Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Sun Dec 11 2022 Valid 12Z Wed Dec 14 2022 - 12Z Sun Dec 18 2022 ...Major storm system will continue to bring possibly heavy snow into parts of the Upper Midwest and possibly the Northeast with a heavy rain/severe weather threat for southern areas around midweek... ...Overview... A deep upper level low will be in place over the central Plains by the start of the period Wednesday and is expected to track steadily eastward into the Northeast by this weekend. The associated surface low pressure system should weaken over the Midwest later this week in favor of a new developing coastal low near the Mid-Atlantic Coast. This system will continue to bring moderate to heavy snowfall across portions of the northern Plains, Upper Midwest, and Great Lakes through Thursday, with increasing confidence for some notable snowfall to impact interior portions of the Northeast as well by Thursday-Saturday. To the south, a heavy rain threat will continue across the Southeast Wednesday before spreading northward into the eastern/coastal Mid-Atlantic and Northeast as the likely nor'easter tracks north. The overall pattern calms down by next weekend, as lower amplitude troughing may linger across the northeastern U.S. leading to some lake effect snow, which combined with uncertain troughing in the West will lead to below normal temperatures across much of the CONUS. ...Guidance/Predictability Assessment... There remains good agreement as the period begins Wednesday for the presence of an upper low and surface low pressure system over the north-central Plains with gradual movement east-northeastward through the latter half of the week. Around Friday into next weekend, today's 00Z/06Z model cycle generally indicated slower movement of those features compared to yesterday's models and the WPC forecast. This would linger low pressure longer across the Great Lakes to Northeast for potentially higher snow totals, shown in this forecast with overall higher probabilities in the days 5-6 Winter Weather Outlook. Recent GFS runs generally had the upper low among the slowest to elongate and move, but were supported by the CMC and GEFS/EC ensemble means. Regardless, recent models continue to show another surface low is likely to originate across the Southeast and then move along the East Coast as a nor'easter, with reasonably good model agreement and a track/timing similar to continuity. The pattern across the Pacific and western North America shows considerable differences from model to model and run to run. An upper low meandering over the eastern Pacific may weaken later this week as it is influenced by an upper high near British Columbia and perhaps track east toward the CONUS. Meanwhile northern stream energy could spill into the west side of the central U.S. upper low and influence the pattern across the western/central U.S. depending on how it evolves. All these interactions of features remain quite uncertain at this time. The WPC forecast utilized a multi-model deterministic blend early in the forecast period but with increasing proportions of the ensemble means to over half by day 7 amid increasing uncertainty. ...Weather/Hazard Highlights... Deep/ample moisture will continue to stream northward ahead of the upper low over the central U.S. to fuel heavy to excessive rainfall into the Deep South and eventually up the East Coast. The latest day 4 WPC experimental Excessive Rainfall Outlook highlights a Slight Risk from the central Gulf Coast into parts of the Southeast/Tennessee Valley/Southern Appalachians, with northern areas sensitive to flooding because of above normal streamflows and soil moisture while southern areas could have higher rain rates due to more instability. The Storm Prediction Center also continues to monitor and highlight the potential for strong to severe convection across the central Gulf Coast on Wednesday. On the backside of the surface low, cold temperatures and wrap-back moisture will continue to support moderate to heavy snowfall continuing across parts of the northern Plains into the Upper Midwest and Great Lakes with some mixed precipitation/freezing rain potential in the transition zone as well as along the eastern edge of the precip shield near/north of the warm front. Then there is increasing confidence for surface low development tracking along/near the East Coast Thursday into the weekend. Even small shifts in this low track could cause differences particularly with precipitation types in the East, so the forecast will continue to be refined. At this point, it looks like coastal areas of the Mid-Atlantic and Northeast may stay rain while interior areas could see several inches of snow. As the system exits the East next weekend, some light lake effect snow showers may continue 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 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 generally 10-20F below normal for high temperatures will be across much of the West and into the northern Plains. Tate/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