Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 300 PM EST Wed Nov 20 2024 Valid 00Z Thu Nov 21 2024 - 00Z Sat Nov 23 2024 ...Back-to-back powerful Pacific storm systems to impact the West Coast through the end of this week with heavy rain, life-threatening flooding, strong winds, and higher elevation mountain snow... ...Heavy snow is likely throughout parts of the central Appalachians beginning on Thursday, with a separate burst of snowfall possible across northeast Pennsylvania and neighboring regions of the Northeast Thursday night into Friday... An active pattern remains in place this week as multiple systems bring heavy rain, life-threatening flooding, and higher elevation mountain snow to the West and the season's first heavy snowfall to the Appalachians over the next couple of days. In the West, back-to-back systems from the Pacific and an accompanying strong Atmospheric River will bring waves of very heavy rain and high elevation snow to the Pacific Northwest/northern California, with the first wave continuing through Wednesday evening into early Thursday and a second wave peaking Thursday night. The most intense, heavy rainfall will focus on northern California/southwestern Oregon, with a Moderate Risk of Excessive Rainfall (level 3/4) in place along the northern California coast through Thursday morning where the heaviest amounts and greatest chance for flooding is expected. A broader Slight Risk (level 2/4) extends inland along the northern Sierra ranges. The second wave Thursday evening is expected to bring even more intense rainfall on top of already saturated soils, with a High Risk (level 4/4) in place along the northern California coast and a Moderate Risk inland along the northern Sierra ranges. Storm total rainfall may reach as high as 12-16", with dangerous flash flooding, rock slides, and debris flows likely. In addition, heavy wet snow is expected for the Cascades and far northern California. Snowfall rates of 2-3"/hr and wind gusts up to 65 mph will result in whiteout/blizzard conditions and near impossible travel at pass level. Snowfall will increasingly focus at higher elevation in northern California as snow levels rise ahead of the continued mild, very moist inflow of Pacific air. Moisture will also spread further inland bringing lower elevation rain and high elevation mountain snow to the northern Great Basin and northern Rockies through Friday. Some locally heavy snowfall totals are most likely in the mountains across central Idaho and along the U.S/Canadian border. To the east, another low pressure system will track eastward from the northern Plains into the Great Lakes region this evening, sweeping a trailing occluded/cold front and a line of showers and thunderstorms eastward through the Appalachians, Mid-Atlantic, and settling into the Northeast by Thursday morning. Meanwhile, a broad area of precipitation continues behind the system under the influence of a deep upper-level cyclone, expanding eastward in tandem with the low pressure system. Heavy snow and blizzard conditions over the northern Plains will wind down through this evening as a wintry mix picks over the Great Lakes overnight Wednesday and into the day Thursday. Some light to moderate accumulations are expected, most likely in vicinity of Lake Michigan. Then, rainfall over New England southwest through the interior Northeast and into the Appalachians will begin to change over to snow for higher elevations as the upper-low moves eastward over the region. Winter weather-related Warnings/Advisories have been issued through the northern and central Appalachians as heavy wet snowfall accumulations are expected through Thursday night and into the day Friday. Winds across the region will also be rather blustery. Elsewhere, conditions will be mostly dry with the exception of some possible showers/thunderstorms for south Florida Wednesday evening. Cooler more seasonable Fall temperatures will return to much of the eastern U.S. following the cold front passage, as highs drop from the 60s and 70s for many locations Wednesday to the 40s and 50s Thursday. In contrast, the West will see rising temperatures as a ridge builds northward over the region, with highs running generally 5-15 degrees above average through the end of the work week. Putnam Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php