will be relatively weak (under 10-15 kts) and warm cloud depths will continue to be relatively high, supporting efficient rainfall producing thunderstorms, supporting a localized/isolated threat of flash flooding. ....Great Lakes to Tennessee Valley... The closed mid/upper level low is expected to be across portions of Minnesota at the start of the period (Sunday morning). Ahead of that feature, an axis of higher moisture is expected to lift northward and wrap into the system, characterized by precipitable water values as high as 1.0-1.25" across portions of Wisconsin into Michigan. While higher instability will be fairly limited this far north and the showers and thunderstorms are likely to be somewhat progressive, some locally heavy rainfall will be possible, particularly by the afternoon/evening hours during peak heating across Michigan. Much of the region has been considerably drier than normal however, and the rainfall will be more welcome than concerning, but any localized intense rain rates (approaching 1"/hr) over sensitive locations and totals up to 2" possibly could lead to flash flooding. Further south into portions of Ohio Valley southward into the Tennessee Valley, the environment will be more conducive for heavy rainfall - characterized by dewpoints in the mid 60s to near 70F, and precipitable water values reaching 1.5"+ as far north as central Indiana. An axis of high instability is expected by afternoon and storms initially developing over the lower Great Lakes into Indiana are likely to grow/progress south and east into the lower Ohio Valley. Storm motions may be relatively quick but colliding storm clusters initially over some wetter than normal soils over Kentucky and southern Ohio support an isolated flash flood risk. Further south across Tennessee into Arkansas, there is more uncertainty how convection will evolve (either continuing from earlier MCS out of IN/KY or separate convection developing ahead of the front across Missouri into southern Illinois) but enough support from various model guidance to keep the Marginal Risk extended south/southwest into portions of Arkansas for the threat of isolated flash flooding. Taylor Day 3 Valid 12Z Mon Jun 26 2023 - 12Z Tue Jun 27 2023 ....THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS MID-ATLANTIC... ....Eastern U.S.... An anomalously deep upper low for late June is expected to be centered over central Michigan during the period with an associated surface trough extending southward into the Ohio Valley and Appalachians. Ahead of those feature, a relatively moist airmass will be in place characterized by dewpoints near 70F and precipitable water values above 1.5". Enhanced upper diffluence combined with the greater moisture and expected daytime heating instability will likely ignite a large area of convection across portions of the Northeast southward through the Mid-Atlantic and into North Carolina. The most recent forecast guidance has clustered the greatest rainfall amounts and potential intense rain rates from central North Carolina into portions of Pennsylvania and New Jersey where pockets of 1-2" and localized amounts up to 3" are possible based on the GEFS/ECENS members. Over the last 7 days, the greatest precipitation anomalies have been across parts of Virginia southward into Georgia though Friday there were pockets of heavy rainfall across the Mid-Atlantic. A Slight Risk was introduced from central North Carolina to southern New Jersey and southeast Pennsylvania where the overlap of higher moisture and instability sets up for the expected afternoon/evening convection and could result in isolated/scattered instances of flash flooding. Within the Slight Risk area, the southern half has certainly been the wettest with 7-day departures above 300 percent of normal and is somewhat more vulnerable to the expected rainfall amounts and intensity. ....Southern Montana into Northern Wyoming... Another shortwave trough lifting through the Intermountain West will reach the Rockies late in the period while higher moisture being advected northward seeps into portions of Wyoming and southern Montana. With favorable forcing for ascent and daytime heating instability, a few complexes of thunderstorms are expected to initiate over southern Montana and northern Wyoming before tracking east/southeast late in the period. The most recent guidance shows potential for pockets of 1-1.5" over areas that have been considerably wetter than normal recently given recent heavy rainfall events and a Marginal Risk was introduced for isolated instances of flash flooding. Taylor Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9Nd3zRPxn8RG1NGaV98TuIHv1rsZpxfSVy_XJuqfH2_s= Mf6pIUvAcPoUPuCIuXBA95y6BKOXzapg8A4oc2QyXxBgVsQ$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9Nd3zRPxn8RG1NGaV98TuIHv1rsZpxfSVy_XJuqfH2_s= Mf6pIUvAcPoUPuCIuXBA95y6BKOXzapg8A4oc2QytMAf7-Q$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9Nd3zRPxn8RG1NGaV98TuIHv1rsZpxfSVy_XJuqfH2_s= Mf6pIUvAcPoUPuCIuXBA95y6BKOXzapg8A4oc2Qy9d63t0U$=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) .