Extended Forecast Discussion NWS Weather Prediction Center College Park MD 315 PM EST Fri Nov 27 2020 Valid 12Z Mon Nov 30 2020 - 12Z Fri Dec 04 2020 ...Dynamic and multifaceted storm to impact the northeast quarter of the CONUS early next week... ...Guidance Evaluation/Preferences... The largest model differences and forecast uncertainty reside with the evolution of the Pacific Northwest energy on Day 3 as it carves out a deeper trough over the Rockies and eventually the south-central Plains later in the period. The ECMWF (and to some degree the UKMET) offer a slower solution with the shortwave energy closing off over the Four Corners region while the GFS runs (06/12Z) show a flatter, progressive solution. Additional uncertainties exist in the northern stream as another shortwave trough could carve out a deeper trough or closed low over the northern tier (as depicted by the latest ECMWF/UKMET). Given the timing and synoptic differences that become more significant in the day 6/7 time frame, there was a heavier weight of the ensemble means utilized as a compromise that blended well with continuity as well. ...Sensible Weather and Hazards... As low pressure occludes over the Appalachians and eastern Great Lakes early next week, a surge of warm/moist air on the heels of breezy southerly flow will result in a large precipitation shield of moderate to locally heavy rainfall across the Mid-Atlantic northward into the New England. The urban corridor along the coast will be most susceptible to flooding in this setup. Wrap around cold air will transition any precipitation over to snow with some locations receiving appreciable amounts. As the low exits through the St. Lawrence Valley, flow over the Great Lakes will wring out additional snowfall or mixed rain/snow. For temperatures, the largest departures from normal during the period will be across southern US, southeast, and Florida where anomalies are forecast to be 10-20F early next week. South Florida may not even break 70 degrees on Tuesday to start the month of December. Incoming system to the Pac NW will be rather moisture-starved with only light amounts for the coastal ranges and Cascades Monday and across the Rockies Tuesday. Path of the upper trough/low will dictate how far west the cooler push of air will travel, favoring along/east of the Rockies per the forecast thinking. As the trough heads east of 100W, increased Gulf moisture will spread northward which favors an expanding area of rainfall along the northwest Gulf Coast into the lower Mississippi Valley late Wed into Thu. This should move eastward into Friday, perhaps through the Southeast into the southern Mid-Atlantic, depending on frontal wave development. Fracasso/Taylor 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 Southern/Central Appalachians, the Tennessee Valley, the Ohio Valley, the Great Lakes, the Southeast, the Mid-Atlantic, and the Northeast, Mon-Tue, Nov 30-Dec 1. - Heavy snow across portions of the Great Lakes and the Ohio Valley, Mon-Tue, Nov 30-Dec 1. - Heavy rain across portions of the Southern Plains, the Lower Mississippi Valley, and the Southeast, Wed-Fri, Dec 2-Dec 4. - Heavy rain across portions of the Central Appalachians, the Mid-Atlantic, and the Northeast, Mon-Tue, Nov 30-Dec 1. - Flooding occurring or imminent across portions of the Southeast. - High winds across portions of the Northeast, Mon, Nov 30. - Much below normal temperatures across portions of the Southern Plains, the Lower/Middle Mississippi Valley, the Tennessee Valley, the Southern Appalachians, and the Southeast, Tue-Wed, Dec 1-Dec 2. 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