Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sat Jan 26 2019 Valid 12Z Tue Jan 29 2019 - 12Z Sat Feb 2 2019 ***Extremely cold weather expected across the north-central U.S. next week*** ...Overview... The overall flow pattern across the continental U.S. and Canada will remain highly amplified going through Thursday with a large scale ridge situated just off the West Coast and a pronounced upper level trough encompassing much of the central and eastern U.S. A large and cold upper low, which is basically a lobe of the polar vortex, will reinforce the trough through Thursday, and after this feature lifts out across southeast Canada, the pattern should not be quite as amplified with the West Coast ridge moving inland and a broad trough becoming established over the eastern Pacific. ...Guidance Evaluation/Preferences... A clipper system crossing the Great Lakes region will be most significant low pressure system during the beginning of the medium range period, with models in good agreement regarding a powerful cold front and perhaps a secondary low forming along the boundary near the Mid-Atlantic region. There was good deterministic model agreement on Tuesday and continuing into Wednesday, with the main difference being that the GFS/GEFS mean are faster with the progression of the vortex across the Great Lakes region, and the ECMWF/CMC/EC mean slower. The eventual decay of the West Coast upper level ridge is where greater model differences emerge as a compact upper low tries to undercut the ridge as suggested by the ECMWF and GFS, but absent on the CMC. The ECMWF becomes stronger with the ridge over the Intermountain West compared to a broader GFS solution. By the end of the forecast period on Saturday, there is a strong model signal for an organized low pressure system off the coast of California, with the GFS near the northern California coast and the ECMWF much farther to the west. The EC mean is close to the model consensus and provides a good starting point for the low position. ...Weather Highlights/Hazards... The strong Alberta clipper system is expected to reach the eastern Great Lakes by the beginning of the forecast period Tuesday morning, along with a very strong arctic front. This will bring a band of light to moderate snow from the central Appalachians to the Northeast U.S., and rain changing to snow for much of the Mid-Atlantic region and coastal New England. After this system departs, more lake effect snow is likely downwind of the Great Lakes with intense cold air advection behind the low. Another thing that will be making weather headlines by the middle of the upcoming week will be an impressive round of frigid temperatures as a lobe of the polar vortex sinks southeastward across Canada and then reaches the northern Great Lakes. Temperatures are expected to plummet well below zero across the Upper Midwest, most of the Great Lakes region, and the northern Ohio Valley by Wednesday and Thursday. Anomalies on the order of 25 to 35 degrees below normal are a good possibility for these regions, and it is quite likely that this would warrant wind chill warnings for many of these areas. Numerous record low temperatures are expected. Although the airmass will modify some by the time it reaches the East Coast, it should still be quite cold for the Mid-Atlantic and Northeast U.S. on Thursday with highs below freezing for most areas. Rain and mountain snow is expected to return to the West Coast by next weekend as a low pressure system works its way towards California. D. Hamrick WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation and winter weather outlook probabilities are found 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