Extended Forecast Discussion NWS Weather Prediction Center College Park MD 200 PM EST Fri Dec 30 2022 Valid 12Z Mon Jan 02 2023 - 12Z Fri Jan 06 2023 ...Winter storm threat for the north-central Rockies/Plains and Upper Midwest early next week as a heavy rain/severe weather threat reemerges for the mid-lower Mississippi Valley/Mid-South then Southeast early 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 lead to a winter weather event from the north-central Rockies/Plains through the Upper Midwest, while heavy rain, flash flooding, and severe weather appear likely across the South Monday-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... The main forecast challenges of the medium-range period included the evolution of an upper-level trough/closed low ejecting out over the Plains from the Rockies and the subsequent effects on the development and track of a surface low that will bring widespread warm sector rain/thunderstorm chances to the Southeast and Midwest and snow to the Plains and Upper Midwest. In addition, there is uncertainty regarding the progression of more subtle shortwaves and troughing over the West Coast with a series of Pacific storm systems that will keep precipitation chances high across the West. In terms of the first system, the initial phasing of the upper-level trough is generally consistent both run-to-run over the individual deterministic and ensemble mean model guidance and across the guidance overall. More differences arise in the development of a closed low, associated embedded energy, and subsequent track northeast across the Plains and into the Midwest. The previous WPC forecast depicted the eventual evolution of a closed low as did the model guidance, albeit with somewhat different timing/progression. The WPC product suite began with a general deterministic model blend (00Z ECMWF/CMC/UKMET and 06Z GFS) plus some continuity from the prior forecast to maintain the broad depiction of the system evolution without emphasizing any more subtle and less predictable differences from the individual guidance. It is clear based on the spread of the individual ensemble members that there is greater uncertainty as to the strength of the closed low as it ejects northeastward over the Midwest. For example, there is a run-to-run trend for a deeper closed low in the GFS compared to the other guidance. Additionally, more subtle details begin to arise with the series of shortwaves and development of an upper-level trough off the West Coast later in the period. A blend of the 00Z ECMWF/CMC and 06Z GFS and introduction of the 00Z GEFS/ECens means follows continuity from the prior forecast with the central U.S. system while also maintaining subtle shortwaves along the West Coast. The 00Z UKMET was phased out as it was an outlier with respect to the progression of the shortwave energy in the West. With respect to the central U.S. system, the updated model blend with a bit deeper closed low that develops earlier resulted in a surface low track that trended a bit northwest of the prior forecast, with subsequent similar effects on the axis of heaviest snow in the Plains and rain in the Mid-South/Lower Mississippi Valley. The small-scale details become more cumbersome to resolve during the mid- to late forecast period with both the evolution of the closed low into the Northeast and embedded energy within the larger-scale trough. The 00Z EMCWF/CMC differed from the 00Z/06Z GFS runs and the prior WPC forecast with maintaining a closed low, while the 00Z UKMET was also out of phase with the other guidance in not including ridging along the West Coast between passing shortwaves. Trended increasingly towards the 00Z GEFS/ECens means given increasing differences in the small-scale features across the deterministic guidance and continued growing spread amongst the individual ensemble members. The 00Z CMC was also removed later into the period as the aforementioned downstream trough in the West was more out of phase compared to the other remaining guidance. The late-period blend of the 00Z GEFS/ECens means while maintaining some of the deterministic 00Z ECMWF/06Z GFS guidance helped to accentuate a bit more amplification in the pattern between the upper-level trough forecast to move over the East Coast as well as ridging over the Plains that precedes energy from the downstream trough over the Pacific moving over the West Coast. ...Weather/Hazard Highlights... Upper trough emergence will support heavy snow across the central Rockies this weekend into Monday. Subsequent surface low pressure consolidation across the central High Plains will then bring a swath of snow across the northern half of the Plains into the Upper Midwest, along with increasingly windy conditions and an icy transitional zone. Inflow east of the low and lingering cold air may also support some later period snow/ice downstream from the northern Great Lakes to a cooled Northeast. 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 lower Mississippi Valley/Mid-South through the Southeast Monday-Wednesday. Scattered instances of flash flooding also look possible as the heavy rain spreads from the Lower Mississippi Valley/Mid-South Monday deeper into the Southeast Tuesday, covered by WPC's experimental day 4/5 "Slight Risk" Excessive Rainfall Outlooks (EROs). Thereafter, a trailing frontal low pressure system could develop under the right entrance region upper jet and then lift over the East. This could spread some moderate to heavy rain up the Mid-Atlantic states and coastal New England. Additional rounds of precipitation are also expected to continue along the West Coast as the week progresses. There is quite a bit of uncertainty with the timing of additional heavy precipitation events at this point given the more subtle shortwave progression, 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. 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. Putnam/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