Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Dec 13 2022 Valid 12Z Fri Dec 16 2022 - 12Z Tue Dec 20 2022 ...Heavy snow threat shifting through the interior Northeast Friday into Saturday, with heavy rain possible across coastal locations... ...Overview... A deep upper level low (the same one responsible for a northern Plains winter storm early this week) will be in place over the Upper Midwest at the start of the period on Friday. Shortwave energy rounding the base of this trough will likely induce coastal low development by early Friday near the Mid-Atlantic, with heavy snowfall for portions of the interior Northeast ongoing as it lifts north. Some accumulating lake effect snowfall will be possible behind the system as well. The initial upper low may wobble over the Great Lakes for a period of time before it weakens and quickly tracks eastward across southeast Canada as the next system drops in from western Canada. There is a lot of uncertainty regarding the specifics and track of this system (a closed low for some period of time) but in general it should send troughing through the Western and central U.S., promoting much below normal temperatures across much of the northern Rockies to northern and central Plains. ...Guidance/Predictability Assessment... There remains good clustering for both the upper low placement over the Upper Midwest as well as the coastal low expected to track northward from the Mid-Atlantic coast as a nor'easter. There remain some uncertainties surrounding individual waves of energy rounding the base of the trough, but a general model blend seemed to mitigate these smaller scale differences. There is better agreement now than previous runs for this low to linger for a day or so over the Upper Great Lakes before sheering off to the East over southeast Canada or New England. By day 5/Sunday, there are increasing uncertainties regarding both track and intensity of the next system from western Canada, which for a time looks to consolidate into a compact closed low over southwest Canada. There is some agreement that this system should eventually send troughing into the central U.S. but the detail differences as early as day 5 results in some much greater larger scale differences by early next week. The past couple of runs of the GFS have been a little weaker with the system skirting the northern tier with an eventual closed low consolidating again over the Upper Great Lakes by Tuesday. The ECMWF lingers the low over southwest Canada before energy sweeps through the northern Plains. And the CMC is much deeper with the low and generally keeps it over western Canada through the period. The ensemble means tend to follow suit with their respective deterministic counterparts further making it difficult to decide a more clear direction. The forecast for tonight started with a general model compromise days 3 and 4, quickly trending towards the ensemble means by later in the period due to uncertainties outlined above. This approach maintained fairly good consistency with the previous WPC forecast as well (through day 6). ...Weather/Hazard Highlights... The likely development of a nor'easter near the Mid-Atlantic coast and subsequent northward track should continue a threat for wintry weather and potentially heavy snowfall across interior portions of the Northeast. There remains plenty of uncertainty in the details of this system, and even small shifts in this low track could cause significant differences in precipitation types/amounts. Closer to the coast, rain could be heavy at times over eastern New England, but with a general lack of instability, is unlikely to create any significant hazards. Elsewhere, lingering wrap-back moisture on the backside of the main upper low will continue to support some light snowfall accumulations across the Upper Midwest with also some lake effect snows downwind of the favorable lakes. A shortwave into California this weekend may spread precipitation into the Southwest. This shortwave will then track across the southern tier and could then influence specifics of moisture that may spread across and north from the Gulf Coast late weekend into next week with some indications for widespread moderate to heavy rain. Some precipitation may also reach the Northwest/northern Rockies as upper troughing descends toward the region. Snow levels could be fairly low over some areas. Behind the system moving off the East Coast, temperatures across most of the CONUS should be near to below normal by Friday and especially next weekend. The most widespread and extreme cold anomalies will likely be across the West into the northern Plains, trending colder with time. By Sunday and especially Monday and Tuesday, daytime temperatures across the northern Plains could be as much as 30-40 degrees below normal, equating to widespread single digit temperatures with some below zero readings across eastern Montana and North Dakota. Recent trends suggest parts of the Northwest could be 10-25 degrees below normal by the start of next week. Santorelli 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, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml