Extended Forecast Discussion NWS Weather Prediction Center College Park MD 143 PM EST Mon Dec 23 2019 Valid 12Z Thu Dec 26 2019 - 12Z Mon Dec 30 2019 ...Overview and Guidance/Predictability Assessment... The latest guidance has an initial synoptic pattern with an amplified trough over the West and ridge over the Southeast that tracks toward the eastern U.S. by the end of the weekend. The northern stream will be fairly progressive while the southern stream will be slower. A consolidated vortex across the Arctic Ocean and an active Pacific wave pattern will keep any significant arctic air outbreaks away from the CONUS, with many locations, especially across the central and eastern U.S. experiencing unusually mild conditions for late December. The main weather feature for the extended period will be shortwave energy ejecting over the Southwest/Southern Rockies that will have an associated surface low pressure system track across the Midwest, the Great Lakes region and into the Northeast. In general, there is a moderate signal within the guidance for this feature however the spread greatly increases for the days 6 and 7 timeframe. The GFS is very progressive which translates to lifting the energy much further north than the rest of the guidance and nearly 12 to 18 hours quicker as well. The ECWMF and the UKMET are on the slower end of the spectrum, but reasonably clustered near the CMC (which seems to be a compromise of the ECWMF and the GFS). Following a similar approach to the previous forecast, at least initially, the model preference incorporated a blend of the 00Z ECMWF/06Z GFS and 00Z UKMET/CMC. Weighting of the ECENS/NAEFS ensemble means by day 5 were increased while decreasing influence of the GFS. This solution still supports the idea of low pressure developing along a front across the Southern Plains Fri night/Sat, quickly moving northeast into the Great Lakes by Sun morning, and the cold front sweeping off the Eastern Seaboard on Mon. Forecast confidence with this system by Sun-Mon remained rather low. ...Weather Highlights/Threats... Within the northern stream, there will be progressive upper-level flow which will maintain the possibility of coastal rain and mountain snow for the Olympic and Cascade ranges by the end of the extended period. Rain and mountain snow is expected for portions of the Desert Southwest as an upper-level low in the southern stream pushes onshore. As the surface low pressure system crosses the Southwest and Southern Rockies rain will become fairly widespread over the Southern and Central plains by the end of the week as Gulf moisture overspreads the quasi-stationary frontal zone. Local areas of moderate to heavy rain will be possible with this system. By next weekend, with potential development of a low pressure system across the central U.S., models indicate the depict a band of moderate to possibly heavy precipitation (possibly in a mixed form, or snow) from the Southern Plains to the Ohio Valley/Great Lakes; however, the specifics are tricky given the uncertainty of evolution. See the medium range Winter Weather Outlook products for more details on the winter weather threat. Temperatures are forecast to be well above average across much of the eastern half of the CONUS through late this week. The largest temperature anomalies are forecast from the Mid-Mississippi Valley to the lower Great Lakes and Ohio Valley on Thu, where high temperatures may approach or exceed 20 deg F above average in some areas. A broad area will see highs 5 to 15 deg above average through Saturday before the arrival of cooler temperatures behind the cold front. Meanwhile, with persistent upper-level troughing in place much of the Great Basin and Southwest will see high temperatures 5 to 10 deg below average into the weekend. Campbell Additional 3-7 Day Hazards information can be found on the WPC medium range hazards 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