Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Mon Jan 13 2020 Valid 12Z Thu Jan 16 2020 - 12Z Mon Jan 20 2020 ...Significant Late Week/Weekend Winter Storm Threat from the Upper Midwest to the Northeast... ...Guidance/Predictability Assessment... Guidance shows progressive large scale flow aloft over the lower 48 for most of the period. By the start of next week consensus indicates a modest mean ridge to building into the West with broad troughing east of the Rockies. Within this pattern the dominant system of interest will be near the West Coast as of early Thu and continue across the lower 48 the rest of the week into the weekend, possibly leading to a strong surface low near New England by early Sun and continuing into the Canadian Maritimes by Mon. Guidance continues to advertise the threat for significant snow and possibly brisk/strong winds from the Upper Midwest into the Northeast while rain may fall over some Mid-Mississippi Valley/Lower Ohio Valley locations where heavy rain fell over the past week. Looking at guidance specifics, the GFS/GEFS mean are south of other guidance for the concentrated surface low off the Pacific Northwest coast on Thu. However as West Coast dynamics progress across the lower 48 there is better than average agreement in principle as surface low pressure reaches the High Plains by early Fri, the Midwest early Sat, and near the New England coast by early Sun. The GFS has exhibited a noticeable slower trend over the past 12 hours of runs while the centroid of model/ensemble surface lows has adjusted a little southward over the past day--increasing the potential for significant snowfall at some locations. The 00Z ECMWF is on the weak side of the spread for the surface low before it reaches New England. Otherwise there is now good agreement with the system initially over the Northeast early Thu (aided by slower GFS/GEFS trends) while the eastern Pacific storm track will shift farther northwest as western U.S. heights aloft trend higher, but with lower confidence in exact details. Based on the array of latest guidance, the updated forecast started with an operational model blend early-mid period with minor manual enhancement for system depth, followed by a model/ensemble mean blend given decent agreement for the mean pattern into early next week. ...Weather/Hazard Highlights... Strengthening low pressure departing from New England on Thu will produce a brief period of wrap-around heavy snow as well as lake effect snow promoted by cold cyclonic flow in its wake. Cold high pressure will build to the south/east across much of the central/eastern U.S. behind the low pressure and trailing cold front. A vigorous low off the Pacific Northwest Wed night into Thu will enhance maritime hazards as amplified upper troughing/leading height falls to the south work inland. This system will spread a moderate swath of precipitation--including a locally heavy and terrain enhanced snow threat--across California, the Great Basin/Southwest and Rockies during Thu-Fri while mostly light activity extends northward. Upstream Pacific systems will bring periods of rain/mountain snow into the northern half of the West Coast though with a general trend for the southeastern edge of the precip shield to retreat northwestward as heights aloft rise. The best-defined event should be from Fri night into the weekend. The initial West Coast upper trough will continue to the east/northeast late this week into the weekend, possibly becoming negatively tilted by the time it reaches the Northeast. With associated deepening low pressure likely to track from the High Plains through the Midwest and then New England and the Canadian Maritimes, the guidance signal is strengthening for the threat of significant snowfall from the Upper Midwest into the Northeast with other wintry precip types also possible to the south of the all-snow area. Some locations should see wintry precip before a changeover to rain. Meanwhile locations in the Mid-Mississippi Valley and Lower Ohio Valley will have to monitor effects from rainfall given the heavy rain experienced over the past week. Other areas from the south-central Plains northeastward could see some runoff issues with locally moderate-heavy warm sector rainfall. Lake effect snow will linger behind the system in the flow of reinforcing cold air. Northern Plains and Upper Midwest will be under the core of coldest air during the period with temperatures 10-30F below normal one or more days. Snow cover from the late week/weekend storm may enhance the cold temperatures. Expect unseasonably warm temperatures over the southern tier of the U.S. east of the Rockies Thu-Sat but then a cooling trend after passage of the front extending from the late week/weekend storm system. Rausch Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indices are 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