Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Fri Dec 03 2021 Valid 12Z Mon Dec 06 2021 - 12Z Fri Dec 10 2021 ...Overview... Expect a broad mean trough aloft to prevail 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 somewhat more active weather pattern than has been the case lately. In particular, parts of the West should see a couple episodes of meaningful precipitation while the intensity/coverage of rainfall should increase over the southeastern quadrant of the country and some snow may fall over northern parts of the East. ...Guidance/Predictability Assessment... Guidance continues to show unusually great spread for the compact system forecast to approach the Pacific Northwest by early Monday. GFS/ECMWF runs and their means have converged for track/timing but GFS runs have been on the deep side of the spread (even with the upper shortwave becoming less closed in the 12Z run) while the latest UKMET/CMC are slower and weaker/elongated due to a weaker shortwave. The past couple GEFS runs show even more spread among individual members, with the weak and strong extremes at least 20-30 mb apart. The relatively small scale of the shortwave keeps predictability unusually low for a day 3 forecast, GFS/ECMWF timing and ECMWF depth looking like the best intermediate approach at this time. On the other hand the models and ensembles are more agreeable for for the strong system tracking into eastern Canada and the trailing front that crosses the eastern U.S., albeit with some ongoing nudges for details. In spite of the issues with shortwave energy initially heading into the Northwest, solutions have trended at least a closer together than in previous days for what happens downstream. Consensus now shows enough amplification into the West to support fairly quick ejection of the leading upper low over Baja California as of early Monday but then decent progression thereafter. However meaningful differences still arise over the East, leading to evolution and track/timing uncertainty for an area of low pressure from the eastern U.S. into the western Atlantic Wednesday-Thursday. One multi-day trend of note is that the forecast surface pattern over the East valid early Wednesday has generally been trending more suppressed (higher pressures). There is fairly good agreement on another bundle of Pacific energy coming into North America Wednesday, with less amplification than its predecessor, producing southern Canada low pressure that anchors a front reaching into the West/Plains/Great Lakes. A trailing feature should amplify more into the West by late in the week with a model/mean blend generally looking good for this feature though the 12Z CMC depth is a bit on the more extreme side. Model preferences led to starting with a composite of 00Z/06Z operational models for about the first half of the period. Then the forecast blend transitioned to a blend operational guidance and the 06Z GEFS/00Z ECMWF mean. ...Weather/Hazards Highlights... A relatively strong surface low pressure system tracking away from the Great Lakes on Monday should support snow over the region along with a broader area of strong winds over the northeastern quadrant of the nation in the tight pressure gradient behind the low. Conditions will be favorable for lake effect snow behind the system through Tuesday. Additionally, expect rain to fall along the trailing cold front associated with the low. Best potential for meaningful totals should extend from the Ohio Valley to south-central U.S. but amounts are likely to be tapering down somewhat compared to Sunday-Sunday night. However, Gulf moisture should increase again by Tuesday into Wednesday 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 could lead to snow across the northern latitudes, with relatively higher probabilities from the Great Lakes into northern New England. 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. Some rain could continue near a wavy front lingering over the extreme Southeast. A couple of shortwaves moving into/across the West will bring increasing precipitation chances there, with favored windward terrain seeing the highest amounts. The Pacific Northwest into much of the Intermountain West and Rockies should see precipitation Monday and clearing out by Tuesday while chances ramp up in the Southwest. Some locations in the Northwest may see fairly low snow levels with the start of the Monday event. Then the next round is likely for the Northwest on Wednesday and spreads into California eastward by Thursday as energy aloft drops southward. 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. Chilly temperatures are likely across the far north-central U.S. on Monday and Tuesday given snow cover from Sunday's system, with temperatures 10-20F below normal. Meanwhile the Eastern Seaboard can expect temperatures about 10-20F above normal on Monday ahead of a cold front that will bring temperatures down to near average by Tuesday. Temperatures should be within about 10 degrees of normal in most places across the lower 48 Tuesday and Wednesday, before a warmup by Thursday and Friday from the Plains eastward, with 10-15F above average temperatures. Cooler temperatures may spread into the West late in the week (highs 5-10F or so below normal by Friday) with the potential for upper troughing to amplify over the region. 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