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HOME> Expert Assessments>Hazards Outlook

U.S. Hazards Outlook - Made August 17, 2018

 Days 3-7Days 8-14Probabilistic Days 8-14
Precipitation No Hazards
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Categorical Outlooks
Experimental Probabilistic Outlooks (Information)

Valid Monday August 20, 2018 to Friday August 31, 2018

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 17 2018

Synopsis: 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.

Hazards Detailed Summary

For Monday August 20 - Friday August 24: 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 August 31: 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 percentile.

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.

Forecaster: Melissa Ou

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