Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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... The medium range period begins Wednesday with a broad mean trough across much of the lower 48, with embedded shortwaves pushing low pressure/frontal systems across the country, leading to a continuing active weather pattern. Expect one system to spread widespread rain to the Southeast on Wednesday while northeastern parts of the U.S. may see snow. As energy drops southward through the western side of the broad trough Thursday into Friday, troughing in the West will amplify and move into the central U.S. by the weekend. This will bring the potential for meaningful precipitation and heavy mountain snows in the West, with precipitation spreading into the central and eastern U.S. by late in the week along and ahead of a strong cold front. ...Guidance/Predictability Assessment... While the overall large scale pattern remains agreeable among model guidance, embedded shortwave uncertainties continue to lower confidence in some important details. One notable example is with shortwave energy located near the north-central U.S. early Wednesday and its depth and evolution as it tracks eastward late Wednesday into 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. GFS runs continue to be among the slowest and deepest with the midlevel shortwave and thus the surface low tracks farther west closer to the Northeast coast compared to consensus. This was particularly true with the 12Z run but the 18Z and 00Z runs are somewhat west as well. The overall trend (including the new 00Z ECMWF) has been for a more suppressed solution with a low track flatter and farther offshore, meaning less precipitation indicated for the northeastern quadrant of the CONUS overall, though also a colder trend, so precipitation could be more likely to fall as snow. Favored an intermediate solution with some component of the 18Z GFS but combined with the 12Z ECMWF/CMC/UKMET for the early part of the medium range period. Other southern stream shortwave differences in the Southwest/eastern Pacific early in the forecast period fortunately have less sensible weather impacts. Considering the high latitude/low predictability origins of shortwave energy dropping southward serving to amplify the trough in the West, model guidance is pretty agreeable with the overall idea of the trough deepening on Friday and pushing eastward through the weekend. However individual runs/ensemble members do still vary considerably for exactly how energy will be distributed within the overall trough, and while there is fairly good consensus for the position of a cold front ahead of the trough, positions of low pressure waves along the front vary within a corridor between the Great Lakes and Southern Plains on Saturday and in the eastern U.S. by Sunday. As the period progressed, transitioned to a 60% deterministic and 40% ensemble mean (EC and GEFS) blend to temper individual model differences in the forecast. ...Weather/Hazards Highlights... Gulf moisture in place by Wednesday ahead of frontal systems will lead to widespread rainfall across the Southeast. Showers are forecast to linger there through the latter part of the workweek as a wavy front slowly lifts northward. Moisture spreading farther north into cooler air may produce snow across the northern latitudes on Wednesday, with the highest probabilities of snow across parts of the Northeast. There is still considerable uncertainty regarding 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 the extent of notable snow. 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. Temperatures Wednesday will be about 10-15F above average for the High Plains, and expect temperatures to only warm from there 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 are forecast to be 15-25F above normal, with highs nearing or exceeding 80F in parts of Texas, which could approach records. Mild temperatures with highs 15-25F and lows 15-30F above normal will spread into the eastern U.S. for the weekend, while central U.S. temperatures should moderate back to normal behind the potent cold front. Meanwhile, cooler than average temperatures by around 10-15F are forecast for parts of the West particularly on Friday and Saturday given the amplifying upper trough. 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