Extended Forecast Discussion NWS Weather Prediction Center College Park MD 441 PM EST Thu Jan 14 2021 Valid 12Z Sun Jan 17 2021 - 12Z Thu Jan 21 2021 ...Overview and Guidance/Predictability Assessment... A deep cyclone will exit New England to the Canadian Maritimes Sunday into Monday as an amplified upper trough digs into the south-central U.S. that then shifts across the East. Meanwhile, upper level ridging in the West gets replaced by a shortwave entering the region by Monday. The Western U.S. shortwave drops into the Southwest by Tuesday and the bulk of guidance shows potential for robust amplification of this shortwave, resulting in a closed low over the Desert Southwest. This system works into Baja and vicinity days 6/7 and there is lingering uncertainty on subsequent lead impulse ejections within southern stream flow that may prove critical with respect to a threat of heavy rains over the South in conjuncture with approaching fronts and Gulf of Mexico moisture return. Overall, guidance now shows better forecast clustering on this overall pattern, albeit with lingering timing/strength differences with various features. Accordingly, the WPC medium range product suite was primarily derived from a 06 UTC GFS and 00 UTC ECMWF/UKMET/Canadian composite model blend Sunday-Tuesday before transitioning to best clustered guidance from the associated GFS/ECMWF and GEFS/ECMWF ensembles means days 6/7 amid growing forecast spread. Latest 12 UTC guidance seems reasonably in line with this forecast strategy. ...Weather/Threats Highlights... Low pressure exiting New England on Sunday will allow for lingering gusty winds across the Northeast, lake effect snow showers around the Great Lakes, and snow showers in the central Appalachians. The next system entering the West should bring light rain or mountain snows to parts of the Pacific Northwest and northern Rockies on Sunday, and the central/southern Rockies and southern High Plains on Monday. Precipitation should begin to expand into the Four Corners regions and west-central Gulf Coast early next week associated with the amplified system in the Southwest and downstream response. Greater moisture transport ahead of this system would increase the potential for widespread rainfall from East Texas into the Lower Mississippi Valley/Tennessee Valley and vicinity Tuesday into Thursday. The WPC Day 3-7 Hazards Outlook highlights this threat. Uncertainty is high in the details of this system, but there is increasing model signal for the potential for moderate to heavy rainfall. Upstream closer to the Southwest to Baja upper trough, there will be a likelihood for some widespread rains and some mountain snows back into Arizona and New Mexico. Schichtel Additional 3-7 Day Hazard information can be found on the WPC medium range hazards outlook chart at: https://www.wpc.ncep.noaa.gov/threats/threats.php Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Tue-Thu, Jan 19-Jan 21. - Heavy snow across portions of the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun, Jan 17. - Heavy snow across portions of the Southern Rockies, the Central Rockies, the Central Great Basin, and the Southwest, Tue, Jan 19. - Heavy snow across portions of the Plains, the Central Rockies, the Northern Rockies, and the Southern Rockies, Mon, Jan 18. - Flooding occurring or imminent across portions of the Southeast and the Pacific Northwest. - High winds across portions of the Central Great Basin, California, and the Southwest, Mon-Tue, Jan 18-Jan 19. - High winds across portions of the Central Rockies, and the Northern/Central Plains, Wed, Jan 20. - Enhanced wildfire risk across portions of California, Mon, Jan 18. 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