WPC Day 1 Excessive Rainfall Outlook (2024)

Updated: 0830 UTC Thu Jun 22, 2023
Valid: 12 UTC Jun 22, 2023 - 12 UTC Jun 23, 2023

Forecast Discussion

 Excessive Rainfall DiscussionNWS Weather Prediction Center College Park MD427 AM EDT Thu Jun 22 2023 Day 1Valid 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 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt