Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sun Jan 27 2019 Valid 12Z Wed Jan 30 2019 - 12Z Sun Feb 3 2019 ***Impressive arctic airmass to affect the north-central U.S. with record breaking temperatures expected*** ...Overview... An amplified flow pattern will exist across the continental U.S. at the beginning of the forecast period on Wednesday with a lobe of the polar vortex crossing Lake Superior and southern Ontario, and a broad upper ridge near the West Coast. With the vortex lifting out across eastern Canada by the end of the week, the pattern is not quite as amplified with a broad ridge over the Intermountain West and a broad trough over the eastern half of the nation. By next weekend, there will likely be a developing low pressure system across the northern plains along with a trailing cold front. ...Guidance Evaluation/Preferences... Deterministic models are in decent overall agreement regarding the deep upper low (lobe of polar vortex) tracking across southern Ontario and Quebec early in the forecast period. 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 not apparent with the CMC. The ECMWF becomes stronger with a southern stream shortwave crossing the Desert Southwest compared to the other operational guidance. By the end of the forecast period on Sunday, there is a strong model signal for a developing surface low across the northern plains and Upper Midwest, but varies substantially regarding the location of the low. The GEFS/EC means provided a good starting point for this and the surface high near the East Coast. The forecast was based mainly on the GFS/ECMWF/CMC early on, and then transitioning to increasing use of the EC mean for placement purposes later in the forecast period along with some previous WPC continuity. ...Weather Highlights/Hazards... The main thing making weather headlines will be the impressive surge of arctic air sinking southward across the north-central U.S. by mid-week as a lobe of the polar vortex tracks across the northern Great Lakes and Ontario. 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 expected for these regions, and some areas could be 40 degrees or more below average for Wednesday! Wind chill values could reach 50 below at times from northern Illinois to Minnesota! Numerous record low temperatures are expected, along with record low maximum temperatures. Some places may come within a few degrees of all-time records. 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 the Pacific Northwest. Heavy rain and mountain snow are expected from northern California to western Washington, and some of this moisture will make its way inland across the Intermountain West. There will likely be some rain for the Gulf Coast region as return flow from the Gulf advects moisture inland across Texas and Louisiana. 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