Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Fri Dec 31 2021 Valid 12Z Mon Jan 03 2022 - 12Z Fri Jan 07 2022 ...Multi-day coastal heavy rain and mountain focusing heavy snow threat for the Pacific Northwest and northern California to the northern Great Basin/Rockies next week... ...Arctic blast to focus over the north-central U.S. Tuesday-Friday with snow likely for the chilled Great Lakes states through the interior Northeast... ...Guidance/Predictability Assessment... The WPC medium range product suite was mainly derived from a composite of best clustered guidance from the 00 UTC ECMWF/UKMET/Canadian days 3/4 (Monday-Tuesday) along with the 13 UTC NBM in a pattern with overall above normal predictability. Model differences near the surface become increasingly problematic Wednesday-Friday despite a generally compatible larger scale pattern evolution aloft. This is not unusual during the cold season in periods with stark baroclinic zones and phasing uncertainties. The WPC medium range product suite in this period was accordingly derived from a composite of better clustered guidance from the 06 UTC GEFS mean and 00 UTC NAEFS mean that have a cooler Arctic airmass over time than the 00 UTC ECMWF ensemble mean. Some added input from the best system compatible 00 UTC ECMWF and additional manual adjustments added some detail consistent with the upper flow, pattern recognition and continuity. ...Weather Pattern and Weather/Hazards Highlights.. A cold Canadian surface high will settle far south across much of the central/eastern U.S. in the wake of the strong cold front to exit the East Coast by Monday whose coastal low lifts offshore the Mid-Atlantic. Surface cyclogenesis ensues over the north-central U.S. Tuesday/Wednesday in response to an amplifying upper trough, with yet another surge of frigid air in it's wake. Trailing frontal low genesis offers some moderate trailing frontal focusing rain potential along with a heavy snow threat spreading mainly from the Great Lake states to interior New England next Thursday/Friday with windy/wrapped flow and cold air over the lakes enhancement. Protracted periods of heavy coastal rains and heavy mountain focusing snows will develop from the Northwest and northern California into the northern Great Basin through early to later next week as energies from a multi-day Pacific storm system press inland. Unlike recent events, latest guidance trends suggest this storm episode is not expected to produce heavy precipitation for central/southern California. A multi-day heavy snow threat is also likely for the north-central Rockies/High Plains as further enhanced by a favorable post-frontal upslope Arctic airmass fetch with temperatures as cold as 20-40 degrees below normal over the northern Plains along with dangerous wind chills. Hamrick/Schichtel 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, 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