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U.S. Week-2 Hazards Outlook - Made August 10, 2020 | About the Hazards Outlook
| Composite Map Day 8-14 |
Experimental Probabilistic Outlooks |
 |
Valid Tuesday August 18, 2020 to Monday August 24, 2020
US Hazards OutlookNWS Climate Prediction Center College Park MD
300 PM EDT August 10 2020
Synopsis: Upper-level high pressure is forecast to strengthen over the West during the next week, with an increasing risk of a long duration heat wave across the Desert Southwest. During the next two weeks, multiple tropical cyclones (TCs) are expected to develop over the East Pacific. Despite this increase in TCs, intensifying drought is likely across the Southwest through at least the early part of week-2. The favorable large-scale environment for TC development is expected to overspread the Atlantic basin during late August.
Hazards
- High risk of excessive heat for the lower deserts of the Southwest, Tue, Aug 18.
- Moderate risk of excessive heat for parts of the Southwest, Great Basin, and California, Tue-Fri, Aug 18-21.
- Slight risk of excessive heat for parts of the western U.S. and southern high Plains, Tue-Mon, Aug 18-24.
For Thursday August 13 - Monday August 17: WPC Days 3-7 U.S. Hazards
For Tuesday August 18 - Monday August 24: A multi-day heat wave is becoming more likely across the Desert Southwest, beginning later this week and persisting into week-2. The ECMWF ensemble mean depicts an amplifying 500-hPa ridge over the western U.S. during the next week with heights near or above 594-dm across the Four Corners region early in week-2. Although the GFS ensemble mean is slightly lower with 500-hPa heights, its model guidance would also support anomalous heat during week-2. On August 18, a high risk of excessive heat is posted for the lower deserts of the Southwest where the ECMWF reforecast tool indicates that maximum temperatures have more than a 60 percent chance of exceeding the 85th percentile. This tool also shows that maximum temperatures have around a 20 percent chance of exceeding the 98th percentile across much of the Southwest, implying the potential for record high temperatures early next week. A moderate risk of excessive heat extends through August 21 across a broader region of the Southwest, Great Basin, and California since the ECMWF reforecast tool depicts maximum temperatures having at least a 40 percent chance of exceeding the 85th percentile for a few more days. An even larger spatial area with a slight risk of excessive heat covers the West and extends through the entirety of week-2, supported by both the GEFS and ECMWF reforecast tools and the anomalous ridge aloft. The heat wave across the Desert Southwest is expected to peak this weekend into early next week when maximum temperatures are near or above 115 degrees F across the lower deserts.
A major climate-related hazard is the expanding and intensifying drought across the Southwest due to a weak North American Monsoon. Since early July, precipitation has averaged less than 25 percent of normal across southern and western Arizona along with southwestern Utah. According to the U.S. Drought Monitor valid on August 4, moderate to severe drought covers 81.68 percent of Arizona. Based on the 500-hPa height pattern, below normal precipitation is favored to continue during the next week to ten days. A predicted increase in the number of tropical cyclones across the East Pacific will have to be closely monitored for northward surges of enhanced moisture. If this occurs, then monsoon rainfall may begin to increase across the Desert Southwest during late August.
A weakness in the subtropical ridge is expected to contribute to enhanced daily convection across the Gulf Coast States during week-2. However, a heavy rain hazard is not posted since scattered convection with locally heavy rain is not expected to be hazardous and trigger flooding at this time.
Many model solutions feature an upper-level ridge persisting over the Aleutians and Bering Sea through week-2. The presence of this ridge aloft favors near to below normal precipitation for much of Alaska. Therefore, no hazardous precipitation is anticipated for Alaska.
Forecaster: Brad Pugh
$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.
Resources
Week-2 Probabilistic Extremes Tool
GFS Ensemble Forecasts