Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Fri Mar 24 2023 00:51:11 FOUS30 KWBC 240051 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 849 PM EDT Thu Mar 23 2023 Day 1 Valid 01Z Fri Mar 24 2023 - 12Z Fri Mar 24 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL OVER A PORTIONS OF THE MID/SOUTHERN MISSISSIPPI VALLEY AND LOWER OHIO VALLEY.. 01Z Update... Satellite and radar imagery shows convection that is becoming increasingly organized and expansive in nature across central to northeast OK and through a large area of southwest and central MO. Over the next couple of hours, convection is likely to spread downstream across areas of southern IL and eventually into southern IN where already there are a couple of broken clusters of showers and thunderstorms. Overall, there is no real change to the big picture synoptically with the heavy rainfall threat overnight as a substantial amount of moisture will be transported north into a quasi-stationary front with the aid of a strengthening low-level jet, and with increasing levels of deep layer ascent fostered by strong upper-jet dynamics and convergence along the aforementioned front. The primary change compared to earlier today is the expectation of convection developing and expanding downstream into the Ohio Valley a bit quicker than what the 12Z HREF CAM solutions were advertising. Recent runs of the HRRR have been a bit underwhelming and slow it appears as well in comparison to the radar and satellite trends. The 18Z HREF QPF did support a bit more of a heavier rainfall signal for areas of southeast MO through southern IL in particular, and based on this and the latest observational trends, the Moderate Risk area has been expanded across much of south-central to southeast MO and into southern IL. Very heavy rainfall rates and training/repeating cells along the front in the area are expected, and areas of flash flooding (some of which may locally be significant) will be likely. Please consult WPC's latest MPD products overnight for more details. The Slight Risk area has also been expanded farther east along and adjacent to the Ohio River involving southern IN, northern KY and southern OH where heavier convective rains and concerns for training/repeating cell activity will likely increase in the 06Z to 12Z time frame. Finally, based on the expectation of the QPF footprint being relatively tight overnight, the Marginal Risk area was narrowed/tightend up a bit from the middle MS Valley east through the OH Valley. Orrison Previous discussion... An abundant Gulf moisture will be streaming northward ahead of a deep-layer trough that ejects from the Southern Rockies and results in strengthening low level flow. At about the same time, upper level divergence from an upper level jet aids in surface low pressure forming along a quasi-stationary front extending from Oklahoma northeastward into the Ohio/Tennessee Valley. A quick look at the 12Z NAM and the latest runs of the HRRR still fundamentally show precipitation breaking out along the same axis as earlier runs although the timing of heavy rainfall may be just a tad quicker to develop than shown in the past couple of runs.=20 12Z soundings showed an axis of stronger flow (30+ kts) from the Gulf coast region towards the ArkLaTex. Models still point to the axis of an anticyclonic upper jet approaching the region that increases upper level divergence by late afternoon and early evening...so the potential for 1 to 2 inch/hour rain rates still looks to be on-track. While this area is not as saturated as it was a couple of weeks ago...there is local concern given that ground conditions are still fairly moist. Overall...there were a few minor adjustments to the on-going outlook areas based on the latest guidance but those changes do not reflect a major shift in forecast reasoning. Bann Day 2 Valid 12Z Fri Mar 24 2023 - 12Z Sat Mar 25 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FROM THE OZARKS TO THE OHIO VALLEY... Showers and thunderstorms will be on-going at the start of the forecast period in proximity to the sprawling frontal boundary with moisture flux convergence increasing across parts of the Mississippi Valley and into the Ohio Valley as a surface low pressure with trailing cold front moves east across the Southern Plains. By 25/00Z a PW fetch of 1.5 inches will span from the Gulf Coast to the MO boot heel, feeding right into the frontal boundary, which is some (+2 to +2.5 standard deviations greater than climatology for late March. It is expected that multiple storms will track across areas soaked from recent rains and will likely surpass local FFGs early into the Day 2 period. The latest model runs have been persistent with areal averages of 2 to 4+ inches amounts. There is always a concern that convection will build a bit farther south towards the instability than suggested by the models...but confidence remains above normal in this cycle given run to run consistency in both placement and rainfall amounts. The Marginal Risk area was trimmed once again just a bit across the Lower Mississippi Valley. Bann Day 3 Valid 12Z Sat Mar 25 2023 - 12Z Sun Mar 26 2023 The probability of rainfall exceeding flash flood guidance is less than 5 percent. Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!73lYtJxHIuKLUcs2VBKr057lAXGRfoutcw8x6Q1hnaIS= r3rLfiZ20gZOeVjdl8x71YedGASi2skXADiATMaPe5wz_LM$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!73lYtJxHIuKLUcs2VBKr057lAXGRfoutcw8x6Q1hnaIS= r3rLfiZ20gZOeVjdl8x71YedGASi2skXADiATMaP2kOjWbk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!73lYtJxHIuKLUcs2VBKr057lAXGRfoutcw8x6Q1hnaIS= r3rLfiZ20gZOeVjdl8x71YedGASi2skXADiATMaP7w8Bsj4$=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) .