Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Tue Jun 20 2023 09:06:29 FOUS30 KWBC 200906 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 505 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 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. Even being a day closer now, still hard to pin down the details of where the greater flash flood risk will be. 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. The 00z guidance appears to be keying in on a shortwave that is in the northeast Gulf of Mexico as of 06z Tuesday. Guidance is intensifying this feature as it then moves up the east coast Tuesday night into Wednesday. It is looking increasingly likely that this 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. Meanwhile we should still have the main closed low 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. Same difficulty can be said further north into the western NC and southwest VA. Not a slam dunk that these area see heavy rainfall this day, as really will come down to where the upper low and instability is by this time...and model solutions vary quite a bit with their QPF output. 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 CO into ND on Wed. Over 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. PWs 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 CO into western NE and southwest ND. To start, 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. The persistence of the 850mb moisture transport into the stationary front should be supportive of a backbuilding and/or training area of convection, which may be enough to result in some area of flash flooding...and supports a Slight risk. We did expand the Slight risk northeastward into the Dakotas with this update. 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. The Marginal risk was extended south into portions of north TX as well. A trough will extend southward into this region...and while weaker forcing should keep convective coverage lower than further north...slow moving cells near the boundary may still lead to a localized flash flood risk. Guidance appears to be trending higher with this threat, and can not rule out a Slight risk upgrade over a portion of this area on later update. Chenard Day 3 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... We will still be dealing with a closed low over the Southeast on Thursday, although it should be drifting more northeastward by this time. Seeing decent model spread with the placement of the closed low and embedded shortwave energy rotating around it by this time. Overall the general moisture and forcing across the Mid-Atlantic and Southeast would be supportive of heavy rainfall and flash flooding...just going to come down to where exactly the better instability and forcing is by this time. Overall did not change continuity all that much with this ERO update. We have one Slight risk over the southern Appalachians into central NC/SC. The best model clustering would suggest the closed low is still positioned favorably to bring heavy rainfall to this corridor, with the 00z GFS/EC and their ensembles continuing to favor this area for additional heavy rainfall. Certainly not a guarantee though, as both the 00z UKMET and CMC are displaced to the east and actually give very little rainfall to the Slight risk area. Opted to maintain the Slight given the aforementioned GFS/EC solutions, and the fact that portions of this area should be increasingly saturated by this time suggesting some increased susceptibility to additional flash flooding. Areas to the east and north of this Slight risk could also see some flooding concerns, but confidence is lower...areas east are not favored as much by the GS/EC solutions and may not be as saturated by this time...while areas to the north will have instability questions to deal with. Thus thought Marginal risks should suffice over these areas for now. Will note that the GEFS based CSU machine learning output actually has a small MDT risk over VA...with the caveat that this is GEFS based and no guarantee the GEFS is handling this evolution correctly. However it does seem to indicate that the flash flood potential Thursday over portions of the Appalachians and Mid-Atlantic could be higher than what pure model QPFs might otherwise indicate...probably picking up on the general favorable ingredients as a whole. A second Slight risk area was maintained across portions of the Gulf Coast. Model fields show continued strong southwesterly to westerly flow off the Gulf on the southern periphery of the closed low. Seems likely that convection streaming in off the Gulf will remain possible through this time frame. Wind fields will remain conducive to backbuilding, and this pattern has produced several backbuilding convective complexes already over the Gulf Coast region. The potential will be there again Thursday and/or Thursday night, and with some of this area likely having experienced one or more recent rounds of heavy rainfall by this time...conditions may be a bit more susceptible to flash flooding by this time. ....Plains... The pattern remains favorable for western Plains convection Thursday. Might not be quite as organized of a convective signal as Wednesday, but should still have plenty of activity stretching from TX to ND. Per recent GFS/ECMWF/NAM runs, the best focus currently looks to be across portions of eastern CO and western NE/KS, so will carry a Slight risk here. Convection may be ongoing to start the day, with additional development then likely late in the day into the overnight hours. Not seeing the strongest mid/upper forcing or most intense low level jet increase into the front, which may be why the convective signal seems a bit lower than Wednesday. Nonetheless, the persistent front/trough/dryline convergence will be enough to trigger convection...and some broad upper level divergence ahead of the west coast trough should provide some additional support as well. Thus should be another active convective day, with isolated to scattered flash flooding possible over a broad geographic extent. Chenard Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!5sWclG9P4_t0BR2dmDcMhxYzTOqsOWecqSBJS_ciF8qQ= rpVCQ0Iki2qLbCN6pOwyTFQTLW1O7VH88KD3oRRcTYn-5VU$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!5sWclG9P4_t0BR2dmDcMhxYzTOqsOWecqSBJS_ciF8qQ= rpVCQ0Iki2qLbCN6pOwyTFQTLW1O7VH88KD3oRRcjTXPYfk$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!5sWclG9P4_t0BR2dmDcMhxYzTOqsOWecqSBJS_ciF8qQ= rpVCQ0Iki2qLbCN6pOwyTFQTLW1O7VH88KD3oRRc-T7EEeU$=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) .