Extended Forecast Discussion NWS Weather Prediction Center College Park MD 220 PM EST Tue Dec 24 2019 Valid 12Z Fri Dec 27 2019 - 12Z Tue Dec 31 2019 ...Overview and Guidance/Predictability Assessment... 19Z update... Uncertainty regarding the prospect of cyclogenesis during the weekend over the central U.S. to the Northeast has lessened somewhat this morning as the ECMWF has continued to trend toward more interaction of the cyclone with the northern stream, resulting in a solution closer to that of the GFS. The latest 12Z ECMWF has decidedly shifted toward a full-blown cyclogenesis over the Plains. Meanwhile, the GFS remains steadfast in its idea of the southern stream wave reaching the southern Plains by Friday night, followed by quite a robust cyclogenesis process across the Plains through the Midwest over the weekend (days 4-5). This system bears quite a few similarities to a similar system several weeks ago for which the GFS significantly outperformed much of the other guidance in the medium range, the origin of which (for both) seems to be a shortwave initially over the data sparse North Pacific, south of the Aleutians. Given these factors, opted to lean heavily (70%) toward the GFS (06Z run) during days 3-5, along with a gradually increasing component of the ECENS/GEFS ensemble means as well (both of which provided a significant degree of support to an idea similar to the GFS). Following this idea, the phased system should cross the Northeast late Sun/Mon, with the cold front moving off the Eastern Seaboard by Monday morning. In addition, although the 12Z ECMWF has trended much closer to the GFS solution, its subsequent storm track remains much less progressive across the eastern U.S. compared with the GFS. The more progressive GFS solutions seem more reasonable. Models continue to show an additional trough/upper low digging southward along the West Coast Sun-Mon, with most deterministic solutions showing another cutoff low near/across southern California by Mon. Deterministic solutions are quite similar with the timing and evolution of this feature, although a look at ensemble members shows that spread is perhaps a bit higher than would be indicated by deterministic guidance alone. Given some general consensus among the guidance, a gradual trend toward heavier use of ECENS/GEFS ensemble means continued through days 6-7 (Mon-Tue). ...Weather Highlights/Threats... The upper low crossing the Southeast on Fri is expected to bring mountain snow to portions of the Southwest and southern Rockies. Ahead of the upper low, an overrunning setup across the southern to central Plains is expected to result in development of an area of precipitation, which should expand northward and eastward across the Plains and eventually the Midwest/Mississippi Valley through time as the eventual low pressure system spins up. Locally heavy rain will be possible for some areas, while in the northern portion of this precipitation shield, sufficient cold air may exist for precipitation to fall as a wintry mix/and or snow. The winter weather potential appears to stretch from the southern Rockies to the central High Plains on Fri, extending into the Midwest/Upper Great Lakes by Sat. Given at least somewhat improved confidence in this scenario, boosted winter weather outlook probabilities across the Midwest/Great Lakes Sat/Sat night. Please see the medium range winter weather outlook products for more details on the winter weather threat. As the cyclone center is forecast to track near the Canadian border of the Northeast, mild air from the eastern U.S. should keep wintry weather across the Great Lakes into interior New England from Saturday night into Monday. Rain is expected from southern New England down through much of the eastern and southeastern U.S. into Florida, followed by gradual clearing behind a strong cold front next Tuesday. Temperatures are forecast to be well above average across the central and eastern U.S. Fri-Sun ahead of the low pressure system. High temperatures of 10 to 20 deg F above average are forecast across a large area. The cold front forecast to arrive over the weekend will bring an end to the warm temperatures from west to east across the country, 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 Fri-Sun, with temperatures moderating somewhat by early next week. Kong/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 Hazards: - Heavy precipitation across portions of the Central Plains, the Central Rockies, the Central Great Basin, the Southern Rockies, the Southern Plains, and the Southwest, Fri-Sat, Dec 27-Dec 28. - Heavy precipitation across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Sat-Sun, Dec 28-Dec 29. - Heavy precipitation across portions of the Northeast and the Great Lakes, Sun-Mon, Dec 29-Dec 30. - Heavy precipitation across portions of the Southwest, Fri, Dec 27. - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Tennessee Valley, the Great Lakes, the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio Valley, Fri-Sat, Dec 27-Dec 28. - Heavy rain across portions of the Mid-Atlantic, the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Northeast, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Sun-Mon, Dec 29-Dec 30. - Flooding possible across portions of the Southeast and the Mid-Atlantic. - Flooding occurring or imminent across portions of the Pacific Northwest and the Northern Plains. 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