Extended Forecast Discussion NWS Weather Prediction Center College Park MD 329 PM 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... ...Very cold temperatures likely to settle over and expand from the northern Plains Sunday onward... ...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 latter system (a closed low for some period of time, and possibly followed by additional energy) 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... The early part of the forecast continues to feature decent clustering among most guidance but a stray run or two for the initial Upper Midwest-Great Lakes upper low and the surface system tracking along the Northeast coast. The 00Z UKMET was the odd model out for the upper low, pulling it north/northwest of consensus after Friday. Latest GFS runs (including the 12Z version) have been tracking the coastal system farther offshore than most other guidance. The 00Z UKMET tracked it farthest inland Saturday onward but the new 12Z run shifted to the east. Preference based on the 00Z/06Z guidance sided more with the 00Z ECMWF/CMC, while the 00Z GFS/06Z GEFS mean were a little closer to this idea than the 06Z GFS by Sunday. Guidance continues to have trouble in resolving important details of trailing flow across western/southern Canada and vicinity from about Sunday onward. During the weekend there is agreement in principle upon digging western Canada energy downstream from a strong upper ridge that builds over far western Alaska. However differences in details for exactly how quickly an upper high closes off northwest of Alaska (by Sunday-Monday) and the character of energy to its southeast lead to the increasing spread that models/ensembles exhibit. Given the fairly high uncertainty, the updated forecast attempted to maintain a reasonable degree of continuity. By late in the period this approach reflected a tilt toward the ensemble means which show energy from the upper low dropping into southwestern Canada ultimately translating eastward across far southern Canada early next week. Latest 12Z model runs are all showing potential for a fairly deep upper low to persist over or near British Columbia during the first part of next week while leading energy could be somewhat more sheared than depicted in the means. Needless to say, confidence an any particular solution is low and this is reinforced by the historical tendency for low predictability to arise from cases when there is an influence from a closed upper high at mid-high latitudes. Across the southern tier, GFS runs have been a little more amplified with the Pacific upper trough (reaching the Southwest around Sunday) associated with an opening upper low and also slower to open up the low itself. Ahead of this feature, GFS runs are also a bit on the strong/slow side with a western shortwave that ultimately brushes the southern U.S. to produce more rainfall along the Gulf Coast and over the Florida Peninsula. The 12Z UKMET has a weaker shortwave but still produces considerable rainfall though. Based on the above preferences, the early part of the forecast employed an operational model composite with greatest weight on the 00Z ECMWF and the least on the UKMET (which was phased out after day 3 Friday). The forecast then trended toward a model/ensemble mean mix (and switching the GFS from the 00Z to 06Z run late based on comparisons to consensus) while also including some continuity by day 7 Tuesday. This provided fairly good consistency while waiting for any potentially significant shifts in the best consensus. ...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, 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 though. 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 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. 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. 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. Rausch/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 Hazards: - Heavy rain across portions of the northern Gulf Coast, Mon-Tue, Dec 19-Dec 20. - Heavy snow across portions of the Mid-Atlantic, the Northeast, and the Great Lakes, Fri-Sat, Dec 16-Dec 17. - Flooding occurring or imminent across portions of the Lower Mississippi Valley and the Southern Plains. - High winds across portions of the Northeast, Fri-Sat, Dec 16-Dec 17. - Much below normal temperatures across portions of the Northern and Central Plains, the Northern and Central Rockies, the Northern Great Basin, and the Upper Mississippi Valley, Sun-Tue, Dec 18-Dec 20. - Much below normal temperatures across portions of the Great Plains, and the Upper and Middle Mississippi Valley, Tue, Dec 20. - Much below normal temperatures across portions of the Alaska Panhandle and mainland Alaska, Sat-Tue, Dec 17-Dec 20. 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