Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 PM EDT Sat Sep 4 2021 Valid 12Z Tue Sep 7 2021 - 12Z Sat Sep 11 2021 19Z Update: The 12Z guidance is in very good agreement on the overall pattern through Thursday with good continuity from the previous forecast. By the end of the forecast period next Saturday, the CMC is considerably stronger with the central U.S. upper ridge building northward, whereas the ECMWF/GFS are indicating a shortwave trough crossing the Upper Midwest. There is still a good deal of uncertainty regarding the potential development and path of a tropical disturbance over the southern Gulf of Mexico, with the guidance suggesting at most a weak surface low associated with it. The WPC forecast was derived from a multi-deterministic model blend through early Thursday, followed by some of the ensemble means for Friday and Saturday. The previous discussion follows below for reference. /Hamrick ...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 will move into the Northeast Tuesday out of southern Canada which 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 12Z/18Z cycles, the models and ensembles continued to show relatively good clustering through the period, with expected differences that grew with time. Large uncertainty remains surrounding flow pattern over the northeast Pacific/Gulf of Alaska region which translates into the West toward the end of the period. There have been enough run-to-run inconsistencies in all the models to continue favoring a majority blend towards the ensemble means (GEFS + ECENS mean) which have been more stable at a cost of some detail. Overall the 12Z ECMWF was favored over the GFS, which has been quite varied out of the Pacific. 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. Some GFS runs have been more aggressive in bringing tropical moisture and/or a system into Mexico and across the border, but continued to favor a drier solution rather than wetter. In the Gulf, tropical development remains uncertain but moisture transport northward/northeastward continues to look more likely into the northern/northeastern Gulf. Please see the NHC outlooks for more information. ...Weather/Hazard Highlights... Rainfall will focus over the Great Lakes Tuesday as an area of low pressure deepens over southern Canada. Some moderate to locally heavy rainfall is possible across Michigan and parts of the Lower Great Lakes, perhaps into New England on Wednesday as the warm front lifts northward. Rainfall along the tail-end of the cold front may be lighter into the Southeast. Increased tropical moisture into the Gulf by midweek may bring the threat for moderate to heavy rainfall along the northern/northeastern Gulf Coast region, but models continue to show significant uncertainty regarding exact locations, timing, and amounts. Maintained a modest signal pending short term trends with the system. Temperatures will be near to above normal over most areas, especially in 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 Great Lakes/Northeast will be near to a bit below normal thanks to generally northwest flow behind the cold front. Fracasso 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