Extended Forecast Discussion NWS Weather Prediction Center College Park MD 245 PM EDT Fri Oct 08 2021 Valid 12Z Mon Oct 11 2021 - 12Z Fri Oct 15 2021 ...First winter storm of the season possible for parts of the central and northern Rockies by early to middle of next week... ...Heavy rain and severe thunderstorms possible across portions of the southern/central Plains Tuesday into Wednesday... ...Overview... After a couple of upper-level shortwaves and surface low pressure systems track quickly across the central U.S. Monday into Tuesday, the dominant system of note during the medium range period will be a potent low pressure system evolving over the central Rockies/High Plains before tracking into southern Canada, bringing significant mountain snow to parts of the Rockies as well as areas of heavy rainfall to portions of the northern Plains. The trailing cold front may also serve as a focus for locally heavy and strong to severe convection over the southern Plains. The amplified upper pattern will bring a pronounced temperature contrast with well below normal readings over the West and multiple warm days over the East. ...Guidance Evaluation/Preferences... Model guidance has reasonably good agreement with the early part of the forecast regarding shortwave and surface low positions across the central U.S. and south-central Canada, as well as with the digging trough in the western U.S. that is expected to close off an upper low likely late Monday into Tuesday. Meanwhile, models continue to forecast persistent ridging across the southeastern quadrant of the U.S., for which there is good model agreement. As the main upper low lifts northeastward toward the Northern Plains Tuesday through Thursday, model guidance remains consistent with its general evolution, but with some meaningful differences in track and timing. Much like the previous cycle, the 00Z/06Z GFS runs remain on the eastern side of the guidance envelope, while the 00Z CMC initially tracks with the slower/farther west ECMWF, but ends up still somewhat to the northwest by around midweek. The 00Z UKMET was closer to the GFS runs but still a reasonable solution closer to a middle ground. Overall, the best compromise for the upper and surface low tracks appeared to be the position of the 00Z EC ensemble mean, which for the deterministic guidance was closest to the ECMWF. Thus today's forecast employed a multi-model deterministic blend early, then eliminating the CMC by around Wednesday, in favor of the EC ensemble mean and eventually some of the 06Z GEFS mean. Higher weighting was given to the ECMWF and EC ensemble mean solutions. This forecast blend maintained reasonable continuity with the previous forecast, with perhaps the main low system moving a touch faster. While there are some differences in timing/amplitude of shortwaves coming into the Northeast Tuesday-Wednesday and into the West for the latter part of the week, this solution worked for those areas as well. ...Weather/Hazards Highlights... The amplifying western trough and closed upper low development over the Southwest/Great Basin into early-mid next week will bring much below normal temperatures behind a leading cold front/low pressure, with broad coverage of daytime highs 20-30 degrees below normal across much of the interior West next Monday-Wednesday. This colder air will lower snow levels in the mountains with potential for heavy snow accumulations in the higher terrain of the northern-central Rockies as low pressure consolidates Tuesday-Wednesday and then tracks northeastward. This would be the first significant winter storm of the season for this area. Away from the mountains, an axis of heavy rainfall is possible around and to the north of the surface low with some mixing of snow not out of the question during the colder overnight hours. Gusty winds may also be possible across the central and northern Plains by Wednesday and Thursday as the surface low deepens. Across the central and southern Plains, moderate to locally heavy rainfall is possible and the Storm Prediction Center is monitoring another threat for severe weather on Tuesday and/or Wednesday depending on exact timing of the front that will extend from the northern Plains low. By Thursday-Friday there is some potential for eastern Pacific tropical moisture to stream northeastward and interact with the front to produce areas of heavy rainfall likely over Texas, but confidence is low for resolving the location and magnitude specifics. Elsewhere, a leading system may bring a band of enhanced rainfall to parts of the Midwest Monday as it lifts northeastward. The Pacific Northwest should see mostly light precipitation with upstream energy coming in from the Pacific through midweek, before an approaching late week frontal system may increase amounts somewhat over the Olympics and far northern Cascades. The West will see its cold temperatures moderate by Thursday-Friday with gradually shrinking coverage of minus 10-20F anomalies for highs and morning lows. Within the eastern U.S. warmth, highest anomalies for max temperatures will be over the Great Lakes/Northeast with highs tending to be 10-15F or so above average. An overwhelming majority of the East will see plus 10-20F anomalies for lows with some locations over/southwest of the Great Lakes possibly even warmer Monday-Tuesday. Such readings may achieve daily records for warm lows over some areas. Tate/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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