Extended Forecast Discussion NWS Weather Prediction Center College Park MD 216 PM EST Fri Dec 04 2020 Valid 12Z Mon Dec 07 2020 - 12Z Fri Dec 11 2020 ...Overview... An upper low is forecast to drop south and settle west of Baja California next week, cut off from the westerly zonal flow/low amplitude ridging in the Northwest before getting pulled back inland toward the Southwest in some form by Thu-Fri. Troughing over the East is expected to de-amplify as the week progresses. Fairly quiet weather is expected in terms of precipitation. ...Guidance Evaluation/Preferences... Regarding the trough in the East, at the beginning of the period Mon, most deterministic guidance (other than the 00Z UKMET) show a closed upper low within the broad trough centered over the Great Lakes region/Ohio Valley, somewhat of a change from previous forecasts. A surface low is likely to spin up in the western Atlantic and move quickly northward Mon-Wed given this upper pattern, potentially spreading light precipitation to portions of the Eastern Seaboard, but the heaviest precipitation should remain offshore given the track. A multi-model deterministic blend was initially utilized for these features as guidance was well-clustered. Increasing model spread comes by midweek with increasing differences in the timing and placement of shortwave energy dropping through the west side of the trough. The 00Z ECMWF is most aggressive with this energy and eventually creates a closed low over Florida by Thu, with little ensemble support. The 00Z CMC was not preferred either as it pinched energy off into the Southern Plains Wed within the broad ridge. Preferred a solution closer to the GEFS/EC means for now to minimize model-to-model and run-to-run differences. In the West, guidance has been persistent with the cutoff upper low settling west of Baja Tue-Wed. GFS runs remain on the eastern side with their position, with the 00Z UKMET a bit west and the 00Z ECMWF/CMC offering a seemingly good compromise position. This round of guidance generally has the low remaining closed through Thu, with the exception of the CMC. Differences continue to abound later in the week with the amount and timing of northern stream energy coming from the northeastern Pacific that will interact with the upper low, so once again a GEFS/EC mean blend seemed like the safest solution for now. ...Sensible Weather and Hazards... As mentioned above, there could be light precipitation over portions of the Mid-Atlantic and eastern parts of the Northeast Mon given the Atlantic surface low. Florida could also see showery conditions through Tue associated with a frontal system. The most notable precipitation is forecast for the Pacific Northwest, with some modest amounts especially in the Coastal Range and Cascades around Tue, and rounds of light amounts through the week. Snow levels could be relatively high, but the higher elevations may see some snow, with light amounts spreading to the Northern Rockies at times too. Later in the week, rain could develop across the Southwest and Southern Plains as the upper low/trough approaches. Ridging will promote milder than normal temperatures by 15-25 degrees across the High Plains to the Upper Midwest/Corn Belt. Actual temperature readings could approach 60 degrees as far north as South Dakota on Tuesday. Colder than normal temperatures are favored for much of the East for the early part of the week, before trending back toward more typical values. Tate Additional 3-7 Day Hazard 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 Pacific Northwest, Tue, Dec 8. - Flooding possible across portions of the Southern Appalachians, the Mid-Atlantic, the Northeast, and the Central Appalachians. - High significant wave heights for coastal portions of the Pacific Northwest, Mon, Dec 7. 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