US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
420 PM EDT Fri Aug 20 2021
 
Valid Monday August 23 2021 - Friday August 27 2021
 
Hazards:
- Heavy rain across portions of the Great Lakes, the Upper Mississippi Valley, and the Northern 
Plains, Tue, Aug 24.
- Heavy rain across portions of the Great Lakes and the Upper Mississippi Valley, Fri, Aug 27.
- Heavy rain across portions of the Mid-Atlantic and the Northeast, Mon, Aug 23.
- Flooding occurring or imminent across portions of the Southeast, the Great Lakes, the 
Mid-Atlantic, the Northeast, and the Southern Appalachians.
- Excessive heat across portions of the Lower Mississippi Valley, the Tennessee Valley, the Middle 
Mississippi Valley, the Southeast, the Southern Plains, and the Ohio Valley, Mon-Tue, Aug 23-Aug 24.
- Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Tue-Wed, Aug 24-Aug 25.
- Heavy rain across portions of mainland Alaska, Mon, Aug 23.
- High significant wave heights for coastal portions of mainland Alaska, Thu, Aug 26.
 
Detailed Summary:

The synoptic scale pattern over the Lower 48 during the medium range period (Mon, Aug 23 - Fri, Aug 
27) can be summarized by a building areas of ridging aloft over the southern U.S. while a steady 
stream of Pacific disturbances track from the Northwest to the Upper Midwest. The weather system in 
the spotlight at the very start of the period is Henri, which is forecast to be somewhere over New 
England or even as far west as upstate New York. Precipitable water levels ~2-2.5 STDs above normal 
and ongoing southerly flow that could become oriented in a favorable upsloping manner would help to 
generate excessive rainfall rates in higher elevations. Much of these areas have also received 
heavy rainfall from remnants of Fred where overly saturated antecedent soil conditions are present. 
Given the latest forecast shows a slowing forward motion and pivot in Henri's track, flash flooding 
and areal flooding will likely remain a serious problem for portions of New England on Monday. 
River and stream flooding in wake of Henri may linger into the middle of the week. Coastal impacts 
should start to wain by Monday morning as the storm quickly weakens over land and the pressure 
gradient relaxes Sunday night, but some lingering coastal flooding may persist into early Monday so 
long as brisk onshore flow is present.

Across the remainder of the CONUS, hazards include an Excessive Heat area in the South-Central U.S. 
where heat indices are forecast to soar as high as 110-115 degrees. On the northern periphery of 
the expansive southern U.S. upper ridge, the pattern could behave in a "ring of fire" type manner 
as the aforementioned disturbances track just north of the Midwest. Areas of thunderstorms, some of 
which could be severe, have the potential to produce heavy rainfall rates and lead to flash 
flooding concerns. The area most at risk is the Upper Mississippi Valley where they reside closest 
a series of frontal systems passing through the region. The days that currently feature the most 
risk for heavy rainfall are Tuesday (8/24) and Friday (8/27). There could be other portions of the 
Northern Plains and Midwest that see rounds of showers and storms at differing times throughout the 
week.

The Gulf of Alaska coast is expected to deal with a deluge of Pacific moisture brought upon by a 
robust upper level trough tapping into rich subtropical moisture. An occluded low pressure system 
will introduce a steady barrage of precipitation out ahead it the first half of the week. 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. 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. Lastly, a quick moving wave of low pressure could generate high waves and 
swells along the northwest coast on Thursday. A significant waves area has been introduced and 
could be expanded or re-positioned should the wave height forecast change on Monday.

Mullinax