Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Sep 06 2023 20:26:22 FOUS30 KWBC 062026 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 PM EDT Wed Sep 06 2023 Day 1 Valid 16Z Wed Sep 06 2023 - 12Z Thu Sep 07 2023 ....THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF THE GREAT LAKES AND ALSO THE ARKLAMISS/MID-SOUTH/MIDDLE TENNESSEE... ....16z Update... The only real meteorological change from the previous forecast was an enhanced precipitation field located over southern MI this morning, moving northeast with embedded heavy rainfall as noted by 1 and 3 hr precipitation obs across SW MI. 12z HREF probabilities highlight a small area to the north of Detroit for non-zero probability of 1" or greater of rainfall in a short time frame while the organized convection moves through. Given trajectory and radar evolution, decided to keep out Detroit proper as the bulk of the precip should lie to the north and west, but some rainfall will still impact southeast MI prior to convective threat ending. Elsewhere, just some minor adjustments in the northern and southern fringes of the MRGL over the Southeast where rates have been generally between 1-2"/hr in the heaviest precipitation according to MRMS and subsequent obs within the corridor impacted. A secondary round of convection will fire along the cold front pressing from the west later this afternoon with multiple deterministic and ensemble outputs pegging rates around 2-3"/hr in the heaviest thunderstorm activity. Adjustments were made based on 12z HREF neighborhood probabilities and mean QPF alignment within the ensemble blend. Kleebauer ....Previous Discussion... ....Great Lakes... Mid-level shortwave entering WI this morning will continue eastward with a preceding cold front and surface low. Air mass remains well-saturated with dew points in the mid-60s to near 70F over Wisconsin into Michigan early this morning with precipitable water values around 1.75 inches (+2 to +2.5 sigma). CAM guidance indicates enough instability to support some localized 1-2"/hr rainfall rates over northeastern WI into the U.P. of Michigan as the system moves through, near the flash flood guidance values. By late this afternoon, the system will shift into Canada and lose its moisture connection to the south. ....ArkLaMiss/Mid-South/Middle Tennessee... A surface cold front will slowly but steadily sink through Arkansas with another round of convection possible this afternoon after the early morning activity out of Missouri weakens and the atmosphere has time to recover. Despite weaker dynamics overall but sufficient moisture in the column ahead of the front, some heavier rainfall rates of 2-3"/hr are possible as storms progress southeastward. May need to re-evaluate later this morning to assess atmospheric recovery/stability. Risk outline continues into Middle TN where FFG values are a bit lower but so are the probabilities of higher rainfall rates >2"/hr as well as a bit lower areal coverage.=20 Fracasso Day 2 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 AND MID-ATLANTIC... ....2030z Update... Previous forecast remains on target with the expected evolution of the mid-level shortwave still within only minor deviations from previous forecast guidance. Main addition to the MRGL was an extension into the Mid-Atlantic, including portions of MD/PA and the spine of the Appalachians and Allegheny front into western NC. Cold front will aid in steep boundary layer forcing as it presses east into the higher terrain across NC and WV. Latest hi-res deterministic and ensemble guidance (HREF) have bumped up local QPF across the general corridor within the mountains as the front taps into a moderately unstable airmass in place ahead of the mean trough. MLCAPE on the order of 1500-2000 J/kg within the mountains and southern PA into northern MD indicate a higher prospect of convective impact within a zone of deep moisture thanks to increasing backed flow advecting well-above normal PWs into portions of the Mid-Atlantic and Northeast U.S (As much as 1.5-2 standard deviations above climatological norm). Tail end of the 12z HREF probabilistic 2"/hr rate potential shows large area of non-zero indicators, including a 20-30% area confined to the southern Appalachians in NC. This gave a higher confidence in at least some localized flash flood concerns within the terrain, and within the urbanized corridors between DC/Baltimore. Kleebauer ....Previous Discussion... ....Northeast and Interior Mid-Atlantic... A mid-level shortwave over Michigan early Thursday will lift into Ontario and weaken as its surfaced cold front continues eastward through the Upper Ohio Valley. Surge of moisture ahead of the front -- precipitable water values 1.50-1.75 inches (+1 to +2 sigma) -- and increased instability in afternoon heating will support scattered showers/storms over the region. 00Z HREF neighborhood probabilities show 10-30% chance of exceeding 1-hr FFG values from WV northward to VT (though at times driven by a couple wet FV3 runs), but the ingredients are there for a localized flash flood threat. FFG values are relatively lower across the Catskills/Southern Tier in NY and into NEPA, though the entire region has seen little rainfall in the past week. Nonetheless, the more complex terrain from eastern WV northeastward into PA and upstate NY into VT may be a factor in more sensitive areas should heavier rain rates develop. Maintained the Marginal Risk area with a small extension into VT which aligns close to the D2 SPC Outlook as well. Fracasso Day 3 The Day 3 outlook will be updated by 2030Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-eq6P34xcYHV_3mEmymOeVztj7y9NcHDbl6uwFeA-4fl= zPp8CZCHdxUH19ypfOUCxrvbpYsWLG8CBSKBGPxVewDIzIU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-eq6P34xcYHV_3mEmymOeVztj7y9NcHDbl6uwFeA-4fl= zPp8CZCHdxUH19ypfOUCxrvbpYsWLG8CBSKBGPxVOMjg8YY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-eq6P34xcYHV_3mEmymOeVztj7y9NcHDbl6uwFeA-4fl= zPp8CZCHdxUH19ypfOUCxrvbpYsWLG8CBSKBGPxVKlpQaGE$=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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