Extended Forecast Discussion NWS Weather Prediction Center College Park MD 254 PM EST Wed Dec 02 2020 Valid 12Z Sat Dec 05 2020 - 12Z Wed Dec 09 2020 ...Guidance Evaluation/Preferences... The most common theme among latest guidance is for varying degrees of eastern North America mean troughing downstream from an eastern Pacific/western North America mean ridge that should gradually trend flatter with time. Within and around this pattern there are numerous complications that will or could have an influence on sensible weather over some areas. The evolution over the eastern U.S./western Atlantic into the Canadian Maritimes through this weekend continues to have less confidence than is typically the case for the first half of the medium range period. The 00Z ECMWF/ECMWF mean have maintained their prior 12Z cycle adjustment toward aggressive amplification of Great Lakes flow and phasing with the upper feature tracking to the south--leading to a deep/westward solution over New England. Meanwhile other models/means through the 06Z cycle show less phasing and a farther east surface low track--including the GEFS mean which has trended eastward from 24 hours earlier. The 00Z UKMET was the farthest offshore and dry over all but extreme eastern New England. On the one hand there appears to be some attempt at convergence toward the proportion of closed low energy emerging from the Plains/Mississippi Valley relative to what pulls off southwestward into northern Mexico, but the forecast seems as uncertain as ever for specifics of Great Lakes flow. Peripheral influence of upstream energy dropping out of southern Canada could play a role and this energy has had considerable spread. The 00Z CMC was quite the western extreme while other models varied over the eastern half of the lower 48. Ultimately adjusted the forecast toward a compromise among the 06Z GFS/GEFS mean and 00Z ECMWF/ECMWF mean as the GFS cluster's evolution was somewhat in the majority in principle (minus finer timing/track issues) while ECMWF continuity merited some introduction of its concepts. In the new 12Z cycle the UKMET/CMC have both trended toward more aggressive Great Lakes amplification/phasing with faster surface progression, albeit with a track east of the ECMWF. The new 12Z ECMWF continues to show significant phasing but a surface track a bit east of the old run. The GFS maintains its lack of phasing aloft. The model/mean blend favored for the eastern system during the first half of the period downplays any individual solution for the aforementioned southern Canada energy dropping into the eastern half of the country, awaiting better clustering before depicting a more defined evolution. Meanwhile the past day has seen a pronounced trend toward more rapid eastward ejection of the upper low that should drop into northwestern Mexico as of early Sat. There is a moderate amount of spread for this ejection and some dependence on details of amplifying flow to the north. There are two primary issues over the West. The first is the shortwave expected to reach the Pacific coast during the weekend. Latest guidance is suggesting that some energy will continue around the northern side of the upper ridge but a meaningful bundle of remaining energy will drop southward in the form of a closed low whose center should reach west of Baja California by next Mon or Tue. Recent GFS runs have been the deepest with this low while the GFS/ECMWF have been a little east of the UKMET/CMC. Then toward the end of the period GFS runs have been faster/more amplified with a shortwave reaching the Northwest next Wed. While not an outlier the GFS is somewhat on the extreme side of the full ensemble spread and some solutions like recent CMC/ECMWF runs and 00Z CMC mean have maintained broad ridging instead. The favored solution reflecting a GFS/ECMWF and GEFS/ECMWF mean blend into Mon followed by gradual reduction of GFS influence and increase of the means provided a conservative starting point for the southward-moving upper low (likely better defined now per 12Z model consensus) followed by flat flow over the Northwest by day 7 Wed. ...Sensible Weather and Hazards... The system affecting the East this weekend is most likely to track from near the Mid-Atlantic northeastward along or a little offshore the New England coast into the Canadian Maritimes. A majority of the precipitation should be rain (potentially at least moderate intensity at some locations) while higher elevation snow could be possible from the central Appalachians into western/northern New England. Specifics will depend on the precise low track which remains uncertain. Behind this system, flow off the Great Lakes may promote some lingering light rain/snow and trailing energy aloft could generate one or more areas of light/scattered precipitation. The front trailing from the leading system will bring some rain of varying intensity to central and southern Florida. The shortwave reaching the Northwest coast this weekend (along with weakening leading front) will bring mostly light rain and high elevation snow. Most guidance suggests that the upper low developing from this energy and dropping south should be fairly dry, with the exception of the deepest side of the envelope for the upper low. Gradual flattening of the western upper ridge should allow more moisture and a front to reach the Northwest by Tue-Wed, with more widespread rain/snow over favored terrain from the coastal ranges into northwestern Montana. Pattern evolution during the period will lead to a gradual increase/expansion of warm temperature anomalies anchored between the northern High Plains and Upper Midwest. This area should see readings up to 10-15F above normal this weekend followed by some plus 15-25F anomalies Mon-Wed, with double-digit anomalies gradually extending southward into the central Plains. Less extreme warmth will prevail over the West with some localized plus 10F or greater anomalies possible by next week. Moderately below normal temperatures (mostly single-digit negative anomalies) will tend to prevail over the South and East. Rausch 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 Central Appalachians, the Great Lakes, the Ohio Valley, the Mid-Atlantic, and the Northeast, Sat-Sun, Dec 5-Dec 6. - Heavy rain across portions of the Mid-Atlantic and the Northeast, Sat, Dec 5. 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