US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 452 PM EDT Tue Jun 15 2021 Valid Friday June 18 2021 - Tuesday June 22 2021 Hazards: - Heavy rain across portions of the Southeast, the Southern Appalachians, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Fri-Wed, Jun 18-Jun 23. - Heavy rain across portions of the Middle Mississippi Valley, the Great Lakes, and the Upper Mississippi Valley, Sun-Mon, Jun 20-Jun 21. - Flooding possible across portions of the Southeast and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Flooding occurring or imminent across portions of the Southeast, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley. - Excessive heat across portions of the Central Plains, the Central Great Basin, the Northern Plains, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Fri-Tue, Jun 18-Jun 22. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Fri-Mon, Jun 18-Jun 21. Detailed Summary: The medium range hazards forecast period (June 18th-June 23rd) continues to feature dangerous heat across the Western U.S., while heavy rain threatens the Gulf Coast beginning this weekend ahead of an area of disturbed weather located over the Gulf of Campeche. Additional heavy rain is possible over parts of the Midwest early next week ahead of a series of frontal systems. Echoing the sentiment of previous discussions, the most immediate and widespread weather hazard over the CONUS is the continuation of oppressive heat over the Western U.S. associated with an anomalously strong mid-upper level ridge. Forecast 500 millibar heights will hover around the upper 90th percentile compared to climatology for much of the region. Warming associated with strong subsidence coupled with dry soils will perpetuate excessively hot temperatures throughout much of the forecast period. Record breaking high temperatures 10-15 degrees above average are likely this weekend, with some locations in Southern California and Arizona hitting 120 degrees. By the 22nd, the mid-level ridge axis is forecast to retrograde northwest before splitting as broad troughing establishes across the Upper Midwest and a shortwave trough approaches the California coast. Temperatures will remain warm across California into Arizona next Tuesday, but are expected to be more seasonal. Thus, it was decided to split the Excessive Heat area into two areas, with the northern Great Basin Excessive Heat area running until Tuesday the 22nd, where temperatures are 15 degrees above average. Other areas, including New Mexico and Southwest Texas may see highs approaching 100, but given the more seasonal nature of these temperatures decided against introducing an Excessive Heat area for now. Our attention turns to the Gulf of Campeche, where an area of disturbed weather has an 80 percent chance of tropical development through the next five days, per the National Hurricane Center. By Friday the 18th, this system is forecast to reside somewhere over the western Gulf of Mexico, bringing with it a plume of deep tropical moisture and subsequent heavy rain potential. Confidence is low as to where and when exactly the system impacts the coast beyond that time; users are encouraged to monitor the latest forecasts from the National Hurricane Center as confidence increases regarding the track and timing of the system. Regardless, the coastal region remains particularly sensitive to heavy rain in light of recent flooding events, with highly saturated soils extending from Southeast Texas into the Florida Panhandle. Ahead of the potential tropical system, there is some concern regarding the potential for heavy rain over portions of the Midwest and Ohio Valley from the 20th onward, as the tropical moisture interacts with a series of frontal systems ahead of a pivoting shortwave in the Upper Midwest. The uncertainty surrounding the timing and placement of these features precludes an additional heavy rain area over the Ohio Valley for the 21st, but will be monitored in subsequent updates. No hazardous weather is expected for Alaska across the current medium range hazards forecast period. Asherman