Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1055 AM EST Fri Jan 25 2019 Valid 12Z Mon Jan 28 2019 - 12Z Fri Feb 01 2019 ...Significant cold air outbreak expected next week across from the Midwest to the Mid-Atlantic/Northeast... ...Overview... Breakdown of the polar vortex ongoing during the short range will allow a lobe of strongly negative height anomalies/cold temperatures to spread southward into the north central/northeastern CONUS during the medium range. Persistent upper ridging off the West Coast expanding north into the arctic during the medium range will ensure that this highly amplified long wave pattern will persist for a while, as numerous arctic shortwaves dive southeast across central Canada and reinforce the broad trough across central/eastern North America. ...Guidance Evaluation/Preferences... A clipper system crossing the north central/northeastern U.S. will be most significant low pressure system during the medium range, with models suggesting the potential for development of a low along the Mid-Atlantic coast Tue/Tue night. Models showed relatively good consensus during days 3-5 (Mon-Wed), and a general blend of recent deterministic solutions served as a good forecast starting point. From late Wed onward models continue to struggle with the exact structure of the deep vortex diving southward across Canada into the Great Lakes, as well as with Pacific shortwave energy attempting to erode the ridge, having some potential impact on the upper flow across western portions of Canada and the CONUS. The 00Z ECMWF was the most aggressive with undercutting the ridge, and was not necessarily the preferred solution at this time. Rather, an increase in ensemble mean weighting after day 5 should allow for some reflection of weakening shortwave energy to approach the U.S. West Coast, but not to the degree shown by the ECMWF. A more ensemble-based approach was also preferable farther east in the vicinity of the deep upper vortex, as models show fairly large variability with arctic shortwave energy rotating around the low. Some degree of modest consensus was noted among models/ensembles that Pacific shortwave energy could result in a surface frontal wave developing across the southern plains by next Fri, although confidence in the specifics is very low at this time. ...Weather Highlights/Hazards... Clipper low pressure system is expected to bring a period of moderate to perhaps heavy snow to portions of the Midwest/Great Lakes Mon-Tue. Development of a low along the Mid-Atlantic coast late Tue could bring a period of enhanced snowfall potential to the Mid-Atlantic and especially the Northeast Tue night into Wed. In the wake of this system, deep upper low digging south toward/into the Midwest/Great Lakes will serve as the epicenter of a formidable cold air outbreak. Temperatures 20-30 deg F below average (for both max and min temps) are forecast to spread into the Midwest (where max temps may remain in the double digits below 0) by Tue and into the Ohio Valley/lower Great Lakes Wed. The air mass may moderate slightly by Thu-Fri as it reaches the Mid-Atlantic/Northeast with widespread temperatures 15-20 deg below average. Wind chill values in the wake of the deepening low pressure system across the along the Eastern Seaboard by Tue-Wed could result in extremely cold wind chill values well below zero across portions of the Midwest and Great Lakes. Farther west, precipitation may spread into areas along the West Coast by Thu-Fri as an upper trough approaches. Ryan 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