US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
317 PM EDT Mon Aug 17 2020
 
Valid Thursday August 20 2020 - Monday August 24 2020
 
Hazards:
- Heavy rain across portions of the Southeast and the Lower Mississippi Valley, Fri-Sun, Aug 21-Aug 
23.
- Heavy rain across portions of the Southeast, the Mid-Atlantic, and the Southern Appalachians, 
Thu-Fri, Aug 20-Aug 21.
- Flooding possible across portions of the Upper Mississippi Valley and the Northern Plains.
- Excessive heat across portions of the Central Great Basin, the Pacific Northwest, the Northern 
Rockies, and the Northern Great Basin, Sat-Mon, Aug 22-Aug 24.
- Excessive heat across portions of the Central Great Basin, California, and the Southwest, 
Thu-Mon, Aug 20-Aug 24.
 
Detailed Summary:

The medium range period (Thurs, Aug. 20 - Mon, Aug. 24) 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 deep upper level trough in the northeast Pacific will aid in the development of a 
strong jet stream just north of the Pacific Northwest and over southwest Canada. The abnormally 
strong ridge out 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 amplify by the weekend, meaning 
after a brief reprieve in the excessive heat over the Northwest and northern Rockies late week, 
sizzling hot temperatures are poised to return. The Pacific Northwest this time around will be more 
under the influence of onshore flow from the upper trough in the northeast Pacific, meaning it will 
be mainly interior regions of the West that experience the hottest conditions. Due to the ongoing 
severe drought conditions and lack of rich moisture aloft, dry thunderstorms are likely to 
transpire across the Southwest and Rockies, which 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 will promote upper level divergence over 
the Southeast. A stalled frontal boundary will 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 the second half of the week. 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 shows it could 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 for tropical disturbances with the 
potential to become named systems. 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, a potent occluded storm system in the northeast Pacific will continue to make for rainy 
and dreary conditions late week. Rainfall amounts are likely to remain below heavy rain threshold 
but it is just more rainfall over an area where the summer has been quite wet. Rivers and streams 
will likely be monitored not just this week but in the weeks to come thanks to the much above 
normal rainfall amounts this season. 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. Ensemble model spread remains quite high and thus confidence in rainfall totals are low 
at this time. This system will be monitored in future forecasts should confidence in heavy rainfall 
impacts increase. Temperatures are expected to remain warmer than average across 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.

Mullinax