Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 246 AM EST Thu Nov 21 2024 Valid 12Z Thu Nov 21 2024 - 12Z Sat Nov 23 2024 ...Strong atmospheric river continues to impact northern California with heavy rain and life-threatening flooding through Friday... ...Developing storm system forecast to bring another round of gusty winds to the Pacific Northwest on Friday with heavy mountain snow spreading toward the northern Rockies this weekend... ...Unsettled weather expected across much of the Northeast and Great Lakes over the next few days, including the likelihood of heavy snow in the central Appalachians and higher elevations of northeastern Pennsylvania and southern New York... Impactful and for some place dangerous weather conditions will continue through early this weekend as two separate storm systems impact the Lower 48. Starting in the West, a strong atmospheric river currently impacting northern California is forecast to remain relatively stationary over the next few days and produce an additional 6-12 inches of rainfall over regions with already saturated terrain. The atmospheric river is expected to peak in intensity today, but with moderate bouts of rain lingering through much of Friday and snow levels finally lowering somewhat on Saturday. In the meantime, dangerous flooding, rock slides, and debris flows are likely, which has prompted a High Risk (level 4/4) of Excessive Rainfall to be issued across the northern California coastline today. Be sure to check conditions before traveling and never drive across flooded roadways. Aiding the surge of atmospheric moisture into northern California and the Northwest through the end of the week is a developing storm system forecast to swing off the Oregon and Washington coastline on Friday. A punch of gusty winds are expected, mainly along coastal regions, could produce rough surf and additional isolated power outages. As precipitation lifts northward and inland along a draped stationary boundary stretching from the northern Rockies to British Columbia, heavy snow is possible from the Washington Cascades to the western Montana, Idaho, and northwestern Wyoming mountain ranges through early Sunday. November snowfall is also in the forecast for parts of the Northeast and Great Lakes thanks to a potent upper-level low swinging over the region. At the surface, a compact area of low pressure is currently looping around the Great Lakes with an eventual southward trajectory over Lake Michigan later today. Parts of eastern Wisconsin could see precipitation fall as snow along with wind gusts up to 40 mph. Winter Weather Advisories have been issued for snowfall amounts up to 3-4 inches. Meanwhile, a separate area of low pressure developing near Long Island tonight is also expected to track in a looping orientation across the Northeast through Friday, while also producing periods of heavy snow. The greatest chances for at least 6 inches of snowfall is found across northeast Pennsylvania and southern New York, including the Pocono and Catskill mountains. Additionally, a long-duration upslope snow event is underway across the central Appalachians and anticipated to linger through at least early Saturday. Up to a foot of snow is possible across the higher terrain of West Virginia, Maryland, and southwest Pennsylvania. Light snow is also expected to reach as far south as the higher ranges of North Carolina. The central and southern U.S. can expect much more tranquil conditions as high pressure creates a void in the unsettled weather impacting both the East and West coasts. However, below average temperatures are forecast from the Mississippi Valley to the Southeast as breezy northwest flow ushers in a colder airmass. This may lead to early morning frost and/or freeze potential across the Southeast and Lower Mississippi Valley. Snell Graphics available at https://www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php