Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Mon Dec 20 2021 Valid 12Z Thu Dec 23 2021 - 12Z Mon Dec 27 2021 ...Pattern Overview and Guidance/Predictability Assessment... The large scale pattern continues to show better than average agreement and predictability. Shortwaves tracking around a very strong central Pacific into Bering Sea/Alaska upper ridge will periodically dive into and reinforce an amplified trough whose axis should be near the West Coast, possibly drifting slightly inland later in the period. Fast flow downstream--between low heights over Canada and a ridge tracking over Mexico and the southern Gulf of Mexico--will carry along ejecting features that should support a couple surface systems during the period. Expect the West Coast upper trough to support a broad area of precipitation over the West for multiple days with areas from western Oregon into the Sierra Nevada likely to see the highest five-day totals. Also of note, a strong temperature contrast will set up between much above normal temperatures over and east of the central/southern Plains and well below normal temperatures over the extreme northern Plains. Even with the above average confidence in the large scale pattern, embedded details have lower predictability so some specifics for the lower 48 forecast remain uncertain. The first major feature of note will be a West Coast shortwave that ejects eastward after early Thursday and likely produces a Plains through southeastern Canada/Northeast U.S. system around Friday and Christmas Day/Saturday. Over recent days there has been a fair amount of variability for the character of this energy near the West Coast and how quickly it will continue onward. An average of 12Z/18Z guidance eventually trending toward a mix of models and means provided a good intermediate solution early in the period and held close to continuity later as the 18Z GFS/12Z ECMWF ended up trending slower than in the previous cycle while the ensemble means and CMC continue to reflect a faster solution per continuity. Overall the new 00Z model runs have become less amplified with the West Coast shortwave as of early Thursday and differ for details of separate offshore energy, affecting coverage/timing of precipitation over southern parts of the West. This change has allowed the GFS and ECMWF to return to a faster surface progression over eastern North America though. The aforementioned strategy also worked well for resolving lower confidence details within and downstream from the persistent West Coast trough after ejection of the first shortwave. There is a reasonable signal in the models and ensemble means that another general area of low pressure should evolve over the central U.S. around Sunday-Monday. ...Weather/Hazards Highlights... A broad area of the West will see lower elevation rain and mid-high elevation snow with a persistent flow of moisture directed onshore ahead of shortwaves digging into the West Coast mean trough aloft, with moisture and terrain enhancement extending as far east as the Rockies. There will be periods when the precipitation will be moderate or heavy which may increase soil saturation and sensitivity to local runoff; however uncertainty on timing and shortwave interactions will temper confidence in resolving the exact location and intensity of lower elevation rainfall with respect to local runoff/flooding issues over California. The Oregon Cascades and parts of northern/central California are likely 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. Favored terrain over the Great Basin and northern/central Rockies will likely see meaningful precipitation as well. The timing and amounts of precipitation over southern parts of the West have become more of a question mark with latest adjustments in the models. Colder air pushing into Northwest may lead to very low snow levels by next Sunday-Monday while other parts of the West may also see declining snow levels as the upper trough axis nudges eastward. A late week into weekend system should spread precipitation across the northeastern quadrant of the country. Best potential for some snow will be across the extreme northern tier including the Upper Great Lakes and parts of New England. Confidence is only moderate at best for system details given model variability thus far. Another area of precipitation (some snow in northern areas) may develop over the northern and east-central U.S. toward early next week with another area of low pressure. The central and southern Plains will see much above normal temperatures for most of the period with plus 15-25F anomalies. Highs could even reach or exceed 30F above normal at some locations on Friday. Flow ahead of the late week system will push some of this warmth into the East as well. Daily records for highs/warm lows will be possible where temperatures are most anomalous. In contrast, temperatures that are 15-30F or more below normal should settle into parts of Montana and North Dakota. The West will trend gradually colder with time, leading to some highs 10-15F below normal by the weekend and early next week. Rausch/Campbell 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