Extended Forecast Discussion...amended NWS Weather Prediction Center College Park MD 228 AM EST Thu Dec 22 2022 Valid 12Z Sun Dec 25 2022 - 12Z Thu Dec 29 2022 ***Unsettled conditions for the West Coast with heavy rain and heavy mountain snow with an atmospheric river event early next week*** ...Synoptic Overview... The overall weather pattern improves going into Christmas weekend across the eastern half of the nation as the core of the intense low lifts farther northward across central Canada and weakens with time. However, there will still be some lingering lake effect snow, mainly downwind of Lakes Erie and Ontario. The impressively cold airmass will begin modifying as the surface high settles across the southern tier states. Looking ahead to early next week, an Alberta clipper system tracks across the central/northern Plains, and a southern stream disturbance tracks eastward near the Gulf Coast. Multiple shortwave passages near the Pacific Northwest will keep things unsettled from northern California to Washington state with an atmospheric river event likely during the Monday-Tuesday night time period. ...Guidance/Predictability Assessment... The latest 00Z model guidance suite indicates very good synoptic scale agreement on Sunday on the overall upper level pattern that features a West Coast ridge and a gradually weakening eastern U.S. trough. However, model differences increase as early as Monday with the shortwave dropping south across the Plains, with the GFS indicating a much faster solution, and the GFS shows a stronger upper trough in the vicinity of the southeast coast by early Tuesday, whereas the ensemble means farther inland and weaker, and the ECMWF is on the slower side of the guidance. There is still not much phasing between this and the northern stream shortwave, so this should limit the extent of any surface cyclogenesis. Out West, model agreement has improved across the eastern Pacific. The larger scale trough expected to move into the West by day 6 Wednesday, along with the fairly strong Pacific surface low earlier, have some detail/timing spread but with a reasonably consistent signal overall. The WPC fronts/pressures forecast relied mainly on a ECMWF/CMC/WPC continuity blend along with a lesser percentage of the GFS through day 4, and then increased use of the ensemble means going forward into the middle of the week. ...Weather/Hazard Highlights... 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. Sunday will still be quite cold over the eastern half of the country with a broad area of readings 15-25 degrees below normal. The theme for next week will be a steady 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 eastward to the Rockies. The primary focus during the weekend is forecast 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 the week as a more amplified upper trough and leading 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 noteworthy precipitation persists over the Northwest. These systems may bring periods of high winds to some areas near the West Coast. Elsewhere, a weak Alberta clipper may bring an area of light snow from the northern Plains into the Midwest during Sunday-Monday. Gulf Coast shortwave energy and associated stationary front should spread some moisture across the Gulf of Mexico and western Atlantic early-mid week, with Florida possibly seeing some rainfall during that time. It remains uncertain about how much, if any, of this moisture reaches the Gulf/Southeast Coast areas though. 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