Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 PM EST Wed Nov 20 2019 Valid 12Z Sat Nov 23 2019 - 12Z Wed Nov 27 2019 ...Pattern Overview and Guidance/Predictability Assessment... Models and ensemble indicate a fairly active synoptic pattern going into the medium-range period across the U.S. Decent model agreement is now shown regarding a developing low pressure system moving fairly quickly across the South on Saturday and into the Eastern Seaboard Saturday night. By Sunday morning, the system will likely be moving off the East Coast where some additional cyclogenesis appears likely. Nevertheless, models continue to show a trend toward more phasing of this system with the northern stream energy. This has resulted in a storm track that is edging closer and closer to the New England coast Sunday into Sunday night. The latest WPC forecast charts show the storm center clipping Cape Cod Sunday morning before departing into the Canadian Maritimes early on Monday. Meanwhile, the next cold front associated with a fairly vigorous shortwave will likely be pushing rapidly southeastward across the western U.S. early next week. By around Tuesday of next week, models begin to show differences in the forward motion of this shortwave and the amount of energy ejecting into the central Plains shortly afterward. Despite the model differences and some expected run-to-run fluctuations in the forecast storm track and intensity, it appears that there is a decent chance for a significant low pressure system to form over the central Plains then moving toward the Great Lakes region during the mid-week period. The 06Z GEFS agrees very well with the 00Z EC mean on this scenario. However, the 00Z GEFS was much slower in comparison. The WPC medium-range grids and forecast charts were derived mainly from a blend of the 06Z GFS/GEFS and the 00Z ECMWF/EC mean together with a small percentage of the 00Z CMC. A smaller percentage of the deterministic 06Z GFS than usual was included due to the presence of an anomalous shortwave trough diving south across the northern Plains during the weekend, which looks spurious by comparison with other models. A large percentage of the 00Z EC mean and 06Z GEFS were used to handle the uncertainty associated with potential central U.S. cyclone midweek next week. ...Weather Highlights/Threats... The low pressure system crossing the eastern U.S. on Saturday is expected to bring showers and some thunderstorms to areas from the Southeast and southern Appalachians to the Mid-Atlantic, with locally heavy rain possible. Greater uncertainty exists farther north due to the variability surrounding the surface low track off the Northeast coast. But with the model trending toward a storm track closer to the New England coast, chance of snow and/or mixed precipitation is now indicated for the interior sections of the Northeast Saturday and Sunday. For the western U.S., rain and mountain snow are expected across the Pacific Northwest and northern Great Basin/northern Rockies Sat-Sun as the first of the two significant shortwaves and cold fronts move inland. The snow should linger into early next Tuesday over the central Rockies as a low pressure system begins to develop across the central U.S. It appears that showers and some thunderstorms will be likely ahead of the system as they move up the Mississippi Valley into the Ohio Valley next Tuesday and Wednesday. On the back side of the system, there could be potentially significant winter weather spread across the northern Plains into the upper Midwest. Please refer to WPC Day 4-7 Winter Weather Outlooks for specifics on the medium range winter weather threat. Finally, as the next low pressure system reaches the Pacific Northwest toward the middle of next week, expect a renewal of potentially coastal heavy rain and inland snow for that region. Expect chilly temperatures to overspread the eastern U.S. Sat-Sun, with highs 5 to 10 deg F below average. A warming trend will bring high temperatures 5 to 10 deg above average to the northern Plains and Upper Midwest over the weekend. Meanwhile, as the upper trough digs into the West by early next week, expect temperatures to drop, with highs 5 to 15 deg below average across much of the interior western U.S. Below average temperatures should spread east into the central U.S. by the middle of next week behind the cold front and the strengthening low pressure system. Kong 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 Hazards: - Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Pacific Northwest, the Northern Great Basin, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley, Tue, Nov 26. - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Ohio Valley, Fri-Sat, Nov 22-Nov 23. - Flooding occurring or imminent across portions of the Northern Plains. - Heavy precipitation across portions of the Alaska Panhandle, Fri, Nov 22. 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