Extended Forecast Discussion NWS Weather Prediction Center College Park MD 204 AM EST Wed Dec 28 2022 Valid 12Z Sat Dec 31 2022 - 12Z Wed Jan 04 2023 ...Multiple rounds of precipitation are likely in the West with lower elevation heavy rain/flooding and higher elevation heavy snow... ...A low pressure system could spread winter weather from the central Rockies/High Plains and the Midwest while a heavy rain threat reemerges over the Lower Mississippi Valley in early 2023... ...Overview... Over the weekend, the upper-level pattern will be characterized by an upper trough lifting from the central U.S. to the Great Lakes region producing mainly rain across the East, while another trough digs into the West. Another atmospheric river taking aim at the West just ahead of the latter trough will lead to additional heavy rain and possible flooding across parts of California in particular. The western trough is forecast to move into the central U.S. as the week progresses, while at the surface, models are coming into reasonably good agreement for a low to consolidate in the central High Plains by late Sunday-Monday and track northeastward through the Midwest/Great Lakes region. Moisture return from the Gulf of Mexico is likely to produce heavy rainfall across the Lower Mississippi Valley into the Tennessee Valley and Southeast Monday-Tuesday, while on the backside of the low there are increasing chances for snow/wintry weather across the central Rockies into the northern half of the Plains and the Upper Midwest. ...Guidance/Predictability Assessment... Recent model cycles have kept with the pattern of relatively good agreement with the large scale pattern described above. But there are still some differences in the details like exact shortwave/low pressure/frontal placements that are certainly within range of typical medium range spread. These include some variations in the main central U.S. surface low track Monday-Wednesday. While the differences are relatively minor for the time range, the exact track will matter in terms of precipitation types for any particular area, and this may take additional time to resolve. The WPC forecast approach was to use a blend of the 12/18Z deterministic models early on 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... Another persistent atmospheric river will span from Friday (now in the short range period) into Saturday, taking aim at California with heavy precipitation. Heavy rain could cause additional flooding along coastal areas as well as the foothills of the Sierra Nevada, so a Slight Risk remains in place in the experimental Day 4 Excessive Rainfall Outlook. These areas are likely to be hydrologically sensitive by that time due to the ongoing strong AR and the rainfall on day 3, and burn scars present especially in southern California could be particularly vulnerable to flooding. Additionally, significant snow amounts are likely in higher elevations of the West into the weekend, shifting east with time. Model guidance still advertises a brief break in the precipitation along the West Coast for Sunday as the trough axis shifts east, but another round of precipitation is forecast once again by Monday-Tuesday. The eastern U.S. is likely to end the year 2022 with rainfall as moisture streams in ahead of the initial central U.S. upper trough. Most precipitation should stay rain but with some chance of minor snow accumulations for the higher elevations of the interior Northeast. 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 the northern half of the High Plains and into the Upper Midwest. Meanwhile farther south, the cold front associated with this low could provide a focus for another bout of heavy rain across parts of the Mississippi Valley into the Tennessee Valley and Southeast by Monday-Tuesday. Milder than average temperatures for winter will be widespread across the eastern half of the country for the weekend into early next week. Lows of around 15-30F above average and highs around 10-20F above average, locally higher in the Ohio Valley by Monday-Tuesday, will be common. Daily records for warm lows could be widespread as the warmest air reaches the East, while a few isolated daily record highs cannot be ruled out. 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 until early next week as 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 as well. 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