Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 152 PM EST Sat Nov 21 2020 Valid 00Z Sun Nov 22 2020 - 00Z Tue Nov 24 2020 ...Low pressure system producing rain over the Ohio and Mississippi Valleys tonight heads for the Northeast and eastern Great Lakes Sunday.. ...Air Stagnation Advisories over parts of Oregon; rain and mountains snow returns to the Pacific Northwest Sunday & Monday... ...Milder southern tier, cooler in parts of the Plains, Great Lakes, and Northeast... The headliner in the short term is a developing wave of low pressure along a frontal boundary that becomes draped across the mid-Mississippi and Ohio River Valleys today. Moisture pooling along the front is leading to a swath of rain with embedded thunderstorms in these aforementioned regions this afternoon. As the low moves northeast tonight, areas of rain and a wintry mix develops over portions of the lower Great Lakes. By Sunday morning, periods of snow may lead to light accumulations in parts of northern Indiana, northern Ohio, and southern Michigan. As the low moves farther northeast, rain will move into parts of the Northeast late Sunday with some snowfall possible in some parts of the eastern Great Lakes. The storm system strengthens Sunday night into Monday morning with showers across much of the Northeast and Mid-Atlantic. Some areas of snow/wintry mix are possible across portions of northern New England. Once the cold front passes through Monday morning, drier conditions return the second half of the day with only some lingering lake effect rain/snow showers downwind of Lakes Erie and Ontario Monday evening. In the Pacific Northwest, an upper-level ridge axis passing overhead and high pressure at the surface is causing a strong low level inversion to trap pollution in low lying areas. As a result, Air Stagnation Advisories have been posted over parts of Oregon through Sunday morning. This period of poor air quality for some will be short lived as another upper trough and associated frontal system takes aim at the Pacific Northwest the second half of the day on Sunday. Coastal/valley showers is likely along with mountain snow in the Olympics and Cascades. A second Pacific cold front heads for the area on Monday with areas of precipitation being a little more widespread compared to Sunday. Temperatures will generally be on the mild side the next couple days in the Southeast and Southwest. The coolest areas versus normal are likely to transpire from the south-central Plains and Great Lakes to the interior Northeast. As a cold front passes over the East Coast on Monday, expect cooler than normal temperatures to return to the Carolinas and Mid-Atlantic on Tuesday, while another passing cold front cools down the Four Corners states. Mullinax Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php