Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 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 upper trough will support a vigorous surface evolution with low pressure over northern latitudes and a trailing strong cold front pivoting eastward then northeastward over the eastern U.S. as the trough aloft takes on a neutral or slightly negative tilt. This system will likely generate areas of warm sector heavy rain and possibly embedded thunderstorms while lake effect/cold sector snow will be possible in its wake. A period of strong winds will be possible as well. 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 energy (possibly in combination with northern stream flow) could contribute to a surface system of currently uncertain character over the central-eastern U.S. by Sun-Mon. As has been the case in recent cycles, the GFS and in some places the GEFS mean were on the faster side of the full 12Z/18Z guidance spread for multiple features during the Thu-Mon forecast period. Thus the updated forecast continued to tilt more toward the ECMWF/UKMET/CMC late week into early weekend and then incorporated more ECMWF mean relative to the GEFS (while still maintaining some operational model input) into Mon as detail uncertainties increase. 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. As an example of one trend that does argue for at least modest inclusion of some GFS/GEFS ideas, for the upper trough forecast valid early day 4 Fri/Christmas the 12Z Dec 20 ECMWF mean actually trended close to what the 12Z Dec 19 GEFS mean had forecast. There is continued uncertainty over shortwave specifics that may influence wave development, currently most likely to occur over the Mid-Atlantic/Northeast. The potential for convection along portions of the cold front is another wild card. Farther west the new 00Z GFS appears to have adjusted closer to the slower majority for leading height falls reaching the Northwest late this week. There has been good clustering for a shortwave to enter the West around the start of the weekend. 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. Lack of definition in the means reflects the uncertainty. At the very least GFS runs through 18Z seemed 12+ hours fast while the new 00Z run has trended slower--but with stronger development than other solutions. Finally, a number of recent GFS runs have also been on the fast side with the upper trough forecast to approach California by early Mon. The 18Z/00Z GEFS mean runs settle in with the majority cluster for this feature. ...Weather/Threats Summary... Some timing and detail uncertainties persist but 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 Thu-Fri 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 Fri/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 precip 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. 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