Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1059 AM EST Mon Dec 21 2020 Valid 12Z Thu Dec 24 2020 - 12Z Mon Dec 28 2020 ...Threat of heavy precipitation and strong winds over the eastern U.S. late this week... ...Overview and Model Guidance Evaluation/Preferences... An amplified western ridge/eastern trough pattern aloft will cover the lower 48 late this week. The mean pattern will trend flatter by the weekend as Pacific energy flows inland. Toward the end of the period early next week a somewhat more amplified upper trough may evolve near the West Coast. These features will bring a return of precipitation to parts of the West. Meanwhile, the leading Pacific system (possibly in combination with northern stream flow) could contribute to a surface system over the central-eastern U.S. by Sunday-Monday. For the major system affecting the East, consensus has actually trended somewhat faster for frontal progression due to becoming deeper with the core of the upper trough and somewhat more rounded but continues to be slower than the GFS. Farther west, there has been good clustering for a shortwave to enter the West around the start of the weekend with some slight progression spread, with the GFS slowing down somewhat. However guidance has been more ambiguous about what happens as this energy continues downstream into Sun-Mon, with specifics of northern stream flow also becoming a question mark. Thus confidence is fairly low regarding details of central-eastern U.S. low pressure on Sunday and next Monday. Overall, the guidance is in reasonable agreement with the 06z GFS somewhat quicker with system progression, yet within the overall spread of possibilities. A compromise of the 00z UKMET, 00z ECMWF, 00z Canadian, and 06z GFS were used for the pressures, fronts, QPF, and wind grids early before becoming more 00z NAEFS/ECMWF inclusive later on. The remainder of the grids used a more even deterministic/ensemble mean mix. ...Weather/Threats Summary... The guidance and pattern evolution continue to provide decent confidence in a heavy rain threat over a broad area extending from the South into the Northeast during Thursday-Friday Christmas Eve/Christmas Day. The cold front extending south from Great Lakes/Canadian low pressure will initially draw upon Gulf moisture and then likely encourage a strengthening feed of Atlantic moisture as it pivots into the Northeast by Fri. Moderate to heavy rainfall should accompany this combined flow of moisture. Areas with a deep snow cover in the Northeast will need to monitor this event carefully due to potential flooding issues (which will depend in part on temperatures/dew points for the extent of snow melt) or sensitivity of some structures to the added weight of snow absorbing the rain. Strong wind gusts and sharp temperature changes will also be possible over the East. Expect some lake effect snow, and frontal wave development could produce one or more other bands of snowfall between the Upper Ohio Valley/Appalachians and Northeast. Consult the days 4-7 Winter Weather Outlook for the latest depiction of areas most likely to see significant snowfall. Parts of the West will see moisture returning Friday/Christmas day with episodes of rain and mountain snow continuing into next week. Activity from late this week into the weekend should focus along the northern two-thirds of the West Coast states with lesser totals extending into the northern half of the Rockies. The next Pacific system forecast to arrive early next week should provide more focus over California with highest totals over coastal and Sierra Nevada locations. Depending on system details some of the moisture may reach farther south than most prior systems so far this season. Leading energy passing through the West may contribute to a system that crosses the central/eastern U.S. by Sun-Mon. Specifics of this system and precipitation coverage/amounts are fairly uncertain at this time. Any precipitation in northern areas may be of a wintry type. Expect well above normal temperatures ahead of the front crossing the East, with highs generally reaching up to 10-20F beyond climatology on Thu. Depending on frontal timing, New England could see lows up to 20-35F above normal on Fri morning. Cold air behind the front will bring highs down to 10-20F below normal over many areas from the Mississippi Valley eastward Thu-Fri followed by steady moderation toward normal into early next week. The Plains will likely see a brief period of highs 5-15F above normal around late week into the weekend followed by a trend back toward normal. Most of the West should see readings within 5-10 degrees on either side of normal during the period. Roth/Rausch Additional 3-7 Day Hazard information can be found on the WPC medium range hazards 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