Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 PM EST Mon Dec 20 2021 Valid 12Z Thu Dec 23 2021 - 12Z Mon Dec 27 2021 ...Pattern Overview and Guidance/Predictability Assessment... During the extended period there will be multiple shortwaves tracking around the very strong ridge over the central Pacific Ocean into the Bering Sea/Alaska which will dive into and reinforce an amplified trough whose axis should be near the West Coast, possibly drifting slightly inland later in the period. These ejected impulses are depicted to be carried downstream that will support the development of a pair of low pressure systems. With a broad trough near the West Coast, a large area of precipitation will be maintained for several days. Also of note, a strong temperature contrast will set up between well above normal temperatures progged for the Central and Southern Plains while well below normal temperatures settles in over the extreme Northern Plains. 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. There has been a notable variability with the timing/strength/progression etc the past few models runs but is favoring a less amplified solution. 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.To maintain a since of continuity, the WPC model preferences included the 00Z/06Z GFS, 00Z ECWMF/CMC/UKMET with increasing weighting of the EC ensemble, the NAEFS and GEFS means by the latter periods. ...Weather/Hazards Highlights... A persistent plume of moisture will be directed onshore the West Coast ahead of multiple shortwaves digging into the West Coast mean trough aloft with moisture and terrain enhancement extending as far east as the Rockies and to southern California/Great Basin. Moderate to heavy precipitation can be expected over the western half of the Pacific Northwest and portions of California, which will increase the 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. Abundant lift and moisture into the favored upslope areas of the Oregon Cascades and the Sierra Nevada will likely result in the highest accumulations and liquid equivalents. Meaningful precipitation will likely be observed across the highest terrain of the Great Basin and the Central and Northern Rockies. 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 filtering into the 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. Across the northern tier, an initial system is expected to spread spread precipitation across the northeastern section of the country. Another low pressure system will pass through the northern states by next week ushering precipitation with some spots of snow to northern states and also the east-central U.S. Temperatures will be colder than seasonal average, with many locations in the Dakotas and Montana averaging 15 to 30 degrees F below normal for late December. Additionally, the West will begin to trend cooler by the end of the extended forecast with daily highs around 10 to 15 degrees F cooler. For the Central and Southern Plains there won't be a White Christmas because temperatures will be quite toasty, with daily maximums around 15 to 25 degrees F above the mean for late December. A few locations may actually surpass 30 degrees F above average Friday 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. Daily records for highs/warm lows will be possible where temperatures are most anomalous and this warmth will spread into the East U.S. 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