US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 322 PM EDT Mon Aug 02 2021 Valid Thursday August 05 2021 - Monday August 09 2021 Hazards: - Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Northeast, Thu, Aug 5. - Heavy rain across portions of the Southwest, Sat-Sun, Aug 7-Aug 8. - Excessive heat across portions of the Central Great Basin, California, and the Southwest, Thu, Aug 5. - Excessive heat across portions of the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, the Ohio Valley, and the Tennessee Valley, Sun-Mon, Aug 8-Aug 9. - Much above normal temperatures across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Fri-Mon, Aug 6-Aug 9. - Much above normal temperatures across portions of the Northeast, the Central Appalachians, the Middle Mississippi Valley, the Mid-Atlantic, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Sun-Mon, Aug 8-Aug 9. - Heavy rain across portions of mainland Alaska, Thu-Fri, Aug 5-Aug 6. Detailed Summary: The synoptic evolution during the medium range period can be summarized as general troughing over the East giving way to rising upper level heights aloft, which combined with a series of short waves tracking into southwest Canada will allow for more hotter than normal conditions to return from the Northern Plains to the Northeast. In addition, an intense North Atlantic surface high pressure will build west towards the Gulf Coast. Working in tandem with an anomalously lower pressure field throughout the Heartland and Canadian Prairies, this in turn generates a tightening pressure gradient and ushers in moist southerly flow throughout the eastern half of the Lower 48. In short, sultry conditions should return across much of the Plains and eastern U.S. this weekend and into early next week. Heat indices could range between 105-110 degrees in parts of the Lower Mississippi Valley. This also unfolds during what is climatologically the hottest time of the year in the Lower Mississippi Valley, further giving credence to what could be a stretch of excessive heat. Meanwhile, ridging over the Rockies and Desert Southwest can expect a trend toward a troughing pattern the second half of the period. Temperature-wise, this will favor at least one day of excessive heat in portions of the Desert Southwest before temperatures trend below normal by the weekend. Precipitation-wise, a quasi-stationary frontal boundary looks to remain draped over the Southeast and along the central Gulf Coast on Thursday. This should lead to at least one more day with numerous showers and thunderstorms, some of which could be either severe and/or contain heavy rainfall rates the lead to flash flooding from the Florida Panhandle to the Mid-Atlantic coast. Some heavy showers and storms could extend as far north as the southern New England coast. As the front undergoes frontolysis Friday and into Saturday, the coverage and intensity of these showers and storms will gradually dissipate. In the Southwest, as the upper level ridge aloft weakens, vorticity lobes rotating around the Four Corners region combined with an influx of monsoonal moisture is set to develop more showers and thunderstorms in Arizona. Given the atmospheric moisture available and anomalously moist antecedent soil conditions, flash flooding will be possible this weekend. Lastly, there is concern for rounds of heavy showers and thunderstorms in the Upper Midwest late Thursday into Friday and again late Saturday into Sunday (the weekend set up holds the most rainfall potential). Confidence in how much and where the worst convection lineups however remains unclear at this time. Future heavy rain areas may be added in the coming days should confidence in heavy rainfall impacts increase. In Alaska, the 500mb pattern across the North Pacific is highly amplified at the start of the period with an upper low near the northeast coast of Russia that the latest 12Z GFS shows being nearly 3 STDs below normal. A lobe of upper level ridging near the north pole is helping to cause a blocking pattern near Alaska, which results in a chance for heavy rainfall on Thursday and perhaps lingering into Friday. Precipitable water values of 2-3 STDs above normal and a favorable synoptic ascent setup could yield heavy rainfall rates over west-central Alaska. The same upper low to the west is set to help in the development of another North Pacific frontal system that is forecast to track through the Aleutians late Saturday into Sunday, then into south-central Alaska by early next week. There remains a good amount of model uncertainty in both storm track and available moisture, so no heavy rain/precipitation area was drawn at this time. Should confidence increase in the coming days, a heavy rain/precip area may be necessary for portions of the southern mainland. Mullinax