Extended Forecast Discussion NWS Weather Prediction Center College Park MD 158 AM EST Tue Nov 27 2018 Valid 12Z Fri Nov 30 2018 - 12Z Tue Dec 4 2018 General Weather Pattern ----------------------- Quasi-zonal flow aloft encompasses much of the continental U.S. after the departure of the large storm system from the Northeast states late on Thursday, with a low amplitude perturbation crossing the Plains. This quickly changes going into Friday as a potent Pacific disturbance crosses the Intermountain West and then reaches the central plains by Saturday. After a brief respite, a second storm system will likely affect the Pacific Northwest and allow for the trough to rebuild across much of the West by Sunday, which may eventually affect the central to eastern U.S. by the end of the forecast period next Monday. Model Guidance Evaluation ------------------------- The evolution and track of the Pacific low pressure system crossing the Intermountain West on Friday and then the central U.S. is becoming better refined by the deterministic guidance compared to 24 hours ago. The CMC, ECMWF and the EC mean are on the northwest side of the guidance spread, and the GFS and the GEFS mean on the eastern side and more progressive. There is also some indication of a cold air damming set-up near the Appalachians ahead of this storm system, which the guidance is likely under-doing. There is also growing consensus for a second Pacific storm system to affect the northwestern U.S. by Saturday, despite some placement and timing differences. The CMC is slightly stronger than the GFS and ECMWF regarding the upper level trough by Sunday morning over the Intermountain West. The ensemble means provided a reasonable placement for this feature for the Sunday and Monday time frame. Sensible Weather ---------------- Across the western U.S., the first round of heavy precipitation is expected to abate by the beginning of the medium range period on Friday as the upper level trough moves across the Great Basin and the Rockies. The second storm system that will likely affect the Pacific Northwest this weekend is expected to produce widespread mountain snow across the northwestern quadrant of the nation, and heavy rain for the coastal region. Active weather is also anticipated from the southern plains to the Ohio Valley Friday night into Sunday as a surface low is forecast to develop and intensify as it tracks northeastward. There is an increasing signal in the model guidance for a band of moderate to heavy rain in the warm sector of this low, and SPC is monitoring the potential for severe weather given the strong dynamics aloft that are expected in combination with enhanced moisture advection. Snow is becoming more likely to the northwest of the low track across the central plains and Upper Midwest, however placement differences limit confidence as to where the band of heaviest snow would become established. Windy conditions in the wake of the departing storm system also appear likely for parts of the Plains by Sunday. Temperatures are expected to be on the order of 5 to 15 degrees above average ahead of the western system Friday through Sunday, beginning over the southern plains and then across the Mississippi and Ohio River valleys before reaching the eastern U.S. A renewed surge of colder weather will filter in behind it as troughing settles across the central states. Temperatures may be on the order of 10-20F below average in the interior west by next week. 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