Extended Forecast Discussion NWS Weather Prediction Center College Park MD 341 PM EST Wed Dec 28 2022 Valid 12Z Sat Dec 31 2022 - 12Z Wed Jan 04 2023 ...Multiple rounds of precipitation for the West with lower elevation heavy rain/flooding and higher elevation/inland heavy snow... ...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 U.S.... ...Overview... The upper-level pattern over the weekend 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 additional troughing works into the West. An atmospheric river will lead to additional heavy rain and possible flooding across parts of California in particular. The western trough is forecast to work across an unsettled West into the central U.S. as next week progresses, while at the surface, models are coming into reasonably good agreement for a low to consolidate/deepen over the central High Plains by late Sunday-Monday and track northeastward through the Midwest/Great Lakes region. 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, while on the backside of the low there are increasing chances for heavy snow/wintry weather across the central Rockies into the northern half of the Plains and the Upper Midwest. ...Guidance/Predictability Assessment... Latest 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 best clustered 00 UTC GFS/ECMWF/UKMET deterministic models early on with increasing proportions of the 00 UTC 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 a less certain another round of heavy precipitation may develop again by by Tuesday/Wednesday. 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 an increasingly stormy/windy northern half of the High Plains and into the Upper Midwest where there will 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. 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, especially by midweek, 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. Schichtel/Tate Hazards: - Heavy precipitation across portions of California, Sat, Dec 31 and Mon-Wed, Jan 2-Jan 4. - Heavy precipitation across portions of the Central and Northern Great Basin, Sat, Dec 31. - Heavy rain across portions of California, Sat, Dec 31 and Tue-Wed, Jan 3-Jan 4. - Heavy rain across portions of the Lower and Middle Mississippi Valley, the Tennessee Valley, the Ohio Valley,the Southeast, and the Southern Appalachians, Mon-Tue, Jan 2-Jan 3. - Heavy rain across portions of the Southeast and the Southern Appalachians, Sat, Dec 31 and Wed, Jan 4. - Heavy snow across portions of the Northern Great Basin, Sat, Dec 31. - Heavy snow across portions of the Eastern Great Basin and Northern and Central Rockies, Sat-Sun, Dec 31-Jan 1. - Heavy snow across portions of the Central and Northern High Plains, Sun-Mon, Jan 1-Jan 2. - Heavy snow across portions of the Central and Northern Plains and Upper Midwest, Mon-Tue, Jan 2-Jan 3. - Heavy snow across portions of the Southwest, Sun, Jan 1. - Severe weather across portions of the Lower and Middle Mississippi Valley, Mon, Jan 2. - Flooding occurring or imminent across portions of the Pacific Northwest. - High winds across portions of the Southern High Plains, Mon, Jan 2. 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