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Climate Prediction Center

U.S. Hazards Outlook - Made Aug 18, 2017

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Day 3-7 Outlook Day 8-14 Outlook Day 8-14 Probabilistic Temperature Hazards

Valid Monday, August 21, 2017 to Friday, September 01, 2017

Summary of Forecasts and Hazards

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 18 2017

Synopsis: 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.

Hazards
  • 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, Aug 22.
  • 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 Plains.
  • 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 Mississippi Valley.
Detailed Summary

For Monday August 21 - Friday August 25: 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 eastern CONUS.

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 22-25.

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 September 01: 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.

Forecaster: Stephen Baxter

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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.


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