Extended Forecast Discussion NWS Weather Prediction Center College Park MD 154 AM EST Sat Nov 24 2018 Valid 12Z Tue Nov 27 2018 - 12Z Sat Dec 1 2018 Synoptic Scale Overview ----------------------- The medium range period begins on Tuesday with a large scale upper level gyre centered over the Great Lakes region with multiple embedded shortwaves pivoting around it, and broad troughing over the eastern Pacific with a downstream ridge axis over the Intermountain West. The large closed low slowly exits the Northeast U.S. on Thursday with quasi-zonal flow likely becoming established over much of the central U.S. and a large scale trough is expected to move inland across the western U.S. Once the trough axis reaches the Plains, a downstream ridge over the eastern U.S. may try to build by next weekend. Model Guidance and Uncertainty Assessment ----------------------------------------- Deterministic models and the ensemble means are in relatively decent synoptic scale agreement on Tuesday across the continental U.S. However, noteworthy differences in the deterministic guidance become apparent as early as Wednesday, with the 12Z CMC indicating a much stronger upper trough crossing the western U.S. where the other models are indicating quasi-zonal flow, and then is out of phase across the central U.S. by Friday where the remaining model consensus is indicative of weak upper level ridging. The 12Z ECMWF becomes more amplified with a shortwave pivoting around the Northeast U.S. upper level gyre, and affects the Ohio Valley to the Northeast U.S. with a stronger surface low on Thursday morning near Maine. By the end of the medium range period Friday and into Saturday, speed and amplitude differences are apparent as the upper trough from the western U.S. reaches the central plains, with the 12Z ECMWF being slower and more amplified, and the GFS more progressive by a few hundred miles with its trough axis. The 18Z GEFS mean appears to offer a more realistic depiction of the trough, whereas the EC mean is considerably broader. There is good model support for a downstream ridge east of the Mississippi River, with the ECMWF showing a rather pronounced ridge axis compared to the broader GFS ridge. Differences are even more apparent across southern Canada leading to below average confidence in the forecast by the end of the forecast period. Taking these considerations into account, the WPC forecast was based primarily on the ECMWF/GFS/UKMET through Thursday morning, and then primarily GEFS mean/EC mean with some deterministic GFS/ECMWF for Friday and into Saturday. Sensible Weather ---------------- The departing storm system over the northeastern U.S. early in the forecast period is expected to produce lake effect snow through Wednesday evening, with heavy snow possible downwind of Lake Erie and Lake Ontario, and lighter snow for northern Michigan. The heaviest precipitation is expected from central California to western Washington as two Pacific storm systems are expected to move onshore and bring enhanced moisture into these areas, with an atmospheric river event appearing likely on Tuesday for northern California. Heavy snow is likely for the higher mountains from the Sierra Nevada to the central and northern Rockies, with 1 to 2 feet of snow possible through Friday night. Temperatures are expected to be on the order of 10 to 20 degrees below average from the northern plains to the Ohio Valley on Tuesday and Wednesday before a moderation trend ensues by later in the week. Areas with fresh snow cover across the Midwest in the wake of the winter storm may have overnight lows dropping well into the single digits for the early to middle part of the upcoming work week, with some readings possibly falling below zero where winds are light and skies clear out. Temperatures should generally be within 5 degrees of climatological averages for the western third of the nation. D. Hamrick 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