Extended Forecast Discussion NWS Weather Prediction Center College Park MD 210 AM EST Sat Dec 04 2021 Valid 12Z Tue Dec 07 2021 - 12Z Sat Dec 11 2021 ...Notable snow in the interior Northeast is currently expected Wednesday, with wintry impacts potentially reaching southward into the I-95 corridor and possibly the Mid-Atlantic states, 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... 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 meaningful precipitation 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... After multiple days of GFS runs had been significantly stronger with shortwave energy coming into the West Day 3/Tuesday, the 18Z and 00Z GFS have come in weaker with the feature, more in line with other guidance. So there seems to be an increasing consensus with this shortwave, but uncertainty is still somewhat high given the energy stems from the less predictable high latitudes. The uncertainty in the details of the forecast continues Wednesday onward despite reasonable consensus for the overall pattern. Differences in the handling of this shortwave energy as well as northern stream/Great Lakes energy lead to evolution and track/timing uncertainty for an area of low pressure from the eastern U.S. into the western Atlantic Wednesday-Thursday. Overall the low pressure system continued its overall trend more suppressed, but with considerable variability in its track and timing with the 12/18Z model cycle and the incoming 00Z cycle, which leads to uncertainty with precipitation amounts across the Mid-Atlantic and Northeast. Overall 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. The 12Z ECMWF appears to be the outlier with this surface low, faster/farther east compared to consensus and the previous model cycles. Then, energy is yet again expected to dig southward and amplify troughing in the West by Friday, though the degree of amplification differs among guidance, with the 12Z CMC depth a bit on the more extreme side. Model preferences led to starting with a composite of 12Z/18Z operational models for about the first half of the period. Then the forecast blend transitioned to a blend of the operational 12/18Z GFS and 12Z ECMWF and the 18Z GEFS/12Z ECMWF mean. ...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 lead to snow across the northern latitudes, with the highest probabilities of notable 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. Fairly light precipitation is forecast for the Pacific Northwest and the Four Corners states on Tuesday, before precipitation chances 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. 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 its first rainfall in over a month. Chilly temperatures are likely across the far north-central U.S. on Tuesday given snow cover from Sunday-Monday's system, with temperatures 10-20F below normal. Other than that, temperatures should be within about 10 degrees of normal in most places across the lower 48 Tuesday and Wednesday before a warmup by Thursday through Saturday from the Plains eastward, with 10-20F above average temperatures. Cooler temperatures may spread into the West late in the week (highs 5-15F or so below normal by Friday and Saturday) with the potential for upper troughing to amplify over the region. Tate/Rausch 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