US Day 3-7 Hazards Outlook NWS Weather Prediction Center College Park MD 400 PM EDT Wed Aug 18 2021 Valid Saturday August 21 2021 - Wednesday August 25 2021 Hazards: - Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern Plains, Mon-Wed, Aug 23-Aug 25. - Heavy rain across portions of the Northeast, Sun-Mon, Aug 22-Aug 23. - Flooding possible across portions of the Southern Plains. - Flooding occurring or imminent across portions of the Southeast and the Great Lakes. - Flooding likely across portions of the Southern Plains. - 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 significant wave heights for coastal portions of the Northeast, Sat-Mon, Aug 21-Aug 23. - Heavy rain across portions of mainland Alaska, Sun, Aug 22. - Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Mon-Tue, Aug 23-Aug 24. Detailed Summary: Over the last 24 hours the biggest development has been forecast guidance suggesting Tropical Storm Henri could come dangerously close to the Northeast coast. Recent trends have included model guidance realizing Henri is strengthening and being more susceptible to mid-upper level ridging and northerly steering flow, thus initially ushering the storm to the west-southwest. Eventually, steering flow becomes more southerly and takes Henri north, well east of the Mid-Atlantic coast by Saturday. From there, Henri's future track is dependent upon a couple key factors. The first is an upper trough over the east-central U.S. that has also been trending more amplified over the last 24-36 hours. The second is the placement and strength of an upper level ridge axis oriented north-south over New England and Quebec. A deepening trough over the Mid-Atlantic, which along with a slightly more south and west track in the short range, suggests Henri could be more prone to feeling the upper trough "tugging" on the cyclone and bring it closer to the Northeast coast. Most 12Z guidance has come in more west with Henri's track while the Euro and Canadian operational guidance initialized considerably weaker with Henri. This could be one of the big reasons these particular models have not trended more closer to the coast (a weaker Henri means it feels less influence from northerly mid-upper level flow). In terms of hazards, it remains to be seen to what extent parts of the Northeast and even Mid-Atlantic deal with rain, wind, and coastal impacts due to the ever changing and uncertain track of Henri. Given most guidance depicts a strong hurricane off the coast, coastal impacts such as high waves and swells could induce coastal flooding and substantial beach erosion. Based on recent trends in guidance, I chose to put southern New England at risk for significant waves and heavy rain for the time being. Battering surf and swells could begin as early as this Saturday. It is important to mention this situation is fluid. Henri could still stay more offshore either by being too far east to produce impacts such as high winds and heavy rain, resulting in mainly coastal impacts. The opposite could also be true; a more north and west track would lead to interior flooding potential and more detrimental impacts from strong winds. In summary, guidance has trended west enough with Henri to the point where residents in the northern Mid-Atlantic and Northeast should keep a close eye on the latest forecast information from the National Hurricane Center in to the upcoming weekend and have a plan of action ready. Elsewhere, the synoptic pattern to start the period shows a deep upper trough over the Northeast and an amplifying ridge over the Lower Mississippi Valley. As the weekend closes and next week begins, 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, 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. The most notable hazard for Alaska 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, especially along the eastern Gulf of Alaska coast, might result in copious amounts of rainfall. Some of the highest elevations could even contend with 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