Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Dec 04 2021 Valid 12Z Tue Dec 07 2021 - 12Z Sat Dec 11 2021 ...Notable snow possible in the interior Northeast around Wednesday, with wintry impacts potentially reaching southward into the I-95 corridor/northern Mid-Atlantic, while locally heavy rain may occur over the Southeast... ...Considerable mountain snows are likely across the Cascades, Sierra Nevada, Intermountain West, and Rockies during the latter half of next week... ...Overview... Latest guidance maintains a broad mean trough aloft across much of the lower 48 next week, with an axis generally extending from central/east-central Canada into the western U.S. Embedded shortwaves will push low pressure/frontal systems across the country, leading to a more active weather pattern than has been the case lately. In particular, parts of the West should see meaningful precipitation--possibly extending into parts of the Plains late in the week--while the intensity/coverage of rainfall should increase over the southeastern quadrant of the country and some impactful snow may fall over northern parts of the East. ...Guidance/Predictability Assessment... Embedded shortwave uncertainties continue to lower confidence in some important details within a much more agreeable mean pattern. In terms of the combined guidance spread and associated impacts of the varied solutions, one of the greatest issues in latest cycles has been with the ultimate evolution of shortwave energy that progresses from the northern Plains/western U.S. as of early Tuesday into the East by late Wednesday/early Thursday. There is a clear signal for the shortwave's existence but subtle low-predictability differences lead to very different surface solutions for low pressure that tracks off the East Coast along with associated coverage/intensity of precipitation and location of the rain-snow line. Latest GFS runs have been on the strong side of the spread with the shortwave and on the deep/west side at the surface by early Thursday while the 00Z ECMWF/ECMWF mean are on the weak/eastern side. Low confidence with fine details for this feature would favor an intermediate solution for strength and track, closest to a deeper version of the 06Z GEFS mean. One upstream uncertainty involves a bundle of energy that originally comes from an upper low over the mid-lower latitudes of the Pacific, with differences over whether it drops around the east/southeast side of the Pacific upper ridge (ECMWF/UKMET/CMC) or progresses along within the lower 48 mean trough (GFS). There is also some spread for details of northern stream Pacific energy that should reach western North America by Wednesday and support southern Canada low pressure with trailing front that crosses the western/central U.S. At least relative to typical behavior for 6-7 days out in time, models and ensemble means are generally better behaved than average for the upper trough that should dig into the West late in the week and then push into the Plains. However individual runs/ensemble members do still vary considerably for exactly how energy will be distributed within the overall trough and by next Saturday for details of corresponding low pressure along a mean front within a corridor between the Great Lakes and southern Plains. Guidance comparisons/preferences led to starting the forecast with a 00Z/06Z operational model composite for days 3-4 Tuesday-Wednesday, followed by somewhat earlier input of 06Z GEFS/00Z ECMWF means than average by Thursday (30 percent total) and then 50-60 percent means for Friday-Saturday. ...Weather/Hazards Highlights... Gulf moisture should increase by Tuesday into Wednesday ahead of frontal systems and cause moderate to possibly locally heavy rainfall from the Lower Mississippi Valley into the Tennessee Valley and Southeast. Moisture spreading farther north into cooler air should produce snow across the northern latitudes, with the highest probabilities of meaningful snow across the Northeast. There is still considerable uncertainty over how low pressure from the eastern U.S. into the western Atlantic will evolve/track, so confidence is fairly low for exactly how much precipitation may fall over the Northeast, and how far south significant snow reaches. Some rain could continue late next week near a wavy front lingering over the extreme Southeast. Expect fairly light precipitation over the Pacific Northwest and the Four Corners states on Tuesday, before precipitation chances and amounts increase from midweek onward. As shortwave energy and troughing drop southward through the West, precipitation will spread from the Pacific Northwest and Northern Rockies Wednesday southward to California into the Great Basin to Central Rockies Thursday and Friday. Favored terrain in Arizona could also see some enhanced activity. Generally expect higher elevation snow and lower elevation rain, with snow levels likely to decline later in the week as upper troughing amplifies over the West. Parts of Southern California may see the first rainfall in over a month. Then as the western upper trough approaches/pushes into the Plains toward the end of the week, rainfall may become moderate to heavy over parts of the east-central U.S. by next Saturday with the aid of strengthening Gulf inflow ahead of a wavy front. The character of one or more frontal waves will determine precipitation amounts and possible winter weather impacts over the Plains and Midwest. Chilly temperatures are likely across the far north-central U.S. into the Great Lakes on Tuesday given snow cover from Sunday-Monday's system, with temperatures 10-20F below normal. Highs of 10F or so below normal could reach as far south as the Ohio Valley. The negative anomalies will moderate as the airmass continues into the Northeast U.S. on Wednesday. Farther upstream, the West will see mostly above normal readings through midweek (some pockets of plus 10-15F anomalies) with more pronounced warming likely over the Plains on Thursday and progressing into the East by next weekend. There should be fairly broad coverage of plus 10-20F anomalies for highs/morning lows, and in addition highs over parts of the southern Plains on Friday plus morning lows from the Mississippi Valley northeastward on Saturday may reach 20-25F above normal. A few places in the far southern tier (primarily Texas) could approach daily record highs. Cooler temperatures should spread into the West and eventually High Plains late in the week (highs 5-15F or so below normal by Friday and Saturday) under an amplifying upper trough. Rausch/Tate 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