Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Fri Dec 24 2021 Valid 12Z Mon Dec 27 2021 - 12Z Fri Dec 31 2021 ...Potential for heavy mountain snows in the West continues through early next week... ...Bitterly cold temperatures are forecast across portions of the Northwest to Northern Plains next week... ...Overview... A rather stagnant pattern is forecast for the medium range period led by a persistent and highly amplified northern Pacific ridge/upper high. Downstream, deep troughing is expected over the western U.S. as a likely closed mid-upper low drifts eastward over southwestern and south-central Canada, while shortwaves eject from the upper trough and are carried along in fast west-southwesterly mean flow, which will support a series of low pressure systems developing and tracking over the central and northeastern U.S.. Meanwhile, another upper high moving from the Gulf of Mexico early week into the Bahamas by midweek will spread ridging and above normal temperatures to the southeastern quadrant of the country south of the storm track. ...Guidance/Predictability Assessment... The latest model guidance remains in more or less good agreement with the large scale pattern through much of next week. There are some uncertainties in the details, particularly pertaining to a couple of short waves rotating out of the Western U.S. trough. The 12z/yesterday UKMET was much weaker with an initial shortwave on Monday-Tuesday into the Upper Great Lakes and eventually the Northeast than the consensus so it was not included in the WPC blend. Mid week, energy should drop down the West Coast with some guidance (such as the 12z and 18z yesterday ECMWF and GFS runs) indicating potential for a closed low over or off the central California coast by later next week. There is plenty of variability in this forecast with regards to timing and evolution so the forecast remains highly uncertain at this time. For the WPC blend, the guidance presented enough agreement for days 3-5 to warrant a purely deterministic model blend between the GFS, ECMWF, and CMC. After that, the ensemble means were introduced to help mitigate the smaller scale details differences which will take several days to resolve still. This approach provided good continuity with yesterdays WPC forecast as well. ...Weather/Hazards Highlights... Rounds of precipitation should continue into the West as the medium range period begins Monday as shortwave energy rounding the persistent upper trough periodically enhances moisture inflow. The highest snow totals are likely for the typical terrain enhanced areas of the Sierra Nevada, and portions of the Rockies Monday and Tuesday. The best moisture/precipitation focus should shift southward with time into southern California and the Southwest on Wednesday into Thursday. Generally progressive low pressure systems will spread periodic precipitation across the north-central and northeastern U.S. through next week. The best snow chances should be confined to the Great Lakes and interior Northeast, with rain to the south. The associated trailing cold front slowly sinking southeastward could provide a focus for enhanced rainfall across the Ohio Valley around Tuesday-Wednesday and toward the Tennessee Valley and Southern Appalachians/Southeast by Thursday-Friday of next week, but with uncertainty remaining regarding the amounts and axis of the heaviest rain. Mild to warm weather is expected across the south-central U.S. northeastward through next week. The southern Great Plains will be at the center of the greatest warm temperature anomalies on Sunday (20-35F above normal), with record daily high minimum and maximum temperatures likely. Anomalous to record warmth may press slightly eastward as the week progresses, with temperatures 20+ degrees above average expected to spread into parts of the Mississippi and Tennessee Valleys. Highs in the 70s will be widespread in the Gulf Coast states, with some temperatures exceeding 80F. Meanwhile, frigid temperatures are likely in parts of the northwestern and north-central U.S., spreading from Montana Sunday into more of the Northwest and Northern Plains through midweek. With temperatures 20-40 degrees below average, even high temperatures could remain below 0F for Montana and North Dakota around Tuesday and Wednesday. Temperatures are also expected to trend colder in the rest of the western U.S., with expanding coverage of highs 10-25F below normal next Monday-Thursday. Increasing numbers of locations along and just inland from the West Coast could see daily records for low maxima Sunday through Wednesday. Santorelli/Tate 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