Extended Forecast Discussion NWS Weather Prediction Center College Park MD 140 AM EST Fri Nov 22 2019 Valid 12Z Mon Nov 25 2019 - 12Z Fri Nov 29 2019 ...Guidance/Predictability Assessment... The medium range period begins with a vigorous shortwave/surface low exiting the Northeast, with various shortwaves rotating through broad troughing across the remainder of the CONUS. For Day 3-4 (Mon-Tues), models overall show very good agreement and thus a majority deterministic model blend sufficed, between the 18z/Nov 21 GFS, the 12z/Nov 21 ECMWF, and the 12z/Nov 21 UKMET. Beyond this, one of the biggest forecast concerns continues to be regarding the evolution of a shortwave as it digs southward through the Western U.S. and ejects into the Central U.S. by day 4. Models continue to struggle with respect to both timing and intensity of this system, which looks to spin up a surface low in the south-central Plains on Tuesday, and track it quickly towards the Northeast by Wednesday/Thursday. The GFS has been very consistent in deepening this surface low early on as it tracks through the Midwest, while the ECMWF shows much more run-to-run variability ranging from a solution close to that of the GFS to maintaining a rather weak wave through the Midwest and not showing much intensification until the low reaches the Northeast on Wednesday. The CMC also continues to exhibit high variability with each new model run and the latest run barely shows a shortwave at all and is so widely different from any other model solution and the ensemble means that it was not considered. Timing of this system also remains in question with the GFS on the faster side of the guidance envelope and the ECMWF/UKMET on the slower side. In addition, scatter plots of various ensemble member surface lows shows better clustering towards the slightly faster GFS. Based on this, and given the uncertainty remaining with this system beyond day 5, a blend of the ensemble means (ECENS/GEFS) was preferred which results in a moderately intense storm and a slightly faster track much closer to that of the GFS. This also maintains very good continuity with the previous WPC forecast. Meanwhile, another vigorous shortwave follows on the heels of this first one reaching the West coast by late Tuesday. Models agree that this wave should become quite amplified by mid to late next week, with some solutions wanting to close off an upper low somewhere along the Southwest coast. Run-to-run variability in the deterministic models, both with respect to timing and closed low vs amplified troughing, remains high with this system and thus more of the better clustered ensemble means was used in the blend for days 6-7. ...Weather Highlights/Threats... The developing surface low in the Central U.S. on Tuesday may bring a period of heavy precipitation from parts of 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, potentially significant snows are possible across northern/interior New England, though given the uncertainty with this system, it is much too early for specifics regarding snowfall amounts. 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. 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