Subj : HVYRAIN: Excessive Rainfall Discussion To : wx-storm@lists.illinois.edu From : COD Weather Processor Date : Thu Jun 22 2023 08:34:22 FOUS30 KWBC 220834 QPFERD Excessive Rainfall Discussion NWS Weather Prediction Center College Park MD 433 AM EDT Thu Jun 22 2023 Day 1 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 vertically stacked low over the Tennessee Valley this morning will slowly drift northward into the Ohio Valley by Friday morning. As it does so, it will continue pumping plentiful Gulf moisture northward up the Eastern Seaboard and into New England. Meanwhile, shortwave disturbances wrapping around the low will remain prolific at supporting additional rounds of showers and thunderstorms across the area. Both their slow movement and abundance of available moisture will support heavy rains that could cause additional flash flooding. The Slight Risk area is considered a high-end one in three separate portions of the Slight risk area. Starting in the south, the eastern Florida Panhandle through the Big Bend area will likely see yet another round of slow moving and backbuilding storms that will train over the same areas being hard-hit now and have been for the past several days. With no relief in sight in this area, and despite the high thresholds for flash flooding which include the very favorable antecedent conditions, the expected storms are likely to result in flash flooding yet again. The ongoing MCS over the area will gradually weaken through the afternoon, with many areas of north FL getting a brief break in the storms, but by afternoon the storms will redevelop over the Gulf and track over the Big Bend area again, taking well into the evening to finally drift far enough south to allow for a longer break in the rainfall. The area from east of Tallahassee to Gainesville has seen anywhere from 200-350% of their normal rainfall over the last 2 weeks according to AHPS data, so soils in the area cannot handle any more rainfall. Further north across south GA, an advancing front trying to bring a bit drier air to the region behind the low will force another round of storms to develop ahead of it across this region. Just like down in FL, this area has seen 200-400% of normal rainfall over the past 2 weeks, and over the last week over 600% of normal towards Albany. Most of the guidance is hinting that the area of rain will be just south and east of Albany, but rainfall amounts there are still 200-400% of normal over the past week, so soils in this region are also very saturated. The front will help to move the storms along towards the northeast, so fortunately no one area will see stationary convection get stuck, but since the front will be stalling out, it will act as a corridor for storms to train over many of the same areas over and over again. General consensus is for the storms to develop in the late afternoon, around 20Z, and persist through around midnight. Finally up towards the central Carolinas and southern VA, scattered showers and thunderstorms will be ongoing at the start of the period and continue well into the overnight tonight. The most organized convection will be in the morning associated with an upper level shortwave rotating around the low, becoming more widely scattered through the afternoon and overnight as the storms pop up and quickly move northward. While training convection is likely, the timing between individual storms will vary widely across this region. The Slight risk was maintained across the Carolinas, which have seen upwards of 200% of normal rainfall over the past week, and are therefore more susceptible than up into southern VA. The Slight risk area was expanded northward into southern VA to include Richmond and the Hampton Roads area due to locally lower FFGs due to urbanization and recent rainfall in this area. As with areas further south, moisture will remain plentiful with most areas staying at 1.75 inches PWATs, or up to 2 sigma above normal. ....Plains... In CO, the ongoing convection at the time of this writing will continue moving slowly eastward across the High Plains in the eastern part of the state. These storms have been responsible for numerous Flash Flood Warnings, several of which have are highlighting considerable flash flooding. Once this MCS finally moves off and begins turning southward into KS, there may be a break in CO for some of the morning hours. With daytime heating however, new storms are likely to develop along the Front Range, but fortunately should by this point be moving eastward fast enough to make the flash flooding threat considerably lower than last night. Nonetheless, the storms will continue to have ample moisture and the local dry line will provide additional forcing, so the storms are likely to be strong, even if they're more isolated than previous days. Given the ample rainfall the area has seen with parts of CO between Denver and Colorado Springs seeing over 600% of normal rainfall the past 2 weeks, any storms even if more isolated and faster moving have potential to result in localized flash flooding. Northeast of there into South Dakota, a cool front will be advancing southeastward out of MT and ND. Ample Gulf moisture from a strong 40 kt LLJ will supply the storms with plenty of energy. The storms will develop in the late afternoon and continue into the evening as they organize into an MCS over eastern SD. While this area hasn't had nearly as much rain as eastern CO has, there is good potential for training and repeating storms as the MCS grows upscale during the evening, which should overcome the drier conditions in eastern SD with rates as high as 3 inches per hour possible. Wegman Day 2 Valid 12Z Fri Jun 23 2023 - 12Z Sat Jun 24 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF THE HIGH PLAINS AND THE FLORIDA PANHANDLE INTO SOUTHERN GEORGIA... ....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 between 2 separate jet streaks. 