US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
410 PM EDT Tue Sep 01 2020
 
Valid Friday September 04 2020 - Tuesday September 08 2020
 
Hazards:
- Heavy rain across portions of the Central Plains, the Lower Mississippi Valley, the Great Lakes, 
the Middle Mississippi Valley, the Upper Mississippi Valley, the Southern Plains, and the Ohio 
Valley, Mon-Tue, Sep 7-Sep 8.
- Heavy rain across portions of the Southern Plains, Fri, Sep 4.
- Heavy rain across portions of the Great Lakes, Sun, Sep 6.
- Flooding possible across portions of the Lower Mississippi Valley and the Southern Plains.
- Excessive heat across portions of California and the Southwest, Fri-Mon, Sep 4-Sep 7.
- Excessive heat across portions of California, Sat-Tue, Sep 5-Sep 8.
- Much above normal temperatures across portions of California, the Central Great Basin, the 
Pacific Northwest, the Northern Rockies, and the Northern Great Basin, Fri-Mon, Sep 4-Sep 7.
- Much above normal temperatures across portions of California, the Pacific Northwest, and the 
Northern Great Basin, Fri-Tue, Sep 4-Sep 8.
- Heavy rain across portions of the Alaska Panhandle and mainland Alaska, Sat-Mon, Sep 5-Sep 7.
 
Detailed Summary:

The medium range period (Sept. 4-8) can be summarized by an expansive upper level ridge along the 
west coast of North America, an amplifying downstream trough over the Canadian Prairies and central 
U.S., and a far reaching ridge across the North Atlantic. The resulting temperature disparity 
across the Lower 48 will be quite pronounced with more autumn-like temperatures making their first 
appearance of the season in the northern and central High Plains. A few locations may even have a 
shot of seeing patchy frost, but most areas will be abnormally cool. From a hazards perspective, it 
is the Desert Southwest and throughout California's Great Valley that stand out most with record 
breaking high temps likely this weekend and into early next week. The Northwest will also contend 
with hotter than normal conditions with high temperatures making a run at triple digits in the 
Willamette Valley Sunday and Monday. How long the heat wave persists in the wave will be determined 
based on the position of an intense shortwave trough diving into the Plains early next week. 
Temperatures should cool down by day 7 in the Intermountain West but the heat may linger along the 
immediate West Coast. The dome of upper level high pressure over the West will unfortunately lead 
to favorable conditions for ongoing wildfires to rage and potentially the start of new fires. 
Expect wildfire smoke and poor air quality to continue throughout the period.

As the pattern becomes increasingly amplified, the threat for heavy rainfall will become more 
apparent across the Nation's Heartland. To start the period, a weak low level disturbance over 
north Texas will have plenty of moisture to work with that combined with weak steering flow aloft 
could lead to heavy rainfall. As the upper level ridge builds across the West this weekend, a 
developing area of low pressure tracking across southern Canada will generate showers and 
thunderstorms across the Midwest and Great Lakes with heavy rain possible in the Upper Peninsula of 
Michigan. By the start of next week, an intense upper trough will traverse the central U.S. and tap 
into Gulf moisture. This will lead to a swath of heavy rainfall that could extend from the southern 
Plains to the Great Lakes. There is also the potential for heavy rainfall in the High Plains and 
central Rockies, but model spread remains high due to the unclear position of the upper level 
disturbance. A heavy rainfall hazard may be requires should guidance suggest the trough aloft sets 
up farther west.

The theme of an amplified jet stream pattern also exists over the North Pacific, meaning Alaska 
will also contend with hazardous weather. A pair of intense storm systems look to funnel 
subtropical moisture northward, resulting in soaking rainfall from the Aleutians to the Panhandle. 
The first storm system is responsible for the initial wave of precipitation with locally heavy 
rainfall along the more elevated locations of the southern coast. The second and more formidable 
storm is forecast to track across the Aleutians Sunday and then reach the Bering Sea on Monday. 
This storm contains rich subtropical moisture that could lead to excessive rainfall over the 
southern mainland late Sunday and into Monday. Multiple inches of rainfall are possible along with 
gusty winds and rough seas along the coast. Heavy rainfall may linger into next Tuesday but it 
depends upon how long the fetch of subtropical moisture can persist.

Mullinax