Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM 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 across the West 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. For the eastern Pacific upper trough the 12Z/18Z models and means have improved their relative clustering thanks to the 12Z ECMWF finally adjusting to a more compatible solution versus the previous couple runs that were questionably progressive with Gulf of Alaska energy that digs southward Tuesday-Wednesday. The current consensus for the mean trough aloft, as well as the best precipitation focus over northern/central California, compare well to teleconnections relative to the upstream ridge's positive height anomaly center. However for specific day-to-day details there is still a fair amount of ensemble spread and complexity over interaction among features and their exact position. This includes not only the initial offshore upper low and the aforementioned Gulf of Alaska energy but yet another strong bundle of energy that may drop into the mean trough late in the week. In the new 00Z model runs the CMC has strayed significantly for some details but the GFS/UKMET remain fairly close to consensus. The ECMWF has gone back to a faster scenario. The progressive nature of flow to the east of the Rockies may continue to cause some trouble for resolving specifics of individual waves/fronts. Recently there has been improved definition of a northern Plains wave that tracks across the upper Great Lakes and southeastern Canada Tuesday onward but continuity/clustering have been lacking for trailing features whose dynamic support are increasingly likely to come from energy that ejects from the eastern Pacific upper trough. Meanwhile agreement and to some degree continuity continue to be poor for the precise structure, timing, and track of the southern stream system expected to be over or near northern Florida as of early Tuesday and continue into the Atlantic thereafter. New 00Z model clustering has generally gravitated to the slower side of the prior envelope, closest to recent GFS runs. A blend of 12Z/18Z models from Tuesday into early Wednesday eliminated the UKMET after the latter time due to straying fast with the western Atlantic system and gradually trended toward a late-period solution of 40 percent total 18Z GEFS/12Z ECMWF means with remaining input coming from the 18Z GFS and 12Z ECMWF/CMC. ...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. 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. 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