Extended Forecast Discussion NWS Weather Prediction Center College Park MD 309 AM 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... Guidance agrees upon a persistent mean ridge aloft from the Gulf of Mexico into the eastern U.S., while amplifying western U.S. energy should close off an upper low over or near the southern Great Basin by early Tuesday and then eject northeastward into the northern Plains. The upper low will support next week's dominant storm system that will evolve 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. A leading wave tracking northeastward through the Midwest may produce a band of enhanced rainfall early in the week. 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... Recent guidance has been consistent with the general evolution of the upper low that closes off over the West and then tracks into the northern Plains, but with some meaningful differences in track and timing. The past 24-36 hours of GFS runs have adjusted somewhat eastward versus prior runs and remaining guidance once the system reaches the central U.S. though the 18Z run came back a bit relative to the 12Z run with the new 00Z run holding onto a similar solution as the 18Z run. The 12Z CMC/CMC mean were on the other extreme with a farther northwest track, leaving the ECMWF/ECMWF mean as closest to an intermediate solution (aside from the 12Z ECMWF being a bit south of the preferred path) and WPC continuity. Among other new 00Z runs, the CMC is getting closer to other solutions but remains on the northwest side of the spread by Wednesday-Thursday while the UKMET has trended somewhat faster the make the GFS less of a minority. The 00Z ECMWF has adjusted northward over the Plains to be closer to initial preference. As for other aspects of the forecast, CMC runs have been on the amplified side of guidance with trailing trough energy that reaches the West--which may be playing a role in its forecast with the aforementioned system of interest. A combination of other guidance looks reasonable for this energy. Ahead of the main system, guidance has been showing some difficulty in resolving the precise surface evolution over the Northeast as an ejecting Midwest shortwave/possible upper low weakens over the Great Lakes/Northeast as it flows into/around the mean ridge. Low pressure that may initially be near the North Carolina coast (with only low probability of any tropical or subtropical development per the National Hurricane Center's Tropical Weather Outlook) will likely drift away from the coast with time. The updated forecast employed a multi-model blend early followed by a trend to a model/mean mix consisting of slightly more 12Z ECMWF/ECMWF mean relative to the 18Z GFS/GEFS mean late. This provided the intermediate solution desired for the Rockies/Plains storm and maintained reasonable continuity. ...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 lowering snow levels in the mountains with potential for potentially heavy snow accumulations in the higher terrain of the northern-central Rockies as low pressure consolidates Tuesday-Wednesday and then track 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. However at this time confidence is low for resolving the location and magnitude of any rainfall enhancement from this tropical moisture. Elsewhere, a leading system may bring a band of enhanced rainfall to parts of the Midwest as it lifts northeastward. The Pacific Northwest should see mostly light precipitation with upstream energy coming in from the Pacific through midweek. 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. 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