Extended Forecast Discussion NWS Weather Prediction Center College Park MD 328 AM EDT Fri Aug 25 2023 Valid 12Z Mon Aug 28 2023 - 12Z Fri Sep 01 2023 ...Hot and humid conditions could persist across the Gulf Coast into Monday before some relative relief with moderating temperatures as the week progresses... ...Increasing chances for a tropical system to move through the eastern Gulf of Mexico and affect Florida with at least heavy rainfall... ...Overview... Upper troughing over the East for the first half of next week will help drive multiple frontal systems across the region, producing a heavy rainfall and flash flooding threat likely focused around the Carolinas. These cold fronts could eventually push through the Gulf Coast states and help to lessen the heat and humidity there after Monday. Meanwhile farther south, the National Hurricane Center is monitoring a disturbance in the Caribbean that shows a high chance of tropical development. This system could move through the eastern Gulf of Mexico and possibly track into Florida, with heavy rainfall and flooding likely to be threats there. Rain will also return to the Pacific Northwest mid-next week as an upper level low moves onshore, while periodic surges of monsoonal moisture affect the Four Corners states with rain. ...Guidance Evaluation/Predictability Assessment... Model guidance is generally in good agreement with the synoptic-scale flow, featuring amplifying troughing across the East Monday-Wednesday and shifting eastward, while an upper low over the eastern Pacific on Monday tracks eastward into the Northwest and north-central U.S. through the week, with ridging in between and southward into the southwestern U.S. (albeit weaker than most of the ridging that has occurred this summer). However, there are certainly some model differences in the details--for example, with frontal positions in the Northeast around Tuesday-Wednesday that produce large differences in the associated QPF. The western upper low begins the medium range period in good agreement but some timing differences arise by around midweek--the 12Z ECMWF tracks the low slightly more quickly east than other guidance, and the ensemble means were generally even slower than the rest of the deterministic guidance. Models diverge further with the flow upstream from this feature in the Pacific by the latter part of next week. The 12Z CMC closed off an upper low more quickly than other guidance, but most models do close it off eventually near the Pacific Northwest, while the 12Z ECMWF seemed to be on the eastern side of the guidance envelope. Overall thought the GFS had a good solution here so maintained some of it through day 7. But mainly transitioned from a multi-model deterministic blend early in the medium range period to half by day 6 and just over half day 7 considering the increasing model spread. Meanwhile some of the most notable differences in the model guidance occur with the tropical disturbance. The non-NCEP guidance has all shown a tropical system for several runs in a row, while GFS runs have had a considerably weaker feature if any. This is intriguing particularly considering GFS tendencies toward having tropical systems that other models do not. But even setting the GFS aside, there have been some considerable variations among and between model runs with the track of the feature and even more so its timing. CMC runs have been among the fastest to track the system northeast. The 12Z ECMWF and UKMET were rather agreeable in a slower timing and the forecast for the track leaned more toward this instead, along with the EC mean that was slightly slower. The forecast points were coordinated with the National Hurricane Center and we agreed on a closed low rather than a spot low, given the high chance for development into a tropical cyclone. The 00Z GFS does finally show more semblance of a tropical feature in the eastern Gulf of Mexico for Tuesday, though weaker and tracking farther east. The 00Z ECMWF slowed down, as did the 00Z CMC, so the timing will continue to be refined in future forecasts. ...Weather/Hazards Highlights... As the workweek begins, a stalling front with moisture pooling along it will likely produce a heavy rainfall threat from the central Gulf Coast through the Southeast and into the Mid-Atlantic, remaining into Tuesday and potentially spreading into the Northeast Tuesday as well depending on frontal positions. A primary focus is likely to be across the Carolinas, and per coordination with the local offices a Slight Risk has been added to Monday's ERO. Despite dry conditions overall in the past few weeks there, a wet day on Day 3/Sunday will likely prime the area to experience some flash flooding issues. Also by Monday and especially Tuesday, the possible tropical system in the eastern Gulf could begin bringing heavy tropical rains to parts of Florida, for a Marginal Risk in southern Florida on Monday and a Slight Risk from the Florida Big Bend southward along the Gulf Coast as the feature could be approaching. The moderate speed of the system could limit too much flash flooding, but tropical downpours with high rain rates should pose some flash flooding threat. The system could track across northern Florida or so into the western Atlantic, which could spread heavy rainfall to the coastal Carolinas by Wednesday but with uncertainty with how much falls onshore. Storms could linger across Florida into late week with a frontal boundary in a moist environment. Above normal moisture is in place across portions of the central/southern Rockies to the Front Range on Monday, and scattered to widespread showers and storms that may be slow-moving could lead to localized flooding. A Marginal Risk was maintained in this area for Monday. Much of Texas could get some welcome rain as well near a frontal system. Some convection may remain into Tuesday with lesser amounts overall. By Wednesday monsoonal convection could focus farther west likely into the Mogollon Rim, with increasing coverage once again by Thursday for the Four Corners states. In the Northwest, the strong upper low coming through for the first half of the week will provide support for rainfall there. There does look to be some instability in place, so localized rain amounts/rates may be high enough to cause a few instances of flash flooding. Another day of hot temperatures and high humidity is possible for the Gulf Coast region on Monday. Additional high maximum and minimum temperatures are likely to be set while heat indices rise well above 110F. Finally some relief is in store by Tuesday and Wednesday with near to just above normal highs expected, but considerably lower heat indices as a front or two come through. Meanwhile heat is forecast to build again across the lower elevations of the Desert Southwest with some 110s likely on Monday and Tuesday, but this should also abate some by Wednesday and beyond. Multiple fronts crossing the East will keep temperatures somewhat cooler than average there for much of next week. The central U.S. could start off somewhat cooler than average but with a warming trend through the week. Tate Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation forecast, excessive rainfall outlook, winter weather outlook probabilities, heat indices and Key Messages are at: https://www.wpc.ncep.noaa.gov/medr/5dayfcst500_wbg.gif https://www.wpc.ncep.noaa.gov/medr/5dayfcst_wbg_conus.gif https://www.wpc.ncep.noaa.gov/5km_grids/5km_gridsbody.html https://www.wpc.ncep.noaa.gov/qpf/day4-7.shtml https://www.wpc.ncep.noaa.gov/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw