Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 PM EST Thu Dec 03 2020 Valid 12Z Sun Dec 06 2020 - 12Z Thu Dec 10 2020 ...Overview... A surface low/nor'easter is forecast to be pulling off the Maine coast Sun after causing potentially heavy precipitation to the Mid-Atlantic and Northeast over the weekend. Troughing should persist across the East through the period but lessen in magnitude. An upper trough is forecast to drop southward across the western U.S. early in the week, close off, and settle west of Baja California underneath zonal flow/ridging over the Northwest. This feature will likely get pulled back inland into the Southwest in some form around Thu. ...Guidance Evaluation/Preferences... 00Z/06Z model guidance continues to have some compatibility issues with various areas of the forecast, despite semi-good agreement with the longwave pattern. First, with the Eastern Seaboard low, a multi-model blend seemed to work fairly well. In general the trend with that low may be a little faster by Sun than in previous days. Preferred the 06Z GFS deterministic run rather than the 00Z run; the latter was more suppressed south with its surface and 500 mb lows, and the incoming 12Z guidance supports this conclusion. The trough over the eastern half of the CONUS into early next week seemed to have reasonable agreement in the guidance, though did not prefer the 00Z CMC, since Mon its axis was shifted eastward compared to consensus and then additional energy broadened its trough Tue. Though, it is not out of the question given the differences in shortwave energy dropping through the western side of the trough. With the trough/low in the West, early in the week the 00Z/06Z GFS deterministic runs as well as the bulk of GEFS members have a more northeastward upper low position than the non-NCEP guidance, and settles a bit east into midweek as well. Tended to prefer the well-clustered 00Z EC/CMC/UKMET/EC mean guidance there. Then, questions emerge regarding the timing and position this feature will phase back with the northern stream. Around Wed, differences increase regarding shortwave troughing coming through the eastern Pacific toward the Northwest in the northern stream flow. GFS runs and the CMC bring a shortwave into the Pacific Northwest, whereas the 00Z EC has troughing much farther west in the Pacific. Meanwhile, given the spread, the ensemble means end up pretty flat, but with some hint of troughing in the GEFS mean near the CMC/GFS trough. This feature affects the fate of the closed low; the 00Z EC and its mean both keep more stream separation Thu, while the other guidance phases the streams as the northern stream trough position is in a position to do so. For now, by day 7/Thu, just blended the GEFS and EC means as a compromise and will see how models trend. ...Sensible Weather and Hazards... Potentially impactful system exiting the Northeast will pull away Sunday, leaving the CONUS in a relatively dry pattern for the period. Periods of precipitation look to occur mainly in the Northwest, with light to modest amounts of higher elevation snow and valley rain, and over Florida for the first part of the week as fronts move across. Chances of light rain could also increase across the Southwest toward Texas ahead of the upper low/trough by the latter part of the week. Ridging will promote milder than normal temperatures for the High Plains to Upper Midwest/Corn Belt into the central Plains, perhaps by 15-25 degrees. Actual temperature readings will approach 60 degrees as far north as South Dakota on Tuesday. Colder than normal temperatures are favored for much of the East to start the period (Sun-Tue) 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 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