Extended Forecast Discussion NWS Weather Prediction Center College Park MD 213 AM EST Sat Dec 31 2022 Valid 12Z Tue Jan 03 2023 - 12Z Sat Jan 07 2023 ...Winter storm threat for the Upper Midwest to linger Tuesday as a heavy rain/severe weather threat works across the Southeast through midweek... ...More rounds of moderate to heavy precipitation for the West with focus across California next week... ...Overview... A deepened low pressure system forecast to develop into early next week over the central 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 into Tuesday/Wednesday. Meanwhile the West should see additional rounds of moderate to heavy precipitation next week, with highest amounts mainly across California next week. ...Guidance/Predictability Assessment... Models and ensembles offer a reasonably similar larger scale pattern evolution next week. However there remains ample uncertainty regarding the progression of shortwaves/troughing into the West Coast with a series of Pacific storm systems that will keep precipitation chances high across the West. Model run to run consistency is subpar with low developments off the West Coast through medium range time scales, especially in recent runs of the ECMWF. Run to run output is uncommonly not well aligned with ECMWF ensembles. Best low clustering in the GEFS/NAEFS/ECMWF ensembles and the other models has been slower than the ECMWF to work system energy and lead atmospheric river plume inland next week, but all guidance does eventually show that multi-day threat. The GFS/UKMET/Canadian hold systems offshore longer than the ECMWF, but they have also been showing less than stellar run to run continuity. Accordingly, the WPC medium range product suite was primarily derived from a blend of the GEFS/NAEFS/ECMWF ensemble means through this forecast period and manual adjustments were applied to the smoothed blend to offshore low centers to ensure reasonable depth considering favorable upper support. ...Weather/Hazard Highlights... A deepened low storm track across the north-central U.S. will shift an axis of commahead snows across the Upper Midwest Tuesday, along with increasingly windy conditions and an icy transitional zone. Guidance is converging on a more common solution with this feature by then, 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 spread widespread heavy rain, along with a threat of severe weather as per SPC, across parts of the South into Tuesday/Wednesday. Flash flooding is also possible as covered by WPC's experimental day 4/5 "Slight Risk" Excessive Rainfall Outlooks (EROs). Thereafter, a trailing frontal low pressure system could develop and then lift over the East to spread some moderate to heavy rain up the Mid-Atlantic states to coastal New England. 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 translatiion. 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. 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