Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1105 AM EST Fri Jan 18 2019 Valid 12Z Mon Jan 21 2019 - 12Z Fri Jan 25 2019 ...Overview... Models are continuing to lock onto a large scale pattern change where flow over the East Pacific Ocean and western North America becomes amplified thus setting up a mean trough over the eastern half of the continent. The increasingly large scale of the eventual mean pattern should lead to better than average predictability overall but embedded shortwaves will be smaller in scale and still provide meaningful uncertainty for some sensible weather details. ...Guidance Evaluation/Preferences... The winter storm system impacting the Lower 48 states in the short range period will be tracking northeast through the Canadian Maritimes by the beginning of the extended range. A fairly vigorous western U.S. shortwave, along with interacting northern stream flow, will most likely support Plains through Great Lakes low pressure over the course of next week. Some of the western energy may also produce waviness along the trailing front as it crosses the East. Confidence in any solution is still only moderate at best at this point as individual solutions remain sensitive to stream interaction and exact small-scale details of shortwave energy. There is even more uncertainty regarding what waviness may exist on the trailing front, with dependence on how initial western energy evolves and eventually some indirect influence from upstream energy reaching western North America by Tue-Wed. . Forecast preference used a compromise of the operational guidance early followed by gradual increase of ensemble mean weight (00Z GEFS and ECMWF mean) from 20 percent early to nearly 50 percent late in the period. ...Weather Highlights/Hazards... Cold, blustery conditions are expected for much of the Great lakes, Ohio Valley, Mid-Atlantic and Northeast regions early in the week as the strong winter storm system pulls away from the Coast. Some locations may have daily maximum temperatures 15-30F below normal into early. Currently, the best potential for any daily records to be challenged would be for cold highs over the northern Mid-Atlantic/southern New England on Monday. Expect the system emerging from the West to spread mostly snow across parts of the Great Basin and Rockies, as well as, the Northern Plains on Monday. The system will draw moisture from the Gulf once it progresses eastward which will increase the potential for heavy rainfall across the Lower Mississippi Valley and areas to the northeast Tuesday onward. Beyond the Upper Mississippi Valley the highest probability for significant snowfall should extend across the Great Lakes and Northeast with a possible transition of wintry mix between this area with rain to the south. Also note that snow potential could extend farther south depending on how a possible frontal wave evolves. Parts of the central U.S. may see a brief period of warmth from this system midweek with some locations having daily maximums 10 to 20F warmer than normal. A focused band of shortwave energy and moisture will bring a period of enhanced snow and low elevation rain to areas from the Pacific Northwest into the Northern/Central Rockies, mostly in the Tuesday-Wednesday time frame. Additional shortwaves could produce light snow over/near the Rockies later in the week but with low confidence in location and timing. Approach of the building eastern Pacific ridge aloft will likely lead to moderately above normal temperatures over the West Coast states by mid-late week but Central/Southern Rockies should remain somewhat below normal through Friday. Campbell/Rausch WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes 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 https://www.wpc.ncep.noaa.gov/heat_index.shtml