Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 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 in the mean for the rest of the period but contain moderately amplified troughs that will provide active weather over some areas. Once southeastern Canada low pressure pushes a sharp cold front through the Northeast on Fri/Christmas Day (GFS still somewhat faster than other guidance by varying degrees), the focus will turn to the West Coast as sharpening shortwave energy likely develops low pressure just off the Washington coast and Vancouver Island. For this system most guidance has been fairly well clustered over the past couple cycles aside from the farther south UKMET. The larger scale shortwave will progress through the West and into the Plains during the weekend. Guidance is showing a decent signal that this energy, along with yet-to-be resolved potential input of northern stream flow from Canada, will support an area of surface low pressure tracking across the central/eastern U.S. Sun-Mon with consolidation into an East Coast or western Atlantic storm by early Tue. At that time the latest GEFS/ECMWF means have been signaling such a system but with enough spread among ensemble members to keep the low fairly weak, while recommending a latitude between the southern ECMWF runs (through 12Z) and northern GFS. The 00Z CMC has come in with a low position close to the 12Z ECMWF mean while the 00Z GEFS mean is a bit more offshore. A preferred model/ensemble mean blend provides a low position closest to the 18Z GEFS/12Z ECMWF means while yielding intermediate depth. Significant change in the new 00Z ECMWF (farther north) highlights the uncertainty that still exists for this system. The next feature of interest will be an amplifying upper trough that should reach California by day 6 Mon and continue inland thereafter. An associated surface low should also approach or reach the California coast on Mon. This low has decent clustering as the 00Z GFS has reverted back toward its 12Z run versus the faster 18Z run. Low pressure should then head toward the central Rockies by Tue. This system may bring an episode of precipitation to parts of California and the southwestern U.S. that have not had much precip so far this season. By Mon-Tue a multi-run blend of GFS/ECMWF runs and their means looks good as over the past day respective runs have shuffled around a bit for timing around a common solution. ...Weather/Threats Summary... During Fri/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 cover areas from the northern two-thirds or so of the West Coast 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. Energy from the first western system, possibly combined with flow from Canada, will likely contribute to central U.S. to western Atlantic low pressure Sun-Tue with an associated moisture shield spreading across the eastern half of the country. System specifics are not especially clear so far, keeping confidence in the lower half of the spectrum for details of precipitation coverage and intensity. At least some precipitation in northern latitudes and higher elevations of the Appalachians may be of a wintry type. There may be significant precipitation/wind effects over parts of the East by next Tue depending on exactly how/where the system develops. The most anomalous temperature readings will be early in the period, during Fri/Christmas Day into Sat. Morning lows over New England early Fri may be 15-35F above normal 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 Fri with modest moderation on Sat (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 Fri-Sat before a cold front brings temperatures down toward normal. 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 Mon into Tue. 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