Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 259 AM EST Tue Nov 20 2018 Valid 12Z Tue Nov 20 2018 - 12Z Thu Nov 22 2018 ...Heavy rain possible for parts of Northern California and Pacific Northwest Coast with a slight risk for excessive rainfall over parts of Central/Southern California... ...Heavy snow possible over the Sierras and light snow over the Cascades and the Upper Great Lakes into parts of the Northeast... ...Air Quality Alerts for parts of California and Air Stagnation Advisory for parts of Washington State/Idaho... ...Temperatures will be 10 to 15 degrees below average over parts of the Upper Great Lakes... Upper-level energy over the Great Lakes/Ohio Valley will move eastward off the Northeast Coast overnight Tuesday. The energy will aid in developing snow over the Great Lakes into Northern New England and rain over parts of the Central Appalachians into Southern New England on Tuesday. The rain will move off Southern New England Coast by Tuesday evening as the rain over the Central Appalachians changes over to snow ending overnight Tuesday. The snow over the Lower Great Lakes will end for short time overnight Tuesday, too. The snow over Northern New England will end by late Wednesday morning. In addition, a second area of upper-level energy over Western Ontario will move eastward to Northern New England by Wednesday evening. Snow will develop over parts of the Northern Plains/Upper Mississippi Valley that will move into the Upper Great Lakes on Tuesday evening. The snow will continue to move eastward to the Lower Great Lakes/Northeast by Wednesday afternoon and extend into Northern New England by Wednesday evening. Meanwhile, a third area of upper-level energy over the Western Gulf Coast/Lower Mississippi Valley will move eastward off the Southeast Coast overnight Tuesday. The energy will produce rain and thunderstorms over parts of the Western Gulf Coast into parts of the Southeast and will, likewise, end overnight Tuesday. Additionally, a fourth area of upper-level energy over parts of Northwestern Mexico will move eastward to Texas by Wednesday evening. The energy will produce rain over parts of Western Texas overnight Tuesday. The energy will also draw moisture from the Gulf of Mexico over the Western Gulf Coast early Wednesday morning into early Thursday morning (Thanksgiving). Rain will redevelop over the Western Gulf Coast on Wednesday expanding into parts of the Central Gulf Coast on Thanksgiving morning. Furthermore, Air Quality Alerts are in effect over parts of Southern California until the fires are extinguished. Light wind and a strong inversion will persist through Wednesday leading to Air Stagnation Advisory across North Idaho as well as Central and Eastern Washington. The arrival of light precipitation and a subtle increase in winds Wednesday and Wednesday night may improve pollution dispersion for some areas across the Inland Northwest. Meantime, a deep upper-level trough will move onshore over the West Coast early Wednesday into Thursday morning (Thanksgiving). Rain will develop along the West Coast early Wednesday morning and expand inland to the Northern Intermountain Region/Great Basin by Thanksgiving morning. There is a marginal with embedded slight risk of excessive rainfall over California from Wednesday morning into Thursday morning. As the trough moves inland the snow levels will lower over the Cascades and the Sierras. Snow will begin over the higher elevations of the Cascades and Sierras by Wednesday afternoon continuing into Thursday morning. Overnight Wednesday, snow will develop over parts of Northern California and the Southern Cascades. Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php