Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Wed Jan 01 2020 Valid 12Z Sat Jan 04 2020 - 12Z Wed Jan 08 2020 ...Overview/Guidance Assessment... The pattern across the CONUS during the medium range period (Sat-Wed) looks to be progressive with an active stream of shortwave troughs traversing the country. An amplified upper trough initially in place across the Ohio/Tennessee Valleys on Saturday will move off the East Coast by Sunday, resulting in a deepening low pressure system across the Great Lakes and Northeast. Minor timing differences with this trough (or closed low), with the GFS/CMC a hair faster than the ECMWF/UKMET days 3 and 4 and the ensemble means in the middle. To the West, a series of systems moving onshore and tracking eastward through time continues to show increasing spread in the guidance, especially toward the end of the forecast period. The first shortwave enters the West coast day 4, with another following quickly on day 5. It's with this second one where models continue to struggle with evolution as it ejects eastward out of the Rockies and into the Central U.S. day 5 and beyond. The ECMWF has been quite consistent in showing a much more amplified trough especially days 6 and 7, which would bring a rapidly deepening surface low from the south-central Plains on Day 5, into the Northeast on day 7. The CMC shows a similar evolution, with the GFS quite a bit flatter/weaker. There is not a ton of ensemble support yet for that strong of a low as depicted by the ECMWF, but given the good run-to-run consistency from the ECMWF, and the pattern set-up, it is not completely unreasonable. Prefer a blend of the ensemble means at this point, with small contributions from the ECMWF for some added definition from the means. A third shortwave moves into the West on day 7, but there is high spread with timing and strength of this low, that the ensemble means were heavily favored. ...Weather Highlights/Threats... Surface low moving through the Northeast on day 3, will bring precipitation from the Lower Great Lakes into New England, possibly in the form of snow. Nearly daily bouts of precipitation are likely for the Northwest into the Northern Rockies during the period, focusing more over coastal locations and the Coastal Range/Cascades. Depening on evolution of the trough/surface low development across the Central U.S. day 5-7, some snow may be possible north and west of the low across parts of the Ohio Valley to the Northeast end of the period. ECMWF and CMC show a much better chance for this, though confidence is low at this time. Above normal temperatures across the Northeast on Saturday should moderate to near normal following passage of the cold front. Above normal temps shift across the Great Plains towards the Midwest days 3-5, with much of the country near normal by days 6 and 7. Santorelli Additional 3-7 Day Hazards 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