Extended Forecast Discussion NWS Weather Prediction Center College Park MD 150 AM EST Tue Jan 12 2021 Valid 12Z Fri Jan 15 2021 - 12Z Tue Jan 19 2021 ...Overview... The medium range period begins Friday with a well defined closed low over the Great Lakes and amplified troughing over the eastern half of the country. Surface low pressure associated with this trough may be fairly deep, tracking into the Upper Great Lakes and Canada, pushing a cold front into the East on Friday. This will spread an area of precipitation (more snow than rain) into the Great Lakes/Northeast and vicinity late this week into the weekend. The upper trough should slowly lift away from the East this coming weekend which may favor lake-effect/enhanced snow on the backside. Meanwhile, upper ridging over the West will give way to a series of shortwaves moving into the Pacific Northwest, with amplified elongated troughing again establishing itself over the Central U.S. early next week. ...Guidance/Predictability Assessment... The latest models and ensembles were mostly agreeable and consistent with the large-scale/longwave pattern evolution, but continue to show differences in the track/depth of the embedded systems. The trend over the past day has been towards a slower/deeper upper pattern unfolding over the East, resulting in a wrapped up system through the Upper Great Lakes, into Southeast Canada Fri-Sun. The ECMWF/UKMET position of the upper low on Friday continues to be a bit south of the GFS/CMC consensus, but better than previous cycles. A general deterministic model compromise served fairly well for this system resulting in a position in between the two camps and consistent with the previous WPC forecast. This will drive a strong cold front through the East late this week, and a more wrapped up system like this may support a wave of low pressure along the front on Saturday, enhancing precip across the Northeast. Though these details continue to remain unclear. The next Pacific shortwave should arrive into the West around Friday-Saturday, amplifying as it moves across the Rockies and into the Central U.S. next weekend. Slower pace of the Eastern system would shorten the wavelength spacing enough to preclude much development of this system heading toward northern Mexico and the northwestern Gulf by next week, as some previous runs of the GFS/CMC may have suggested. With upper ridging expected to build again near 130W by the end of the period, further digging of the trough is possible. Again, details on individual shortwaves remains very uncertain at this time frame, so weighting more towards the ensemble means is preferred. ...Weather/Threats Highlights... Leading closed low energy into the Great Lakes would support a combination of synoptic and lake effect/enhanced snows from the Great Lakes into the northern/central Appalachians, with precipitation also extending into parts of the Northeast. Expect rain in the southern part of the moisture shield along the leading cold front into the East as well. In the West, some light precipitation is expected along and over favored terrain as various shortwaves move through. As troughing again deepens over the Rockies/Plains this weekend, precipitation may begin to push out of the mountains and expand into parts of Texas and the lower Mississippi Valley early next week. Above normal temperatures should be fairly widespread across much of the northern half and southwestern portions of the lower 48 on Thursday, though temperatures +5-10 degrees should persist across California and the Desert Southwest through the period. In the East, the mild temperatures on Thursday will be replaced by more typical mid-January values by the end of the period, while parts of the Southeast and Rockies will see near to below normal temperatures by about 5 degrees. Another round of above normal temperatures should also move into the Northern Plains by next week, with daytime highs 10 to 15 degrees above normal. Santorelli/Fracasso Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook 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