Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Tue Dec 04 2018 Valid 12Z Fri Dec 07 2018 - 12Z Tue Dec 11 2018 ...Significant storm system to bring potential winter weather to areas from the southern plains to the eastern U.S. late this week... ...Overview... Upper-level flow across the CONUS during the medium range continues to lie in a transition zone from a highly amplified/blocked flow regime across the Pacific to more progressive flow in the Atlantic. Upper ridging initially across western Canada with downstream cyclonic northwesterly flow from central Canada into the eastern U.S. will ensure an ample supply of cold air. A number of shortwaves will traverse this northwesterly flow, while farther south an active southern stream transports Pacific energy from Baja California eastward across the U.S. southern states. In response to this southern stream energy, a wave of low pressure is expected to develop along the polar front along the western Gulf Coast by early Sat, then move east across the Gulf Coast states into early Sun. Guidance continues to show varying degrees of phasing/interaction between this southern stream wave and northern stream energy, which affects the eventual track of this low pressure system as it moves off the East Coast likely on Sun and either takes a more suppressed track out to sea or turns and moves northeastward closer to the coast. Farther west, guidance continues to suggest that the upper ridge will begin to move east toward the Rockies/High Plains by early next week, allowing relatively strong height falls to reach the West Coast, ushering in a period of stormy conditions for that region. ...Guidance Evaluation/Preferences... Models are reasonably well clustered during the first half of the medium range, and a blend of the ECMWF/GFS/FV3 served as a good forecast starting point. Confidence with respect to the southern stream system is relatively good through Sat night as the surface low moves across the Gulf Coast states and model/ensemble solutions are relatively well-clustered. Forecast confidence declines after 12Z Sun as to whether the low pressure system takes a more easterly trek out to sea or northeasterly close to the coast, or perhaps even slows/stalls for a period of time. The eventual outcome seems to hinge largely on interaction with a relatively small-scale northern stream shortwave currently well out over the data sparse North Pacific, evident on water vapor satellite imagery near 31N/175W. This feature will deamplify as it rounds the apex of the western Canada ridge before amplifying once again as it enters cyclonic flow across central Canada Fri-Sat. Some consensus has emerged as to the eventual track of the Southeast U.S. low pressure system after Sun, with some degree of ensemble clustering around a low perhaps 50-75 miles east of the Outer Banks by Mon morning. This should be taken somewhat with a grain of salt, however, given the current location of the critical feature and the complex degree of interaction that will occur with the southern stream system. Confidence is a bit better along the West Coast by early next week with broad consensus that strong height falls should arrive by Sun-Mon. While timing is similar across most guidance, solutions continue to differ as to how amplified the trough will be. With these factors considered, increased weight was placed toward ensemble means (ECENS/GEFS) after Sun, with almost exclusive use of ensembles by day 7 (Tue). ...Weather Highlights/Hazards... The system crossing the southern U.S. is expected to produce widespread precipitation north of a surface warm front as abundant Gulf of Mexico moisture streams northward. Models continue to show a signal for potentially heavy rainfall from portions of the southern plains to the lower Mississippi Valley on Fri into early Sat. On the northern periphery of the precipitation shield, sufficient cold air should be in place to produce wintry precipitation, with some areas seeing potential accumulations of snow and/or ice. As the system shifts eastward Sat into Sun, precipitation will also focus across areas of the Southeast, Tennessee Valley, and perhaps portions of the southern Mid-Atlantic region. Models have shown a persistent signal for a potentially significant winter weather event for portions of the southern Appalachians late Sat into Sun. Depending on the eventual track and intensity of the low pressure system, accumulating snows may also be possible east of the Appalachians from the Carolinas north into the southern Mid-Atlantic states, although confidence in this aspect of the forecast remains relatively low. See Day 4-7 Winter Weather Outlooks for further details on the winter weather threat associated with this system. Along the West Coast, expect an increase in precipitation from Sun onward as heights begin to fall and a cold front approaches. Current indications are that areas from northern California north into coastal Oregon and Washington will see the potential for the heaviest precipitation amounts (rain and mountain snow) through early next week. Below average temperatures will prevail across much of the central and eastern U.S. during the medium range, with high temperatures ranging from 5 to 15 deg F below average for many areas. The Rockies will see gradually warming temperatures through the weekend as upper ridging slowly shifts east. Temperatures may reach several degrees above average across portions of the central/northern Rockies and the northern High Plains from Sun onward. Ryan WPC medium range 500 mb heights, surface systems, weather grids, quantitative precipitation, winter weather outlook probabilities and heat indexes are found 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