Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Sat Dec 31 2022 Valid 12Z Tue Jan 03 2023 - 12Z Sat Jan 07 2023 ...Winter storm threat across the upper Midwest on Tuesday as a heavy rain/severe weather threat works across the Southeast in Wednesday... ...More rounds of moderate to heavy precipitation for the West with focus across California... ...Overview... A deepened low pressure system with a forecast track across the northern U.S. will likely support a lingering heavy snow threat for the Upper Midwest Tuesday, while heavy rain, flash flooding, and severe weather appear likely across the South Tuesday into Wednesday. Meanwhile, the West should see additional rounds of moderate to heavy precipitation next week with highest amounts mainly across California. ...Guidance/Predictability Assessment... Models and ensembles offer a reasonably similar larger scale pattern evolution through the medium-range. However, the ECMWF has shown quite a bit of uncertainty regarding whether a deep cyclone will form and track off the coast of the Pacific Northwest by about Thursday. The most two recent runs have settled into a cyclogenesis scenario in good agreement with the rest of the guidance. Across the northern U.S., the ECMWF has been consistently forecasting a more southerly track with the cyclone associated with the winter storm. Other guidance have trended toward the ECMWF solution. Across the Northeast, the ECMWF continues to support more surface troughing on Thursday which will lead to wintry weather implications. Across the Southeast, models have continued to show a downward trend in the QPF ahead of a cold front Tue-Wed. It appears that the threat of heavy rain has decreased farther inland, leaving the Florida Panhandle to be the area most susceptible to heavy rain due to interaction with a weak coastal front. Accordingly, the WPC medium range product suite was primarily derived from a blend of the 00Z ECMWF, 06Z GFS, and 00Z CMC and their ensemble means but leaning toward the 00Z ECMWF/00Z EC means through the medium-range forecast period with manual adjustments near cyclone centers to ensure reasonable depth. ...Weather/Hazard Highlights... A deepened low storm track across the north-central U.S. will shift an axis of comma-head snows across the upper Midwest Tuesday, along with increasingly windy conditions and an icy transitional zone. Guidance is converging on the ECMWF southerly track solution, bolstering forecast confidence. Inflow east of the low and lingering cold air may also support some snow/ice downstream across the upper Great Lakes and northern New England. Meanwhile farther south, the frontal system associated with this low and favorable Gulf moisture/instability and right entrance region upper jet lift should support moderate to heavy rainfall but recent model QPFs have trended downward in this area. Thereafter, a trailing frontal low pressure system could develop and spread some moderate to heavy rain up the Mid-Atlantic states to coastal New England. Some wintry precipitation is also possible from the central Appalachians into the interior Northeast. Additional rounds of precipitation are also expected to continue along the West Coast as the week progresses, but primarily with a focus on California. There is quite a bit of uncertainty with the timing of additional heavy precipitation events at this point given the varied shortwave progressions and surface system focus, but amounts look to trend upward by Wednesday-Thursday across California as a stronger frontal system approaches from the Pacific with another potential Atmospheric River event given the signal for a strong fetch of moisture with the system. Many areas are likely to be sensitive to additional rainfall by then given the ample recent rainfall and additional rain during the short range period. There is also a multi-day threat for heavy snow that will focus over the Sierra, but also to a lesser extent spread northward through the Cascades and with some gusto inland across the Intermountain West toward the Rockies over time with system translation. Milder than average temperatures for winter will be widespread across the eastern half of the country for early next week. Lows of around 15-30F above average and highs around 10-20F above average will be common, even reaching 20-30F anomalies in the Ohio Valley/Great Lakes Tuesday and the Northeast on Wednesday. Daily records for warm lows could be widespread for parts of the central and eastern U.S., while daily record highs are possible as well especially in the Ohio Valley. Meanwhile in the West, the overall wet/snowy pattern will tend to keep daytime temperatures around 5-10F below normal. As the large-scale upper trough moves through the western U.S. into the Plains, near to below average temperatures are expected to work their way from west to east across these areas. Kong/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 WPC medium range 500mb heights, surface systems, weather grids, quantitative precipitation, experimental excessive rainfall outlook, 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/#page=ero https://www.wpc.ncep.noaa.gov/wwd/pwpf_d47/pwpf_medr.php?day=4 https://www.wpc.ncep.noaa.gov/heat_index.shtml