Extended Forecast Discussion NWS Weather Prediction Center College Park MD 411 PM EST Tue Jan 05 2021 Valid 12Z Fri Jan 08 2021 - 12Z Tue Jan 12 2021 ...Overview... A progressive synoptic pattern will prevail over the CONUS through the period with a tendency for mean flow aloft to evolve toward a larger scale West Coast (or a little inland) ridge and eastern Canada into southern Plains trough by next Tue. Primary systems of interest will affect the Northwest, South, and Southeast. Eastern Pacific systems should track into Canada with their fronts producing daily light to moderate precipitation for especially coastal Washington and Oregon. Over northern areas the amounts could trend a little heavier toward the end of the period. Lead system in the Southeast Friday will move off the coast and out to sea as a southern Rockies system heads for the Texas coast. The second system may bring moderate to heavy rain to areas near the Texas/Louisiana coast. Its front will move eastward through the Gulf and perhaps off the Southeast/Mid-Atlantic coast by next Mon-Tue. Pacific origin of most flow reaching the lower 48 will keep any arctic air in Canada for the next week but the mean trough aloft extending into the southern Plains and succession of southern tier systems will promote cooler than normal temperatures over much of the southern U.S. ...Guidance/Predictability Assessment... Ensemble means continue to provide good agreement and continuity for the overall pattern evolution but recent operational model runs show a lot of uncertainty for a number of embedded features, tempering confidence in specifics--especially by next week. Even earlier in the period there has been some disagreement over eastern Canada flow that could drop a compact upper low into northern New England, though seemingly with fairly modest influence on specifics of the leading system tracking into the Atlantic. As for the evolution near the East Coast by next Tue, recent ECMWF runs have been the most stable even in spite of some pronounced variability in details farther west (seen in other models as well). Perhaps not a surprise, the new 12Z ECMWF has changed its details yet again with pronounced effects at the surface by next Tue. Fast GFS runs have been trending slower over the past 12 hours with the new 12Z run getting closer to the 00Z ECMWF timing. Very ill-defined surface reflection in the ensemble means thus far and 12Z CMC adjustment of dampening this low in favor of a low well to the north further highlight the continued uncertainty. Over the West the primary notes are a recent CMC tendency to exaggerate its amplitude of shortwave energy and then late in the period a 12Z GFS adjustment away from the amplified shortwaves seen in the 00Z/06Z runs. Otherwise, resolving the character of shortwaves encountering the western ridge aloft is a low-confidence endeavor. Guidance comparisons led to starting with an operational model blend early followed by a steady increase of 06Z GEFS/00Z ECMWF mean input, reaching 60 percent total by day 7 Tue. At that time operational model weight leaned more toward the past two ECMWF runs. The resulting forecast provided good continuity from the previous cycle. ...Weather/Threats Highlights... The Pacific Northwest will continue to see periods of precipitation about every day or so Fri-Tue. Current guidance is not yet unanimous but there is a decent signal that heaviest rain/high elevation snow will fall next week. Low pressure near the southeastern coast late this week will turn northeastward and spread some precipitation over the Carolinas and southern Mid-Atlantic region Fri-Sat. The northern/western part of the moisture shield may contain snow. The Winter Weather Outlook probabilities depict the most likely area of meaningful snowfall, though potential is on the lower side of the spectrum in absolute terms. The most likely track of this system would keep precipitation away from New England except for perhaps far eastern areas. Next system out of the southern Rockies will tap the Gulf for moisture reaching into the Texas and Louisiana coasts around late Sat through at least Sun night. This could lead to a heavy rain threat for east-central Texas to the central Gulf Coast though the heaviest rainfall may be just offshore. To the north, temperatures may be cold enough for some light wintry weather near the ArkLaTex. Confidence is low for precipitation coverage and type over the East toward the end of the period. Northern tier locations, especially from Montana into Minnesota, will be consistently well above normal for temperatures. Expect some plus 10F or greater anomalies each day. Morning lows should generally be farther above normal than the daytime highs and plus 20F or greater anomalies are possible for Fri-Sat lows. California will tend to be above normal but with only single-digit anomalies. The South will likely see below normal highs most of the period, with one area of minus 5-10F or so anomalies for highs behind the first system and then another from the central Rockies through the southern Plains/South in association with the second system. Rausch/Fracasso 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, Mon-Tue, Jan 11-Jan 12. - Heavy rain across portions of the Lower Mississippi Valley and the Southern Plains, Sun, Jan 10. - Heavy snow across portions of the Central Rockies, the Central Plains, and the Southern Plains, Sat, Jan 9. - Flooding occurring or imminent across portions of the Southern Plains, the Mid-Atlantic, and the Pacific Northwest. - Flooding likely across portions of the Mid-Atlantic. 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