Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Sep 01 2023 Valid 12Z Mon Sep 04 2023 - 12Z Fri Sep 08 2023 ...Idalia to linger over the western Atlantic next week .. ...Summer Heat to spread slowly across the Plains and Midwest toward the East next week... ...Wet pattern to work from the Intermountain West to the northern Rockies/Plains early next week... ...Overview... An amplified upper trough over the West will lift out through the north-central Intermountain West/Rockies and 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 areas of locally excessive rainfall and localized flooding concerns. Downstream, yet another hot summertime upper ridge over the central U.S. will gradually shift eastward through next neek. This pattern will support well above seasonal normal temperatures across the Plains, Upper Midwest before approaching the East and also back again into the Southwest. Meanwhile, scattered showers and storms could persist across the Gulf Coast region into Florida next week with a lingering and slow moving shortwave trough and frontal boundary. The ultimate eastward progression of the aforementioned upper ridge and rounding impusles may prove critical with respect to the later next week track of resilient Idalia over the western Atlantic. ...Guidance Evaluation/Predictability Assessment... The WPC medium range product suite was primarily derived from a composite of reasonable clustered mid-larger scale guidance of the GFS/ECMWF/UKMET/Canadian for early next week for much of the lower 48. Significant emerging differences with the mid-later next week handling of flow interactions over the lower 48 and Idalia over the western Atlantic prompted a switch to the ECMWF ensemble mean and especially the GEFS ensemble mean for these longer time periods to best maintain continuity and to avoid individual model variances from the NHC forecast for Idalia as some recent runs of the ECMWF and Canadian offer more of a turn toward the Northeast/Canadian maritimes than the GFS. The 00 UTC ECMWF and Canadian have trended way from that potential. ...Weather/Hazards Highlights... As a main upper trough lifts from the West early next week, precipitation should expand/push through the northern Rockies/Plains while lingering over parts of the Intermountain West. This is likely as flow to the east of the trough axis offers instability and channeled monsoonal moisture with precipitable water values well above normal. A Marginal Risk remains in place for Monday across the Northern Rockies/High Plains and into central Utah as moisture levels remain above normal. Rain chances are expected to spread east into the Midwest by 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 for 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. 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. 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