Extended Forecast Discussion NWS Weather Prediction Center College Park MD 445 PM EDT Fri Sep 03 2021 Valid 12Z Mon Sep 06 2021 - 12Z Fri Sep 10 2021 ...Overview... Upper ridging will drift out of the Great Basin early next week toward the southern Plains as troughing moves into the Pacific Northwest. Troughing in the Northeast on Monday will lift out by Tuesday but will be quickly replaced by incoming troughing from southern Canada. That trough should exit the East Coast around the end of next week, steering Hurricane Larry in a more northerly direction out over the open Atlantic. The pattern is largely quiet and drier than average after a very active and wet several weeks. ...Guidance Evaluation and Preferences... Through the 00z/06Z cycle, the models and ensembles continue to show relatively good clustering through the period, with expected differences that grew with time. Large uncertainty remains surrounding flow pattern over the Pacific/Alaska region which translates into the West days 6-7. The 00z ECMWF has backed off on amplification of a shortwave nearing the coast, while the GFS remains more amplified. The CMC would also suggest troughing along the coast on Day 7, and that's consistent with the ensemble means as well. Enough run to run inconsistencies though in all the models to continue favoring a majority blend towards the ensemble means, with smaller contributions from the deterministics just for a little added system definition. Over the Southwest, the movement of the upper high southeastward may allow for an increase in moisture into the lower CO River basin late next week. GFS/GEFS solutions remain a bit wetter than the ECMWF/Canadian and their ensembles. Favored a slightly drier scenario for now. In the Gulf, some of the deterministic models may be a little bit more robust on potential tropical activity, but the ensemble solutions continue to exhibit a wide array of spread. Large uncertainties remain regarding timing and potential impacts to the Gulf Coast late week (if any), so prefer a more modest approach at this time. ...Weather/Hazard Highlights... Rainfall will focus over the Upper Midwest/Great Lakes Monday in advance of a system out of the northern Plains, which may promote some moderate to locally heavy rainfall on Tuesday-Wednesday across Michigan and parts of the Lower Great Lakes. Rainfall along the cold front may be lighter and continued southward into the Southeast in a warmer and more humid environment. Increased tropical moisture into the Gulf by mid to late week may bring the threat for moderate to heavy rainfall along the central Gulf Coast region, but models continue to show significant uncertainty regarding exact locations and timing. Temperatures will be near to above normal over most areas, especially the West to the Northern Rockies ahead of the incoming Pacific trough. Temperatures 5-15 degrees above normal are possible as the area of warmth moves into the southern/central Plains late next week. Temperatures in the Northeast will be near to a bit below normal thanks to generally northwest flow. Santorelli/Fracasso Hazards: - Flooding possible across portions of the Northeast. - Flooding occurring or imminent across portions of the the Lower Mississippi Valley and the Mid-Atlantic. - Flooding likely across portions of the Upper Mississippi Valley. 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