Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1039 AM EST Tue Dec 22 2020 Valid 12Z Fri Dec 25 2020 - 12Z Tue Dec 29 2020 ...Threat of heavy precipitation and strong winds over parts of the eastern U.S. to extend into Christmas Day... ...Overview and Model Guidance Evaluation/Preferences... Amplified upper flow late this week, featuring a western ridge and eastern trough, will trend flatter for the rest of the period but contain moderately amplified troughs that will provide active weather over some areas. The guidance is in excellent agreement today, so a compromise of the 06z GFS, 00z Canadian, 00z UKMET, and the 00z ECMWF was utilized with some contribution from the 00z NAEFS/ECMWF ensemble means. ...Weather/Threats Summary... During Friday/Christmas Day the primary focus for heavy rainfall will be over New England ahead of the strong cold front pushing northeastward across the region, with a feed of Atlantic moisture likely to enhance amounts. Expect brisk to strong winds both ahead of the front as well as back in the cold sector, with lake effect snow late this week and into the early weekend. The first event to affect the West should bring highest rain/mountain snow totals to northern California and the southwest corner of Oregon late this week with some precipitation extending farther inland during the weekend. The broader moisture shield should extend inland into the northern half of the Rockies. The late weekend/early next week system will focus more of its precipitation over California including southern areas where flow aloft may be favorable for terrain enhancement. Precipitation should continue to spread across the Great Basin/Southwest and into the Rockies and possibly the Plains early next week. Some areas likely to be affected by this system have been well below normal for precip thus far this season, so precipitation would be welcome. Energy from the first western system, possibly combined with flow from Canada, will likely contribute to central U.S. to western Atlantic low pressure Sunday into next Tuesday with an associated moisture shield spreading across the eastern half of the country. At least some precipitation in northern latitudes and higher elevations of the Appalachians may be of a wintry type, particularly in NY state. There may be significant precipitation/wind effects over parts of the East by next Tuesday depending on exactly how/where the possible nor'easter develops. The most anomalous temperature readings will be early in the period, during Friday/Christmas Day into Saturday/Boxing Day. Morning lows over New England early Friday may be 20-35F warmer than average ahead of the front heading into the region. Behind the front, a fairly broad area of minus 10-20F anomalies for highs will cover the East on Friday as temperatures likely fall during the day with modest moderation on Saturday (except for Florida which could still see highs up to 15F below normal), followed by readings closer to normal. At the same time the Plains will see highs 10-20F above normal Friday-Saturday before a cold front brings temperatures down towards late December averages. Over the West, expect temperatures to be near normal over central/northern areas while the Southwest will be moderately above normal until clouds and precip bring a cooling trend Monday into Tuesday. 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