Extended Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM 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 over 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, while some accumulating lake effect snows will be possible behind the system as well. There remains considerable uncertainty regarding the character of a deep upper trough (with likely embedded low) expected to evolve over western Canada, affecting flow across the northeastern Pacific and lower 48. In general the forecast confidence is highest with respect to this pattern supporting well below normal temperatures across much of the northern Rockies to northern Plains, while confidence is lower for the coverage of cold air beyond this most likely core. This cold pattern should persist well beyond the end of the medium range period, per Climate Prediction Center outlooks. Meanwhile a couple shortwaves interacting with a wavy front from the Gulf of Mexico to just south of Florida may produce episodes of rainfall, the first mainly over Florida during the weekend and the second across more of the Gulf Coast region early next week. ...Guidance/Predictability Assessment... The forecast over and near the Northeast from the weekend into Monday still has some complexity to it aloft, with east-west elongating energy drifting toward Nova Scotia and then followed by upstream flow (some combination of the parent upper low and/or surrounding energy possibly closing off another low over the Canadian Maritimes. Recent consensus trends for the surface low near the New England coast have been more offshore after early Saturday (a nod to earlier GFS runs) and then the 12Z GFS/CMC/ECMWF suggest the evolution aloft by Monday could ultimately pull back the surface low a bit to a position just northeast of Maine. Models/ensembles continue to vary with the evolution of the deep upper trough digging into western Canada, with influence from exactly how upper ridging evolves over far western Alaska and closes off to the northwest of the state--and depiction of shortwave energy to the southeast of the upper high--along with western Pacific ridging. One noticeable trend over the past day is for leading energy that reaches western Canada during the weekend to be somewhat less amplified as it progresses eastward. While GFS runs have been erratic over the past days and GEFS means have tended to have higher heights over southwestern Canada and the western U.S. versus other guidance, there has been a general trend toward a more persistent upper low over western Canada that would support fairly low heights extending into the Northwest for a time. Then the overall trough axis would begin to press eastward by around midweek. The new 12Z GFS compares better to this trend while the 12Z CMC has favorably modified its 00Z run that was fairly extreme with its westward position of the upper low and was similar to the 00Z ECMWF in being on the amplified side of the spread over the West. The new 12Z ECMWF has also toned down its western U.S. troughing. The 12Z GEFS mean has trended lower with its southwestern Canada/northwestern U.S. heights, bringing it closer to the 00Z ECMWF mean. As the guidance seems to be narrowing its spread for the pattern aloft somewhat, there is a gradually increasing signal that Pacific flow may carry a system into the Northwest by around Tuesday, with a front possibly continuing into the West thereafter. The updated forecast started with a 00Z/06Z operational model composite for about the first half of the period and then transitioned to a blend of operational models and 00Z ECens/CMCens and 06Z GEFS means. This approach provided some elements of continuity while leading to adjustments where suggested by a significant majority of guidance. Examples of the latter include the farther east track of the initial New England coastal system, a moderate trend toward lower heights over the Northwest after the weekend, and a more suppressed wavy Gulf front toward midweek. ...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/northeastward track should continue a threat for wintry weather and potentially heavy snowfall across interior portions of northern New England through Saturday. There is still some 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. Recent guidance trend for a slightly farther offshore low track does offer the potential for some snow to extend a little farther southeastward than in some earlier forecasts. 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 still 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 (whether moisture reaches as far south as portions of California and the Great Basin) due to guidance spread for important specifics of the upper pattern. Snow levels could be fairly low over parts of the Northwest depending on pattern specifics. 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 northern Plains from Sunday onward, with daytime temperatures as cold 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. Surrounding areas should see less extreme anomalies of 10-20F below normal. The West will tend to see readings 5-15F below normal during the weekend followed by some moderation over central/southern areas. The East should be coolest over the weekend with some highs up to 10-12F below normal, with slight moderation to single-digit negative anomalies early-mid 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 Southeast, the Lower Mississippi Valley, and the Southern Plains, Mon, Dec 19. - Heavy snow across portions of the Northeast, and the Lower Great Lakes, Sat, Dec 17. - Flooding occurring or imminent across portions of the Lower Mississippi Valley. - High winds across portions of the Northeast, Sat, Dec 17. - Much below normal temperatures across portions of the Northern/Central Plains, the Northern/Central Rockies, the Upper/Middle Mississippi Valley, and the Northern Great Basin, Sun-Wed, Dec 18-Dec 21. 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