Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1056 AM EST Tue Dec 04 2018 Valid 12Z Fri Dec 07 2018 - 12Z Tue Dec 11 2018 ...Significant storm system to bring locally heavy rain and 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 near the western Gulf Coast by early Saturday with a large area of potentially modest and locally heavy rain. This will then move eastward across the Gulf Coast states into early Sunday and slowly deepen. 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 just off the East Coast on Monday. Farther west the upper ridge will begin to move eastward toward the Rockies/High Plains by early next week allowing relatively strong height falls to reach the West Coast and ushering in a period of stormy conditions for that region. ...Guidance Evaluation/Preferences... Models are reasonably well-clustered during the first couple days of the medium range (Fri-Sat), and a blend of the 00Z/06Z deterministic runs 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 in the details after 12Z Sunday when timing differences between the northern/southern stream amplify. The eventual outcome seems to hinge largely on interaction with a relatively small-scale northern stream shortwave currently over the North Pacific. This feature will de-amplify as it rounds the apex of the western Canadian ridge before amplifying once again as it enters cyclonic flow across central Canada Fri-Sat. Strong surface high pressure (1036-1040mb) across the Midwest will likely outrun the surface low to the south which will play a role in how far north the precipitation shield will get (working against the typically seen NW drift in dprog/dt). Trailing shortwave (of mid-latitude Pacific origin) will be yet another source of model/forecast error as it catches up and overtakes the initial northern stream shortwave which may act to slow the trough and tug the system closer to the coast as it attempts to exit to the east-northeast or northeast. Trend has been for a slower exodus next Mon/Tue but that may not be enough to overcome the cold/dry air to the north. With all the given unknowns, specifics will have to wait another couple of forecast cycles but the best agreement on at least modest snowfall lies over SW Virginia and NW North Carolina. 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 (mostly ECENS with some GEFS) after Sun as the 00Z/06Z GFS/GEFS lagged behind the 00Z ECMWF/Canadian/FV3-GFS ...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 a couple inches of snow and/or some 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 and potentially significant snow 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 the WPC Day 4-7 Winter Weather Outlooks for further probabilistic 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 into the High Plains will see gradually warming temperatures through the weekend as upper ridging slowly shifts east. Fracasso/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