Extended Forecast Discussion NWS Weather Prediction Center College Park MD 325 PM EST Mon Dec 13 2021 Valid 12Z Thu Dec 16 2021 - 12Z Mon Dec 20 2021 ...Overview... A fairly progressive weather pattern is expected in the medium range period especially in the northern stream, as an initial deep surface low lifts quickly northward into eastern Canada, though its trailing front could stall across parts of the central and eastern U.S. and produce heavy rainfall late this week. A series of troughs will make their way across the northern tier of the U.S. during the period, while a southern stream trough may separate from the northern prevailing flow and move slowly across the Southwest/northern Mexico this weekend. Ridging over the southeast quadrant of the country should get suppressed early next week and bring temperatures closer to normal after a mild/warm spell. ...Guidance/Predictability Assessment... Deterministic and ensemble models for the 00/06Z cycle remain fairly well clustered with the overall pattern described above. Early in the period, GFS runs were slightly west of consensus (including the 00Z ECMWF/UKMET/CMC) with their positions of the surface low north of the Great Lakes early Thursday, so those were not weighted as heavily. But overall a blend of the deterministic guidance worked well for the WPC forecast, with emphasis especially on the 00Z ECMWF. By the weekend, there are some initially minor differences with the aforementioned possibility of stream separation in terms of timing and whether an embedded closed low may form, and with strength of energy within northern stream shortwave troughs as well. No particular trend was seen with the southern stream trough as models are waffling with the details. In the northern stream, the 00Z CMC was notably stronger than consensus with another shortwave tracking through the Midwest/Great Lakes by day 5/Saturday, so weighted it less by that point. Farther upstream, another energetic shortwave looks to dive into the Pacific Northwest over the weekend, but questions remain with its strength and how far south it may dive and interact with the southern stream feature by Monday. Thus by day 6/7, the WPC pressures/fronts forecast used about half ensemble mean guidance (from the 06Z GEFS and 00Z EC) to temper the individual model differences. This was in good agreement with the previous WPC forecast products. ...Weather/Hazards Highlights... A potent upper trough along with a strong surface low will lift quickly northeastward through the north-central U.S. Wed into Thu, with some lingering snow and high winds possible in the Upper Great Lakes region Thu. Southeast of the deep low track, upper ridging should support some record warm temperatures ahead of the cold front, with positive anomalies upwards to 15-25F shifting from the east-central U.S. to the East Thu-Sat. The trailing portion of the front will meanwhile stall across the South, with an axis of enhanced rainfall and potential for local runoff and flooding/flash flooding issues from the southern Plains through the Mid-South/Ohio Valley with return flow from the Gulf of Mexico. A developing frontal wave may also bring a swath of wintry precipitation from the Great Lakes through northern New England into the weekend as the enhanced rainfall axis drops southward across the Gulf Coast and Southeast as cooling high pressure builds across the Plains and to the East. Upstream, a moisture surge with moderate precipitation will work into California and across the Northwest Thu ahead of an approaching Pacific system. This system will gradually weaken into Friday. Moisture from another Pacific system may then impact the Pacific Northwest with moderate to possibly locally heavy precipitation amounts Fri into the weekend. Tate/Schichtel 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 Hazards: - Heavy precipitation across portions of the Pacific Northwest and the Northern Great Basin, Fri-Sat, Dec 17-Dec 18. - Heavy rain across portions of the Lower Mississippi Valley, the Central Appalachians, the Tennessee Valley, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Thu-Fri, Dec 16-Dec 17. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sat-Sun, Dec 18-Dec 19. - Heavy snow across portions of the Northern Rockies, Sat, Dec 18. - High winds across portions of the Great Lakes, Thu, Dec 16. - Heavy precipitation across portions of mainland Alaska, Sat-Sun, Dec 18-Dec 19. - Heavy snow across portions of mainland Alaska, Sun, Dec 19. - High winds across portions of mainland Alaska and the Aleutians, Sat-Sun, Dec 18-Dec 19. - Much below normal temperatures across portions of mainland Alaska, Thu, Dec 16. 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