Extended Forecast Discussion NWS Weather Prediction Center College Park MD 319 PM EST Fri Dec 17 2021 Valid 12Z Mon Dec 20 2021 - 12Z Fri Dec 24 2021 1830 UTC Update... No major changes were needed for todays update to the WPC medium range progs. The 00z/06z suite of models remained fairly consistent to their previous runs and so only minor changes were made to frontal structures and placements from the overnight shift. The forecast was good enough for an operational model blend through about day 5, quickly increasing incorporation of the means thereafter to account for forecast variability and instability, particularly regarding details and evolution of Western U.S. trough energies. The 00z ECMWF was still considerably faster on day 7, with a trough making it all the way into the Plains, while the better consensus favored something more back towards the West Coast. For this reason, limited ECMWF was used for days 6 and especially 7. See the previous discussion below for additional details. Santorelli Previous Discussion below issued at 0658 UTC... ...Pattern Overview and Guidance/Predictability Assessment... The majority of guidance suggests that an amplified mean trough aloft over the far eastern Pacific will be the dominant feature during the period, bringing multiple days of precipitation to parts of the West Coast states with lesser amounts farther inland. To the east a southern stream shortwave tracking eastward from the southern Plains Monday onward will support a western Gulf into eastern Atlantic system spreading rain across the Gulf Coast and Southeast. Expect mostly light precipitation with one or more northern tier waves/fronts carried along by the progressive mean flow. The large scale pattern will likely favor well above normal temperatures over parts of the central/southern Plains but below normal temperatures over the extreme northern Plains. For about the first half of the period a composite of 12Z/18Z operational models represented the most common ideas of guidance, though lingering uncertainties exist in multiple places. Latest solutions have adjusted a bit faster than previous forecast for the southern tier system while the models are still in the process of resolving specifics of a couple shortwaves within the southern Canada/northern tier U.S. stream. There is decent agreement in principle for the upper low off the West Coast, with latest trends adjusting the feature a little more offshore corresponding with the upper ridge over the Northwest/Interior West being stronger. Around midweek a strong bundle of energy diving south from Alaska begins to add to the forecast complexity, with recent individual model runs and ensemble members showing considerable spread for what happens. Possibilities include this energy merging with the initial upper low, forming a new upper low while ejecting energy from the initial low eastward, or itself becoming a mere progressive shortwave while the first upper low remains offshore (12Z and new 00Z ECMWF). Latest ensemble means have been providing a good middle ground among these various options, keeping an upper trough axis close to the West Coast into day 7 Friday--a reasonable idea based on teleconnections relative to the upstream ridge's positive height anomaly. Thus the forecast transitioned toward about half ensemble means (18Z GEFS/12Z ECens) while splitting the lingering GFS input between its 12Z/18Z runs and eventually switching ECMWF input to the 00Z/16 run that was slower with the West Coast trough (and keeping a little 12Z CMC). ...Weather/Hazards Highlights... The upper low initially offshore the West Coast and then digging trough energy from Alaska should provide favorable ingredients for periods of moderate to heavy precipitation over portions of the West Coast states with lesser amounts extending inland through the Great Basin and Rockies. Currently northern and central California have the best potential for seeing the highest five-day totals, aided by slow motion of the initial offshore system, a developing moisture plume, and favored southern/western facing terrain enhancement. Some guidance shows sufficiently heavy rainfall at lower elevations that there could be some local runoff/flooding issues while snow would be heavy over Sierra Nevada/Shasta areas. Confidence in the details of coverage and intensity of precipitation after midweek decreases though. Elsewhere, low pressure tracking from the western Gulf of Mexico through Florida and into the Atlantic will spread rain across portions of the South/Southeast. Some locations over the Southeast/Florida could see moderate to locally heavy rainfall but there is still a fair amount of uncertainty for intensity at any particular location as well as the northward extent of moisture. One or more northern tier waves/fronts may spread mostly light snow from the extreme northern Plains through the Great Lakes and New England. Upper ridging over the west-central U.S. will support persistently warm temperatures over the southern two-thirds of the Plains next week. Expect the coverage of plus 10-20F anomalies for highs to expand during the first half of the week and remain broad through Friday. A few locations may see highs even greater than 20F above normal. On the other hand the extreme northern Plains should see below normal temperatures. Monday should be the coldest day with highs 10-15F or so below normal over parts of Montana and North Dakota. 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 Hazards: - Heavy precipitation across portions of California, the Pacific Northwest, and the Northern Great Basin, Fri, Dec 24. - Heavy precipitation across portions of California, the Central Great Basin, and the Pacific Northwest, Mon-Tue, Dec 20-Dec 21and Thu, Dec 23. - Heavy snow across portions of the Central Great Basin and California, Tue-Fri, Dec 21-Dec 24. - Heavy snow across portions of the Central Great Basin, California, and the Southwest, Wed-Fri, Dec 22-Dec 24. 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