Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 PM 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 Louisiana with system genesis. Meanwhile, benign weather and gradually warming temperatures are forecast for the West. ...Guidance/Predictability Assessment... Models and ensemble guidance continues to show fairly good agreement and as a result higher confidence in the overall large scale pattern through next week. Especially mid to late period though, there are some smaller scale differences particularly with shortwaves working to reinforce Eastern U.S. troughing and the specifics/frontal timing of a more defined surface low/system through the Great Lakes mid next week. There is also some uncertainty with the timing/orientation of a very slow moving southern stream elongated trough/closed low near the Southwest U.S. which has implications for rainfall amounts across south-southeast Texas much of the week. WPC progs for today used a general model blend for the first half of the period, trending towards half of the ensemble means later in the period. This maintained good continuity with the previous WPC forecast as well. ...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 with lake enhanced snow possible again on the backside. 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. At this point, rainfall amounts are not high enough to warrant even a marginal risk on the Days 4 and 5 Excessive Rainfall Outlooks, but this can continued to be monitored given anomalous moisture and modest instability. Initial upper ridging will keep the West mainly dry, but precipitation chances may gradually increase into the Pacific Northwest later next week. Santorelli/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