Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 AM 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 off the Mid-Atlantic coast bringing a threat for heavy snowfall to portions of the central Appalachians and interior Northeast as it lifts north, with some accumulating lake effect snowfall on the backside of the system as well. The primary upper system should shift north and weaken as it interacts with another system dropping into central Canada. Weak shortwave energy looks to rotate around these systems through the U.S. this weekend (with some entering the West Coast as well), eventually resulting in mean troughing and below normal temperatures across much of the CONUS by early next week. ...Guidance/Predictability Assessment... Models show fairly good agreement with the upper low in place over the Midwest on Thursday but struggle thereafter on the details of various pieces of shortwave energy rounding the base. There is also increasing confidence on a surface low originating over the Southeast to live along the East Coast as a nor'easter with model guidance continuing to show relatively good consensus for track and timing. By this weekend, there are also bigger questions and differences regarding how the main upper low interacts with possible energy to the north. Through the 12/18z Dec 11 runs, the ECMWF and UKMET both show more phasing of these two systems due to a stronger Canadian shortwave, but the GFS and CMC are slower and more elongated with the upper low as it moves over the Great Lakes. Models disagree with how/when to get there, but there is some consensus for another upper low to become anchored over central Canada allowing for mean troughing across much of the CONUS by early next week. There remains a lot of uncertainty out West still too as energy or an upper low tries to undercut a ridge in the Gulf of Alaska. A quick look at the new 00z runs (available after forecast generation) shows continued uncertainties and wild variation in the details across the board. Despite the uncertainties as early as day 4, a general model blend sufficed through day 4. By day 5, the UKMET was removed from the blend as it took the upper Midwest low too north too quickly. A quick transition to the ensemble means seemed prudent the last half of the period with a lot of forecast uncertainty mid to late period. Did blend in smaller amounts of the ECMWF and GFS through 7 though just for some added system definition. This maintained good agreement with the previous WPC forecast through day 6. ...Weather/Hazard Highlights... Deep/ample moisture will continue to stream northward ahead of the upper low over the central U.S. to fuel moderate to heavy rainfall into the eastern U.S. Thursday and Friday. The likely development of a nor'easter off the Mid-Atlantic coast brings a threat for wintry weather and potentially heavy snowfall across interior portions of the Northeast and Mid-Atlantic, 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 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. There may also be some light precipitation moving into California and the Southwest next weekend, while Gulf moisture return could produce some showers over the western-central Gulf Coast. Above normal temperatures are expected ahead of the cold front as it slides through the East later this week but by Friday and especially next weekend, much of the CONUS should be near or below normal. The greatest and most widespread readings of below normal temperatures will be across much of the West and into the northern Plains, trending colder with time. By Sunday and Monday, daytime temperatures across the northern Plains could be as much as 20 to 30+ degrees below normal, equating to widespread single digit temperatures with some below zero readings across eastern Montana and North Dakota. 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