Extended Forecast Discussion NWS Weather Prediction Center College Park MD 214 PM EST Sun Dec 19 2021 Valid 12Z Wed Dec 22 2021 - 12Z Sun Dec 26 2021 ...Pattern Overview and Guidance/Predictability Assessment... The upper-level pattern continues to favor a slower evolution through the end of the weekend. During this time, a very strong ridge will be anchored over central/east-central Pacific Ocean. There will be impulses tracking through the flow on the north side of this ridge, which help reinforce a mean trough over the eastern Pacific that drifts near/along the West Coast. 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. Well-above normal temperatures are expected across much of the Southern Plains before spreading to the east later in the extended period. In contrast, temperature will trend colder across the northern tier of the country. 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. 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 WPC blend began with a mix of the 00Z ECWMF/CMC/UKMET/GFS(and 06Z) and with time increased weighting of both the EC ensemble and GEFS means while decreasing the use of the CMC and UKMET. ...Weather/Hazards Highlights... Lower elevation rain and mid-high elevation snow can be expected for a broad area of the western states as a persistent flow of moisture is directed onshore ahead of an eastern Pacific/West Coast upper trough. There will be periods where the precipitation will be moderate or heavy which may increase soil saturation and increase sensitivity to local runoff; however given some of the uncertainty on timing and interactions, the exact location and intensity of lower elevation rainfall with respect to local runoff/flooding issues over California. Parts of northern/central California is expected to have some of the highest accumulations, especially heavy snow in the Sierra Nevada. These snows may have high liquid equivalents in the favored upslope areas of the Sierra Nevada range. 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. Temperatures ranging 15 to 25 degrees F warmer than seasonal average will be present over much of the Southern Plains will spread into the Mississippi Valley later in the period - a few record highs may be possible over Texas. A warm up of 10 to 20 degrees F above the mean may reach the East by the weekend. 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. A more organized system may spread snow and rain spread from the Northern High Plains to the Northeast. The far northern portion of the Plains will have temperatures plummet- with 15 to 30 degrees F 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. Campbell/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