US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
309 PM EDT Tue Aug 18 2020
 
Valid Friday August 21 2020 - Tuesday August 25 2020
 
Hazards:
- Heavy rain across portions of the Southeast, the Lower Mississippi Valley, and the Southern 
Plains, Mon-Tue, Aug 24-Aug 25.
- Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, 
Fri, Aug 21.
- Heavy rain across portions of the Southeast, Fri-Tue, Aug 21-Aug 25.
- Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains.
- Excessive heat across portions of the Central Great Basin, California, and the Southwest, 
Fri-Tue, Aug 21-Aug 25.
- Excessive heat across portions of the Central Rockies, the Central Great Basin, the Northern 
Rockies, the Northern Great Basin, and the Southwest, Fri-Mon, Aug 21-Aug 24.
- Heavy rain across portions of mainland Alaska, Sun-Mon, Aug 23-Aug 24.
 
Detailed Summary:

The medium range period (Fri, Aug. 21 - Tues, Aug. 25) can be summarized by the ever present upper 
level ridge over the Southwest while an upper level trough is positioned over the Deep South. In 
addition, a couple tropical waves may develop into tropical cyclones and impact the Gulf Coast 
region and/or Florida by the beginning of next week. The abnormally strong ridge in the West means 
the ongoing heat wave is likely to continue across the Desert Southwest and into the Great Valley 
of California. The ridge is forecast to further amplify by the weekend after briefly relaxing on 
Thursday, meaning after a brief reprieve in the excessive heat over the Intermountain West and 
central Rockies late week, sizzling hot temperatures are poised to return and persist into early 
next week. Due to the ongoing severe drought and lack of rich moisture aloft, dry thunderstorms are 
likely to transpire across the Southwest, Rockies, and interior Northwest. This, combined with a 
potentially breezier wind pattern aloft could lead to increased fire weather potential across these 
regions. There could be a slight increase in monsoonal moisture this weekend over the southern half 
of the Four Corners states, making for slightly wetter conditions in the higher elevations.

Precipitation-wise, the upper trough over the Deep South favors upper level divergence over the 
Southeast. A dying stalled frontal boundary should act as a trigger to initiate scattered showers 
and thunderstorms throughout the region. The area most at risk for heavy rain is located over the 
southern Appalachians on Friday. There could be some locations across the Southeast from the 
Carolina beaches on south to the eastern Gulf Coast where locally heavy rainfall is possible. The 
upper trough should weaken as it tracks west towards the western Gulf, allowing for high pressure 
to build back in over Florida. That said, tropical activity will need to be closely monitored this 
weekend as NHC highlights a pair of tropical disturbances with the potential to become named 
systems.  Whether these systems develop into tropical cyclones or not, the position of the 
upper-level ridges across the western Atlantic and Southwest favors plentiful tropical moisture and 
the potential for locally heavy rain along the entire Gulf Coast Monday and Tuesday. Heavy rain 
areas in this region may be more distinguishable once the future of the tropical disturbances 
becomes clearer. Lastly, guidance is keying in on an unstable air mass out ahead of a cold front 
tracking through the Upper Midwest Friday and into the start of the weekend. Segments of showers 
and thunderstorms, some of which could be severe, could lead to excessive rainfall in areas where 
soil is overly saturated. There is currently not enough confidence in the position/timing of 
mesoscale features or rainfall totals to issue a heavy rain area, but it could be issued in future 
charts.

In Alaska, focus turns to a strong area of low pressure tracking through far northeast Russia as it 
guides a tongue of elevated moisture across the western and central mainland. Ensembles are 
highlighting an area of potentially heavy rain along the western and central Alaska range. 
Temperatures are expected to remain warmer than average through Sunday with highs in the 70s and 
even nearing 80 degrees over much of the mainland. The coolest conditions versus normal should 
transpire up and down the Panhandle.

Snell