Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Wed Dec 14 2022 Valid 12Z Sat Dec 17 2022 - 12Z Wed Dec 21 2022 ...Heavy snow threat will linger into Saturday across northern New England, with heavy rain possible along the coast... ...Bitterly cold temperatures likely to settle over the northern Plains from Sunday onward... ...Overview... A coastal low will be lifting along the Northeast coast by the start of the period on Saturday, continuing a threat for heavy snows for mainly interior locations, and some moderate to heavy rain along the coast. The parent upper low will be over the Great Lakes at the same time with its energy eventually lifting north and weakening over Canada, and with some accumulating lake effect snows possible behind the system as well. There remains considerable uncertainty regarding the specifics and track of a strong shortwave/closed low dropping through Western Canada, but in general it should send troughing/height falls through the Western and central U.S., promoting well below normal temperatures across much of the northern Rockies to northern and central Plains. This cold pattern is likely to persist well beyond the end of the medium range period, per Climate Prediction Center outlooks. Meanwhile shortwave energy and a wavy front may produce an episode of enhanced rainfall near the Gulf Coast region early next week. ...Guidance/Predictability Assessment... On the large scale, there's good agreement the early part of the period, but still questions in the details. The GFS (including the new 00z run tonight) remains a bit farther offshore with the coastal low in the Northeast than the better consensus. There are also differences with the parent upper low... the GFS and UKMET pull it north seperate from a developming compact low off the Canadian maritimes while the ECMWF and CMC favor more of a combination of the two. Either way, the result is the same over the eastern portion of the CONUS, rising heights and more zonal flow. Despite these differences, a general model blend sufficed for the first half of the period. Guidance continues to have trouble in resolving important details of trailing flow across western/southern Canada and vicinity by early next week. The GFS and ECMWF are weaker with initial energy skirting northern tier but do show some bit of amplification of the flow over the Northwest as a second piece of energy drops in. The CMC is much more amplified (and west) with the initial upper low and also tracks the second energy more east into the northern Plains while the GFS and ECMWF brings it south. A look at the ensembles show a lot of spread as well so the forecast out here remains highly uncertain. Regardless, there is a general trend towards much more amplified flow towards the very end of the medium range period (and beyond). Across the southern tier, GFS runs are more amplified with energy reaching the Southeast this weekend, and slower to open it up as it slides inland. It becomes more of an outlier as it keeps some energy farther south (weakening northern energy through the southern U.S.) while the CMC, ECMWF, and UKMET are stronger with that northern energy. This of course has implications for QPF along the central Gulf Coast by early to mid next week. The WPC forecast this cycle used a general model blend through day 5 of the latest deterministic model runs. After this, the CMC was dropped due to its differences across the Northwest in favor of the ensemble means which seemed more inline with the GFS/ECMWF solutions. By day 7, a nearly equal deterministic/ensemble mean blend was utilized which toned down some of smaller scale details, but with more overall definition to the pattern. This also maintained good agreement (through day 6) with the previous WPC forecast as well. ...Weather/Hazard Highlights... The likely development of a nor'easter near the Mid-Atlantic coast at the end of the short range period and subsequent northward track should continue a threat for wintry weather and potentially heavy snowfall across interior portions of northern New England through Saturday. 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 down east Maine, but with a general lack of instability, this rain is unlikely to create any significant hazards. The gradient between this system and high pressure well to the north could produce a period of strong onshore winds as well. Elsewhere, favorable westerly flow on the backside of the main upper low will continue to support some lake effect snows over favorable downwind areas. A shortwave reaching into the Southwest this weekend may produce a little light/scattered precipitation. 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. A leading shortwave brushing the southern tier may produce some rainfall during the weekend, most likely confined to Florida, but there is even a fair degree of uncertainty with amounts for this episode. Flow to the south of a deepening western Canada trough/upper low may produce some terrain-enhanced precipitation over the Northwest/northern Rockies from Sunday onward. Confidence is currently fairly low for amounts and southward extent due to guidance spread for important specifics of the upper pattern although 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 this 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 beyond, daytime temperatures across the northern Plains could be as much as 30-40 degrees below normal, equating to widespread and bitterly cold single digit temperatures with some below zero readings across eastern Montana and North Dakota. Morning lows could fall to at least 10-20 degrees below zero. Less extreme anomalies of 10-20F below normal should spread over the central Plains and Upper Midwest. Parts of the Northwest may 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