Extended Forecast Discussion NWS Weather Prediction Center College Park MD 120 PM EST Sat Dec 18 2021 Valid 12Z Tue Dec 21 2021 - 12Z Sat Dec 25 2021 ...Pattern Overview and Guidance/Predictability Assessment... From Tuesday into Christmas Day/Saturday, most guidance continues to show that impulses rounding an impressively strong central Pacific upper ridge should provide periodic reinforcement of a deep upper trough aligned just off the West Coast. This trough will likely contain one or more upper lows, and a persistent flow of moisture ahead of the trough axis will likely produce a multi-day period of significant precipitation over parts of the West Coast (northern/central California in particular) with meaningful amounts also extending into the Rockies. Southern Canada/northern tier U.S. flow will carry along a couple waves with modest precipitation before lower 48 flow becomes more dominated by what energy may eject from the eastern Pacific trough. Farther south, a southern stream system tracking from northern Florida or vicinity into the Atlantic may bring some rain to the Southeast early in the forecast period. Expect a broadening area of above normal temperatures over the southern two-thirds of the central U.S. while the extreme northern Plains will tend to see below normal temperatures. The eastern Pacific upper trough does show better agreement on the large scale compared to previous days, but day to day details regarding feature interactions and timing continue to be problematic, especially beyond day 4. The 00z CMC at this time is the outlier as it separates energy dropping south from the Gulf of Alaska and accelerates it much faster into the Western U.S. than the other models. The 00z ECMWF is also a little faster with the troughing (more phased) while the 06z GFS is the most amplified and thus slowest. The ensemble means would favor a solution more in between the GFS and ECMWF which seems reasonable at this time. Trouble continues further downstream with the details of individual waves and fronts moving through the northern Plains and upper Great Lakes, the dynamics of some likely dependent on energy ejecting from the eastern Pacific upper trough. Meanwhile, large scale agreement is decent for the southern stream low moving across northern Florida or the Southeast into the western Atlantic with some timing and structure questions remaining. The updated WPC medium range progs for today used a blend of the operational models (with more emphasis towards the GFS/ECMWF over the CMC/UKMET) for days 3-4. After that, increasingly added more weighting to the ensemble means, eliminating the CMC due to problems discussed above. Kept some degree of ECMWF and GFS just for a little added system definition. This allowed for fairly good continuity with the previous shift with no major changes or adjustments needed. ...Weather/Hazards Highlights... Expect the western U.S. to see a broadening area of rain and mountain snow downstream from the persistent upper trough just offshore the West Coast, with some of this activity likely to be moderate to heavy. There are still question marks for some of the details due to uncertainty over individual systems and their interaction, but guidance signals and teleconnections for the mean pattern are consistent in showing northern and central California as having the highest precipitation potential through the period--including heavy snow over Sierra Nevada/Shasta areas. This region should see the most consistent moisture flow and experience southern/western facing terrain enhancement. It may take into the shorter-range time frame to asses the location and intensity of lower elevation rainfall with respect to local runoff/flooding issues. Effects from the multi-day accumulation of precipitation will also require monitoring even if it is not intense on any particular day. Some moisture may extend as far south as southern California and Arizona by mid-late week, while the Interior West and Rockies should see meaningful precipitation as well. Low pressure tracking from over or near northern Florida into the Atlantic will spread rain across portions of the Southeast on Tuesday and possibly into Wednesday. Some locations over the Southeast/Florida could see moderate to locally heavy rainfall. Some uncertainty persists for the intensity and northward extent of rainfall though latest trends are increasing the potential for meaningful totals over parts of the extreme Southeast. A leading northern tier system should spread mostly light snow from the extreme northern Plains eastward through the Great Lakes and New England during Tuesday-Wednesday. The system's accompanying front should be relatively dry. Any trailing features could produce some precipitation but again likely with fairly light amounts. Above normal temperatures focused over the central/south-central High Plains on Tuesday will steadily expand eastward across the southern two-thirds of the central U.S. with time and gradually become more anomalous. Through midweek highs in this region will tend to be 10-20F or slightly more above normal. Toward the end of the week there should be more locations with highs 20-30F above normal and some record highs could be challenged over the southern Plains by Christmas Day/Saturday. On the other extreme, the far northern Plains should see below normal temperatures except for perhaps being near normal Wednesday in between systems. A stronger push of cold air may reach Montana and the Dakotas next weekend. The unsettled pattern over the West should lead to near or above normal lows and near to below normal highs through the entire week. Santorelli/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