Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 22 2023 01:01:14 FOUS30 KWBC 220101 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 900 PM EDT Wed Jun 21 2023 Day 1 Valid 01Z Thu Jun 22 2023 - 12Z Thu Jun 22 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND PLAINS... ....Southeast... 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 from now through tonight, where a Slight Risk remains. A bit more support for organized convection here, with some training/backbuilding potential within the deep layer southwesterly flow. Guidance indicates the potential of a more narrow training convective band streaming in off the Gulf from now into early Thursday. The pattern would support a narrow convective axis tonight as convergence increases and tightens to the south of the closed low. ....Plains... It has been an active convective day across the Plains, which should continue into tonight. A stationary front will stretch from WY into ND, and a dryline from CO into TX...and both features have acted as triggers for convective development. Sufficient CAPE, with low level convergence strong enough to act as a convective trigger, as 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. 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. Roth/Chenard Day 2 Valid 12Z Thu Jun 22 2023 - 12Z Fri Jun 23 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND HIGH PLAINS... ....Southeast... The risk of flash flooding will linger across parts of the Southeast U.S. on Thursday as a broad mid-level closed low meanders towards the northeast. Increasing model agreement first noted overnight has persisted into the daytime suite of model runs. It looks like the center of the closed low will be centered somewhere over TN/AL/GA, shifting north northeast into the OH Valley by Thursday night. corridor of enhanced southerly moisture transport along the east coast, along with a pretty well defined low level front/convergence axis, should focus and sustain showers and thunderstorms during the period. It will be tough to pin down exact details at this lead time, but the overall ingredients appear equally favorable for flash flooding stretching from the FL Big Bend northeastward into southern Virginia, and so we will maintain the Slight risk with few changes. The ingredients appear slightly less favorable over the western Carolinas into southwest VA, but this area has saturating soil conditions and will be close to the center of the closed mid level low...so extending the Slight risk into those areas seems warranted as well. ....Plains... The pattern remains favorable for western Plains convection Thursday. A stationary front will stretch from western Nebraska into Minnesota, and a trough and/or dryline from Colorado into Texas. Just like on Wednesday, both areas of enhanced convergence will be a trigger for convective development Thursday as well. Will likely have some activity ongoing to the start the day, with additional development again likely during the afternoon/evening hours. At least a localized flash flood risk likely exists over a broad geographic region stretching across most of the Plains from Texas to Minnesota. The better chance for a more concentrated risk may again be near the intersection of the stationary front and dryline/trough across eastern Colorado, southeast Wyoming and western Nebraska...with activity also eventually getting into western KS. Thus the inherited Slight risk was not moved much. Model QPF forecasts show a bit more spread by this time...but the Slight risk area is still in a relative max in most guidance. Chenard/Bann Day 3 Valid 12Z Fri Jun 23 2023 - 12Z Sat Jun 24 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE SOUTHEAST AND HIGH PLAINS... ....Plains... Impressive synoptic forcing will move into the northern Plains Friday as a strengthening shortwave intensifies into a closed low by Friday night. The flow aloft should be highly divergent with dual left exit and right entrance upper jet support. In the lower levels a developing low pressure and associated frontal structure will provide plenty of lower level convergence to go along with the mid and upper level ascent. PWs are not off the charts anomalous, but still plenty above average for late June. Thus many ingredients are coming together to support a heavy rainfall event over portions of Montana, Wyoming, South Dakota and North Dakota. The magnitude of the flash flood threat is likely going to come down to the placement of the low level front and higher instability, and how progressive things end up being. This does look like a pretty quick moving system, which should put an upper bound on total rainfall potential. Opted to remain with a Slight risk and not upgrade to a Moderate risk at this point. The question of rainfall rate longevity and placement of the highest risk precludes still lingers so will call this a higher end Slight risk. The greatest flash flood risk may actually end up a bit southeast of where the model and WPC QPF max is located. The QPF max over central MT may be a combination of convective and stratiform rain behind the low...with the highest rate potential probably over southeast MT into northeast WY and the western Dakotas. But we're still 3 days out and there is inherent uncertainty in forecast details at that lead time. So for now a broad Slight risk covers the threat. Farther south a broad Marginal risk extends over most of the remainder of the Plains. The weaker forcing should lead to less convective organization...but there should still be plenty of instability to work with. ....Eastern U.S... The eastern U.S closed low will continue drifting northward on Friday and begin to lose some of its structure as it does so. So likely looking at a bit weaker southerly low level flow, and not quite as concentrated lower level convergence as compared to Thursday. This idea is supported by the lower QPF output we're generally seeing in the models by this time. This gradual weakening of forcing and questionable instability supports a lower flash flood risk compared to Thursday, so will just continue to carry a broad Marginal risk. With that said, this is the type of system that will likely produce low topped warm rain dominant convective cells. And while less defined than Thursday, the unidirectional southerly flow and convergence axis is still around. So even though model QPFs are not that impressive...this would appear to be a setup where localized higher rainfall is quite possible...which could result in some areas of flash flooding. The central Gulf Coast into northern FL will remain in an environment conducive for southwest to northeast moving convection off the northern Gulf. Quite possible a Slight risk may need to be introduced here on future updates...but do note a slightly lower model/ensemble QPF signal by this time.. Chenard/Bann Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!9F1yXfOyE9KdlNtMJn3VpCXMt3-7ZV5Pqd0lEhlicy1v= TDfICwO7NhDkaHunrj-nZr3tG0sDhtcoBWzKQj_zSz1-rlw$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!9F1yXfOyE9KdlNtMJn3VpCXMt3-7ZV5Pqd0lEhlicy1v= TDfICwO7NhDkaHunrj-nZr3tG0sDhtcoBWzKQj_zNyKsd2c$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!9F1yXfOyE9KdlNtMJn3VpCXMt3-7ZV5Pqd0lEhlicy1v= TDfICwO7NhDkaHunrj-nZr3tG0sDhtcoBWzKQj_zEK_iKl0$=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) .