Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sun Dec 05 2021 Valid 12Z Wed Dec 08 2021 - 12Z Sun Dec 12 2021 ...Some snow is possible on Wednesday for portions of the Northeast and northern Mid-Atlantic, potentially including the I-95 corridor... ...Considerable mountain snows are likely across the Cascades, Sierra Nevada, Intermountain West, and Rockies during the latter half of next week... ...Overview... Expect the start of the medium range period on Wednesday to feature a broad mean trough across much of the lower 48, supporting an active weather pattern with embedded shortwaves pushing low pressure/frontal systems across the country. One system should bring widespread rain to the Southeast on Wednesday while parts of the Northeast/Mid-Atlantic may see snow. By Thursday-Friday amplifying energy entering the western side of the mean trough will develop a more pronounced upper trough over the West, with the trough likely to progress steadily onward into the central and eastern U.S. during the weekend. This system will bring the potential for meaningful precipitation and heavy mountain snows over the West and then organized precipitation spreading into the central and eastern U.S. by late week through the weekend along and ahead of a strong cold front. Guidance is advertising a change in the pattern around the end of the period and beyond with the next trough amplifying over the eastern Pacific. ...Guidance/Predictability Assessment... During the first half of the period, the overall large scale pattern remains agreeable among model guidance but embedded shortwave uncertainties continue to lower confidence in some important details. The most notable example continues to be with a shortwave forecast to be over the east-central U.S. early Wednesday and northern New England/western Atlantic by early Thursday. Over multiple runs subtle low-predictability detail differences have led 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. Within the broad spread of guidance, recent GFS runs have been among the slowest and deepest with the midlevel shortwave, leading to surface low tracks farther west closer to the Northeast coast compared to consensus. The 00Z/06Z runs were still on the western side of the spread in spite of some eastward trend recently, while ensemble trends over the past day show a dramatic trimming of members on the western side of the envelope. The new 12Z GFS has continued its recent trend of gradual suppression of the track and moisture shield. The GFS and overall guidance trend toward a more suppressed/offshore low track is leading to less precipitation forecast to fall over the northeastern quadrant of the CONUS overall, though a corresponding colder trend would support a farther south rain-snow line. Upstream energy reaching the Northwest and Southwest Wednesday and moving quickly east thereafter, along with the associated surface reflection, have also exhibited some detail differences but with less influence on sensible weather impacts. For this part of the forecast the preference continues to be with an intermediate solution derived from a 00Z/06Z model composite. The general consensus remains fairly good for the upper trough amplifying into the West by late in the week and continuing eastward thereafter, but with lingering spread for specifics of how energy will be distributed within the trough and frontal wave details. Trends over the past couple days have generally been somewhat faster but the new 12Z ECMWF has reversed that trend a bit. Meanwhile CMC runs and some ensemble members have been trying to pull off some energy into a slower southern stream feature, with 00Z CMC runs tending to be more extreme versus other models/means and the 12Z runs adjusting eastward closer to other guidance. Behind this system, essentially two clusters develop for the upper trough expected to amplify over the eastern Pacific--the somewhat faster and more amplified GFS/GEFS mean versus slower/broader ECMWF/CMC and their ensemble means. Differences for these two major features are generally within the realm of guidance error for the late extended time frame (though understood to be very important for the character of precipitation over the Pacific Northwest next weekend), leading to transitioning the forecast toward a blend of 00Z/06Z GFS and 06Z GEFS runs along with the 00Z ECMWF/ECMWF mean. ...Weather/Hazards Highlights... Around midweek wavy fronts over the Southeast and low development into the western Atlantic will produce rainfall over the Southeast and spread some moisture into cooler air farther north where precipitation may fall as snow. Models are still having difficulty in resolving important subtle details that will determine precipitation coverage, type, and amounts. Recent trends for a more more suppressed surface low track are bringing the potential rain-snow line gradually southward over the Mid-Atlantic but also lowering the amount of snow that may fall to the north over New England. Overall the probabilities for significant amounts of snow have declined in the past day. As shortwave energy and upper 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 over the eastern half of the country and possible winter weather impacts from the Plains through the Midwest/Great Lakes. Strong flow ahead of a developing eastern Pacific upper trough may bring an atmospheric river event into the Pacific Northwest starting around Friday night or Saturday and continuing through the weekend. There is still a fair degree of uncertainty over the progression/southward extent of moisture but this event requires monitoring due to the potential for some areas to see heavy precipitation. Temperatures Wednesday will be about 10-15F above average for the High Plains, and expect temperatures to trend even warmer across the central and eventually eastern U.S. as the pattern amplifies. By Friday temperatures in the Southern Plains and the Lower/Middle Mississippi Valley should reach 15-25F above normal, with highs nearing or exceeding 80F in parts of Texas into the Lower Mississippi Valley, which could approach or reach daily records. After a chilly start Wednesday, the eastern U.S. will see a pronounced warming trend as highs 15-25F and lows 15-30F above normal reach into the eastern U.S. for the weekend. Behind the front, the West should see a couple days of highs 5-15F below normal late in the week but this cool air will modify closer to normal by the time it reaches the Plains. The upcoming change in the pattern toward the end of the period should return temperatures to 10-20F above normal across the northern two-thirds of the Plains by next Sunday. 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