Extended Forecast Discussion NWS Weather Prediction Center College Park MD 159 AM EST Sun Nov 24 2019 Valid 12Z Wed Nov 27 2019 - 12Z Sun Dec 01 2019 ...Several Major Storms for the Thanksgiving Holiday Period... ...Guidance/Predictability Assessment... The latest models and ensembles show better than average forecast clustering/predictability regarding the evolution of a shortwave trough exiting the Central Rockies at the end of the short range period, and steadily deepening as it lifts into the Great Lakes to begin the medium range period on Wednesday. This should spin up a rather impressive cyclone at the surface which tracks from the Great Lakes to the Northeast on Wednesday-Wednesday night. The CMC is a hair faster with the upper shortwave/closed low as it exits the Northeast early Thursday and so a blend of the better clustered GFS/ECMWF with minor contributions from the ensemble means was used, which also maintains good continuity with the previous WPC forecast. The biggest forecast differences in the medium range is with the evolution of highly amplified troughing along the West Coast as it slowly inches inland and shortwave energy eventually ejects into the Central Plains around day 6 to day 7. Through day 5, models show reasonable agreement/clustering, but its after this time where timing and intensity differences begin to arise. The GFS (and to some degree the GEFS mean) are much quicker to eject energy from the Southwest into the South-Central Plains, depicting a wrapped up closed upper low over the Central Plains by as early as day 6. Most other models and ensembles hold the energy back a bit, not depicting a closed upper low to emerge until the system reaches the Midwest on day 7. Given the uncertainty at the longer range time scale, opted to go for a blend of the ECENS/GEFS means, along with some percentage of the ECMWF which is better in line with the ensemble means. Regardless of timing though, the models do agree that another potent storm system could affect portions of the north-central U.S. by the end of the period potentially with a very similar track of the earlier storm days 3-4. ...Weather Highlights/Threats... Moderate to locally heavy precipitation is likely Tuesday into Wednesday on the north and west side of the surface low from the Central Plains to the Upper Midwest. Some accumulating snows may be possible on the northern edge of the precipitation shield. As the storm lifts into the Northeast on Wednesday, wintry precipitation is possible across northern/interior New England. A period of high winds appears likely to accompany this system as well. Meanwhile, heavy mountain snows are likely across the Great Basin and Rockies Wednesday-Friday with snow levels expected to drop significantly. Another round of potentially heavy rainfall with a threat of runoff problems are also especially likely across Southern CA and the Southwest (including burn scars) by mid to late next week. Heavy rainfall is also possible from portions of the Southern Plains to the lower Ohio Valley as multiple shortwave impulses round the base of the Western U.S. trough and tap some mid-upper level moisture from the tropical east Pacific. The northwest edge of this precip shield across eastern New Mexico/Texas Panhandle may also see accumulating snowfall with surface high pressure and sufficiently cold air diving southward across the Plains. With the next potential storm system winding up in the Central U.S. into next weekend, accumulating snows (potentially heavy or significant) are possible to the north and west of the surface low from the Northern Plains to the Upper Mississippi Valley next Friday and Saturday. The biggest temperature anomalies will focus across the Western U.S. through much of the period with daytime highs averaging 10 to 20 degrees below normal. In response to this trough, ridging builds across the south-central U.S. which should bring a return of above normal temps (anomalies +5 to +10 degrees) to this region by the end of the period. In the East, above normal temperatures on Wednesday should moderate back to normal or slightly below normal at the end of the week. 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