US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 316 PM EDT Mon Mar 15 2021 Valid Thursday March 18 2021 - Monday March 22 2021 Hazards: - Heavy rain across portions of the Central Appalachians, the Mid-Atlantic, the Southern Appalachians, the Southeast, the Great Lakes, and the Ohio Valley, Thu-Fri, Mar 18-Mar 19. - Severe weather across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, Thu, Mar 18. - Flooding possible across portions of the Central Plains, the Middle Mississippi Valley, the Lower Mississippi Valley, and the Ohio Valley. - Flooding occurring or imminent across portions of the Central Plains, the Middle Mississippi Valley, the Southern Plains, and the Ohio Valley. - Flooding likely across portions of the Central Plains and the Middle Mississippi Valley. Detailed Summary: The medium range forecast period (Thursday, March 18th to Monday, March 22nd) features an impactful storm system exiting the East Coast on Thursday and Friday. Meanwhile, a separate system and associated upper-level trough is expected to enter the western U.S. and reach the Great Plains by Monday. Starting in the East, by Thursday morning an area of low pressure is forecast to begin weakening across the Midwest as an associated cold front pushes across the Southeast. Heavy rain (mixing with light snow) could be found across the Midwest and Ohio Valley, but will sharply dissipate by Thursday evening. Light snow will also remain possible across the northern Mid-Atlantic and southern New England as a cold front swings through the region on Friday. The greatest threat for heavy rainfall totals over an inch will be from the southern Appalachians to Mid-Atlantic on Thursday into early Friday morning. Widespread flooding/flash flooding is not expected given recent dry conditions, but localized flooding will be possible if thunderstorms align and repeat over similar areas. This is mainly a concern across western North Carolina and southern Virginia. Speaking of thunderstorms, the Storm Prediction Center has highlighted a large area from the Southeast to southern Mid-Atlantic where severe thunderstorms are possible on Thursday. The combination of an approaching cold front, anomalously high atmospheric moisture content, and instability will make conditions ripe for severe weather from northern Florida to southern Virginia. The next system to traverse the Lower 48 is expected to enter the West Coast on Thursday and bring a mix of lower elevation rain and mountain snow from central California to Washington. Following the Pacific cold front, light-to-moderate precipitation should enter the Intermountain West and Rockies by Friday and Saturday. As the upper-level trough enters the central U.S. by Sunday, model spread increases regarding where/if the greatest chances for heavy rain exist. In fact, it appears most likely that efficient moisture transport from the western Gulf of Mexico doesn't occur until after Monday. Therefore, no heavy rain areas were added to the hazards graphic for Monday, March 22, but an area could very well be added over the next few forecast cycles. No hazardous temperatures are expected during the medium range time frame. Cooler than average temperatures are forecast along much of the West Coast and south-central U.S., as well as the Southeast following the cold frontal passage on Friday. High temperatures around 10 to 20 degrees above average could be found across much of the northern tier, spreading into the Great Lakes and Northeast by Sunday. For Alaska, relatively tranquil weather is expected through early next week. Temperatures will remain a few degrees below average across much of the state. The only notable precipitation will be found throughout the southeastern Panhandle as multiple waves of low pressure meander across the Gulf of Alaska. Precipitation amounts are expected to remain below hazardous threshold. Snell