Subj : HVYRAIN: Excessive Rainfa To : All From : Mike Powell Date : Tue Jun 20 2023 16:28:00 FOUS30 KWBC 202021 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 420 PM EDT Tue Jun 20 2023 Day 1 Valid 16Z 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 U.S. will continue to be a focus for heavy rainfall and potential flash flooding through tonight. Maintained the Moderate risk area that had been introduced across portions of western North Carolina and northwest South Carolina. Environmental ingredients remain conducive to heavy rainfall across this area with precipitable water values around the climatological 90th percentile, with wet bulb zero heights around the 75th percentile per 12Z GSO sounding. Soundings have the look of efficient warm rain processes with a deep saturated layer and skinny CAPE. Persistent east to southeasterly upslope flow should increase with time 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 expected to gradually drop southward, bringing instability and convergence along with it...thus the heavy rain threat should also 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 farther north should stabilize with time being on the cool side of the southward drifting front. From a model QPF perspective the 12Z HREF keeps 20-60% neighborhood probabilities of exceeding 5" over the MDT risk area...with recent HRRR runs have also been showing upwards of 3-7" of rainfall somewhere within the MDT risk. Overall, expect scattered to numerous instances of flash flooding to be possible within he MDT risk area, some of which could be locally significant in nature. In addition...there is increased risk of landslides and/near the Blue Ridge Escarpment in North Carolina as a result of the underlyng geology being primed by the rainfall which fell overnight combined with the additional rainfall expected through 12Z Wednesday. Portions of Polk, Rutherford and McDowell counties are especially suseptible to landslides from 5 inches or more rainfall in 24 hours...which is in the realm of possibilities given the 12Z run of the HREF. A Slight risk extends eastward into central NC/SC and portions of northeast GA and it was expanded a bit westward to into extreme eastern Tennessee. The central Carolinas will see a similar setup as the western Carolinas, but a weaker to even a complete lack of an 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. The westward expansion of the Slight Risk area was made in deference to placement of the mid-and upper-level dynamics that has at least some potential to mitigate any downsloping of low level flow. The 06Z and 12Z HREF neighborhood probabilities have at least enough of a signal in the 1- and 3-hourly amounts and the probability of 3-hour rainfall amounts exceeding flash flood guidance to warrant nudging the boundary of the Slight Risk area westward. A Slight risk was maintained across portions of central and southeast LA with this update. Organized convection is expected to redevelop this afternoon and persist into the evening. 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 instances of severe flash flooding. In these instances convection has exhibited pretty extreme backbuilding characteristics into the westerly flow. Could potentially see that occur again later 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 back builds. But given this pattern has been favoring the latter, think keeping the Slight risk was warranted. ....Northern Plains... A Marginal risk was maintained across portions of central North Dakota/South Dakota 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/Bann Day 2 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 HIGH PLAINS... ....Southeast... The closed mid/upper level low persisting over the Southeast will bring another day of heavy rainfall and potential flash flooding to much of the region although confidence in details lacking. The area of greatest flash flood potential will be dependent on which areas have the most saturated conditions by this time, and exactly where the better forcing/instability overlaps...with the latter often hard to predict with closed lows of this nature. The closed low will have lobes of embedded energy rotating around it, and we should still have a low level boundary and instability gradient in the area...but trying to predict these features at this lead time is difficult. It is looking increasingly likely that a system making its way northeast from the Gulf will drive a heavy rainfall threat along the coastal plain of the Mid Atlantic...although will also say that this is a pretty subtle feature at the moment, and likely open to additional changes in its exact evolution. There is also a question as to the degree of instability that will exist with this feature, so think a Marginal risk should be enough at this time. Confidence is better that the main closed low will be hanging back over the Southeast. This is expected to drive a heavy rainfall threat as well, although model differences in the placement of the closed low and associated boundaries/instability gradients makes this a difficult forecast. It seems like somewhere over eastern TN into GA/SC should see a more concentrated flash flood risk with these features, but nailing down the specifics at this point is difficult...with some potential for overlap with areas soaked on Day 1. That could prolong concern about landslides along the Blue Ridge Escarpment due to waterlogged soils if the guidance proves to be too quick to sweep out the instability. Some chance this region is in between the coastal wave and Southeast closed low, resulting in a relative lull in rainfall. Given this uncertainty think the best course of action is staying pretty close to continuity with a broad Slight risk from the FL Panhandle into the southern Appalachians. Hopefully by later today or tonight we can get a bit better idea of convective evolution by this time and be able to better focus our risk areas...but from a probabilistic sense the broad Slight still seems warranted. ....Plains... Stronger mid/upper level forcing moving over a stationary boundary will trigger convective development from northeast Colorado into North Dakota on Wed. Numerical guidance still depicts more than 2000 J/KG of CAPE should be present near the boundary, with weak embedded shortwave energy ahead of the approaching trough and pretty substantial upper level divergence helping trigger convective development along the low level convergence axis. Precipitable water values are also forecast upwards of the climatological 90th percentile...so plenty high enough to support a heavy rainfall threat. While heavy rainfall is possible anywhere along the boundary, there appears to be a pretty good signal for excessive rainfall over portions of northeast Colorado into western Nebraska and southwest North Dakota. Mean winds will be parallel to the front supporting some training...and with time southeasterly 850mb flow increases into the boundary and persists. This will support some slow eastward propagation of convection as it grows upscale in nature. These areas might not see quite the organization of activity as further south, but still looking at potential training given mean flow parallel to the stationary front...and this will be the 2nd day in a row of possible slow moving convection over this area, suggesting some increased susceptibility to flash flooding is possible. The 00z CSU GEFS based machine learning first guess field depicts a solid 25% plus area across most of the Slight risk area as well...which is typically indicative that we will at least see some decent FFG and ARI exceedance across this corridor. Chenard/Bann $$ --- SBBSecho 3.14-Linux * Origin: Ilink: CCO - capitolcityonline.net (454:1/105) .