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Valid Monday August 20, 2018 to Friday August 31, 2018
US Hazards Outlook
NWS Climate Prediction Center College Park MD
EDT August 17 2018Synopsis
: Two stationary fronts are forecast
to span across much of the lower 48 states at the beginning of the period. A
surface low is anticipated to be centered over the Middle Mississippi Valley
along one of these fronts. By Aug 21, this low is expected to track to the
Great Lakes and exit the Northeast by Aug 22. These fronts may bring unsettled
weather to parts of the eastern half of the lower 48 states. Mid-level low
pressure is predicted to form over the Aleutians and move eastward into the
Pacific Northwest by the end of the 3 to 7 day period. Mid-level low pressure
across the northwestern Contiguous U.S. is expected to persist throughout much
of week-2, with mid-level high pressure dominating the eastern two-thirds.
Hurricane Lane is forecast to tracks West to the Central Pacific.
Summary For Monday August 20 - Friday August 24:
- Heavy rain across portions of the Middle
Mississippi Valley, the Great Lakes, the Ohio Valley, and the Upper Mississippi
Valley, Mon-Tue, Aug 20-Aug 21.
- Heavy rain across coastal parts of the Carolinas, Wed-Thu, Aug 22-Aug 23.
- High winds across portions of the Central Plains and the Southern Plains,
Mon, Aug 20.
- High winds across portions of the Great Lakes and the Ohio Valley, Wed, Aug
- Heavy rain across the South Coast of Alaska and South Alaska Range,
Mon-Tue, Aug 20-Aug 21.
- Slight risk of excessive heat for portions of the Great Plains, Upper and
Middle Mississippi Valley, Great Lakes, Northeast, Mid-Atlantic, and Central
Appalachians, Sat-Fri, Aug 25-Aug 31.
- Slight risk of heavy precipitation for portions of the Central Plains, the
Northern Plains, the Middle Mississippi Valley, the Upper Mississippi Valley,
the Great Lakes, and the Ohio Valley, Sat-Wed, Aug 25-Aug 29.
- Flooding occurring or imminent across portions of the Lower Mississippi
Valley and the Southern Plains.
- Severe drought for parts of the middle to lower Mississippi Valley, Great
Plains, and western U.S., and the Big Island of Hawaii.
A surface low is anticipated to form over the Middle Mississippi Valley
along a stationary front extending from the Southwest to the Mid-Atlantic at
the beginning of the period. The stationary front may support high winds
(sustained wind speeds of 20 knots or greater) across parts of the Central
Plains and the Southern Plains, Aug 20 and the Great Lakes and Ohio Valley, Aug
22. Heavy rainfall is expected for parts of the Middle Mississippi Valley, the
Great Lakes, the Ohio Valley, and the Upper Mississippi Valley, Aug 20 to 21,
as this low tracks northeastward. As its trailing front propagates eastward
across the CONUS, parts of the coastal Carolinas may receive heavy rain Aug 22
to 23. These highlighted areas may receive 1 to 2 inches of rainfall in
24-hours, locally more.
Numerous large wildfires continue to burn throughout the western U.S. The
largest wildfire ever recorded in California, the Mendocino Complex Fire, has
burned more than 370,000 acres and is currently 76 percent contained. The Carr
wildfire in Shasta County of northern California continues to increase with
more than 215,000 acres burned. The large number of wildfires is likely to
contribute to poor air quality at times across the western U.S. through late
August. An amplifying upper-level ridge (500-hpa heights near 588-dm) is
expected to result in anomalously warm temperatures (maximum temperatures in
the low to mid 90s across parts of the Pacific Northwest on Aug 20 and 21.
However, since the temperature anomalies are expected to be localized, an
excessive heat area is not specified at this time.
An amplifying upper-level trough is likely to result in an intensifying low
pressure system across the north Pacific by the weekend. Heavy rain (2 to 3
inches), due to enhanced onshore flow, is forecast to bring heavy rainfall to
parts of the South Coast of Alaska and South Alaska Range, Aug 20 to 21.
As of 8am PDT on Aug 17, Hurricane Lane is located at 11.2N/132.9W in the
East Pacific. Low wind shear and sufficiently warm waters are expected to
provide a favorable environment for strengthening and Lane is forecast to
intensify into the weekend. This storm may move near or south of the Big Island
of Hawaii by next Wednesday. An increase in surf along with heavy rainfall is
possible for the Hawaiian Islands next week. For Saturday August 25 - Friday
The ensemble means are in good agreement regarding mid-level
troughing across the northwestern CONUS and amplified ridging across the
eastern two-thirds of the CONUS throughout most of Week-2. This troughing is
expected to supress northward migration of the Southeast subtropical ridge,
providing less favorable conditions for enhanced Southwest Monsoon flow.
Therefore, the slight chance for heavy rainfall across the Desert Southwest has
been removed from the map. Persistent ridging across the eastern two-thirds of
the CONUS favors a slight chance of excessive heat across parts of the Great
Plains, Upper and Middle Mississippi Valley, Great Lakes, Northeast,
Mid-Atlantic, and Central Appalachians throughout Week-2. These areas may
experience air temperatures or heat index values reaching the 85th to 90th
Surface low pressure is predicted to form across the central CONUS ahead of
upper-level troughing across the northwestern quarter of the CONUS at the
beginning of Week-2. This feature supports a slight chance of heavy rainfall
across parts of the Central Plains, the Northern Plains, the Middle Mississippi
Valley, the Upper Mississippi Valley, the Great Lakes, and the Ohio Valley, Aug
25 to 29. The GEFS and ECMWF Reforecast Tools indicate a 20 percent chance of
these areas receiving 3-day precipitation accumulation amounts exceeding the
85th percentile compared to normal.
The U.S. Drought Monitor, valid on August 14, shows a slight decrease in
severe drought coverage to 20.02 percent from 21.06 percent last week. Drought
improvement is noted across the Southern Plains.
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.