Extended Forecast Discussion NWS Weather Prediction Center College Park MD 454 PM EDT Thu Sep 03 2020 Valid 12Z Sun Sep 06 2020 - 12Z Thu Sep 10 2020 ...Intense heat in the West to relent as Thanksgiving-like cold air plunges through the Rockies next week... ...Overview and Guidance/Predictability Assessment... A robustly amplified upper pattern will unfold over the CONUS next week, set in part by the extratropical transition of tropical cyclone Maysak as it lifts into northeastern Asia. This will set 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 models have been inconsistent in how to handle the trough--whether or not to dig it southwestward into a closed low or remain more progressive--but the latest 00Z ECMWF and 06Z GFS were closer than in previous days in favoring a less-progressive solution. This was evident in the GEFS mean as well with an overall slower solution. Parallel GEFS members were perhaps a bit quicker than the operational members to slow the trough. However, the quicker 00Z UKMET/Canadian limit confidence in the flow. By next Wed-Thu, 06Z GFS represented a modestly slower solution not as slow/west as the 00Z ECMWF nor as far east as the Canadian with a closed low over the Southern Plains. Ensembles were washed out E-W with varying solutions of quicker/slower which again limits confidence. ...Weather/Hazard Highlights... Ongoing western heat will start to ease Monday as the strong cold front exits Montana. Records highs will be likely for areas ahead of the front, especially Sun-Mon over California, the Great Basin, and the Desert Southwest/Four Corners. Highs will be well into the 100s for many areas in the Central Valley and deserts. By Tuesday, much colder air will push through the northern/central Rockies and Plains where temperatures may change by 30-40 degrees in 12 hours. Highs in some places on Tuesday will be more typical of late November rather than early September (20-40 degrees below normal). In addition, 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. Record cold temperatures are probable for parts of the Plains as the cold air sinks toward Texas. Rainfall will accompany and precede the front and could increase over Texas as the front nears Tuesday. Again this will be dependent on how the upper pattern evolves. Rainfall will push eastward with the front into the Great Lakes and Ohio/TN Valleys midweek, then toward the Appalachians/East Coast by next Thursday. 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 Hazards: - Heavy rain across portions of the Central/Southern Plains, and the Middle/Lower Mississippi Valley, Tue-Wed, Sep 8-Sep 9. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sun-Tue, Sep 6-Sep 8. - Much above normal temperatures across portions of California, the Central Great Basin, the Pacific Northwest, the Northern Great Basin, and the Southwest, Sun-Mon, Sep 6-Sep 7. - Much above normal temperatures across portions of the Northern/Central Plains, and the Upper/Middle Mississippi Valley, Sun, Sep 6. - Much below normal temperatures across portions of the Plains, the Rockies, the Mississippi Valley, the Central Great Basin, the Great Lakes, the Ohio Valley, and the Southwest, Mon-Thu, Sep 7-Sep 10. 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