Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Wed Jun 21 2023 08:58:04 FOUS30 KWBC 210857 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 457 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 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 broad mid level closed low will continue to pose a flash flood risk as it drifts northward on Thursday. Models have come into better agreement on the structure and placement of the system by this time. It looks like the center of the closed low will be centered somewhere over TN/AL/GA, shifting northeast into the OH Valley by Thursday night. To the east of this we should have a corridor of enhanced southerly moisture transport along the east coast, along with a pretty well defined low level front/convergence axis. This corridor will likely be a focus for periods of convection through the period. Going to 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 VA, and so we will carry a broad Slight risk. 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. The drawn Slight risk also matches up pretty well with model QPF fields and the CSU ML ERO forecast. ....Plains... The pattern remains favorable for western Plains convection Thursday. A stationary front will stretch from western NE into MN, and a trough and/or dryline from CO into TX. 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 TX to MN. The better chance for a more concentrated risk may again be near the intersection of the stationary front and dryline/trough across eastern CO, southeast WY and western NE...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. Combine this with the above ingredients and the CSU ML ERO also showing a Slight risk in this location...and little change was needed to the Slight risk in effect. Chenard 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. The mid levels will have a strengthening shortwave, intensifying into a closed low by Friday night. The upper levels are very 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 MT, WY, SD, ND. 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. This event has MDT risk potential, but at this time going to stay with a Slight risk. Think the question of rainfall rate longevity and placement of the highest risk precludes introducing a MDT at this time...instead will call this a higher end Slight risk for portions of MT/WY and into the western Dakotas. 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. Further south a broad Marginal risk extends over most of the remainder of the Plains. Less convective organization expected here given the weaker forcing...but still expect some activity with 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 CSU ML ERO (which is GEFS based) actually shows a 25% area over the Mid-Atlantic, which is a higher end Slight risk. This is despite only modest GEFS QPF output, so clearly it is picking up on the favorable ingredients in place. So will need to continue to monitor in the coming days. However, also worth noting that antecedent conditions are very dry as you go northward in the Mid-Atlantic, so will be hard to get anything more than localized (mainly urban) flash flooding with this system it would seem. 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...and the increasing distance from the closed low would also suggest a bit lower risk than previous days. So for now kept a Marginal, as we have plenty of time to consider an upgrade based on model trends and antecedent soil conditions over the area. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!_HRe9LAxEd5RoGMzqyAAWrvjixPMfNB5qOP15q1QRION= ugrdBE_P8D6879aOrAgxcp3D6_gTOLkM4LSlB7C8kcH3d04$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!_HRe9LAxEd5RoGMzqyAAWrvjixPMfNB5qOP15q1QRION= ugrdBE_P8D6879aOrAgxcp3D6_gTOLkM4LSlB7C8iNTIYHg$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!_HRe9LAxEd5RoGMzqyAAWrvjixPMfNB5qOP15q1QRION= ugrdBE_P8D6879aOrAgxcp3D6_gTOLkM4LSlB7C8hZdbXN8$=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) .