Extended Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EDT Sat Sep 02 2023 Valid 12Z Tue Sep 05 2023 - 12Z Sat Sep 09 2023 ...Summer Heat next week to spread to 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 back into Florida next week with a lingering and slow moving shortwave and front. ...Guidance Evaluation/Predictability Assessment... Model and ensemble solutions are generally well clustered over much of the lower 48 next week in a pattern with seemingly above normal predictability. There are some greater differences notably with Pacific energies working inland into the West, albeit with limited rainfall, but in particular with the eventual track of Idalia and the proximity and affect to the East Coast from additional western Atlantic impulses/upper troughs. There remains some suggestion (though lowering potential) in some guidance that Idalia could turn back to the Canadian Maritimes and this potential relies heavily on details of energies digging to the lee of the amplipied building ridge in the East. Prefer the broad model and ensemble blend for the West to minimize smaller scale variance. For the western Atlantic it seemed prudent to exclude the 12 UTC ECMWF from the broad blend for Idalia through next midweek to best match the latest guidance from NHC that remains significantly farther to the east. ...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/Wednesdayas 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. But this will push above normal temperatures 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 high heat from the southern Plains to the Southwest and southern Great Basin/Rockies through later next week. 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