Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 PM EST Wed Dec 21 2022 Valid 12Z Sat Dec 24 2022 - 12Z Wed Dec 28 2022 ***A major winter storm exiting into eastern Canada will likely continue to cause strong winds on Saturday near the Great Lakes, along with bitterly cold temperatures for Christmas weekend*** ...Synoptic Overview... After an extremely impactful winter storm hammers the Eastern U.S. during the short range period, conditions will start to improve over the Christmas weekend as the core of the low lifts northward across Quebec and then Hudson Bay. However, it will still be quite windy across the Great Lakes region and the Northeast on Saturday with a strong pressure gradient still in place, along with some lake effect snow. The impressively cold airmass will begin modifying as surface high pressure settles across the southern tier states. Meanwhile, the initial West Coast ridge/eastern trough upper pattern will promote rain/mountain snow over the Northwest as northeastern Pacific systems brush the region. Flatting of the mean upper pattern as an eastern Pacific trough approaches/moves into the West will spread precipitation farther south and east across the West next week while continuing a warmer trend for temperatures across most of the lower 48. ...Guidance/Predictability Assessment... An operational model composite during the first half of the period followed by a transition toward a half model/half ensemble mean blend by day 7 Wednesday again provided a reasonable approach to depict the most common elements of guidance through the period while toning down the typical detail uncertainties that arose for various features. Consensus looks good for the storm eastern Canada storm during the weekend. Behind this system, there is still a general signal for a shortwave and associated surface wave Sunday-Monday, but with continued spread for specifics. Guidance has been having difficulty in resolving what happens to this shortwave as it progresses through the base of the mean trough aloft, leading to spread for the surface/moisture pattern from the Gulf of Mexico into western Atlantic as well as nearby parts of the South and East. The new 12Z GFS has come in with the slowest and most closed upper feature by Tuesday-Wednesday (versus the 06Z GFS that was actually somewhat on the fast side). On the other hand the new 12Z ECMWF is somewhat faster/less amplified than the prior run. This variability favors a blended/mean approach until clustering and continuity improve. Meanwhile there is a lot of spread for exactly what happens from the northern tier into the Great Lakes during the latter half of the period. Over time ECMWF runs have been gradually backing off with the amplitude of central Canada energy reaching the Great Lakes while latest GFS runs show minimal influence from that region and instead bring Pacific energy into the region. A conservative approach closer to the means seems best for this aspect of the forecast. Eastern Pacific into western North America shortwaves and associated surface systems during the weekend generally appear to exhibit typical detail and timing spread. Then by late Monday the 06Z and 12Z GFS runs stray farther southeast than other solutions with low pressure reaching the coast. The 12Z GFS becomes quite pronounced in how it flattens the downstream ridge over the West by early Tuesday. The larger scale trough expected to move into the West by day 7 Wednesday, along with the fairly strong Pacific surface low the prior day, have some detail/timing spread but with a reasonably consistent multi-day signal overall. ...Weather/Hazard Highlights... The departing winter storm is still likely to produce strong and potentially damaging winds from Michigan to Upstate New York through the day Saturday before gradually abating by Saturday night. Intense cold air advection over the warmer lake waters should produce bands of lake effect/enhanced snow, some of which could be locally heavy. Frigid wind chills will also be a significant hazard from the Midwest to the Ohio/Tennessee Valley, and then extending eastward to the East Coast (though not quite as severe as over locations west of the Appalachians). Expect the widespread snow from earlier in the event to produce a white Christmas for many locations. Saturday will feature widespread temperature anomalies of 20-30 degrees below average for morning lows and daytime highs across much of the central and eastern U.S. Best potential for daily records on this day will be for cold highs over the East. Sunday will still be cold over the eastern half of the country with a broad area of readings 15-25F below normal, but daily records should be fairly isolated. The theme for next week will be a pronounced warming trend as the upper pattern becomes less amplified. Moderately above normal temperatures will become established over the West (lows tending to be a little more above normal than daytime highs) and then spread into the High Plains by next Tuesday-Wednesday with some areas reaching 10-20F above normal. The eastern half of the country will continue to moderate, leading to mostly single-digit negative anomalies by next Wednesday. A series of northeastern Pacific systems should bring a multi-day period of atmospheric river events to the West Coast, leading to significant totals of rain and higher elevation snow. Some of this moisture will likely extend into the Rockies. Expect the primary focus during the weekend to be over the Pacific Northwest and far northern Rockies. There is still some uncertainty over the precise intensity and southward extent of precipitation at any particular time, though at the moment the best signal for highest amounts is over the Olympics and Washington Cascades. Then the precipitation shield will likely push southeastward during the first half of next week as a more amplified upper trough and leading wavy surface front move in. As this occurs, the heaviest rain/mountain snow should focus along the central West Coast into the Sierra Nevada while some moisture extends farther south over California and east into the Great Basin/Rockies, and meaningful precipitation persists over the Northwest. These systems may bring periods of brisk to strong winds to some areas near the West Coast. Elsewhere, a weak wave may spread an area of light snow from the northern Plains into the Midwest during Sunday-Monday. Southern tier shortwave energy and associated wavy should spread moisture across the Gulf of Mexico and western Atlantic early-mid week, with Florida possibly seeing some rainfall in the process. It remains a big question mark how much, if any, of this moisture reaches the Gulf/Southeast Coast areas though. Rausch/Hamrick 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml