US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 509 PM EDT Thu Jun 17 2021 Valid Sunday June 20 2021 - Thursday June 24 2021 Hazards: - Heavy rain across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle Mississippi Valley, the Mid-Atlantic, the Southern Appalachians, the Southeast, and the Southern Plains, Sat-Wed, Jun 19-Jun 23. - Heavy rain across portions of the Central Plains, the Middle Mississippi Valley, the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Sun-Mon, Jun 20-Jun 21. - Flooding possible across portions of the Southeast and the Lower Mississippi Valley. - Flooding possible across portions of the Southeast and the Lower Mississippi Valley. - 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. - 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 Lower Mississippi Valley and the Southern Plains. - Excessive heat across portions of the Southern Plains, the Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Sun-Wed, Jun 20-Jun 23. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Sun-Mon, Jun 20-Jun 21. - Much above normal temperatures across portions of the Central Great Basin, the Northern Rockies, the Central Rockies, California, the Northern Great Basin, the Pacific Northwest, and the Southwest, Sun-Wed, Jun 20-Jun 23. - Heavy rain across portions of mainland Alaska, Wed-Thu, Jun 23-Jun 24. Detailed Summary: The medium range hazards forecast period (June 20th-June 25th) will feature the tail end of excessive heat in the west, while heavy rain threatens the Gulf Coast beginning this weekend ahead of Potential Tropical Cyclone Three in the southwestern Gulf of Mexico. Additional heavy rain is possible over parts of the Midwest and Southeast early next week ahead of a series of frontal systems. After a week of blistering hot temperatures in the West, some relief is on the way in the form of slightly cooler (albeit still warm) temperatures. By early next week, the ridge axis shifts east as a closed low develops off the California coast while a vigorous trough pivots through 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. After Monday, temperatures finally moderate to average (and even below average along the coast) as the upper low inches closer toward land. Meanwhile, a strong cold front associated with the Central U.S. trough will dive south from Canada Monday the 21st, and usher in 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. Given that forecast heat indices fail to break 100 over much of the Great Basin and Pacific Northwest for the remainder of the forecast period, the Excessive Heat area was largely changed to Much Above Normal to reflect above average (but not heat wave magnitude) high temperatures. The one exception is the area east of the Cascades in Eastern Washington/Oregon, where highs touch the upper 90's through the 23rd. 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 Potential Tropical Cyclone Three located in the southwestern Gulf of Mexico, with advisories beginning from the National Hurricane Center shortly after this writing. Some spread exists within deterministic model guidance as to the exact position of the the system once ashore the morning of the 20th, but the GEFS, EPS, and CMC ensembles are all in agreement on a position somewhere close to the Central Mississippi-Alabama border. However, it is emphasized that these forecasts can quickly change; users are encouraged to monitor the latest forecasts from the National Hurricane Center and Weather Prediction Center for the latest information. More certain are the expected significant heavy rain impacts as a plume of tropical moisture accompanies the system to produce heavy rainfall along the Central Gulf Coast, and further inland over the Coastal Plain into the Piedmont. The heavy rain area has been accordingly expanded further into the Carolinas to account for a potential storm track in the area, where orographic enhancement of rainfall could occur along the Piedmont and Blue Ridge mountains. 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 any earlier accumulations from this weekend. Elsewhere, heavy rain is likely from the 20th-21st as convection initiates along and north of a warm front with abundant moisture in place. The Heavy Rain area was adjusted further north to match higher ensemble confidence of exceeding an inch of rain, and to line up with portions of the Missouri River which may be more vulnerable to heavy rain. With trends in forecast QPF, have continued to hold off on a heavy rain area over the Great Lakes with the progressive nature of the system. Further east, some higher forecast totals may bear watching for a Heavy Rain area on the 23rd as the cold front interacts with remnant tropical moisture before a dry airmass settles in. Over Alaska, a shot of heavy rain appears likely from the Kenai Peninsula and further south the 23rd-24th as a deep low approaches the Western Aleutian chain. Ensemble consensus for higher totals prompted the addition of a heavy rain area there. Asherman