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U.S. Hazards Outlook - Made July 27, 2017
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Valid Sunday July 30, 2017 to Thursday August 10, 2017
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EDT July 27 2017
Synopsis: An unseasonably strong mid-level low is forecast to slowly exit the northern mid-Atlantic early in the period, while a trailing front becomes stationary near the Gulf Coast. The tail end of this front is expected to extend back towards the southern high Plains. Mid-level high (low) pressure over the western U.S. (mainland Alaska) is likely to persist through the beginning of August.
Hazards
- Heavy rain for the eastern Big Bend of Florida along with northern and central parts of the Florida peninsula, Sun-Wed, Jul 30-Aug 2.
- Heavy rain for the southern high Plains, Sun-Mon, Jul 30-31.
- Heavy rain shifting southeast to northern and central Texas, Tue-Wed, Aug 1-2.
- Flash flooding possible for parts of the Southwest, Sun-Thu, Jul 30-Aug 3.
- Much above-normal temperatures for parts of the northern Great Basin, northern Rockies, and Pacific Northwest, Tue-Thu, Aug 1-3.
- Flash flooding possible for parts of the southwestern U.S., Fri-Thu, Aug 4-10.
- Slight risk of much above-normal temperatures for much of the western U.S. and northern Great Plains, Fri-Sun, Aug 4-6.
- Flooding occurring, imminent, or possible for parts of the Midwest.
- Severe Drought across parts of the Great Plains, Arizona, California, and Hawaii.
For Sunday July 30 - Thursday August 03: A vigorous closed low at 500-hpa initially over the mid-Atlantic, a pattern more typical during the cold season, is likely to result in a cold front pushing south to the northern Gulf of Mexico and Florida peninsula by July 30. The presence of this front, a trough aloft, and ample moisture are likely to enhance the coverage of showers and thundershowers across the eastern Big Bend of Florida along with the northern and central Florida peninsula through August 2. Total rainfall amounts of 2 to 5 inches, locally more, are expected in the outlined area for heavy rainfall.
The tail end of a front along with upslope low is likely to promote heavy rainfall (total amounts locally exceeding 3 inches) across the southern high Plains, continuing at least through August 1. The focus for heavy rainfall (more than 1 inch per 24 hours) is expected to shift southeast to northern and central Texas on August 2 and 3. The risk of flash flooding is likely to remain high across parts of Colorado and New Mexico through August 1. Areas with burn scars and locations with saturated soils will remain most vulnerable to flash flooding.
As of July 27 at 8am PDT, Hurricane Hilary is located at 17.6/116W over the east Pacific. Although the interaction between Hilary and Tropical Storm Irwin remains uncertain, the model consensus favors a west to northwest track through the weekend. The deterministic GFS model runs indicate an increase in precipitable water values, partly associated with Hurricane Hilary, across the desert Southwest this weekend. Therefore, a flash flooding hazard is posted for this region.
An amplified ridge aloft is expected to result in much above-normal temperatures, with maximum temperatures 10 to 15 degrees F above normal, across parts of the northern Great Basin, northern Rockies, and Pacific Northwest. The highest temperatures are forecast across eastern Washington where daily maximum temperatures at or above 100 degrees F can be expected. This anomalous heat may begin to expand east to the drought-stricken areas of the northern Great Plains later next week.
Moderate to major flooding continues along multiple rivers across eastern Iowa, northern Illinois, and southern Wisconsin. River flooding is expected to slowly recede during the next due to drier weather.
An upper-level trough is expected to promote scattered areas of rainfall throughout mainland Alaska. Rainfall amounts are not expected to exceed hazards criteria.
For Friday August 04 - Thursday August 10: Ensemble spread among the GFS and ECMWF members increases with the amplitude of the western U.S. ridge during Week-2. Although their ensemble means maintain above normal 500-hpa heights along the West Coast, the positive anomalies are smaller compared to previous days. These smaller positive anomalies are due to the increasing ensemble spread. Based on this increased ensemble spread and poor continuity among the deterministic GFS model solutions, the risk for much above-normal temperatures across the western U.S. is limited to slight.
A variable monsoon flow is expected across the southwestern U.S.as the axis of the subtropical ridge oscillates during early August. A factor in the moisture availability is likely to be east Pacific tropical cyclone activity. The CFS model indicates a robust atmospheric Kelvin Wave propagating east across the Western Hemisphere during the next week to ten days which would provide a favorable environment for additional tropical cyclone development across the east Pacific through mid-August. The risk of flash flooding is expected to be highest across parts of the desert Southwest, southern Great Basin, and Big Bend of Texas due to anomalous southeasterly flow and the potential for enhanced moisture associated with tropical cyclones across the east Pacific later in Week-2. If a trough develops along the West Coast and causes more westerly flow, a rapid drying trend would likely occur across the southwestern U.S.
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
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