Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Fri Jan 18 2019 Valid 12Z Mon Jan 21 2019 - 12Z Fri Jan 25 2019 ...Overview... Guidance agrees quite well upon a significant pattern transition, from one generally featuring a progression of Pacific systems toward an amplified flow regime consisting of a building eastern Pacific/West Coast ridge (extending into northwestern North America) and corresponding mean trough covering 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 strong storm system affecting the Lower 48 in the short range time frame should reach the Canadian Maritimes by early day 3 Mon and continue northeastward. With the departure of this system the next focus will be on a fairly vigorous western U.S. shortwave which 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. The ensemble means up to this point seem to be in an ongoing process of becoming better defined with this system but an average of their timing (ECMWF mean somewhat slower than the GEFS mean) provides a stable anchoring point for the forecast. Recent ECMWF runs have been showing a pronounced faster trend toward this intermediate solution and is still somewhat slower than its mean in the 12Z cycle. GFS runs have been consistently in the faster half of the spread with latest runs a bit faster than those from 24-36 hours ago. Confidence in any solution is still only moderate as individual ensemble members and other models provide other alternatives for specifics--which still appear fairly 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. CMC runs are quite slow versus consensus for the overall western trough while the new 00Z GFS/GEFS mean/UKMET in their own ways are raising the possibility of a slower and/or stronger trailing wave. As for the shortwave(s) reaching the West by mid-late week, teleconnections relative to the East Pacific ridge offer potential for some southwestward elongation while teleconnections relative to Atlantic ridging suggest more progression. This leaves a model/mean blend as a reasonable starting point to resolve this aspect of the forecast. Based on guidance through 12Z/18Z cycles, forecast preferences led to using a blend primarily emphasizing operational guidance early followed by gradual increase of ensemble mean weight (18Z GEFS and 12Z ECMWF mean) from 20 percent early to 70 percent late in the period. ...Weather Highlights/Hazards... The strong storm tracking away from New England will leave in its wake very cold air over the East early in the week, along with windy conditions and low wind chill values over the Northeast. Anomalies may reach 15-30F below normal for min/max readings Mon into early Tue from portions of the Ohio Valley/Great Lakes into the Mid-Atlantic and Northeast. 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 Mon. Expect the system emerging from the West to spread mostly snow across parts of the Great Basin and Rockies as well as northern Plains on Mon. Amplification of the system as it progresses eastward should then promote a period of Gulf moisture inflow and heavy rainfall potential from the lower half of the Mississippi Valley eastward/northeastward from Tue 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 and plain rain to the south. Also note that snow potential could extend farther south depending on how a possible frontal wave evolves. Progression of this system will bring a brief surge of warmth across the central-eastern states Tue-Thu (plus 10-20F anomalies) followed by a return to below normal temperatures. 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 Tue-Wed 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 Fri. 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