Extended Forecast Discussion NWS Weather Prediction Center College Park MD 1109 AM EST Wed Nov 21 2018 Valid 12Z Sat Nov 24 2018 - 12Z Wed Nov 28 2018 ...Overview... A blocking pattern will be favorable through the course of the extended periods as a closed high retrogrades from England to northern Quebec. The eastern third of the U.S. will stay in a trough with elongated negative height anomalies across the mid-latitudes of the Atlantic. In the west, ridging will attempt to reassert itself on Sunday ahead of another system or two next Tuesday/Wednesday. ...Guidance/Uncertainty Assessment... Model guidance is pretty well clustered on Saturday at the beginning of the extended forecast but noticeable departures arise over north-central CONUS as early Monday. The latest runs of the Canadian/UKMET have continued to be somewhat displaced from the GFS/ECWMF/Ensemble means. The 06Z GFS has slowed resulting in the East Coast system staying closer to the Coast and not racing out into Atlantic, which has been the preferred track of late. It was more favorable than its 00Z cycle through Monday, however, it quickly diverges from the cluster on Tuesday making it an unrealistic solution. After a couple quieter days in the west Sun/Mon, troughing will attempt to push into the re-established ridge but prefer to keep the rain threat to mainly coastal areas of WA/OR as an upper low over the eastern Aleutians should be the main player in keeping the downstream ridge mostly in tact. This would still allow some weakening fronts into the Pac NW but likely not into much of CA/NV. ...Weather Highlights/Threats... A system lifting through the Southeast and Mid-Atlantic states will spread precipitation up the coast to Northeast... with some isolated wintry precipitation through the Mid-Atlantic into parts of the Northeast. There continues to be some uncertainty on how far north, precipitation type and its intensity will be due to how the weakening system through the Great Lakes interacts. Still favor the I-81/95 corridors between NC and PA/NJ and maybe southeaster NY for the heaviest rain (over an inch to an inch and a half possible, primarily on Saturday evening into Sunday morning). Potent shortwave in the west on Saturday and attendant surface front will take the widespread rain/snow eastward through the Plains into the mid-Mississippi Valley where precipitation ahead of the front will expand in coverage as Gulf moisture gets drawn northward. Cold front out of Canada to the north will provide enough cold air to support snow or a mixed bag of precip to the north/northwest of the surface low Sunday from NE/SD eastward to the central Great Lakes. As that low lifts toward Lake Erie by early Monday it will get blocked from continuing northeastward and instead likely wrap its triple point low out through southeastern New England as its front pushes off the coast. This pivot could yield a significant rain and inland snow event for the Northeast depending on the evolution of the system. Temperatures will generally be below average over most of the lower 48 by about 5-10F. A couple brief exceptions would be across the southern Plains Saturday ahead of the western system and southern Florida until about Tuesday when a cold front should settle into the Florida Straits. Campbell/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