Extended Forecast Discussion NWS Weather Prediction Center College Park MD 243 AM EDT Thu Oct 07 2021 Valid 12Z Sun Oct 10 2021 - 12Z Thu Oct 14 2021 ...First winter storm of the season possible for parts of the central and northern Rockies by early to middle next week... ...Guidance Evaluation/preferences... For a medium range forecast, with one main exception there's actually really good agreement through the entire period on the overall large scale setup which features a shortwave/closed low lifting through the southern Plains early next week, while troughing amplifies over the West and a well defined low pressure system emerges into the Central Plains by the middle of next week. There remains some relatively minor timing and detail differences, but these kinds of uncertainties may take until the short range to resolve. Overall, the WPC forecast suite was derived from a composite blend of the deterministic models, ensembles and National Blend of Models for days 3-7. This maintains good continuity with the previous WPC forecast helping to increase confidence some in the pattern and relevant weather hazards. The one main exception is over the Mid-Atlantic and Northeast Sun/Mon where the 12 UTC ECMWF/Canadian and to a lesser extent ECMWF ensembles wrap much more QPF inland from a western Atlantic low than the GFS/UKMET/GEFS. WPC continuity, latest NHC guidance, and latest 00 UTC guidance trends do not favor the 12 UTC ECMWF/Canadian solution. ...Pattern Overview and Weather/Hazards highlights... A slow moving frontal boundary and area of low pressure off the East Coast, with limited tropical development probabilities as per NHC, offers potential for some heavy downpours into coastal areas of the Mid-Atlantic to the Northeast, especially Sun/Mon. The WPC product suite minimizes heavier rainfall potential onshore considering latest NHC system guidance and WPC continuity. Upstream, a front lifting through the north-central U.S. will bring moderate to locally heavy rainfall to parts of the northern Plains/Upper Midwest this weekend. Amplifying southern stream upper trough energy should act to focus moisture/instability to fuel an increasingly favorable convection and organized rainfall environment Sun/Mon for portions of the Plains, with the SPC noting a severe weather area across the southern Plains on Sunday in their day extended outlook. The system should then begin lifting and shearing over the Midwest/Great Lakes on Tuesday, with mainly light to moderate precipitation. Out West, moderate to heavy rainfall and mountain snow is possible across parts of the Pacific Northwest this weekend, with moisture shifting southward and inland with a wavy/lead front. Deep troughing and closed upper low development down over the Southwest/Great Basin into early-mid next week will bring much below normal temperatures behind the cold front with daytime highs as much as 25-30 degrees below normal across much of the interior West well into next week. This would support lowering snow levels in the mountains with potential for decent snow accumulations in the higher terrain of the northern Rockies beginning on Sunday. A low pressure system should eject out of the Rockies into the Plains, with increasing confidence in potentially heavy snowfall within an emerging deformation area on the back/west side of the low. This could bring the first winter storm of the season to portions of the central and northern Rockies. An axis of heavy rainfall is possible around and to the north of the 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 of next week as the surface low deepens through the central to northern U.S. and further enhances precipitation. Across the central and southern Plains, moderate to locally heavy rainfall is possible with another threat for severe weather on Tuesday into Wednesday as highlighted in the latest SPC extended outlook. Schichtel 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