Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Mon Jan 28 2019 Valid 12Z Thu Jan 31 2019 - 12Z Mon Feb 4 2019 ***Impressive arctic airmass to affect the north-central U.S. with record breaking temperatures and extreme wind chills expected on Thursday, followed by a moderating trend*** ...Overview... The deep upper level vortex situated over southern Quebec at the beginning of the forecast period Thursday will quickly lift out, and the recent persistent upper level troughing that will be over the eastern U.S. this week will begin undergoing a pattern change in time for the weekend. The flow pattern aloft is forecast to become more progressive through the medium range period, with a southern stream shortwave perturbation over the Desert Southwest quickly tracking eastward towards the northern Gulf of Mexico. Height rises in the wake of the departing polar vortex lobe will gradually allow broad upper level ridging to encompass much of the eastern U.S. by next Monday and a broad trough becoming established over the western U.S. ...Guidance Evaluation/Preferences... Models and ensemble means have improved over the past couple of days regarding placement and strength of the deep vortex crossing southern and southeast Canada late in the week. This also holds true for supporting the compact upper low crossing California and the Desert Southwest early in the forecast period, with the exception of the 12Z UKMET which is slower than the ensemble means, and also slower with the next major Pacific disturbance approaching on Saturday. A blend of the 12Z ECMWF/18Z GFS represented this consensus well, and served as a forecast starting point through Friday. By Day 5 on Saturday, speed and amplitude differences are apparent with the large scale trough near California. The 18Z FV3 GFS is a little slower than the operational GFS and closer to the CMC and ECMWF solutions, and this remains the case going into Sunday as the disturbance approaches the Plains. With that consideration in mind, the WPC forecast favored the slower set of solutions for this region, but not to the degree indicated by the UKMET given its lack of ensemble support. In the northern stream, a number of solutions suggest amplification of a shortwave across the Canadian prairies next weekend, with an associated surface low passing along the U.S./Canada border toward the Great Lakes. Model consensus was a bit better here and ensemble members showed reasonable clustering with respect to the surface low track, even with some timing/intensity variability evident in deterministic solutions. Given these considerations, the forecast during the second half of the forecast period continued to emphasize the ECMWF/FV3 GFS with increasing use of the ensemble means by late in the forecast period. ...Weather Highlights/Hazards... The extreme arctic air intrusion is expected to continue going into Thursday from the Great Lakes and Ohio Valley to the Northeast U.S. Although the coldest temperatures of this event are expected on Wednesday for the Upper Midwest and northern Great Lakes, frigid temperatures are still forecast with highs running about 30 degrees below normal from Minnesota to northern Ohio, and about 20-25 degrees below normal for the big cities of the Northeast U.S. Record low temperatures are likely for some of these areas Thursday morning along with impressive wind chills colder than 40 below at times! Temperatures are forecast to moderate substantially going into Friday and especially by the weekend for the entire eastern U.S. as the deep upper level trough lifts out and milder westerly flow from the Pacific brings conditions closer to seasonal averages or even slightly above. Along the West Coast, the arrival of a couple Pacific systems will keep precipitation (rain and mountain snow) fairly widespread, starting in the Northwest and gradually moving south across California. Rain/snow will gradually spread into the Great Basin and the Rockies by next weekend as the second and larger upper trough moves inland. Rain is also expected to increase in coverage over the Gulf Coast region and extending northward across the Deep South and Tennessee River Valley as southerly flow advects moisture northward ahead of the storm system developing across the Plains by Sunday and Monday. 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