US Day 3-7 Hazards Outlook
NWS Weather Prediction Center College Park MD
455 PM EDT Fri Jul 23 2021
Valid Monday July 26 2021 - Friday July 30 2021
- Heavy rain across portions of the Mid-Atlantic, Mon, Jul 26.
- Flooding occurring or imminent across portions of the Lower Mississippi Valley.
- Excessive heat across portions of the Lower Mississippi Valley and ArkLaTex, Mon-Fri, Jul 26-Jul
- Excessive heat across portions of the Northern High Plains, Mon-Tue, Jul 26-Jul 27.
- Excessive heat across portions of the Upper and Middle Missouri Valley, Mon-Wed, Jul 26-Jul 28.
- Excessive heat across portions of the Lower Missouri Valley and Middle Mississippi Valley,
Wed-Thu, Jul 28-Jul 29.
- Excessive heat across portions of the interior Pacific Northwest, Thu-Fri, Jul 29-Jul 30.
- Much above normal temperatures across portions of the Northern Great Basin, Mon, Jul 26.
- Much above normal temperatures across portions of the Northern and Central Plains and the
Midwest, Mon-Thu, Jul 26-Jul 29.
- Heavy rain across portions of mainland Alaska, Tue-Wed, Jul 27-Jul 28.
A heat wave across the central and western U.S. highlights the hazards outlook for the medium-range
forecast period (Monday, July 26 - Friday, July 30). At the start of the period, a persistent
upper-level ridge will remain anchored in place over the Central Rockies. A combination of high
temperatures and high humidity lead to several Excessive Heat areas through the forecast period.
High temperatures in the mid-100s are forecast for eastern Montana on Monday (July 26) and Tuesday
(July 27). Mid-100s are also forecast for portions of the Upper Missouri Valley Monday through
Wednesday (July 28), with upper 100s possible for central South Dakota. Further to the southeast,
higher humidity will result in hazardous heat indices with dew points in the low to mid-70s in
place throughout the Lower and Middle Mississippi and Missouri Valleys. High temperatures in the
low to mid-100s combined with high humidity will lead to heat indices above 105 for the Middle
Missouri Valley Monday through Wednesday. A back door cold front from the northeast combined with
strengthening troughing to the east, weakening the ridge, should provide a slight cool down for the
Upper and Middle Missouri Valley by Thursday. However, high temperatures and moisture will begin to
increase for the Middle Mississippi and Lower Missouri Valley. Highs in the upper 90s are forecast
on Wednesday and Thursday (July 29), with dewpoints potentially rising as high as the upper 70s.
Widespread heat indices over 105 are expected, with the potential for heat indices over 110 for
urban areas such as St Louis. The southward moving front and strengthening trough over the eastern
U.S. look to provide at least some relief by Friday (July 30) for these areas as well. Further
south, Excessive Heat is expected throughout the forecast period for portions of the Lower
Mississippi Valley and ArkLaTex. Daily highs in the upper 90s to 100 degrees combined with the high
humidity will keep peak heat indices consistently over 105 each day, with locally higher heat
indices of 110 possible.
In addition to the noted Excessive Heat areas, above normal temperatures in the lower to mid-100s
that have been in place across portions of Idaho and the the northern Great Basin look to continue
into Monday. High temperatures across the Northern and Central Plains as well as western portions
of the Midwest will remain much above average from Monday through Thursday as well, with highs
outside of the Excessive Heat areas generally in the mid- to upper 90s.
On Thursday, the ridge will expand to the northwest, bringing hot weather back to the interior
Pacific Northwest. Excessive Heat is expected both Thursday and Friday. On Thursday, high
temperatures in the mid-90s are forecast for the Willamette Valley; around 100 for eastern
Washington, northeastern Oregon, and western Idaho; and into the mid-100s for the Rogue Valley.
Highs will climb into the 100s for eastern Washington, northeastern Oregon, and western Idaho on
Friday. It is important to note that with the upper-level ridge in place, potentially hazardous
heat could be seen on different days in different areas across the central and western U.S. The
designated outlook areas were chosen to highlight regions and times when more widespread, higher
temperatures and heat indices appear most likely.
In addition to the heat, showers and thunderstorms developing along a surface trough and southward
moving cold front are expected to produce heavy rain for portions of southeastern Virginia and
northeastern North Carolina on Monday. A series of fronts progressing slowly across the eastern
U.S. may produce additional instances of heavy rain, particularly for New England on Wednesday.
However, differences in model guidance on the timing and placement of any heavy rain preclude
additional outlook areas at this time.
In Alaska, showers associated with a stationary boundary over the northern mainland are expected to
lead to the threat for heavy rain across portions of the western and northwestern mainland on
Tuesday and Wednesday. The threat appears to be most significant and longer in duration for the
southern Seward Peninsula, where daily rain totals of 1-2 inches are possible. After Wednesday,
model guidance begins to diverge in terms of the location of any additional heavy rain threats.
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