Extended Forecast Discussion NWS Weather Prediction Center College Park MD 126 AM EST Mon Dec 23 2019 Valid 12Z Thu Dec 26 2019 - 12Z Mon Dec 30 2019 ...Overview and Guidance/Predictability Assessment... An amplified flow pattern is expected across the CONUS during the medium range forecast period, with a progressive northern stream and a less progressive southern stream with a slow-moving upper-level low. 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. A shortwave and associated frontal system are forecast to cross the Midwest/Great Lakes/Northeast Thu-Fri night. Models were relatively agreeable here, with differences primarily confined to the strength of the shortwave and the depth of the surface low as it moves into east central Canada. Farther west, an upper-level low is forecast to be initially near or just off the southern California coast early on Thu (day 3). Models have struggled to resolve the timing of this system as it moves east across the Southwest and eventually into the central U.S., and guidance is not much closer to a resolution than this time last night. The ECMWF continues to be on the slow side of the spread, but with a substantial degree of support from a seemingly underdisbursed ECENS. Meanwhile, the GFS remains on the fast side of the spread (albeit at least a bit faster than the GEFS ensemble mean). These described differences worsened through time during the forecast period, ultimately resulting in cascading spread downstream by the weekend as the system approaches/reaches the central U.S., and potentially interacts with progressive northern stream waves. The preference in this forecast was to lean at least a bit more toward the faster timing, using blend of roughly 1/3 each of the 12Z ECMWF/UKMET and 18Z GFS for days 3-4, with increased weight placed on ECENS/NAEFS ensemble means by day 5 due to quickly increasing guidance spread. 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. Models show general agreement that another trough and potential upper-level low should dig along the West Coast by Sun-Mon (days 6-7). Run-to-run differences among guidance were significant enough with this feature to go heavily toward ECENS/NAEFS ensemble means by that time period. ...Weather Highlights/Threats... Rain and mountain snow are expected Thu across the Southwest as the upper low moves onshore. Progressive upper-level flow will keep rain/mountain snow a possibility across the Olympics and the Cascades late this week. Moisture return ahead of the southern stream system will create an overrunning setup north of a frontal boundary across the Southern/Central Plains Thu-Fri, with relatively widespread (and at least locally heavy) rain expected to develop. By next weekend, with potential development of a low pressure system across the central U.S., models indicate the potential for a band of moderate to possibly heavy precipitation from the Southern Plains to the Ohio Valley/Great Lakes, although details remain elusive for reasons described above. Nonetheless, there appears at least some potential for an area of winter weather on the north side of this system (despite the relative lack of significant cold air). Once models get a better handle on the specifics of next weekend's system, a clearer picture will emerge as to what degree of heavy rain and/or winter weather threat exists. At this time, the greatest probabilities associated with this potential low pressure system are indicated across portions of the southern Rockies and southern High Plains Fri/Fri night. 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. Ryan 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