Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Sep 05 2023 20:20:32 FOUS30 KWBC 052020 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 419 PM EDT Tue Sep 05 2023 Day 1 Valid 16Z Tue Sep 05 2023 - 12Z Wed Sep 06 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT ACROSS PORTIONS OF THE NORTHERN PLAINS AND UPPER MIDWEST AS WELL AS THE MID-MISSISSIPPI VALLEY THROUGH WESTERN KENTUCKY AND SOUTHERN INDIANA... ....16z Update... Minor shifts in the edges of the MRGL risk(s) were created based on the latest trend in hi-res guidance, including updated 12 HREF probabilities. Surface low over the northern plains continues to move to the east-northeast with pressures down into the upper 990s as of the latest obs and surface analysis. Western edge of the precip field is moving slowly to the east, but rates have been fairly benign with max of 0.5"/hr in spots over northwestern SD. Precip field on the western extent will shrink with emphasis turned to the convection over MN/WI later this afternoon and evening. Latest HREF probabilities continue the trend of heaviest rainfall focused across the arrowhead of MN where strongest mid-level forcing and anomalous PW axis intersect. 12z sounding out of KMPX was very moist, within a solid 2 standard deviations above normal with guidance pinpointing similar PW indices off to the northeast near Duluth and surround locales. A general 1-2" will be possible over northeastern MN during the convection in place later today, but locally higher amounts are possible given the signature for 2"/hr rainfall rates on this mornings CAMs and HREF probabilities. Areal flooding extent still not anticipated to be enough to warrant an upgrade, a lot attributed to the quick-moving nature of the convection today, as well as antecedent dry conditions in place where precipitation is forecast to be heaviest. Marginal risk over the Mid-Mississippi Valley is still within reason given the latest hi-res trends, but did expand the northeast edge into central KY and southern IN with guidance trending towards a secondary QPF maximum oriented over the area. Deep moisture layer present within a zone of higher MLCAPE extending from Paducah up into Louisville will provide a chance for enhanced rainfall rates where 12z HREF probability shows low, but non-zero chances for 2"/hr rates later this afternoon and evening. There's some indication on a few of the CAMs that the MRGL might need an extension into eastern IN later this evening, but not enough of a consensus to warrant the expansion further north at this time, but bears monitoring. Kleebauer ....Previous Discussion... ....Upper Midwest... A vigorous shortwave over SD this morning will continue eastward into MN by early Wednesday with its attendant surface low and cold front out ahead of it. Precipitable water values over 1.50-1.75" (+1.5 to +2.5 sigma) will precede the front in the warm sector with dew points into the upper 50s to mid 60s, south of a boundary stretching into Ontario. To the southeast, a separate weakening shortwave over the mid-Mississippi Valley will lift into the Great Lakes bringing its own surge of moisture into the region. Afternoon convection is expected to increase over MN today where SPC highlights a Slight Risk of severe weather, with frontogenesis along the northern boundary acting to focus rainfall over northeastern MN/Arrowhead where 1-2" areal average rainfall is forecast, and local amounts of 3-4" possible. 00Z HREF neighborhood probabilities show low (<15%) chances of exceeding 1-hr FFG but a bit higher (15-30%) chances of exceeding 3-hr FFG values. The region has been very dry for the past 1-2 weeks, so with 1-hr rates generally 1-2"/hr at most, expect mainly isolated flash flooding where heavier rain falls over more sensitive areas. Maintained the Marginal Risk area into northern WI and into the U.P. of Michigan where rainfall will move into the region this evening into the overnight hours. Farther west, any rainfall just after 12Z over the central Dakotas may exceed recently depressed FFG values from overnight rainfall. ....Mid-Mississippi Valley/Ozarks... Marginal Risk area is outlined from eastern OK to the Mid-South, out ahead of a surface cold front approaching from the NW. Despite the exiting shortwave in the morning hours into the Great Lakes, southerly flow will supply sufficient moisture into the region with precipitable water values around 1.75 inches (+1 to +1.5 sigma). Instability will increase during the day and support isolated to scattered storms into the