Extended Forecast Discussion NWS Weather Prediction Center College Park MD 355 PM EDT Tue Sep 24 2019 Valid 12Z Fri Sep 27 2019 - 12Z Tue Oct 01 2019 ...Much below normal temperatures expected in the West with winter weather possible for the northern Rockies... ...Much above normal temperatures expected in the eastern third of the nation... ...A couple potential rounds of heavy rain expected across the central U.S... ...Pattern Overview and Guidance/Uncertainty Assessment... Forecast confidence is a bit above average during the medium range due to strong model consensus on the development of a highly amplified pattern with a trough in the West and ridge in the East. The 00Z ECMWF/06Z GFS solutions were clustered quite well within the ensemble consensus, and a blend of these deterministic solutions served as a basis for the forecast during days 3-5 (Fri-Sun). As upper-level shortwave energy traverses the amplifying western trough and moves into the central U.S., multiple potential waves of low pressure should traverse a surface front. The most significant of these waves looks likely to develop Sat night-Sun in the lee of the Rockies, with a significant area of precipitation likely north of the low from the northern Rockies to the northern High Plains (rain and potential areas of snow). By days 6-7 (Mon-Tue) model differences began to increase with respect to whether the western trough moves east more quickly or remains farther west as a slower and more cut off solution. Ensembles remained in much better agreement than deterministic solutions with below average spread, and thus a trend toward heavier ECENS/NAEFS ensemble mean weighting was shown by early next week. Ryan ...Weather Highlights/Threats... Expect significant precipitation over the western states to be focused over/near the northern Rockies and into favored terrain of the Great Basin. Northern Rockies precip should be enhanced by a period of upslope flow. Recent forecasts have been consistent in showing the greatest snowfall potential over western Montana. While the heaviest snow will be confined to the mountains, temperatures may be cold enough to bring some snow even to lower elevations of the northern High Plains. Rain/high elevation snow should be lighter and more scattered over northern California and the Pacific Northwest. Some locations may see a period of relatively stronger winds, most likely from the Great Basin into the central Rockies. Farther east anticipate some moisture extending across the northern Plains while heavier rainfall is likely within an area centered over the Midwest/Upper Mississippi Valley and western half of the Great Lakes. A leading frontal system will likely stall near the southern edge of the threat area and then lift northward in response to the wave forecast to track across the extreme northern tier. Shortwave energy ejecting from the Southwest may enhance activity for a time as well. Evolution of the eastern ridge aloft continues to favor a gradual increase of moisture over Florida as easterly flow becomes established. It is still very uncertain whether the moisture/rainfall will be enhanced by Tropical Storm Karen. Monitor NHC forecasts for the latest information regarding Karen. Much below normal temperatures will settle into northern parts of the West and northern Plains by Sat and then cover more of the West by early next week. There will be a broad area of minus 10-25F anomalies for high temperatures and parts of Montana could see a day or so with highs at least 30F below normal. Morning lows will be less extreme but may still be at least 10F below normal over some areas by Sun-Tue. Locations over the East and even into the central/southern Plains will see plus 10-20F anomalies for min and/or max temps. Best potential for daily record highs will be over the Southeast, possibly extending a little farther westward with time corresponding to motion of the upper high. A few locations farther north could see records as well. Morning lows may reach 20-25F above normal over parts of the central Plains/Midwest by next Mon-Tue. 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 Hazards: - Heavy precipitation across portions of the Northern/Central Rockies, the Northern/Central Great Basin, and the Northern Plains, Sat-Mon, Sep 28-Sep 30. - Heavy rain across portions of the Central Plains, the Upper/Middle Mississippi Valley, the Great Lakes, and the Ohio Valley, Fri-Sat, Sep 27-Sep 28. - Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Sun-Mon, Sep 29-Sep 30. - Flooding possible across portions of the Central/Southern Plains, the Great Lakes, and the Upper/Middle Mississippi Valley. - Flooding occurring or imminent across portions of the Plains, the Upper/Middle Mississippi Valley, and the Great Lakes. - Flooding likely across portions of the Middle Mississippi Valley and the Southern Plains. - Much above normal temperatures across portions of the Central/Southern Appalachians, the Tennessee Valley, the Mid-Atlantic, the Southeast, the Great Lakes, and the Ohio Valley, Fri-Tue, Sep 27-Oct 1. - Much below normal temperatures across portions of the Northern/Central Great Basin, the Northern Plains, the Northern Rockies, California, and the Pacific Northwest, Sat-Tue, Sep 28-Oct 1. 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