US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 408 PM EDT Thu Aug 19 2021 Valid Sunday August 22 2021 - Thursday August 26 2021 Hazards: - Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon-Tue, Aug 23-Aug 24. - Heavy rain across portions of the Middle Mississippi Valley and the Upper Mississippi Valley, Sun, Aug 22. - Heavy rain across portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - Severe weather across portions of the Central Plains, the Middle Mississippi Valley, the Upper Mississippi Valley, and the Northern Plains, Sun, Aug 22. - Flooding possible across portions of the Great Lakes, the Mid-Atlantic, the Northeast, and the Central Appalachians. - Flooding occurring or imminent across portions of the Southeast, the Great Lakes, the Mid-Atlantic, and the Southern Appalachians. - Excessive heat across portions of the Southeast, the Middle Mississippi Valley, the Lower Mississippi Valley, the Southern Plains, and the Tennessee Valley, Sun-Tue, Aug 22-Aug 24. - High winds across portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - High significant wave heights for coastal portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Aug 23-Aug 24. - Heavy rain across portions of mainland Alaska, Sun-Mon, Aug 22-Aug 23. Detailed Summary: Diagnosing Henri this afternoon, the storm is feeling the effects of northerly wind shear displacing the storm's most intense convection just south of the low level circulation center. This bout of vertical wind shear will keep the storm from intensifying through the evening hours, but the storm's westerly movement will soon lead it into an area of diminishing wind shear on Friday. At the same time, an upper level trough over the east-central U.S. will aid in the development of a healthier poleward outflow channel that will continue into the day on Saturday. Combined with the storm remaining over very warm water, Henri is forecast to intensify on Friday and continue to do so into Saturday. Henri's track remains highly dependent upon how intense the storm is, the amount of "tugging" it feels from the upper trough over the Mid-Atlantic, and the strength/position of the ridge over New England and Quebec. Most guidance has come into a little more agreement on the storm slowing down on Sunday, weakening as it tracks over cooler waters, and coming close to a pivot anywhere from just east of the Massachusetts Capes to as far west as Long Island. How strong Henri can become over the next 24-48 hours will play a critical role in its resulting track and what type of hazards it produces over New England. Speaking of hazards, ensemble guidance coming into a little better agreement on a track closer to the Massachusetts Capes increases the odds of seeing significant coastal impacts to the region. Most notable hazards along the southern New England coast would include battering surf, resulting coastal flooding and beach erosion, tropical storm force winds, and torrential rainfall. Battering swells from Long Island to northeast Massachusetts are also likely until Henri substantially weakens Monday into Tuesday. Inland flooding (both flash flooding and river/stream flooding) would become increasingly problematic should Henri track farther west or more inland, especially following the heavy rainfall from the remnants of Tropical Storm Fred in recent days. However, the situation and forecast track remains fluid. Henri could still stay further offshore and reduce potential impacts such as high winds and heavy rain, resulting in mainly coastal impacts due to highs surf. The opposite could also be true; a more westerly track would lead to increased interior flooding potential and more detrimental impacts to more populated areas north and west of I-95. In summary, residents in the Northeast should keep a close eye on the latest forecast information from the National Hurricane Center in to the upcoming weekend, have a plan of action ready, and follow the advice of local officials. Elsewhere, the synoptic pattern on Sunday and Monday depicts a deep upper trough over the Northwest and an amplifying ridge over the Lower Mississippi Valley. By mid-week, upper level ridging will expand into the Southwest and become centered over the Southern Plains. Meanwhile, a series of upper level disturbances look to track across the Northern Plains and into the Upper Midwest. Resulting hazards include a potential Excessive Heat area in the South-Central U.S. where heat indices of 110-115 degrees could transpire. On the northern periphery of the ridge, the pattern could behave in a "ring of fire" type manner as the aforementioned disturbances track across the Midwest. Areas of thunderstorms, some of which could be severe (especially on Sunday), may also produce heavy rainfall rates and lead to flash flooding concerns. The area most at risk is the Upper Mississippi Valley where they reside just north of a quasi-stationary frontal boundary early next week. Alaska's most noteworthy hazard in the medium range is heavy rainfall brought upon by a robust upper level trough tapping into rich subtropical moisture and directing it towards the Gulf Coast. An occluded low pressure system will introduce a steady barrage of precipitation out ahead it. Combined with persistent upslope flow along the higher elevations, copious amounts of rainfall are anticipated along the higher elevations of the Gulf Coast. Some of the highest elevations could even witness some measurable snowfall, although those cases will be confined to the tallest mountain tops. The heavy rain area may need to be expanded into the Panhandle should guidance trend more confident with heavier amounts, but expect unsettled weather to be at the very least in the forecast the first half of next week. Even parts of central Alaska could see scattered showers and thunderstorms as upper level troughing lingers over the state, but rainfall amounts are forecast to remain below threshold. Mullinax