Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 AM EDT Fri Sep 09 2022 Valid 12Z Mon Sep 12 2022 - 12Z Fri Sep 16 2022 ...Overview... As the medium range period begins Monday, a closed upper low and an occluded surface low and frontal system will be present across the Upper Midwest and move into the Great Lakes region Tuesday, before sweeping its cold front off the East Coast by midweek. Farther west, warm ridging is forecast to shift from the Four Corners region into the south-central Plains as next week progresses, while periods of troughing along the West Coast will promote cooler weather in the West. This flow pattern should also direct moisture (related to Kay) into the Intermountain West and cause showers and thunderstorms there. ...Guidance Evaluation/Predictability Assessment... Model guidance has been consistent with showing a closed upper low and surface slowly moving eastward across the Midwest/Great Lakes Monday-Tuesday. The main change in the models over the past day or so has been to accelerate these features more quickly northeastward out of the CONUS by late Wednesday into Thursday, while older model runs had them farther south and lingering longer. The 12Z (yesterday) GFS was still on the slower side, but the 18Z GFS picked up on this trend that the other 12Z guidance showed. Incoming 00Z models are consistent with the faster movement of these features. Models are in good agreement initially for upper ridging across the Four Corners and a shortwave near the Pacific Northwest Monday. The details become more nebulous as that shortwave travels east, possibly disrupting the northern edge of the ridge (as GFS runs show most aggressively) around Wednesday-Thursday, and rounds of uncertain additional energy enter the lower 48 from upstream. Clear trends or outliers were not really evident here but the differences do make the details somewhat uncertain. The WPC fronts/pressures forecast was based on a blend of the 18Z GFS and 12Z ECMWF, CMC, and UKMET early in the period, with increasing weighting of the ECMWF and GEFS ensemble means to about half by the end of the period. This led to a faster eastern trough/low and front compared to the previous forecast, with some changes in the associated QPF moving more quickly across the Northeast Tuesday-Wednesday. ...Weather/Hazards Highlights... While Kay is currently a tropical storm in the eastern Pacific off the coast of Baja California, the National Hurricane Center forecasts Kay to become a post-tropical cyclone and then dissipate through the medium range period. Regardless of Kay's status, increased tropical moisture looks to flow into much of the West with deep layer flow from the southwest. Rain and some thunderstorms are possible in the Desert Southwest Monday, with a northward trend to the forecast rainfall into the central/northern Great Basin and Rockies as next week progresses. Then farther east, the cyclone moving across the Upper Midwest/Great Lakes and its attendant fronts will produce rain and thunderstorms across the northeastern quadrant of the U.S. and across the Eastern Seaboard Monday-Tuesday before the cold front clears precipitation chances out on Wednesday. Isolated flash floods are possible in both the West and East in areas where rain is particularly heavy. The southern end of the eastern front is forecast to linger near the Gulf Coast for the latter half of the week and could cause scattered showers and storms, but most rainfall may stay offshore except across Florida where the front stalls. The western U.S. is forecast to see highs around 5-10F below normal in a likely welcome change from the recent heat. Overnight/morning lows are likely to remain around average to above average there. Above normal temperatures by around 5-15F are expected for the northern High Plains Monday and spreading across much of the Plains Tuesday and lasting into late week as the upper ridge settles there. Meanwhile the east-central U.S. should be cooler than normal behind the cold front Monday-Tuesday but moderate to near normal for the latter half of the week. Tate 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