Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 AM 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... ...Overview and Guidance/Predictability Assessment... During Tue-Wed the model and ensemble guidance continues to show an extremely amplified evolution that would be more typical for November. Strong upper ridges over the West Coast and western Atlantic will bookend sharply digging trough energy reaching into or just east of the Four Corners, possibly forming a closed low. 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 could help to produce significant snowfall over parts of the central Rockies. After about early Wed guidance continues to diverge regarding the timing and path of the possible upper low. As a result there is still considerable uncertainty regarding the duration/coverage/intensity of precipitation from the southern half of the Rockies through the Plains into the east-central U.S. from midweek onward. Looking at available guidance through the 12Z/18Z cycles, the wide array of GEFS/ECMWF/CMC ensemble members highlighted the ongoing uncertainty in the forecast--including possibilities ranging anywhere from an upper low closing off as far west as the Great Basin, to a more progressive trough/low such as offered by the 12Z/18Z GFS and 12Z ECMWF. The latter was a surprising jump to the east given the model's prior multi-run commitment to a Four Corners closed low. The 12Z CMC was the one operational model closest to the Four Corners closed low scenario while the UKMET was a compromise by the end of its run early Thu. Enough GEFS/ECMWF ensemble members favored a fairly slow/western solution to recommend an updated forecast emphasizing the 18Z GEFS/12Z ECMWF means and some aspects of the 12Z CMC mid-late period. The approach of northern stream flow actually helped to bring guidance a little closer together in principle aloft (though not at the surface) by day 7 Sat. Thus the blend reintroduced a little 18Z GFS/12Z ECMWF input at that time. Their inclusion helped to lower surface pressures a bit over the Midwest compared to the means, which seems reasonable given the presence of at least a shortwave trough aloft. The favored blend yielded only modest continuity changes including a slight eastward nudge for the Rockies trough/upper low and a slower trend for the front reaching the East Coast late in the week. By day 5 Thu the new 00Z GFS/UKMET/CMC all favor slower emergence of the trough/upper low into the Plains than forecast by prior GFS runs and the 12Z ECMWF. On the other hand the new 00Z ECMWF has added to the intrigue by trending even farther northeast with the upper low versus the 12Z run. ...Weather/Hazard Highlights... The cold surge continuing southward over 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 at higher elevations, possibly including some high mountain passes, will be from southern Wyoming through Colorado during Tue-Tue night. The upper trough/low expected to track out of the Rockies along with the leading wavy surface front progressing from the Plains into the eastern U.S. may generate areas of locally heavy rainfall from portions of the southern-central Plains into the central-upper Great Lakes. Other areas in the East may see significant rainfall from a combination of this front and leading moist Atlantic flow that is forecast to push a lingering western Atlantic front back toward the East Coast. 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, while lows will likely be somewhat more above normal for most of the period. Temperatures over the East after midweek will depend on increasingly uncertain surface details. 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