Extended Forecast Discussion NWS Weather Prediction Center College Park MD 253 AM EDT Fri Sep 04 2020 Valid 12Z Mon Sep 07 2020 - 12Z Fri Sep 11 2020 ...Intense heat in the West to relent as Thanksgiving-like cold air plunges through the Rockies/Plains next week... ...Overview and Guidance/Predictability Assessment... An extremely amplified upper pattern more reminiscent of late fall will develop over the lower 48 during the first half of next week. The extratropical transition of tropical cyclone Maysak as it lifts into northeastern Asia will have some influence on the amplitude of Pacific and North American flow, setting in motion a downstream wavetrain including a potent system for Alaska Sun-Mon and ultimately a strong trough and possible closed low over/near the Rockies or western Plains. The combination of upper dynamics and southward push of low level cold air could support an unusually early significant snowfall over parts of the northern-central Rockies. Guidance through the 18Z cycle was fairly close up to about early day 4 Tue. After that time solutions rapidly diverged, with the GFS taking all of the upper trough energy eastward into the Plains/east-central U.S. in contrast to numerous ECMWF runs that have been showing an upper low lingering for a time near the Four Corners region. The 12Z UKMET was also fairly progressive while the 12Z CMC was the slowest of the progressive cluster. Meanwhile the GEFS/ECMWF ensembles have been diverse enough for the means not to close off a low, but recent GEFS mean trends have been strongly leading toward the ECMWF mean that has been hinting at the operational ECMWF idea. Teleconnections relative to the upstream ridge's positive height anomaly center support some southwestward elongation of trough energy but are more ambiguous about the specific ECMWF cluster. At the very least the GEFS mean trend and teleconnections favor leaning closer to the ECMWF/ECMWF mean/GEFS mean idea mid-late period, with a little CMC included to account for alternate scenarios. New 00Z GFS/CMC/UKMET runs have all trended slower/more closed to provide more support for the favored cluster. The 00Z ECMWF is close to its continuity. Last fall there were some cases of the GFS being way too open/progressive with amplifying western upper troughs in the mid-late extended range, followed by a trending back to the slower/closed ECMWF (and usually one or more other global models), only to have the then-agreeable cluster adjust a little faster--though still verifying closer to the original ECMWF idea. It will be interesting to see if this evolution fits that template. ...Weather/Hazard Highlights... To the south of the strong cold front dropping through the West early in the week, hot temperatures will continue into Monday with a number of record highs possible from California into the southern High Plains. Highs will be well into the 100s for many areas in the Central Valley and deserts. Much colder air pushing into the northern Rockies/Plains on Monday will reach into the central-southern Rockies/Plains Tuesday-Wednesday, with temperatures possibly changing by 30-40F in 12 hours. Highs in some places on Tuesday will be more typical of late November rather than early September (20-40F below normal). Expect low temperatures to be somewhat less extreme, generally 10-25F below normal. Numerous locations in the Rockies/Plains should see record cold lows and/or highs with this event. Within the broader area of enhanced precipitation dropping south across the Rockies/Plains, snowfall will be likely for higher elevations and even down to 6-7000 ft (maybe lower even to the foothills). Some snow could be heavy at high mountain passes. Highest probabilities for meaningful snow extend from southern Montana into Colorado. Elsewhere, upper ridging will support highs up to 10-20F above normal over western Washington/Oregon with some eastward expansion of the warmth after midweek. Waves along the eastern side of the western-central U.S. cold front will promote areas of rainfall that could be heavy from the Midwest into the Great Lakes. The front will continue into the East by Thursday-Friday, also with some locally heavy rainfall possible. Above normal temperatures will prevail over most of the East ahead of the front, with anomalies for morning lows generally more extreme than daytime highs. A front lingering from the northern Gulf of Mexico into the western Atlantic Monday-Wednesday may provide some focus for rainfall over Florida and close to the southeastern coast. Rausch/Fracasso 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