US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 532 PM EDT Wed May 13 2020 Valid Saturday May 16 2020 - Wednesday May 20 2020 Hazards: - Heavy rain across portions of the Great Lakes into the central Appalachians, the eastern portion of the southern Plains, as well as from northern California to the Cascades and the Olympic Peninsula, Sat-Sun, May 16-May 17. - Heavy precipitation along portions of the Sierra Nevada, Mon-Tue, May 18-May 19. - Heavy rain across portions of Montana, Wed, May 20. - Heavy rain across portions of the central Appalachians into southern New England, Mon-Tue, May 18-May 19. - Flooding possible across portions of the central Plains, the Mid-Mississippi Valley, and the Great Lakes. - Flooding occurring or imminent across portions of the Great Plains into the Midwest, as well as the lower Mississippi Valley. - Much above normal temperatures across portions of the central Rockies eastward into the central and northern Plains, Mon-Wed, May 18-May 20. - Heavy precipitation across portions of southern coastal areas of Alaska, Tue, May 19. Detailed Summary: A major change in the synoptic pattern will be the theme during the medium range period (Saturday 5/16 to Wednesday 5/20). Models are in excellent agreement that a relatively flat upper-level pattern across the U.S. during the weekend will evolve to a highly amplified omega block by the middle of next week. Prior to this pattern change, moisture streaming northward from the Gulf of Mexico will interact with a well-developed warm front to result in a threat of heavy rain across the lower Great Lakes into the central Appalachians during the weekend. By early next week, the threat of heavy rain should shift eastward across the central Appalachians into southern New England as a major upper-level trough is forecast to form along the East Coast. This will be in stark contrast with a major ridge of high pressure forecast to develop over the Great Plains. The current WPC forecast calls for the trough/cyclone to move slowly off the East Coast. However, uncertainty is quite high with this rapidly evolving synoptic pattern. The ECMWF is now indicating the possibility of mutual interaction of the East Coast trough with a subtropical/tropical low pressure system off the East Coast that could result in a lingering upper low centered near the Mid-Atlantic coast by the middle of next week. Models have been indicating the formation of a low pressure system near or east of Florida this weekend as it generally heads northeastward into the Atlantic. The National Hurricane Center continues to monitor this area for the potential of subtropical cyclogenesis. Impacts associated with this system are expected to remain largely offshore, although models have been showing a trend for a better-defined system to track closer to the Southeast U.S. this weekend. As mentioned in the last paragraph, there is a possibility for this system to mutually interact with the rapidly deepening trough forecast to develop along the East Coast by early to the middle of next week. Over the West Coast, a deepening upper-level trough is forecast to approach from the Pacific during the weekend. Moisture ahead of this system is forecast to bring a potential for heavy rain across portions of northern California into southwestern Oregon through the weekend. By next Monday into Tuesday, heavy wet snow could spread into the Sierra Nevada as the upper trough digs further into California. While major troughing develops along either side of the country, a pronounced ridge of high pressure is forecast to develop over the Rockies during the weekend before moving east into the Great Plains early next week. This 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 near 90. Underneath this strengthening upper ridge over the Plains, a noteworthy mesoscale vortex could linger in the vicinity of eastern Texas as a piece of upper-level energy may become trapped underneath the upper ridge and begin interacting with moisture from the Gulf of Mexico. The GFS shows the system spinning closer to the Texas coast and result in a better-defined system along with heavier rainfall amounts. On the other hand, the ECMWF tends to send the system farther to the northeast. The predictability regarding a mesoscale system of this nature is generally rather low for the medium-range. However, the potential for heavy rainfall cannot be ruled out especially if the system meanders closer to the Gulf Coast, which could lead to the possibility of subtropical/tropical development as suggested by the GFS. Over the Pacific Northwest, a frontal system moving onshore is expected to bring localized areas of heavy rainfall this coming weekend for the terrain-favored areas along the Cascades as well as for the Olympic Peninsula. By Wednesday, the threat of heavy rain is forecast to reach portions of Montana ahead of a low pressure system. 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