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Valid Monday August 21, 2017 to Friday September 01, 2017
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT August 18 2017Synopsis
: A cold front is forecast to develop
across the Northern Plains and Upper Mississippi Valley, pushing southeastward
across the country throughout Week-1. Surface high pressure is expected to
build across much of the eastern half of the lower 48 states in its wake.
Periodic disturbances are forecast in the Gulf of Alaska and Chukchi Sea
throughout Week-1. During Week-2, a tendency towards mid-level high pressure
across much of the western U.S. is favored, with upper-level low pressure
across the eastern half of the Lower 48 and south of Alaska.
For Monday August 21 - Friday August 25:
- Heavy rain across portions of the central
Plains, the Middle and Upper Mississippi Valley, and the Great Lakes, Mon-Tue,
Aug 21-Aug 22.
- Severe weather across portions of the Great Lakes and the Ohio Valley, Tue,
- Heavy rain across portions of the eastern Carolinas, Wed-Thu, Aug 23-Aug 24.
- Heavy rain across portions of Florida, Tue-Fri, Aug 22-Aug 25.
- Flooding possible across portions of the Central Plains and the Southern
- Slight risk of much above-normal temperatures for portions of the Great
Basin, the Northern Plains, the Northern Rockies, California, the Pacific
Northwest, and the Southwest, Sun-Fri, Aug 27-Sep 1.
- Moderate risk of much above-normal temperatures for portions of the
Northern Rockies and the Northern Great Basin, Sun-Tue, Aug 27-Aug 29.
- Severe Drought across the Central Plains, the Northern Plains, Hawaii, the
Northern Rockies, the Middle Mississippi Valley, California, and the Upper
A surface low is anticipated to
form over the Northern Plains late this weekend. As this surface low and its
associated cold front (stretching from the Northern Plains to the Upper Middle
Mississippi Valley) move eastward, associated hazards are anticipated. Heavy
rain (>1 inch in 24 hours) is likely for parts of the central Plains
northeastward through the Great Lakes on Monday and Tuesday, while SPC depicts
a severe weather threat for parts of the Ohio Valley and Great Lakes on
Tuesday, August 22. As this cold front approaches the Carolinas and begins to
stall, periods of heavy rain are likely for coastal sections of the Carolinas
on Wednesday and Thursday. In the wake of this cold front, cooler and drier
conditions are expected for the end of the week over much of the central and
The National Hurricane Center (NHC) is currently monitoring Tropical Storm
Harvey as well as two other disturbances in the Atlantic. One disturbance is
forecast to approach Florida during the period, possibly as a tropical
depression. This, along with an approaching cold front, leads to increased
chances for heavy rain for southern parts of the Florida Peninsula for Aug
Tropical Storm Harvey is forecast to make landfall over near Belize during
the period, but could reemerge over the southern Gulf of Mexico by the end of
the week. If so, there is some chance that it could impact parts of South Texas
at the very end of Week-1 or early in Week-2. Interested parties should monitor
the latest NHC forecast advisories.
Locally heavy rain is possible over parts of southeastern Alaska at the
very beginning of the period due to a robust upper-level trough forecast to
move through the Gulf of Alaska. The latest guidance, however, suggests that
heavy precipitation is not likely to reach hazardous criteria over any
widespread region. For Saturday August 26 - Friday
The cold front across the Southeast from the end of the 3 to
7 period is likely to progress further southward. This cold front in addition
to enhanced moisture flow associated with possible tropical waves in the
Atlantic may support heavy rainfall across the Gulf Coast, Florida and coastal
Southeast. The latest ensemble guidance, however, does not support the
depiction of a deterministic hazard, with fairly large discrepancies between
the GEFS and ECMWF systems.
The latest ensemble guidance also suggests a weaker trough moving through
the western CONUS early in Week-2. Large discrepancies likewise exists over the
central CONUS between the ECMWF and GEFS, with the ECMWF and the latest 12z
GEFS being less wet over the Plains. The hazard issued yesterday for this
region is therefore removed in today's forecast, though odds of above-normal
precipitation remain elevated.
In general, broad ridging is expected across the western CONUS throughout
Week-2. This pattern favors a slight chance of much above normal temperatures
across much of the western third of the CONUS Aug 27 to Sep 1. The GEFS
Reforecast Tool shows a 20 percent chance or greater of the highlighted area
reaching the 85th percentile or greater of daily maximum temperatures, and
localized temperatures reaching or exceeding 95 Deg F, with areas across
California and the Southwest reaching 100 Deg F or greater. The best odds for
exceeding the 85th percentile or greater of daily maximum temperatures are over
the interior Northwest, where a moderate risk is denoted for Days 9-11.
The U.S. Drought Monitor valid on Aug 15 indicates that severe to
exceptional (D2-D4) drought coverage decreased slightly from 5.38 percent to
5.33 percent across the continental U.S.
Click here to see a display of the GFS Ensemble Forecasts
Please consult local NWS Forecast Offices for short range forecasts and region-specific information.