Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Sat Sep 02 2023 Valid 12Z Tue Sep 05 2023 - 12Z Sat Sep 09 2023 ...Summer Heat intially across the Plains/Midwest/East before shifting to the South/Southwest... ...Overview... An energetic upper trough over the West will lift out through the northern Rockies to the north-central U.S. early-mid next week as kicked by additional upper energies feeding inland from the Pacific. Height falls and pooled moisture and instability near an associated front will fuel a swath of showers and thunderstorms that could lead to localized areas of excessive rainfall and flooding concerns, particularly across the northern High Plains/Rockies. Downstream, yet another hot summertime upper ridge over the central U.S. will gradually shift eastward through next week. This pattern will support well above seasonal normal temperatures across the Plains and Upper Midwest before reaching the East and also back into the Southwest. Meanwhile, scattered showers and storms could persist across the lower MS/TN Valleys and Gulf Coast region and Florida next week with a lingering and slow moving shortwave and front. ...Guidance Evaluation/Predictability Assessment... On the large scale, model and ensemble solutions are mostly well clustered over the lower 48 during the medium range period. There are some notable differences in the details/timing of energy exiting the West moving across the Upper Midwest, rounding the top of the central-eastern U.S. upper ridge. Eventual downstream details are heavily relient on peristant troughing off the East Coast, which may play a role in steering of currently Post-Tropical Cyclone Idalia, with potential for some impacts to parts of the Canadian Maritimes. Shortwaves into the Northeast should also eventually suppress the upper ridge back to the South, aided by additional shortwave energy moving through southern Canada later next week. Upper ridge across the South should strengthen and become better established as the overall pattern amplifies somewhat by next weekend. WPCs blend today relied mostly on a blend of the deterministic model solutions day 3-5, with additional inclusion of the ensemble means late period to smooth out the harder to resolve late period detail differences. Did not use the 00z CMC after day 4 as it looked slightly too strong and slow with the initial shortwave through the Great Lakes into the Northeast. This maintained good continuity with the previous WPC forecast as well. ...Weather/Hazards Highlights... As a main upper trough ejects from the West early next week, lead precipitation should work downstream through the northern Rockies/Plains as initially fueled by channeled monsoonal moisture with above normal precipitable water values. Rain chances generally spread east Tuesday-Wednesday, but some lingering moisture across the northern High Plains and into the Upper Midwest seems to offer some marginal risk for heavy local convective rains for Day 4/5 Tuesday/Wednesday as the upper trough and cold front move eastward. Farther south, a shortwave is forecast to hover over the Lower Mississippi Valley, which in combination with a frontal boundary will provide support for scattered showers and storms. Similarly, some showers are possible across especially the southern Florida Peninsula. Heat is forecast to build across parts of the central to eastern U.S. next week underneath upper ridging. Temperatures 15 to 25 degrees above seasonal average are expected across the north-central states and record warm maximum and minimum temperatures could be widespread. Highs will be well into the 90s if not possibly even hitting the 100F mark there, with lows potentially staying in the mid-70s with limited overnight relief. The north-central U.S. should see some relief with cooler temperatures pushing west to east by Tuesday-Thursday as an upper trough and surface cold front move through. Above normal temperatures will also extend into the Ohio Valley and Great Lakes region and the Mid-Atlantic and Northeast, with anomalies around +10-15 degrees before moderating late next week. Farther south, temperatures generally 10-15 degrees above average (around 100F for highs) will extend summer for the southern half of the Plains through next week. The western U.S. on the other hand looks to be on the cool side especially for high temperatures particularly early in the week, with some moderation by midweek leading into a renewed re-building of the upper ridge from the southern Plains to the Southwest and southern Great Basin/Rockies through later next week. Santorelli/Schichtel 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