Extended Forecast Discussion NWS Weather Prediction Center College Park MD 513 PM 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*** ***Heavy rain and severe thunderstorms possible across portions of the southern/central Plains on Sunday and also Tuesday into Wednesday*** 18Z Update: The updated forecast remains on track with an amplified upper level pattern featuring a ridge axis from the Gulf of Mexico to the East Coast, and a building trough with developing closed low over the Intermountain West that will sustain a significant low pressure system across the Plains by the middle of next week. Forecast confidence is generally above average on the synoptic scale through Wednesday and then near average by next Thursday. The 6Z and 12Z runs of the GFS become more progressive with the ejecting low from the Rockies to the western High Plains, compared to the slower non-NCEP solutions. Recent ensemble trends support the slower deterministic model solutions by the second half of the forecast period, so the CMC/ECMWF/ECENS was the preferred blend for Wednesday and Thursday, whereas a general deterministic model blend sufficed as a starting point for Sunday into Tuesday. The previous forecast discussion follows below for reference. /Hamrick ...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 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Sun-Mon, Oct 10-Oct 11. - Heavy precipitation across portions of the Central Rockies, the Central Plains, the Northern Plains, and the Northern Rockies, Tue-Wed, Oct 12-Oct 13. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Southern Plains, Sun, Oct 10. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Southern Plains, the Upper Mississippi Valley, and the Northern Plains, Tue, Oct 12. - Heavy rain across portions of the Central Plains, the Upper Mississippi Valley, and the Northern Plains, Tue-Wed, Oct 12-Oct 13. - Heavy snow across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, and the Northern Great Basin, Tue-Wed, Oct 12-Oct 13. - Heavy snow across portions of the Northern Plains, the Northern Rockies, and the Northern Great Basin, Mon-Tue, Oct 11-Oct 12. - Severe weather across portions of the Central Plains, the Lower Mississippi Valley, and the Southern Plains, Sun, Oct 10. - Severe weather across portions of the Southern Plains, Tue, Oct 12. - Flooding possible across portions of the Great Lakes. - Flooding possible across portions of the Great Lakes. - Flooding occurring or imminent across portions of the Southeast and the Tennessee Valley. - Much below normal temperatures across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Northern Plains, the Northern Rockies, the Southern Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Tue-Thu, Oct 12-Oct 14. 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