Extended Forecast Discussion NWS Weather Prediction Center College Park MD 314 PM EDT Sat Sep 05 2020 Valid 12Z Tue Sep 08 2020 - 12Z Sat Sep 12 2020 ...Exceptional cold surge to continue southward over the Rockies/Plains next week with significant snow possible over the central Rockies... ...Increasingly wet conditions expected to develop along the East Coast... ...Overview and Guidance/Predictability Assessment... During Tue-Wed., the model and ensemble guidance continues to trend toward an extremely amplified synoptic pattern that is more typical of the cold seasons. Strong upper ridges over the West Coast and western Atlantic will be in stark contrast with a trough digging sharply through the Rockies and High Plains before possibly forming into a closed low as it lifts toward the Great Lakes later next week. The accompanying cold surge will likely bring widespread record cold max/min temperatures to areas from the Rockies into the central U.S., while the upper dynamics is expected to produce significant snowfall over parts of the central Rockies. After about early Wed., deterministic solutions are beginning to indicate a more consistent signal for a possible upper low to track from the central U.S. toward the Great Lakes. The ensemble means have not shown such a signal yet, indicating there is still considerable model spread between the closed low scenario and the more progressive scenario. A general compromise between the 00Z ECMWF, 06Z GFS, and 00Z CMC, together with a smaller portion from the 00Z EC mean and 06Z GEFS were used for early in the medium-range period given the good model agreement. From Day 5 onward, something close to an even blend of the 00Z ECMWF, 06Z GFS and their ensemble mean solutions was used. This yielded a better defined surface low tracking toward the Great Lakes by the end of next week compared with previous forecasts. ...Weather/Hazard Highlights... The southward plunge of the cold air across the Rockies/Plains Tue-Wed will likely bring daily record cold max/min temperatures to many locations. Record cold highs could extend into the Upper Midwest. The greatest anomalies should be over and near the central High Plains/Rockies with highs up to 30-40F below normal and morning lows 20-30F below normal. Although temperatures will steadily moderate over the central U.S. after Wed there may still be some daily records on Thu and locations with minus 5-15F anomalies into Sat. The best potential for heavy snow will be at the higher elevations from southern Wyoming through Colorado, possibly including some high mountain passes, during Tue-Tue night. Areas of locally heavy rainfall may impact parts of the central to southern Plains Tuesday and Tuesday night as low pressure waves are expected to form along the leading cold front ahead of the upper trough/low forecast to exit the Rockies. Although uncertainty remains high regarding the eventually evolution of the upper trough, areas of rain are expected to track from the southern-central Plains and then linger over the Great Lakes. Along the East Coast, rain chances are forecast to increase and spread further inland as a stationary front off the coast returns as a warm front. In fact, rain could be heavy and persistent from the Mid-Atlantic coast southward down to Florida as an influx of moisture from the Atlantic becomes firmly established. Meanwhile, the upper ridge near the West Coast will support highs up to 10-20F above normal over western Washington/Oregon by midweek (possibly into parts of California on Tue) followed by eastward expansion of the warmth. Gradual weakening of the upper ridge should allow highest anomalies over the Northwest to settle into the plus 10-12F range by next Sat. Expect the eastern U.S. to see moderately above normal highs through Wed-Thu followed by a trend toward normal due to increasing clouds and rains, while lows will likely be somewhat more above normal for most of the period. Kong/Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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