Ozarks where FFG values are relatively lower than surrounding areas and very isolated storms farther east where the models are struggling with the evolution. Fracasso Day 2 Valid 12Z Wed Sep 06 2023 - 12Z Thu Sep 07 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT OVER PORTIONS OF THE GREAT LAKES AND ALSO THE MID-MISSISSIPPI AND OHIO VALLEYS... ....2030z Update... General forecast remains on track for both MRGL risk areas. An expansion through western TN and KY was made in conjunction with better hi-res deterministic and ensemble output indicating a QPF maximum located along the stout theta-e gradient present ahead of the cold front pressing through the Midwest and adjacent Ohio Valley. 12z HREF probs for 1-2" 24 hr QPF have gone up a fair bit from the previous overnight runs with a bullseye of 20-30% located along the I-65 corridor from Louisville down into Nashville. The bulk of the precip forecast is during the 6-9 hr window between 12-21z Wednesday, so this is a good indication that the convective nature of the rainfall within a deep moisture field will present some localized flash flooding concerns for the outlined area. Convergence along the cold front will still be present further to the southwest over Memphis back into southern AR, so have maintained the MRGL outlook over that general corridor, but trimmed the northern extent based on model trends. No discernible changes necessary for the MRGL over the Great Lakes. Target area remains the U.P. of Michigan where the highest PW anomalies will be centered under robust mid-level ascent providing the opportunity for some 1-2"/hr rain rates during the height of impact, backed-up by a continued signal from ensemble probabilities in both global and hi-res deterministic. Kleebauer ....Previous Discussion... ....Great Lakes... Early on Day 2/Wednesday, the mid-level shortwave or weakly-closed low will move across Wisconsin along with the surface low and cold front. CAM guidance indicates enough instability may reside in the moist air mass (precipitable water values around 1.75 inches) to support some localized 1-2"/hr rainfall rates over northeastern WI into the U.P. of Michigan as the system moves through. Introduced a small Marginal Risk area to account for a low-end threat atop lower FFG values before the system shifts into Canada and loses its moisture connection to the south. ....Mid-Mississippi/Ohio Valleys... A surface cold front will slow its progression through Arkansas on Wednesday where another round of convection is possible in the afternoon. With rainfall on the previous two days, FFG values may lower enough to be attainable despite weaker dynamics overall but sufficient moisture in the column. Though additional isolated convection is possible farther northeast into the Ohio Valley, antecedent conditions are quite dry and rainfall rates and duration are lacking. Fracasso Day 3 Valid 12Z Thu Sep 07 2023 - 12Z Fri Sep 08 2023 ....A MARGINAL RISK OF EXCESSIVE RAINFALL IS IN EFFECT FOR PORTIONS OF THE NORTHEAST U.S... ....Northeast U.S... A sharpening mid and upper trough axis will allow for enhanced meridional flow across the Mid-Atlantic and Northeast with a wedge of PW anomalies approaching 1-2 standard deviations above normal. Increased mid-level ascent ahead of the main trough axis will provide a wedge of convective development extending from the Mason Dixon up through NY State beginning Thursday afternoon, carrying through the evening time frame. A better signal for localized heavy rainfall is beginning to show up on global deterministic and ensemble guidance with a general 1-2" maximum showing up over across the terrain in south-central PA and the Hudson Valley to the Capital District in NY. SREF plumes are pretty stout for this range, especially ARW members with a large coverage of ~1" potential situated over the aforementioned area. Given the overall synoptic evolution forecast, ensemble signals, and the already low FFG's across the NE CONUS, have decided to add the MRGL risk to the above area. Kleebauer Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!4XwkV5RGtuKP7EYuq0h4ii5PNyXI8LMQmT1arvU214Ql= Hsc4sg5C9FMNHF-LjMXpgHHcwOhjgT8hRj6SoL9QHlmpXAY$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!4XwkV5RGtuKP7EYuq0h4ii5PNyXI8LMQmT1arvU214Ql= Hsc4sg5C9FMNHF-LjMXpgHHcwOhjgT8hRj6SoL9QSIdblaA$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!4XwkV5RGtuKP7EYuq0h4ii5PNyXI8LMQmT1arvU214Ql= Hsc4sg5C9FMNHF-LjMXpgHHcwOhjgT8hRj6SoL9QYew6FTU$=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) .