Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Fri Nov 29 2024 Valid 12Z Mon Dec 02 2024 - 12Z Fri Dec 06 2024 ...Central/East Arctic cold may moderate then renew next week... ...Significant Great Lake effect snows continue into next week... ...Overview... A large scale upper trough is expected to encompass much of the central to eastern United States next week, with only a very gradual shift east and weakening over time. This pattern with west/northwest winds will be favorable for continued long duration lake effect focusing snows. Some of the snowfall accumulations could be significant in favored to the lee locations. The periodically reinforced trough aloft and cold surface high pressure originally stemming from the Arctic will cause continued colder than average temperatures migrating far southward across central and eastern parts of the country, albeit likely to offer some moderation over time as return inflow from the Gulf of Mexico also brings some enhanced rainfall chances to Texas and Lousiana with system genesis. Meanwhile, benign weather and gradually warming temperatures are forecast for the West. ...Guidance/Predictability Assessment... Model and ensemble forecast spread and uncertainty seem lower than normal for the period from Monday through Wednesday, bolstering confidence. A favored composite of best clustered guidance from the 18 UTC GFS and 12 UTC ECMWF/Canadian along with the 01 UTC National Blend of Models (NBM) provides a solid basis that enjoys great ensemble support and continuity. Forecast spread gradually increases into later next week with the handling of southern stream upper trough/low progression inland as well as with embedded system details and resulting main upper trough amplitude variances over the east-central states. Preferred a composite of compatible GEFS/ECMWF/Canadian ensemble means and the NBM for max product continuity on the amplified side of solutions given upstream flow. Latest 00 UTC models have favorably trended more in line. ...Weather/Hazards Highlights... It generally remains the case that a pronounced upper trough and a cold surface high will remain in place into next week and produce some continued cold weather that could be dangerous in the northern Plains. The cold is forecast to gradually moderate compared to the short range period, but remain below normal by 15-20F Monday in the northern Plains as lows will be in the negative single digits with highs in the low 10s. Meanwhile the cold air mass will spread farther southeast in moderated form, across the Mississippi Valley, Ohio/Tennessee Valleys, and into the Appalachians and Eastern Seaboard. A handful of record low max and min temperatures are possible across the Ohio Valley into the Appalachians, where temperatures are forecast to be around 15-20F below average and stay below freezing for highs in many areas Monday-Tuesday. By midweek, the cold should be gradually lessening in scope and magnitude, becoming more limited to the East Wednesday, but additional cold air may surge in from Canada into later next week. Meanwhile above average temperatures are forecast to spread across the West and reach the Plains into mid-later next week. A favorable pattern for Great Lake effect snow will persist into next week. Exact snow axes and local intensities will depend on reinforcing system passages with wind directions that could vary from northwest to west day to day, but some areas downwind of the lakes could see significant accumulations, likely causing travel disruptions on I-90 between Cleveland and Buffalo and I-81 north of Syracuse. Later, shortwaves and frontal systems may come through the Great Lakes region for some additional light snow that could cover a broader area. Much of the rest of the country can expect mainly dry conditions next week. Return flow of moisture could start to come into the western Gulf Coast region by early next week to fuel some enhanced rainfall, which looks to be slow to spread across the Lower Mississippi Valley and vicinity into mid-later next week. Initial upper ridging will keep the West mainlty dry, but precipitation chances may gradually increase into the Pacific Northwest later next week, but associated Pacific system approach trended slower. 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 forecast (QPF), excessive rainfall outlook (ERO), winter weather outlook (WWO) probabilities, heat indices, and Key Messages can be accessed from: 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml https://www.wpc.ncep.noaa.gov/#page=ovw