Extended Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EDT Thu Sep 8 2022 Valid 12Z Sun Sep 11 2022 - 12Z Thu Sep 15 2022 ...General Overview... A pattern change will be underway across much of the Rockies and the central/northern Plains into the Midwest as a strong cold front heralds a change to much cooler temperatures for the weekend, compared to the anomalous warm weather presently. The upper trough that is accompanying this front is forecast to evolve into a slow moving closed upper low across the Ohio Valley and Great Lakes region by early Monday, with a ridge axis near the East Coast slowing its eastward progression. Moisture associated with what will eventually be the remnants of Hurricane Kay is expected to advect inland across California and the Great Basin, albeit with lighter rainfall by this time. There will likely be another trough approaching the Pacific Northwest by the middle of the week. ...Guidance Evaluation/Predictability Assessment... The 00Z model guidance suite initially has a good overall synoptic scale depiction of the main pattern through about early Tuesday, with the typical mesoscale model differences. As the upper low builds in across the Midwest by Monday, the GFS is initially faster with its eastward progression early on, and then the 00Z ECMWF becomes faster later in the week compared to its earlier runs and the GFS/CMC. By next Thursday, the GFS and ECMWF become more out of phase across south-central Canada with a trough crossing the Hudson Bay region. In terms of QPF, the NBM along with about 15% of the operational 12Z ECMWF was used as a baseline. The WPC fronts/pressures forecast utilized a multi-model deterministic blend through Tuesday, gradually increasing use of the ECMWF and GEFS means to about half by the end of the period as individual model differences increase. ...Weather/Hazards Highlights... The first strong cold front of September is forecast to bring quite a change to the Plains and then the Midwest and Great Lakes region for the weekend. Highs are expected to be only in the 60s across most of the Upper Midwest on Sunday, which is roughly 5-15 degrees below normal for this time of year. Significant abatement of the heat is also expected across California and Desert Southwest, with much of this cool down attributable to clouds and rainfall associated with a weakening Tropical Storm Kay by that time. There will be warmer conditions for the Pacific Northwest for Sunday and Monday, but not to the same magnitude as this past week. Across the eastern U.S., readings should generally be within 5 degrees of climatological averages for highs, while overnight lows near the East Coast should be about 5-10 degrees above normal most nights, owing to higher humidity and more cloud cover. Moisture from the future remnants of Hurricane Kay will likely be in place across the Intermountain West on Sunday, although the most impactful rain should be over by that time. Farther to the east, a cold front that will be in the process of becoming occluded is expected to produce moderate to locally heavy rain from Iowa to southern Wisconsin and northwestern Illinois, and a Slight Risk of excessive rainfall is now in effect for these areas on the Day 4 experimental outlook. The overall upper trough anchored by this upper low and the leading surface system may interact with the moisture initially over the South to spread a broader shield of rainfall over the eastern U.S. into early next week. Hamrick 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, experimental excessive rainfall outlook, 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/#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