Short Range Forecast Discussion NWS Weather Prediction Center College Park MD 343 PM EST Mon Dec 18 2017 Valid 00Z Tue Dec 19 2017 - 00Z Thu Dec 21 2017 ...Heavy snow possible over the Cascades and Northern Rockies... ...Heavy rain possible over parts of the Pacific Northwest Coast... ...Heavy rain possible over portions of the Southern Plains into the Southern Appalachians... Onshore flow ahead of the next system will aid in producing rain and highest elevation snow over parts of the Pacific Northwest and snow and lower elevations rain over parts of the Northern Intermountain Region/Northern Rockies through Tuesday morning. On Tuesday the next system will begin to move onshore over the Pacific Northwest moving inland to the Northern Rockies/Great Basin and parts of Central California by Wednesday. The snow levels will decrease over the Northwest as the associated front moves inland, lowering to the coast over parts of Northwestern Washington State. Rain and higher elevations snow will develop over the Northwest and snow over the Northern Intermountain Region/Northern Rockies on Tuesday continuing into Wednesday. Rain and higher elevation snow begins to move into Northern California by Tuesday evening that will expand into Central California and snow in the Great Basin overnight Tuesday into Wednesday. In addition, snow will begin to develop over parts of the Northern Plains also overnight Tuesday into Wednesday. Meanwhile, an upper-level low over Northwestern Mexico will move northeastward to the Lower Mississippi Valley by Wednesday while weakening. Moisture from the Western Gulf of Mexico will stream northward over the Southern Plains into parts of the Lower Mississippi Valley and into the Southeast Monday evening into Wednesday morning. The moisture and upper-level energy will aid in producing showers and thunderstorms over parts of the Western Gulf Coast/Southern Plains that will begin to expand eastward into the Lower Mississippi Valley by Tuesday morning. On Tuesday, the showers and thunderstorms will continue to expand eastward into parts of the Tennessee Valley/Southeast well into Wednesday morning. Elsewhere, a front over the Upper Midwest will move off the Northeast/Northern Mid-Atlantic Coast by Wednesday morning. Rain and snow will develop along and near the boundary from the Northeast to the Upper Midwest that will end over the Upper Midwest by Tuesday morning. The snow will taper off to snow showers over the Upper Great Lakes and parts of Northern New England by Wednesday morning. Ziegenfelder Graphics available at www.wpc.ncep.noaa.gov/basicwx/basicwx_ndfd.php