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 and the Dakotas. 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. Besides this area of the country having been abnormally wet over the past 2 weeks, an LLJ bringing Gulf moisture northward into the Dakotas, and perhaps a bit towards the west into MT, and some instability being advected into the region, it will be interesting how all of this interacts with the favorable upper level environment. The guidance is hinting at portions of south central MT, near Billings, where leeside troughing may locally amplify the forcing and allow for more persistent heavy rains in the broad southwesterly flow. The Dakotas will have the benefit of more moisture, but being somewhat displaced from the best upper level divergence, may see lesser amounts of rain, though any storms that form over that area still have the potential to produce 2 inch/hour rainfall rates. For the southern Plains, the typical afternoon showers and thunderstorms will redevelop again Friday afternoon, but since they will be a bit more isolated and less organized as the upper level flow diminishes, the flash flooding risk is expected to be more isolated as compared with previous days. ....Eastern U.S... A low-end Slight risk area was introduced for all of the Florida Panhandle and into southern Georgia with this morning's forecast update. The stalled out front that has acted as a corridor of forcing for storms the last several days will remain stuck in place, as a drier air mass over central MS/AL/northern GA tracks east on the north side of the front. Thus, the front will continue to have a significant enough difference between the air masses on either side of the front to continue to provide a source of lifting. With those differences maximized, the winds on the moisture-laden/stormy side of the front across the FL Panhandle should allow the storms that form to be moving a bit more quickly off to the ENE. This should result in less overall rainfall over this area, despite the tendency for storms to backbuild. Nevertheless, given the incredibly wet conditions already present over this area, and the potential for another 1-3 inches of rain, with those higher values falling in a short time remaining very capable of resulting in additional flash flooding, think the potential is high enough for a Slight Risk area. Further north across the Mid-Atlantic, the upper level low will begin to open up into a trough, but will become negatively tilted. This will increase the overall southerly wind flow over the area, allowing for faster storm movement, but the added lift from the divergence associated with the trough will support storms capable of producing heavy rainfall to 2 inches per hour. Given the still overall dry conditions over much of the Mid-Atlantic from a few months of abnormally dry conditions, think any instances of flash flooding will be localized to urbanized and poor drainage areas.=20 Wegman Day 3 Valid 12Z Sat Jun 24 2023 - 12Z Sun Jun 25 2023 ....THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR MUCH OF NORTH DAKOTA AND MINNESOTA... ....ND/MN area... A strengthening shortwave trough over south-central MT Saturday morning will track east to western MN by Sunday morning. This feature will allow the widespread area of rain, with some embedded thunderstorms, to track east across the Slight Risk area on Saturday. A slowly strengthening surface low will remain out ahead of the shortwave, which will both draw moisture from the ever-present LLJ northward, while also dragging the LLJ eastward ahead of its cold front. The front will set off additional showers and thunderstorms across IA/MO and WI, but since the front will be progressive, so too will the storms along the front, so only isolated flash flooding is possible, namely in urbanized areas. With ND and MN still in the best forcing as the low wraps up, training storms are likely to develop along the track of the low. While soil conditions are dry from eastern ND and even drier across northern MN, it's important to note that the abnormally high atmospheric moisture (characterized by PWATs over 1.5 inches) and favorable flow for training storms across this area may allow for the heaviest rain that falls in this region to overcome the antecedent dry conditions and result in flash flooding. If the rain falls hard enough, then the dry soil can't absorb that much water all at once, so there's increased runoff. Expect this to be the case in the Slight Risk area Saturday. ....Northeast... The upper level trough will continue to ever so slowly northeast on Saturday, starting the day in the Ohio Valley and moving to Central NY by Sunday morning. The trough will remain negatively tilted, so the southerly LLJ of tropical Atlantic moisture will continue pumping northward ahead of the the trough. Since the forcing will be shifting north ahead of the trough, a Marginal flash flooding threat will primarily focus from Philadelphia northward. From northern New England and especially into western ME, rainfall totals over the past 2 weeks are 200-300% of normal, so the wet soils in this area will promote flash flooding, especially on the southeast facing slopes of the White Mountains of NH and ME. Very few changes were needed for this morning's update, as the forecast remains on track. Wegman Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/94epoints.txt__;!!DZ3fjg!-MpxrM5LNnuvGWqIDlARyb3oVqqKd8fa88CEVTY_5C6I= Bh9QgEfzYdL21VEikYwBQIgQAhdheQecz4tYW_NctIVx0Dg$=20 Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/98epoints.txt__;!!DZ3fjg!-MpxrM5LNnuvGWqIDlARyb3oVqqKd8fa88CEVTY_5C6I= Bh9QgEfzYdL21VEikYwBQIgQAhdheQecz4tYW_NcIYB1e7o$=20 Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go= v/qpf/99epoints.txt__;!!DZ3fjg!-MpxrM5LNnuvGWqIDlARyb3oVqqKd8fa88CEVTY_5C6I= Bh9QgEfzYdL21VEikYwBQIgQAhdheQecz4tYW_NcPqB8wyE$=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) .