US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 445 PM EDT Thu May 14 2020 Valid Sunday May 17 2020 - Thursday May 21 2020 Hazards: - Heavy rain across portions of the Northeast, the lower Great Lakes, the central Appalachians, northern California, the Cascades in Oregon, as well as southeastern Texas, Sun-Mon, May 17-May 18. - Heavy precipitation along portions of the Sierra Nevada, Mon-Tue, May 18-May 19. - Heavy rain across portions of the interior Mid-Atlantic into the central Appalachians, as well as from the northern High Plains to the northern Rockies, and the Intermountain West, Tue-Wed, May 19-May 20. - Heavy rain across central Montana, Thu, May 21. - Flooding possible across portions of the central and southern Plains, the Great Lakes, and the Mid-Mississippi Valley. - Flooding occurring or imminent across portions of the Great Plains, the lower and Mid-Mississippi Valley. - Much above normal temperatures across much of the northern Plains, the central High Plains, and nearby Rockies, Mon-Wed, May 18-May 20. - Much above normal temperatures across portions of the upper Midwest into the northern Plains, Wed-Thu, May 20-May 21. - Enhanced wildfire risk across portions of the the central Rockies, the central Great Basin, and the Southwest, Mon, May 18. - Heavy precipitation across portions of the southern coastal sections of Alaska, Tue, May 19 and Thu, May 21. Detailed Summary: A highly amplified synoptic pattern will be the theme during the medium range period (Sunday 5/17 to Thursday 5/21). Models are in excellent agreement that a relatively flat upper-level pattern across the U.S. during the weekend will dramatically amplify into an omega block pattern, with a pronounced ridge sandwiched in between two major cut-off lows impacting both the West Coast and East Coast. The biggest change in the forecasts over the past 24 hours has been for the East Coast where models dig down an upper trough much more quickly and aggressively from the Great Lakes into the eastern U.S. This would introduce a threat of heavy rain for portions of the East Coast by the middle of next week. The current assessment of the situation calls for heavy rain to occur from the interior Mid-Atlantic into the central Appalachians next Tuesday into Wednesday. Nevertheless, since models are still in the process of adjusting toward a more stable solution under this highly amplified pattern, the forecast impacts next week from midweek onward over the eastern U.S. will likely subject to change. Prior to the formation of this anomalous upper low over the East Coast, a threat of heavy rain is expected to impact areas from the lower Great Lakes eastward across the central Appalachians into parts of New England ahead of a pair of well-developed frontal boundaries and a low pressure system. This will be in stark contrast with a pronounced ridge of high pressure forecast to develop over the Great Plains. This ridge will ensure a significant warming trend across the northern and central Plains for early to middle of next week with afternoon temperatures likely reaching well up into the 80s to possibly the lower 90s. Over eastern Texas, a noteworthy mesoscale vortex could linger in the vicinity as a piece of upper-level energy may become trapped underneath the strong upper ridge and begin interacting with moisture from the Gulf of Mexico. Heavy rainfall could impact southeastern Texas Sunday into Monday. Since models are now forecasting the upper trough over the Great Lakes to dig much more aggressively into the eastern U.S. next week, models are now in general agreement that the disturbance over Texas will eastward into the central Gulf Coast and then weaken. Models continue to indicate the formation of a low pressure system fairly near the east coast of Florida this weekend. The National Hurricane Center continues to monitor this area for the potential of subtropical cyclogenesis. Models have shown a trend of tracking this system closer toward the southeastern U.S. in the past 24 hours. Impacts associated with this system are expected to remain largely offshore. However, with the major shift toward a major closed low developing over the eastern U.S., there is a possibility for the offshore subtropical system to mutually interact with the deep upper low over the East Coast and thus spreading moisture toward the eastern U.S. Over the West Coast, a deepening upper-level trough is forecast to bring a potential for heavy rain across portions of northern California into southwestern Oregon and the nearby Cascades Sunday into Monday. Meanwhile, heavy wet snow is expected for the higher elevations of the central Sierra Nevada next Monday into Tuesday as the upper trough digs further into California. Over the Pacific Northwest, a frontal system moving onshore is expected to bring localized areas of heavy rainfall for the Oregon Cascades Sunday into Monday. By Tuesday and Wednesday, the threat of heavy rain is forecast to spread across northern Idaho and western Montana ahead of a low pressure system. By Thursday, the heavy rain threat should linger across central Montana as the low pressure system begins to interact with a cold front arriving from western Canada. Wet snow can also be expected over the highest elevations in these areas. Over Alaska, a low pressure system moving across the southern portion of the state could result in heavy precipitation along the southern coastal areas later on Monday into Tuesday. Above normal temperatures are forecast to continue over the western and much of the southern portions of the state, in contrast with colder than normal temperatures over the north and northeast portions of the state. Kong