US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
430 PM EDT Tue Aug 17 2021
 
Valid Friday August 20 2021 - Tuesday August 24 2021
 
Hazards:
- Heavy rain across portions of the Northern/Central Plains, the Tennessee Valley, the Middle/Upper 
Mississippi Valley, and the Ohio Valley, Fri, Aug 20.
- Severe weather across portions of the Northern/Central Plains and the Upper/Middle Mississippi 
Valley, Fri, Aug 20.
- Flooding possible across portions of the Southeast and the Southern Appalachians.
- Flooding occurring or imminent across portions of the Southeast.
- Excessive heat across portions of the Southeast, the Middle/Lower Mississippi Valley, the 
Southern Plains, and the Tennessee Valley, Sun-Tue, Aug 22-Aug 24.
 
Detailed Summary:

On Friday, a front across the Northern/Central Plains will focus showers and thunderstorms over the 
area as moisture streams into the region with a deep upper-level trough and energy move across the 
area. As a result, an area of heavy rain is depicted on the Hazards Chart on Friday that will 
overlap an area of potentially severe thunderstorms that will also be over the Upper Midwest on 
Friday.

The front moves slowly eastward to the Great Lakes by Sunday as the moisture supply ends while the 
trailing part of the boundary will extend across the Upper Midwest.  Rain will continue along and 
near the boundary but will have lower precipitation amounts.  Furthermore, tropical moisture will 
be over parts of the Tennessee and Southern Ohio Valleys on Friday.  The moisture will aid in 
producing showers and thunderstorms over the region. Therefore, an area of heavy rain is depicted 
on the Hazards Chart over parts of Tennessee Valley and parts of the Southern Ohio Valley on 
Friday. 

The previously mentioned upper-level trough will aid in producing below-average temperatures over 
the Northern High Plains and Northern Rockies through Tuesday.  However, an upper-level high will 
develop over parts of the Lower Mississippi/Tennessee Valleys and parts of the Central Gulf Coast 
starting on Sunday through Tuesday. The temperatures will be slightly above average, yet the dew 
point temperatures will be in the low to mid-70s. The temperatures and dew point will combine to 
make the heat index around 110 to 115.  Therefore, as a result, an area of Excessive Heat is 
depicted over parts of the Lower Mississippi/Tennessee Valleys and parts of the Central Gulf Coast 
starting on Sunday going through Tuesday.

For Alaska, the upper-level pattern will be dominated by troughing, which will be conducive to wet 
weather.  At the surface, low pressure and a front will be across the Interior of Alaska on Friday 
and then move into Canada by Saturday, trailing the front across the Gulf Coast on Saturday.  On 
Saturday, another area of low pressure and a front move across the Aleutians and move into the 
Interior and Gulf Coast by Tuesday.  In the meantime, another front will move across the Far North 
on Sunday and move into Canada on Monday while draping across the Interior into Tuesday.  The 
systems will produce rain over parts of the state through Tuesday.  Yet, the precipitation totals 
will not reach the criteria for a hazard for this time of year. There is some uncertainty for heavy 
rain along the Gulf Coast, with the ECMWF being the most robust with the precipitation totals.

Ziegenfelder