US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 506 PM EDT Wed Jun 16 2021 Valid Saturday June 19 2021 - Wednesday June 23 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 19th-June 24th) 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 and Southeast early next week ahead of a series of frontal systems. No significant changes to the thinking regarding the Western U.S.heat wave: dangerous temperatures will continue into next Wednesday throughout the region, associated with strong subsidence and dry soils below an abnormally strong upper ridge. By the 19th, the ridge axis will gradually migrate east before flattening early next week as a closed low develops off the California coast while a vigorous trough digs over the Midwest. However, this transition will occur gradually, and temperatures will remain 10-15 degrees above normal until at least the 21st in California, southern Nevada, and Arizona. In fact, locations in the Sacramento Valley could see 110 degrees this weekend, with even higher temperatures approaching 120 are possible in Southern California and Arizona. As such, a very high heat risk for the entire population extends over the area Saturday and Sunday, per experimental National Weather Service HeatRisk guidance. Temperatures moderate slightly across the southern Excessive Heat area on the 21st with the approach of the closed low, although they are still warm and slightly above average. Elsewhere, a strong cold front associated with the Central U.S. trough will dive south from Canada Monday the 21st, and usher in much below average temperatures to the Northern Plains and Rockies. Post-frontal high temperatures quickly rebound north of the Great Basin to slightly above average on the 22nd, but overall heat indices remaining below 100 degrees prompted the removal of northeast portions from the Excessive Heat area. Hot temperatures continue through the 23rd from the Great Basin into the Pacific Northwest as the pesky ridge axis remains centered in the region, in spite of the overall flattening trend and eastward shift. Additional excessive heat is likely over South Texas beginning Sunday, where ensemble guidance highlighted a greater than fifty percent chance of exceeding 105 degree heat indices for multiple days as a tropical airmass surges out of the Gulf. All eyes remain on Invest 92L located in the Gulf of Campeche, now with a high likelihood of tropical development over the next five days according to the National Hurricane Center. Over the last 24 hours, ensemble and deterministic model guidance has come into better agreement that by Saturday the 19th, this system will reside just offshore of far east Texas or Louisiana, bringing with it a plume of deep tropical moisture and subsequent significant heavy rain as it moves inland. Given the clustering of solutions toward the Texas-Louisiana border, have trimmed portions of the central Texas coastline, where heavy rain impacts appear less likely. However, it is emphasized that these forecasts can quickly change; users are encouraged to monitor the latest forecasts from the National Hurricane Center for the latest forecast information. As it stands, there are increasing concerns for potentially significant flooding along the central Gulf Coast, as heavy rain falls atop inundated soil from earlier flooding. An additional round of heavy rain is possible over the region from the 22nd-23rd as the aforementioned sweeping cold front interacts with remnant tropical moisture, potentially exasperating the situation. North of the potential tropical system, some heavy rain is possible over parts of the Ohio Valley from the 19th-20th as a leading cold front may interact with the rich moisture moving north. Uncertainty regarding the influence of tropical storm track and the progressive front precludes a heavy rain area. More certain is the heavy rain over Corn Belt from the 20th-21st as convection initiates along and north of a warm front with the abundant moisture firmly in place. The Great Lakes will also be monitored for heavy rain from the 21st-22nd as the main cold front moves across the region with potentially favorable upper dynamics, but as of now the overall system appears to be too progressive to warrant a heavy rain area. No hazardous weather is expected for Alaska across the current medium range hazards forecast period. Asherman