Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 258 AM EST Wed Dec 28 2022 Valid 12Z Wed Dec 28 2022 - 12Z Fri Dec 30 2022 ...Stormy and unsettled weather to impact the western United States and Rockies... ...Increasing shower and thunderstorm chances across parts of the southern Plains and lower Mississippi Valley by Thursday... ...Well above average temperatures to surge into the central and eastern U.S. by mid-to-late week... The western U.S. is in transition to an active wet pattern as an atmospheric river comes onshore spreading Much of the active weather over the next few days is expected to be focused around the western half of the Nation as a pattern change ushers in rain and mountain snow throughout the region, while the central and eastern U.S. experiences a rapid warm up. A deep and fast-moving storm system sweeping through the Intermountain West is forecast to continue producing moderate-to-locally heavy rain across the West Coast and parts of southern/central California through early this morning. There may be instances of a few scattered flash floods possible, with the greatest chances for rapid runoff and debris flows near recent burn scars. With snow levels lowering behind the front heavy snow is possible throughout the higher terrain of the Sierra Nevada, Cascades, and for the northern and central portions of the Rockies as moisture flows eastward along a potent Pacific jet streak. Another system is forecast to make its way onshore the West Coast by Thursday ushering in more rain to the Pacific Northwest southward to central California and additional accumulating snow in the favored areas of higher terrain. This unsettled weather pattern is expected to linger into the upcoming weekend as well. For areas east of the Front Range a warm up is in store as return flow from the Gulf of Mexico transports warmer moist air northward ahead of an ejecting upper-level trough over the Great Plains on Thursday. Temperatures across a large portion of the central U.S. will moderate and returning closer to normal for the last days of 2022. As the aforementioned storm systems impact the West and a high pressure system slides into the western Atlantic, downsloping winds off the Rockies and warmer air from the Gulf of Mexico will quickly kick out any lingering arctic air. High temperatures around 20 degrees above average and into the upper 60s and low 70s will first be felt over the southern Plains on Wednesday before the warmer air spreads into the Midwest on Thursday. Mild overnight minimum temperatures are also anticipated to help encourage the late-December thaw. Low temperatures are expected to remain above freezing as far north as the lower Great Lakes and Interior Northeast by Thursday.Showers and thunderstorms are expected to become increasingly widespread ahead of and along the approaching front. The environment will be conducive for some of these storms to potentially turn severe, especially near the Arkansas, Texas and Louisiana juncture to the lower Mississippi Valley. Moderate to isolated areas of heavy rainfall could lead to an elevated risk for flash flooding. SPC has issued a Marginal Risk (level 1/5) of severe thunderstorms while WPC has a Slight Risk (level 2/4) of excessive rainfall also highlighting a similar area. Farther north, a warm front extending across the northern tier from North Dakota to Maine may produce light instances of snow and/or a wintry mix through Thursday. Campbell/Snell Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php