Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Wed Jan 01 2020 Valid 12Z Sat Jan 04 2020 - 12Z Wed Jan 08 2020 ...Overview/Guidance Assessment... Uncertainty regarding the track and intensity of a couple of low pressure systems across the eastern U.S. will continue to be the main concern during the medium-range period. Models general agree that the synpotic pattern across the U.S. will be dominated by a progressive west-to-east flow with a couple of shortwave troughs that could amplify into significant cyclones for the eastern half of the country. The first of such cyclone should track across the eastern U.S. this weekend. Model guidance has continued to slow down the northeastward progression of this cyclone as a deeper upper-level trough/cut-off low is being forecast over the Midwest on Saturday with each successive model run. The ECMWF, being the slowest piece of guidance, continues to slow down the forward speed of the cyclone as of the 00Z run. Meanwhile, the faster and more progressive GFS has successively slowed down the system toward the ECMWF solution. Therefore, the WPC grids for the weekend were based heavily of the 00Z ECMWF solution (70% 00Z ECMWF vs. 30% 06Z GFS) together with some of their ensemble means. The next upper trough of concern is forecast to reach the West Coast on Sunday. Models generally agree that this feature will track across the Rockies and then emerge into the Plains by next Tuesday. However, the ECMWF and the GFS disagree with the amplitude of the wave while it is still in the Pacific. The deeper ECMWF solution eventually leads to the formation of quite an intense cyclone for the eastern U.S. toward the middle of next week. However, an accelerating trend is noted on the ECMWF track for this potential storm. On the other hand, the faster GFS hardly shows any development of this wave over the eastern U.S. at all. Meanwhile, the 00Z ECMWF ensemble mean shows a faster motion of the cyclone than its deterministic run. The WPC medium-range grids used 30% ECMWF, 30% EC mean, 20% 06Z GEFS mean, and 20% 06Z GFS to handle this system. This yields a low pressure system tracking eastward across the Great Lakes to the Northeast from early to middle of next week. ...Weather Highlights/Threats... Surface low moving through the eastern U.S. on day 3 will bring mainly rain across the region ending with some wintry precipitation from the lower Great Lakes into the Appalachians and New England Saturday night into Sunday. 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. Depending on the 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 toward end of the period. Confidence remains low at this time regarding the specifics. With a fast west-to-east flow across the country, no severe outbreak of arctic air is anticipated. This will lead to temperatures averaging near to slightly above normal with relatively short periods of colder than normal temperatures behind departing storms. Kong 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