Extended Forecast Discussion NWS Weather Prediction Center College Park MD 238 PM EDT Thu Sep 2 2021 Valid 12Z Sun Sep 5 2021 - 12Z Thu Sep 9 2021 18Z Update: The 12Z deterministic guidance is in excellent agreement overall at the beginning of the forecast period Sunday. By later in the forecast period, the CMC becomes slower with the trough progression across the Great Lakes and slightly farther west with the upper ridge axis across the Intermountain West, and greater overall model spread with the next shortwave trough approaching the Pacific Northwest. In terms of rainfall prospects, a much quieter forecast period is expected compared to recent weeks. The possible exception to this may be along the western Gulf coast where moisture from a potential tropical wave/disturbance may advect deeper moisture northward by the middle of next week. Scattered showers and storms are also expected from the Great Lakes to the Northeast, but these are expected to be progressive. The WPC fronts/pressures forecast was primarily based on a multi-deterministic model blend through next Tuesday, followed by some increase in the GEFS and ECENS by Wednesday and Thursday. The previous discussion follows below. /Hamrick ...Guidance Evaluation and Preferences... Models and ensembles continue to show fairly good agreement on the larger scale upper flow pattern through much of the medium range period for much of the nation, but still struggle with smaller scale individual features, especially next week. The WPC medium range product suite was primarily derived from a composite blend of reasonably well clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/UKMET along with the compatible 01 UTC National Blend of Models for days 3-5 (Sunday-Tuesday). This blend has broad ensemble support despite the lingering small scale variances, bolstering forecast confidence. Forecast spread grows enough into days 6/7 (Wednesday/next Thursday) to favor a blend of the compatible GEFS/ECMWF ensemble means and best clustered model guidance from the 12 UTC ECMWF. This composite builds overall east-central U.S. upper trough amplitude on the higher side of the full forecast envelope (GFS less amplified). This seems more reasonable considering expected upstream ridge building and growing warmth over much of the the West ahead of Pacific upper trough translation. This maintains reasonably good WPC continuity in line with newer 00 UTC guidance. WPC progs also take into account latest NHC guidance that limits Bay of Campeche to western Gulf of Mexico tropical system development next week due to unfavorable upper level winds. Recent Canadian runs have been especially zealous. ...Pattern Overview with Weather/Hazard Highlights... A series of energetic northern stream upper troughs will force several wavy frontal systems into the Northwest and out though the north-central U.S. and increasingly southward into next week across the central and eastern U.S. with main upper trough reinforcement/amplification. Conditions seem best for some locally enhanced lead system convection from the Great Lakes to the Mid-South and central Plains Saturday, with more modest potential downstream more into the southern and eastern U.S. Sunday into Monday. The next trough to move into the north-central U.S. should bring another chance for organized rainfall across parts of the Midwest and Upper Great Lakes by next Monday into Tuesday, with scattered trailing frontal activity then again settling down over the southern and eastern states through next midweek in response to planned upper trough amplification. 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, winter weather outlook probabilities and heat indices 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml