Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 20 2023 08:24:30 FOUS30 KWBC 200824 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 423 AM EDT Tue Jun 20 2023 Day 1 Valid 12Z Tue Jun 20 2023 - 12Z Wed Jun 21 2023 ....THERE IS A MODERATE RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE WESTERN CAROLINAS... ....Central Gulf Coast-Eastern TN Valley-Southeast-Lower Mid Atlantic... The mid/upper low over the Southeast will continue to be a focus for heavy rainfall and potential flash flooding today into tonight. A Moderate risk has been introduced across portions of western NC and northwest SC, where the flash flood risk appears highest. Environmental ingredients remain conducive to heavy rainfall across this area. PWs are around the climatological 90th percentile, with wet bulb zero heights around the 75th percentile per 00z RNK/GSO soundings. The soundings have the look of efficient warm rain processes, with a deep saturated layer and skinny CAPE. East to southeasterly upslope flow will persist and should actually increase with time today into tonight. Mean deep layer flow supports south to north cell motions early, transitioning to more east to west movement by later in the day into tonight. With low level flow generally aligned with these motions we could continue to see some small scale linear backbuilding/training segments. A stationary front in the vicinity is helping focus lower level convergence and organize convective development. With time today this front is expected to gradually drop southward, bringing instability and convergence along with it. Thus the heavy rain threat should shift southward with time...with the risk initially over most of western NC, and then shifting more into northwest SC and adjacent areas of western NC later today/tonight. Areas further north should stabilize with time being on the cool side of the southward drifting front. From a model QPF perspective the 00z HREF is showing 20-60% neighborhood probabilities of exceeding 5" over the MDT risk area, and location wise this is aligned with QPF from the 00z EC and UKMET. The latter two global models made a pretty significant jump south with their QPF compared to their 12z runs...but this makes sense given the location of the front and instability gradient. Recent HRRR runs have also been showing upwards of 3-7" of rain within the MDT risk. Areas of flooding should be ongoing at 12z to start the period, with the threat continuing through the day, just shifting a bit south with time. Overall, expect scattered to numerous instances of flash flooding to be possible within the MDT risk area, some of which could be locally significant in nature. A Slight risk extends eastward into central NC/SC and portions of northeast GA. The central Carolinas will see a similar setup as the western Carolinas, but the lack of upslope component should keep the flash flood risk a bit lower. Nonetheless, some linear training segments are probable here as well near the slowly southward drifting front. Some of these areas experienced heavy rainfall over the past 24 hours, and 00z HREF neighborhood probabilities of exceeding 3" are generally over 40% over most of the Slight risk area. Expect isolated to scattered flash flooding to be a concern here, but the coverage should be lower than areas further west. A Slight risk was introduced across portions of central and southeast LA with this update. Organized convection is expected to move across this area today/tonight. Very high instability and strong flow aloft supports quick moving convection and more of a severe threat. However the pattern is similar to what we have see the past couple days over the Gulf Coast, which has resulted in severe flash flooding near Pensacola one day, then south of Mobile the next, and at least some flash flooding over northeast LA Monday night. In these instances convection has exhibited pretty extreme backbuilding characteristics into the westerly flow. Could potentially see that occur again today if convective outflow is not strong enough to clear out instability. Confidence is high on convection that will be capable of heavy rainfall rates...but confidence is lower on whether this activity ends up progressing through quickly or hangs up and backbuilds. But given this pattern has been favoring the latter, think a Slight risk is warranted. ....Northern Plains... A Marginal risk was maintained across portions of central ND/SD with this update. A narrow corridor of very strong instability is forecast along a low level convergence axis. Subtle shortwave energy and some right entrance upper jet dynamics should help aid in convective development this afternoon. The narrow nature of the instability pool does suggest cells may not have too much longevity as instability gets eroded fairly quickly. However deep layer shear does support some convective organization...so tend to to think cells will maintain along/near the boundary just long enough to drop some local 2-3"+ amounts. This part of the country is not that susceptible to flash flooding, so think a Marginal risk should suffice, with localized issues possible. Chenard Day 2 The Day 2 outlook will be updated by 0830Z. Day 3 The Day 3 outlook will be updated by 0830Z. Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!659fQOqQ7PnPLjjjna2zK6KyHayF5DDHlvjenRzwcPlm= r-TQTQOV8tLHPimrgjjSMDbWfUff-Zvm8Qf38UQm5wfLrp0$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!659fQOqQ7PnPLjjjna2zK6KyHayF5DDHlvjenRzwcPlm= r-TQTQOV8tLHPimrgjjSMDbWfUff-Zvm8Qf38UQmSYtt-DY$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!659fQOqQ7PnPLjjjna2zK6KyHayF5DDHlvjenRzwcPlm= r-TQTQOV8tLHPimrgjjSMDbWfUff-Zvm8Qf38UQmj2Pyjko$=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) .