Extended Forecast Discussion NWS Weather Prediction Center College Park MD 215 PM EST Fri Nov 22 2019 Valid 12Z Mon Nov 25 2019 - 12Z Fri Nov 29 2019 ...Guidance/Predictability Assessment... The medium range period will begin with a low pressure system/nor'easter exiting New England. Meanwhile, models are in excellent agreement on the steady deepening of a broad scale upper-level trough across the western U.S. as a cold front pushes through the region Monday and Tuesday. A shortwave within the broad upper trough is forecast to exit the central Rockies into the central Plains where a low pressure system is likely to form on Tuesday. The biggest forecast concerns continue to be the run-to-run variability shown on the model forecast track for this system. There has been an eastward shift of the model consensus track for the past few deterministic runs. On the other hand, the ensemble mean solutions show a track that is west of the general deterministic consensus track. A blend of the 06Z GFS/GEFS with the 00Z ECMWF/EC mean yielded a track slightly to the west of the previous WPC forecast package. This basically takes the storm center quickly across the Midwest Tuesday evening and then across the central Great Lakes early on Tuesday. The next area of concern will be along the West Coast as models are showing a large-scale upper-level trough getting firmly established by the middle of next week and then linger in the vicinity into late next week. Models are showing a definite trend for lowering heights to dig deeper into the Desert South for the past couple of days. In response to this huge upper trough, a high amplitude ridge is now indicated through the Great Plains toward the end of next week. This appears to signal a major synoptic pattern change for much of the U.S. by late next week. The WPC morning grid package used a blend of about 60% 00Z EC mean with about 40% 06Z GEFS mean for Days 6 and 7, mixing in with a small percentage of their deterministic solutions to handle the system. This agrees fairly well with continuity but with a slightly deeper surface low near the California coast. ...Weather Highlights/Threats... Despite the run-to-run variability on the forecast track of the developing surface low in the central U.S., it appears that a swath of wintry precipitation will be on the back side of this system from the central Plains to the Upper Midwest/Great Lakes, with snow possible on the northern edge of the precipitation shield. As the surface low lifts into the Northeast on Wednesday, some wintry precipitation is possible across northern/interior New England. Depending on surface low intensity, a period of high winds may also accompany this system. Rainfall will also be likely along the trailing cold front as it moves from the lower Mississippi Valley to the the Southeast Tuesday-Wednesday. Across the West, heavy snowfall is likely in the higher terrain of the Washington/Oregon Cascades, the Sierra Nevada range, and eastward across the Great Basin/Rockies both with the initial shortwave crossing the region Monday, and even more so with the amplified troughing developing over the region by late in the week. Another round of potentially heavy rainfall is possible across the Southwest by mid to late next week, with rainfall also developing by Friday eastward into the Southern Plains/Mid-Lower Mississippi Valley ahead of the upper trough. Colder temperatures across the West will also accompany this system, with daytime highs averaging 10 to 20 degrees below normal across the Great Basin and Northern Rockies. Kong/Santorelli 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 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