Extended Forecast Discussion NWS Weather Prediction Center College Park MD 241 PM EST Tue Dec 14 2021 Valid 12Z Fri Dec 17 2021 - 12Z Tue Dec 21 2021 ...Guidance/Predictability Assessment... The latest guidance and the ensemble means are in fairly good agreement initially and through most of the extended period, thus leading to an above average confidence with this forecast. The WPC medium range product suite was primarily comprised of a blend of the 12Z CMC/ECWMF/GFS/UKMET and the NAEFS, GEFS and EC ensemble means. The inclusion of the means increased during the middle and later periods. The ensemble support was also strong and WPC product continuity was well maintained. These models solutions diverge more significantly early next week for Days 6/7 with the timing/amplitude of embedded systems, but most notably with the multi-stream flow over the East/Western Atlantic where the GFS develops a major coastal low while the ECMWF suppresses development. ...Weather/Hazards Highlights... A strong surface low pressure system will be lifting north through eastern Canada/Canadian Maritimes Thursday and Friday, with its associated trailing frontal boundary draped across the Mid-Atlantic to the Desert Southwest. Showers and thunderstorms along this slow moving boundary is expected to be moderate to heavy at times, which may lead to excessive rainfall and localized flooding concerns from the Southern Plains through the Mid-South/Ohio Valley. Well to the south, upper ridging should support some record warm temperatures ahead of the attendant cold front, with positive anomalies upwards to 15-25F shifting southward over the East to the South/Southeast into Friday and Saturday. A developing frontal low will bring a swath of wintry precipitation from the Great Lake states through northern New England into the weekend as the trailing enhanced rainfall axis drops southward across the Gulf Coast and Southeast as cooling high pressure builds across the Plains and to the East. Some of the guidance is suggesting that by early next week this feature may spin up and track up the East Coast; potentially yielding maritime threats, spreading rain near the Mid-Atlantic and possibly snow for the Northeast. Meanwhile, moisture surging onshore the West Coast is causing moderate precipitation across the Northwest and into California ahead of an approaching Pacific system; which is expected to gradually weaken into Friday. Moisture from another Pacific system may then impact the Pacific Northwest with moderate to possibly locally heavy precipitation amounts into the weekend, with additional energies digging southward over the eastern Pacific early next week that would eventually affect California. Campbell 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, Sat, 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, Fri-Sat, Dec 17-Dec 18. - Heavy rain across portions of the Southern Plains, Sat-Sun, Dec 18-Dec 19. - Heavy snow across portions of the Northern Rockies and the Northern Great Basin, Sat-Sun, Dec 18-Dec 19. 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