Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 257 AM EST Tue Dec 17 2024 Valid 12Z Tue Dec 17 2024 - 12Z Thu Dec 19 2024 ...Another Atmospheric River to bring heavy coastal rains and high elevation mountain snow to the Northwest... ...Unsettled, wet weather expected for most of the eastern U.S. Wednesday... ...Periods of snow for the Northern Plains through mid-week with heavier accumulations most likely Wednesday night... ...Much above average temperatures expected for most of the lower 48 into mid-week... Yet another Pacific system will help to usher in a wave of Pacific moisture/Atmospheric River into the Pacific Northwest and inland over the northern Great Basin/Rockies Tuesday. A more northerly track will help to focus the heaviest lower elevation rainfall along the coastal ranges of northern Oregon and Washington State, with heavy high elevation mountain snow for the northern Cascades. Some moderate snow accumulations will also be possible into the northern Great Basin and northern/central Rockies, aided in part by a leading wave departing the region early this morning. Lower elevation inland locations can expect a wintry mix of rain and snow, with some freezing rain possible to the east of the Cascades in Washington. Precipitation chances should come down through the day Wednesday for the Pacific Northwest while lingering longer in the northern Rockies. A lingering frontal boundary will help to trigger some showers and thunderstorms over portions of the Southern Plains east into the Lower Mississippi, Tennessee, and Ohio Valleys through the day Tuesday. An approaching upper-level wave will help to reinforce the boundary overnight Tuesday and enhance moist, southerly flow from the Gulf, leading to increasing storm coverage and locally heavy rainfall. The boundary will push eastward as a cold front towards the East Coast through the day Wednesday spreading showers and storms into the Southeast and coastal Northeast. Colder air in place to the north over the interior Northeast will lead to snow showers with some light to moderate accumulations for higher mountain elevations. A quick moving upper-level wave will bring some snow showers to portions of the Northern Plains/Upper Midwest today with some light accumulations possible, especially from central South Dakota east through southern Minnesota. These light snow showers will spread eastward into the Great Lakes Wednesday. Then, another more potent wave will approach from the west during the day Wednesday leading to lee cyclogenesis over the northern High Plains and helping to consolidate/strengthen a surface frontal system. This will lead to enhanced lift over the Northern Plains and a quick developing and potentially intense band of snowfall by Wednesday evening. Snow totals of 2-4" can be expected with locally heavier amounts possible through Thursday morning. Some mixed precipitation with light ice accumulations will be possible as well. Elsewhere, persistent shower and thunderstorm chances will continue for Florida, particularly along the Atlantic Coast. Some intense downpours will be possible Tuesday with an isolated threat of some flooding in the urban areas of South Florida. Most of the country will continue to see well above average temperatures through at least mid-week. Forecast highs Tuesday range from the 40s and 50s in the interior West and New England; the 50s and 60s for the West Coast, central Plains, Ohio Valley, and Mid-Atlantic; and 60s and 70s for the Southwest, southern Plains, and Southeast. A cold front will bring some cooler, more seasonable temperatures to the Plains Wednesday as highs fall into the 40s and 50s. One region that will remain much colder will be the Northern Plains/Upper Midwest, with most highs only into the teens and 20s. Putnam Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php