Extended Forecast Discussion NWS Weather Prediction Center College Park MD 157 AM EST Thu Dec 29 2022 Valid 12Z Sun Jan 01 2023 - 12Z Thu Jan 05 2023 ...Winter storm threat for the north-central Rockies/Plains and Upper Midwest early in 2023 as a heavy rain/severe weather threat reemerges for the Mid-Lower Mississippi Valley/Mid-South then Southeast... ...Multiple rounds of precipitation for the West with lower elevation heavy rain/flooding and higher elevation/inland heavy snow... ...Overview... As the medium range period and the new year begin on Sunday, one upper-level trough will be lifting out of the Great Lakes/Northeast while another amplifies in the West. The latter trough will produce precipitation across the Intermountain West before ejecting into the central U.S., while in response a surface low consolidates and deepens across the central High Plains by late Sunday-Monday and then tracks northeastward through the Midwest/Great Lakes region. On the backside of the low heavy snow/wintry weather is forecast across the central Rockies into the northern half of the Plains and the Upper Midwest, while farther south deep layered moisture return from the Gulf of Mexico is likely to produce heavy rainfall and possible severe weather across the Lower Mississippi Valley into the Tennessee Valley and Southeast Monday-Wednesday. Meanwhile the West should see additional rounds of moderate to heavy precipitation next week, with amounts trending upward by midweek. ...Guidance/Predictability Assessment... Model guidance continues to show relatively good agreement with the large scale pattern, namely with the trough coming into the West Sunday that shifts into the central U.S. as the week progresses while upper ridging tracks ahead of it, and with the developing surface low. There are still some differences in the details like the exact low pressure track and frontal placements that, though within range of typical medium range spread, have implications like the precipitation types for any particular area, which will take additional time to resolve. The 12Z ECWMF and many of the EC ensemble members indicated a farther southeast track of the surface low compared to the GFS/CMC suites. The 00Z deterministic ECMWF appears similar to its previous run. This results in a swath of heavy snow across Nebraska rather than from western Nebraska into the Dakotas, for example. The WPC forecast for the low track and associated precipitation and winter weather probabilities attempted to keep a middle ground solution not as far south as the EC, but perhaps some GFS runs have been too far north. The deterministic CMC runs may be a good proxy for an in between solution. There are also some differences with energy diving into the western side of the trough around Tuesday--the 12Z CMC appeared weakest but is stronger in the most recent 00Z run. Some rounds of upstream energy in the Pacific reaching the West show some variations in timing and strength as well. Thus the WPC forecast utilized a blend of the deterministic 12/18Z model guidance early in the period, with increasing proportions of the GEFS and EC means as the period progressed given the increasing (but still within reason, especially considering the overall active/stormy flow) model spread. ...Weather/Hazard Highlights... The northeastern U.S. could see some mainly rain showers (with perhaps some minor snow accumulations in higher elevations) early Sunday with an exiting low pressure system. Then, as the next upper trough emerges and surface low pressure consolidates across the central High Plains after producing possibly heavy snow across the central Rockies, snow could spread into lower elevations as well, particularly for an increasingly stormy/windy northern half of the High Plains and into the Upper Midwest where there could also be a icy transition zone. Lead flow around the low and lingering cold air may also support some snow/ice downstream from the northern Great Lakes to 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 provide a focus for another episode of widespread heavy rain, along with a threat of severe weather as per SPC, across parts of the Mississippi Valley into the Tennessee Valley and Southeast by Monday-Wednesday. The West Coast states can expect a brief break from the heavy precipitation on Sunday as the trough axis shifts east/farther inland. But additional rounds of precipitation are forecast to arise as the week progresses. There is considerable uncertainty with the timing of additional heavy precipitation events at this point, but generally amounts look to trend upward by Tuesday-Thursday as a stronger frontal system approaches. Many areas are likely to be sensitive to additional rainfall by that time given the ample recent rainfall and additional rain during the short range period. 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. Lows should generally be a bit above average Sunday morning before the second large-scale upper trough moves through, bringing lows within a few degrees of normal. This trough and the associated cold front will bring the Plains closer to average by Tuesday-Wednesday, tracking into the east-central U.S. by Thursday. Tate 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