Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Sun Dec 19 2021 Valid 12Z Wed Dec 22 2021 - 12Z Sun Dec 26 2021 ...Pattern Overview and Guidance/Predictability Assessment... Latest models and ensembles continue to show only slow evolution of the upper pattern through the Wednesday-Sunday period. Impulses flowing around the north side of a very strong central/east-central Pacific upper ridge should reinforce a mean trough whose axis should gradually drift from the eastern Pacific to the West Coast or very slightly inland. Fast flow will prevail downstream, between low heights over central Canada and an upper high forecast to migrate eastward across Mexico and the southern Gulf of Mexico. This pattern will favor multiple days of widespread precipitation over the West with the Sierra Nevada likely to see the highest five-day totals. Systems to the east of the Rockies will initially have dynamic support from southern Canada/northern tier U.S. flow and then become more dependent on features ejecting from the West Coast trough. Much above normal temperatures will prevail over the southern half to two-thirds of the Plains with some of this warmth extending eastward with time. Meanwhile the far northern Plains should see a pronounced colder trend by next weekend. Guidance through the 12Z/18Z cycles still displayed some important detail differences for features within the eastern Pacific/West Coast mean trough with eventual implications on the forecast across the lower 48. During mid-late week the primary issue has been the character of digging energy that replaces an initial short-term upper low. GFS runs had been eager to close off another offshore upper low whose energy takes longer to eject inland, thus leading to a slower system reaching the East by next weekend. Aside from one ECMWF run, that model has tended to be more open with the shortwave with faster ejection--but generally not as fast as latest CMC runs. Ensemble means have been providing a good middle ground among these options. By next weekend the means end up gravitating toward the 12Z ECMWF idea of a well-defined system reaching the Great Lakes on Christmas Day/Saturday and the Canadian Maritimes on Sunday. The fast nature of the mean flow over most of the lower 48 would seem to favor this idea versus the slower 12Z/18Z GFS as well. The new 00Z GFS has made a significant adjustment toward the majority cluster, now showing more north-south elongation of the mid-late week energy instead of a slow upper low and then faster ejection thereafter. Later in the period the GFS/ECMWF and their means compare fairly well for the West Coast trough as additional energy reinforces the feature, with differences well within typical error/guidance spread for forecasts that far out in time. Early in the period over the East, consensus has been stable in recent runs for the system crossing southeastern Canada and northeastern U.S. around midweek. However guidance is still having some difficulties in resolving the details of the system expected to be just off the North Carolina coast early Wednesday and continue over the Atlantic thereafter, favoring an intermediate approach. The operational model blend used to start the forecast for Wednesday began to incorporate the 18Z GEFS/12Z ECMWF means by day 5 Friday and then reached 60 percent total ensemble weight by day 7 Sunday. The forecast excluded the CMC by the latter half of the period and 12Z/18Z GFS weight was kept low enough to minimize influence of its slower handling of late week West Coast energy. ...Weather/Hazards Highlights... A broad area of the western U.S. will see rain and mountain snow during the period, supported by a persistent flow of moisture ahead of an eastern Pacific/West Coast upper trough. Some of this activity will likely be moderate to heavy though it will take additional time to resolve the finer details due to uncertainty over individual systems and their interaction. However guidance signals and teleconnections for the mean pattern still suggest that northern and central California should have the highest precipitation potential over the five-day period--including heavy snow over Sierra Nevada/Shasta areas. The greatest liquid equivalent totals should be over favored terrain along the Sierra Nevada. It may still take into the shorter-range time frame to asses the location and intensity of lower elevation rainfall with respect to local runoff/flooding issues over California. Effects from the multi-day accumulation of precipitation will also require monitoring even if it is not intense on any particular day. Some moisture should extend as far south as southern California and Arizona after midweek while favored terrain over the Great Basin and northern/central Rockies will see meaningful precipitation as well. Low pressure tracking away from near the North Carolina Coast may be accompanied by a little rain early Wednesday while passage of a southeastern Canada system and a weak upstream feature may produce light snow over the upper Great Lakes and northern New England. Latest guidance is signaling a more organized system could produce snow or rain depending on latitude between the northern Plains and Great Lakes/Northeast, though differences in evolution keep confidence low for resolving coverage and intensity of precipitation at this time. Above normal temperatures over the southern two-thirds of the High Plains on Wednesday will expand eastward across the Plains and into the Mississippi Valley later in the week, with increasing coverage of highs at least 15-25F above normal. Such readings will likely persist over the southern half of the Plains/Mississippi Valley through next weekend. A few record highs may be possible over Texas during this warm period. Some of this warmth should also reach the East with highs reaching 10-20F above normal by the end of the week or next weekend. On the other hand the far northern Plains should see a pronounced colder trend during the weekend as temperatures drop to 15-30F below normal over parts of Montana and North Dakota. The unsettled pattern over the West should lead to near or above normal lows and near to below normal highs through the entire week. Western highs should gradually trend cooler with time as the axis of the eastern Pacific upper trough reaches at least as far as the West Coast. Rausch 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 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