Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jun 21 2023 08:26:34 FOUS30 KWBC 210826 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 425 AM EDT Wed Jun 21 2023 Day 1 Valid 12Z Wed Jun 21 2023 - 12Z Thu Jun 22 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND PLAINS... ....Southeast... The core of the flash flood risk should shift southward today given the location of the mid/upper level low and low level front/instability gradient. The 00z HREF guidance focuses the threat primarily over portions of southeast AL into much of GA, western SC and into the FL Panhandle...and will carry a Slight risk here. HREF 24hr 3" neighborhood probabilities are over 60% over most of this Slight risk, with 5" probabilities considerably lower, but still as high as 30-50% in spots. The general expectation is that we will have scattered to numerous coverage of convection across this area today underneath/near the closed low. Cells will probably tend to be disorganized and mainly pulse in nature, which in itself is generally a negative for flash flooding. However, coverage should be enough that we will likely have some cell mergers/collisions locally prolonging rainfall duration, and there will also likely be some corridors of enhanced convergence in the sfc-850mb layer that will periodically help focus/train cells. Thus the idea of scattered 3"+ totals that the HREF is depicting seems reasonable. The coverage of these amounts won't be great, but should end up with quite a few localized 3"+ totals. Coverage of 5" amounts will be even lower...but do think the HREF is reasonable in suggesting at least a few locales will exceed that amount. Thus all in all looks like a solid Slight risk, with isolated to scattered instances of flash flooding. The FL Panhandle into adjacent areas of northern FL may have a bit more of a higher end QPF Potential dependent on how things evolve today into tonight. A bit more support for organized convection here, with some training/backbuilding potential within the deep layer southwesterly flow. Guidance indicates some activity already by morning/afternoon...with some indications for the potential of a more narrow training convective band streaming in off the Gulf later tonight into early Thursday. The pattern would indeed support a narrow convective axis tonight as convergence increases and tightens to the south of the closed low. Another area of higher rainfall totals is expected across the Mid-Atlantic from central VA into the Delmarva and southern NJ. This is associated with a mid level wave and surface low moving up the coast resulting in pretty strong easterly low level flow and isentropic ascent. Most of this will likely end up being stratiform rainfall given the lack of instability, thus the flash flood risk is pretty low. However we will need to keep an eye on the coastal plain from eastern NC into eastern VA and possibly into the eastern shore of MD and coastal DE. There are some signs that some weak instability could get into these areas today. Any convection that is able to develop will be low topped and warm rain dominant, thus capable of some pretty heavy rates. Areas north of Virginia Beach have very high FFG, so the risk is likely quite low even with this potential. There may be a slightly better chance of seeing localized flash flooding over northeast NC into southeast VA where FFG is lower, but still not enough confidence for a Slight risk. ....Plains... It will be an active convective day across the Plains today into tonight. A stationary from will stretch from WY into ND, and a dryline from CO into TX...and both features will likely act as a trigger for convective development. The most robust signal continues to generally be from northeast CO into southeast WY and western NE. Would expect MLCAPE to be over 2500 J/KG across this area by afternoon, with low level convergence strong enough to act as a convective trigger, with the right entrance region of an upper jet to the north also aiding development. With increasing low level easterly flow and westerly winds aloft, cells motions should be pretty slow and chaotic. By the evening into the overnight still thinking the increasing low level southeasterly inflow will support upscale growth and a slowly southeastward moving convective complex. Expect this setup to result in scattered instances of flash flooding supporting a Slight risk. Convection is also likely to develop further northeast along the stationary front. Convection is ongoing across this corridor this morning, and should continue to some extent through the day. By tonight the stronger forcing to the northwest clips the region, probably resulting in a more organized convective complex. This complex may be progressive, but convection earlier today will have some training potential given the stationary front in place. The flash flood risk is probably a bit lower here than the areas further southwest described above...however multiple rounds of rain should gradually increase flood susceptibility over this corridor, so still think a Slight risk is warranted. A localized flash flood risk also exists today/tonight across the central and southern Plains. A complex of elevated convection is likely to be ongoing at 12z Wednesday over KS...and the pattern would favor some backbuilding/training of that activity. So some flash flood risk may be ongoing here to start the day 1 outlook period. By this afternoon we will likely see convection form along the dryline in TX, and activity will probably initially stay tied to the boundary, resulting in at least some flash flood risk. It seems possible that an increasing easterly low level jet this evening may allow for some upscale growth of this activity into a more organized convective cluster that could track southeast along what should be a corridor of strong instability. This would likely result in some instances of flash flooding, but the probable quick movement and high FFG keeps this at a Marginal risk. 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!8aQl2YlLnVPnEPoyVpSpArPzDuvgtxV0C7c3269XZ5RT= tayk5yuebMjTiFMfk56tan_bBgplL_Al7JKJskEnd1r2LaU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!8aQl2YlLnVPnEPoyVpSpArPzDuvgtxV0C7c3269XZ5RT= tayk5yuebMjTiFMfk56tan_bBgplL_Al7JKJskEn580_SmI$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!8aQl2YlLnVPnEPoyVpSpArPzDuvgtxV0C7c3269XZ5RT= tayk5yuebMjTiFMfk56tan_bBgplL_Al7JKJskEnmRHvBgM$=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) .