Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Mon Dec 12 2022 Valid 12Z Thu Dec 15 2022 - 12Z Mon Dec 19 2022 ...Heavy snow threat shifts into the interior Northeast Thursday and Friday, with heavy rain possible across coastal locations... ...Overview... A deep upper level low (the same one responsible for a northern Plains winter storm in the short range) will be in place over the Midwest by the start of the period on Thursday with a cold front at the surface spreading moderate to heavy rainfall into Eastern portions of the U.S.. Coastal low development looks likely by early Friday near the Mid-Atlantic, bringing a threat for heavy snowfall to portions of the central Appalachians and interior Northeast as it lifts north. Some accumulating lake effect snowfall will be possible behind the system as well. The initial upper low should track into the upper Great Lakes and then southeastern Canada/New England (with some details to be determined by interaction with Canadian flow). Within the pattern that guidance suggests will bring below normal temperatures to most of the CONUS by the weekend and early next week, there has been some shifting for the details of late-period Canadian into northern tier U.S. troughing and thus for specifics of the coldest air forecast to reach the northern tier. Shortwaves embedded within the mean flow (including energy from an opening Pacific upper low) will provide some medium to smaller scale uncertainties in the forecast. ...Guidance/Predictability Assessment... In general the guidance from the 00Z/06Z cycles appeared to add to the recent improving trend with respect to clustering for the compact coastal wave expected to track from near the Mid-Atlantic from around Thursday night into the weekend. However the new 12Z runs have diverged somewhat, with the GFS adjusting more offshore and the UKMET/CMC straying faster with the northward progression. This divergence maintains a decent amount of uncertainty for coverage/location of various precipitation types for New England. As for the initial upper low, there is still a question of how it evolves after it reaches near the upper Great Lakes by around early Saturday--ranging between continued eastward progress over southeastern Canada and/or New England to a shearing process that yields a separate low to the east, or maybe even both. Interaction with surrounding Canadian flow continues to provide some uncertainty over the course of the period as well. The favored blend from the 00Z/06Z guidance tilted toward the shearing scenario but the new 12Z guidance leans toward one better defined upper low. Meanwhile guidance has been struggling with exactly how to depict another amplifying upper ridge over and near Alaska along with energy on its eastern side, leading to significant differences and trends for Canadian troughing expected to amplify into the northern portions of the lower 48 after Saturday. Recent GFS runs through the 00Z cycle had been showing more high-latitude closing of the upper ridge (as shortwave energy pulls westward over Alaska) to result in a central North America trough axis. GEFS means through 06Z and the 00Z CMC mean sided with this idea as well. On the other hand, recent ECMWF runs have been trending toward a farther west axis of digging energy, with the 00Z CMC and 06Z/12Z GFS runs trending this way as well. The GEFS mean has been slower with its trend but by day 7 Monday the 12Z version has adjusted to the 00Z ECMWF mean (an intermediate solution between latest trends and recent continuity). Finally, the details of this evolution will likely have some influence on how shortwave energy from an initial Pacific upper low may ultimately influence the lower 48 as it becomes embedded in the mean flow. The 00Z-12Z ECMWF and 12Z GFS/CMC are not too far apart in principle aside from typical timing differences. The updated forecast started with a 00Z/06Z operational model blend early in the period, with more GFS/ECMWF weight and least weight on the UKMET (which strayed from consensus for various parts of the forecast, especially to the northwest of the CONUS late in its run). Then the forecast transitioned toward a nearly even blend of the GFS/ECMWF/CMC and the 06Z GEFS/00Z ECMWF means by the end of the period. This provided a meaningful trend to reflect emerging guidance adjustments while not yet going fully to the pure operational model cluster given the newness of the trend. ...Weather/Hazard Highlights... Deep/ample moisture will continue to stream northward ahead of the central U.S. upper low to fuel moderate to heavy rainfall into the eastern U.S. Thursday and Friday. The likely development of a nor'easter near the Mid-Atlantic coast and subsequent northward track would bring a threat for wintry weather and potentially heavy snowfall across interior portions of the Mid-Atlantic and Northeast, with even some ice/sleet potential closer in towards the I-95 corridor early Thursday before the warm air moves in and changes the precipitation to rain. 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 across the East. 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, and especially downwind of the Great Lakes. Details of shortwave energy originating from a Pacific upper low will determine if any light precipitation reaches California and the Southwest next weekend. This shortwave could then influence specifics of moisture that may spread across and north from the Gulf Coast late weekend into next week. 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. Expect above normal temperatures ahead of the cold front sliding through the East later this week, but much 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 Monday, daytime temperatures across the northern Plains could be at least 20-35 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. 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 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