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Climate Prediction Center

U.S. Hazards Outlook - Made Jul 21, 2017

Composite Images

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

Valid Monday, July 24, 2017 to Friday, August 04, 2017

Summary of Forecasts and Hazards

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT July 21 2017

Synopsis: A cold front is forecast to progress into the southeastern U.S. early in the period. An area of upper-level high pressure is expected strengthen over the southern Great Plains and lower Mississippi Valley during the next week and then shift west towards the West Coast by the beginning of August. A tropical cyclone over the East Pacific is forecast to track northwest. A strong area of upper-level high pressure is likely to develop over the Aleutians during the next week.

  • Locally heavy rain for the eastern Carolinas, Mon-Tue, Jul 24-25.
  • Heavy rain shifting southeast across the upper to middle Mississippi Valley, Great Lakes, and Midwest, Tue-Thu, Jul 25-27.
  • Flash flooding possible for parts of Arizona, Colorado, New Mexico, and Utah, Mon-Fri, Jul 24-28.
  • Excessive heat for parts of the lower Mississippi Valley, Oklahoma, and Texas, Thu-Fri, Jul 27-28.
  • Excessive heat continuing for parts of Oklahoma and Texas, Sat-Mon, Jul 29-31.
  • Flash flooding possible for the desert Southwest, Sat-Fri, Jul 29-Aug 4.
  • Slight risk of much above-normal temperatures for much of the western U.S. and Great Plains, Sat-Fri, Jul 29-Aug 4.
  • Flooding occurring, imminent, or possible for parts of the upper Mississippi Valley, Great Lakes, and Midwest.
  • Severe Drought across parts of the Great Plains, Arizona, California, and Hawaii.
Detailed Summary

For Monday July 24 - Friday July 28: An amplifying upper-level trough is expected to result in a cold front advancing south into the southeastern U.S. at the beginning of this period. This front is forecast to bring an increase in thunderstorms with locally heavy rain (more than 1 inch per 24 hours) across the eastern Carolinas on July 24 and 25.

Moderate to major flooding continues along multiple rivers across northern Illinois and southern Wisconsin. Heavy rainfall this weekend is likely to exacerbate ongoing flooding and potentially cause additional flooding throughout the Midwest. Model guidance indicates that a vigorous shortwave trough tracks to the north of the Great Lakes during mid-week. This shortwave trough is expected to result in convection, with heavy rainfall (more than 1 inch per 24 hours), spreading southeast across the upper to middle Mississippi Valley, Great Lakes, and Midwest from July 25 to 27.

Anomalous moisture and a favorable environment for convection are expected to maintain a flash flooding risk across parts of Arizona, Colorado, New Mexico, and Utah during late July. The 6Z GFS ensemble mean on July 21 indicates locally as much as 3 inches (valid from July 24 to 28) in the outlined area for flash flooding.

According to the National Interagency Fire Center on July 21, 44 large wildfires are currently active throughout the western U.S. These large wildfires have burned more than 800,000 acres. The potential for dry thunderstorms is expected to continue across the northern Great Basin and interior Pacific Northwest. Lightning, associated with these dry thunderstorms, could ignite additional wildfires.

The deterministic GFS and ECMWF models on July 21 are in good agreement that a 500-hpa ridge rebuilds over the southern Great Plains and lower Mississippi Valley during the final week of July. Based on the strength of this 500-hpa ridge (heights near 594 dm) and low-level thermal fields, excessive heat is expected to affect the eastern half of Oklahoma, north Texas, and parts of the lower Mississippi Valley on July 27 and 28. Maximum heat index values at or above 105 degrees F are anticipated in the outlined area for excessive heat.

An increase in rainfall is forecast over interior Alaska during this period, due to moist, westerly flow. Since heavy amounts (more than 1 inch) are expected to be localized, a heavy rain hazard is not posted.

As of July 21 at 5am HST, Tropical Storm Fernanda is located at 18.6N/143.6W over the central Pacific. Increased wind shear and entrainment of dry air is likely to weaken Fernanda with it becoming a remnant low in the next 36 hours. The remnant low is expected to bring an increase in moisture and locally heavy rain to the Hawaiian Islands early in this period.

For Saturday July 29 - Friday August 04: The GFS, ECMWF, and Canadian ensemble means agree that an amplified 500-hpa ridge becomes centered over the western U.S. by the end of July. This anomalous ridge supports a slight risk of much above temperatures across much of the western U.S. and Great Plains. The outlined area on the map is generally consistent where the GEFS reforecast tool indicates that maximum temperatures have a 20 percent chance of exceeding the 85th percentile compared to climatology. Excessive heat, with heat index values at or above 105 degrees F, are expected to persist across parts of Oklahoma and Texas through at least July 31. The skill weighted CPC heat index tool indicates that this area has at least a 20 percent chance of the heat index exceeding the 95th percentile compared to climatology.

Tropical Depression 9 developed in the east Pacific (9N/93.5W) on July 21, becoming the 5th tropical cyclone in the east Pacific during July. Favorable environmental conditions are likely to aid in strengthening of this tropical cyclone during the next 5 days. The GFS and ECMWF models remain consistent that the tropical cyclone tracks to the northwest. Given the good model consistency on this track, a northward surge of moisture is forecast for the desert Southwest by the beginning of Week-2. Therefore, this region is expected to be vulnerable to flash flooding during Week-2.

The U.S. Drought Monitor valid on July 18 indicates that severe to exceptional (D2-D4) drought coverage increased to 4.77 percent across the continental U.S. This increase is related to an expansion of severe drought across the northern Great Plains.

Forecaster: Brad Pugh


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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