Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Dec 01 2021 Valid 12Z Sat Dec 04 2021 - 12Z Wed Dec 08 2021 ...Overview... Quasi-zonal upper flow expected to be in place to begin the weekend across the northern tier states should amplify into a broad mean trough thereafter as an upper ridge strengthens over the eastern Pacific. Within this pattern a weak leading shortwave/surface system will reach the East Coast early in the weekend, followed by two more significant storm systems--one tracking from the northern Plains into southeastern Canada/Northeast U.S. from the weekend through Monday and the second tracking from the West through the Plains and into the Ohio Valley/Mid-South Monday-Wednesday. ...Guidance/Predictability Assessment... The relatively greater proportion of uncertainty in the forecast still seems to emanate from question marks within multiple streams of an upper trough over the east-central Pacific. Leading energy reaching the Northwest by early Saturday and the associated surface reflection have trended closer together in the past 24 hours, though the new 12Z GFS run is already a bit more amplified than consensus at that time. Then as this energy amplifies downstream to produce a Plains into eastern U.S./Canada low and frontal system, recent ECMWF/CMC trends have been more in the direction of, though not completely to, the deep and northwest GFS. Over the past day the UKMET has been very inconsistent with the 00Z run quite suppressed for a time versus 12Z runs that have been more like some GFS runs. The character of the next shortwave nearing the Northwest early next week will depend on how much if any interaction there may be between northern stream energy just south of Alaska and a separate upper low farther south, with no clear consensus evident yet for exactly what will happen. Detail differences at this "decision point" ultimately lead to meaningful amplitude and timing differences as the energy drops into the broad mean trough over the lower 48. Additional weaker energy could drop down behind this shortwave as well. On the positive side the more extreme solutions from some earlier model runs have moderated somewhat, leading to somewhat better comparisons among the models and means on average by days 6-7 Tuesday-Wednesday. This tendency has also improved consensus toward longer persistence of the upper low over or near Baja California. The first half of the forecast incorporated the 00Z/06Z GFS and 00Z ECMWF/CMC, with the UKMET excluded due to poor comparison to other guidance for the system reaching the Plains during the weekend. A blend of the aforementioned models along with the 06Z GEFS/00Z ECMWF means represented the pattern well while accounting for lower confidence detail specifics after early Monday. ...Weather/Hazards Highlights... A weak leading system may brush parts of the upper Great Lakes and Northeast with light precipitation (mostly snow) on Saturday. Solutions are gradually coming closer together for the next stronger system from the weekend into Monday, with some potential for a band of meaningful snow from parts of North Dakota into the upper Great Lakes. Precipitation type and intensity over the eastern Great lakes and New England will depend on the exact low track. Moisture increasing ahead of the cold front should yield scattered showers and a few thunderstorms from the Deep South to the Ohio Valley over the weekend. There could be some locally moderate to heavy rain embedded within this area but with fairly low confidence in exactly where at this time. The shortwave and cold front forecast to reach the Pacific Northwest by Sunday night and through Monday should produce a period of moderate to locally heavy rain and mountain snows over western Washington and Oregon, with some of this moisture reaching the northern Rockies by Tuesday. Heavy snow will be possible for the higher elevations of northern Idaho, western Montana, and western Wyoming. Some moisture will extend farther south over favored terrain in the Great Basin and central Rockies. By next Tuesday-Wednesday, Plains into eastern U.S. low pressure and associated fronts are likely to generate another episode of showers and some storms with highest totals between the Ohio Valley and Lower Mississippi Valley. At least some precipitation should be in the form of snow to the north of this rain area. Expect temperatures to be above normal for early December across most of the nation through early next week. The greatest positive anomalies should be over the north-central High Plains and vicinity on Saturday with highs 15-20F or so above average, followed by another episode of similar anomalies over parts of Texas and Oklahoma on Tuesday ahead of the next cold front. On the other end of the spectrum, cold weather will likely continue across the Northeast U.S. with highs running up to 10 degrees below normal through the weekend with a cold upper level trough overhead. Behind the Sunday-Monday storm system and north of the next one, areas along the extreme northern tier from the Plains into New England may see moderately below normal highs from Monday into Wednesday. No major arctic airmass intrusions are expected during this forecast period though. Rausch/Hamrick 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