Extended Forecast Discussion NWS Weather Prediction Center College Park MD 129 AM EST Thu Dec 26 2019 Valid 12Z Sun Dec 29 2019 - 12Z Thu Jan 02 2020 ...Significant winter storm expected to impact portions of the Upper Midwest and Upper Great Lakes this weekend... ...Overview and Guidance/Predictability Assessment... A relatively deep upper-level and surface low are forecast to be in place across the Midwest Sunday morning (day 3), with a potential winter weather event for portions of the Upper Midwest/Great Lakes expected to be ongoing. Model consensus for this system has improved substantially over the past couple days, and all deterministic solutions show the deep surface low moving relatively slowly into the upper Great Lakes by Mon morning and into Ontario by Tue morning. This is a trend toward a slower progression of this feature shown by some of the ECMWF yesterday. Farther east, a triple point low is expected to develop somewhere across the Northeast on Mon, although models differ on the exact timing of this feature, but agree that it should remain relatively week and progressive. Overall the GFS has outperformed the ECMWF during the medium range with this system, however, so was reluctant to lean too heavily on the ECMWF and opted to go with a GFS/ECMWF blend. Farther west, a similar scenario as seen in at least a couple instances recently looks to occur again, with nearly all guidance forecast a shortwave to separate from an active North Pacific jet and dive southward along the U.S. West Coast as it amplifies, with development of a closed upper low expected on Mon. Most guidance shows the system continuing a slow drift southeastward through Tue before eventually coming under the influence of an amplifying trough farther north, and moving east more quickly. The ECMWF continues to be a bit less amplified initially, and slower to close the feature off than the majority of other guidance (and is even a bit less amplified with the wave on Mon than the ECENS ensemble mean - a bit of a red flag for the deterministic solution). 12Z/18Z guidance showed some consensus that a surface low could develop across the Southern Plains by Wed night/Thu as the merged upper trough moves east. The 00Z GFS made a significant change by days 6-7 (Wed-Thu), and is now much slower to move the trough/upper low eastward across the Southwest/northern Mexico and merge it with the northern stream trough, with would keep a phased southern stream low from developing in the manner shown by previous guidance. Overall, models are fairly inconsistent by that time frame, so a more ensemble based blend seemed in order. The WPC forecast was initially based on a blend of the 18Z GFS/12Z ECMWF during days 3-5, with a shift to majority ECENS/GEFS ensemble mean weighting during days 6-7. ...Weather Highlights/Threats... As low pressure develops and begins to deepen moves northeastward across the Midwest/Great Lakes Sun-Mon, an area of precipitation is expected to accompany the system. Potentially significant accumulating snows are expected Sun-Sun night across portions of the Upper Midwest/Great Lakes. Accumulating winter weather will also be possible for northern New England Sun/Sun night, likely continuing into part of Mon before changing to rain. Please see the medium range winter weather outlook products for more details on the winter weather threat. Farther south along the cold front, rain is expected across the Southeast, Appalachians, and Mid-Atlantic regions through Sun night before the cold front moves offshore on Mon. Across the Pacific Northwest, as the shortwave moves onshore early next week, rain and mountain snow are forecast to develop across the coastal ranges and Cascades in Oregon and Washington, potentially spreading farther inland into portions of the northern Rockies by midweek. As the next cutoff southern stream system begins to interact with a northern stream trough and approaches the central U.S. by next Tue-Wed, ample moisture from the Gulf of Mexico is forecast to be pulled northward into the system, with the potential for widespread showers and thunderstorms across portions of the western/central Gulf Coast and lower Mississippi Valley, with the potential for areas of heavy rain. Confidence in this aspect of the forecast is a bit below average at this time, however, given model variability with the forward speed of the cutoff upper low. Temperatures are forecast to be well above average across the central and eastern U.S. on Sun ahead of the low pressure system. High temperatures of 15 to 25 deg F above average are forecast across portions of the Ohio Valley, central Appalachians, and Great Lakes, with a much of the eastern third of the country at least 10 deg F above average ahead of the cold front. Low temperatures across these area on Sunday are forecast to be 20 to 35 deg F above average, with a number of record high minimum temperatures likely. These warm conditions should moderate somewhat by Mon as they shift to the Mid-Atlantic/Northeast. The cold front forecast to arrive by Sun-Mon will bring an end to the warm temperatures from west to east, with temperatures returning to near seasonal norms. Meanwhile, given persistent lower heights across the interior western U.S., highs are forecast to be 5 to 10 deg below average from the Great Basin to much of the Southwest and portions of the Great Basin and central/southern Rockies through much of the forecast period. Ryan Additional 3-7 Day Hazards information can be found on the WPC medium range hazards chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, 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/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml