Extended Forecast Discussion NWS Weather Prediction Center College Park MD 113 AM EST Wed Dec 05 2018 Valid 12Z Sat Dec 08 2018 - 12Z Wed Dec 12 2018 ...Significant storm system to bring heavy rain and potential winter weather to areas from the southern Plains to the eastern U.S... ...Overview... Split flow across the CONUS at the start of the medium range will support a significant low pressure system moving along the Gulf Coast states. The system will likely interact with a northern stream shortwave late Sun into Mon as it crosses the coastal Southeast and moves into the Atlantic Ocean, before following a general northeastward motion into early next week off the Eastern Seaboard. 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, ushering in a period of stormy conditions for that region. Ensemble guidance suggests perhaps a significant pattern change across the northern hemisphere beginning early next week and continuing beyond the forecast period. Strong positive height anomalies which have been persistent across Siberia for a couple weeks should begin to break down, with downstream flow across the North Pacific becoming much more progressive than seen recently. Developing negative height anomalies across Alaska and into western North America as a result of this pattern change will favor upper-level ridging centered in the vicinity of Hudson Bay, which would significantly reduce the flow of polar/arctic air southward into the CONUS, while also resulting in period of much wetter conditions for the Pacific Northwest than seen recently. ...Guidance Evaluation/Preferences... Models remain reasonably well-clustered during the first couple days of the forecast period (Sat-Sun), and a blend of the 12Z/18Z deterministic runs served as a good forecast starting point. Confidence with respect to the southern stream system is relatively good through Sun morning as the surface low moves across the Gulf Coast states and model/ensemble solutions are relatively well-clustered. Forecast confidence remains a bit lower after 12Z Sun 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. A 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 accumulating 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. These differences amplify by Tue-Wed as the system crosses the Rockies and moves into the central U.S., with central Plains cyclogenesis looking probable even if the specifics are unclear. With these factors considered, increased weight was placed toward ensemble means (ECENS/GEFS) Mon onward. ...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 across portions of the lower Mississippi Valley and Southeast Sat-Sun. On the northern periphery of the precipitation shield, sufficient cold air will be in place to produce wintry precipitation, with some areas seeing accumulations of snow and/or some ice. 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. 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 (negative anomalies of 5 to 15 deg F for high temperatures) will prevail across much of the central and eastern U.S. Sat-Sun, with a slow moderation of temperatures possible by early next week beginning across the northern High Plains and gradually spreading eastward in association with the upper-level ridge. Ryan/Fracasso 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