Composite Map Day 8-14

Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Monday August 17, 2026 to Sunday August 23, 2026

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT August 09 2026

Synopsis: Compared to yesterday, today's dynamical model guidance maintains more amplified mid-level high pressure across the south-central and southeastern contiguous U.S. later into week-2, suggestive of a long duration, significant heat event, particularly across the Southern Plains and Lower Mississippi Valley where a high risk for extreme heat has been designated. Antecedent dryness, combined with little precipitation favored over the next two weeks supports a Rapid Onset Drought (ROD) risk for portions of the Southern and Central Plains and Lower Mississippi Valley. A weak but broad area of mid-level low pressure across eastern Canada favors enhanced northwesterly mid-level flow and a progression of enhanced moisture and frontal activity from the Four Corners to the Midwest and Eastern Seaboard where periodic heavy precipitation is possible early in week-2.

Hazards

• High risk of extreme heat for portions of the Southern Plains, Lower Mississippi Valley, and Southeast, Mon-Wed, Aug 17-19.
• Moderate risk of extreme heat for portions of the Central and Southern Plains, Middle and Lower Mississippi Valley, and Southeast, Mon-Fri, Aug 17-21.
• Slight risk of extreme heat for portions of the Central and Southern Plains, Middle and Lower Mississippi and Tennessee Valleys, Southeast, and southern Mid-Atlantic, Mon-Sun, Aug 17-23.
• Slight risk of heavy precipitation for portions of the Four Corners, Northern and Central Plains, Midwest, Tennessee Valley, Appalachians, Northeast, and Mid-Atlantic, Mon-Thu, Aug 17-20.
• Rapid Onset Drought (ROD) risk for portions of the Southern and Central Plains and the Lower Mississippi Valley.

Detailed Summary

For Wednesday August 12 - Sunday August 16: WPC Days 3-7 U.S. Hazards

For Monday August 17 - Sunday August 23: Dynamical model guidance is in good agreement regarding subtropical ridging rebuilding across the south-central and southeastern U.S. late in week-1 and persisting into week-2. The 0z ECMWF, GEFS, and Canadian ensembles depict a 595-596 dm 500-hPa height maximum across the central Gulf Coast states. Unlike yesterday's guidance which showed some weakening later in the period, today's ECMWF and Canadian ensembles maintain or increase the amplitude of the ridge by the end of the period. As a result, confidence is increasing for a long duration heat event centered across the Southern Plains and Lower Mississippi Valley where a high risk for extreme heat is posted, Aug 17-19. Both the GEFS and ECMWF Probabilistic Extremes Tools (PETs) depict 30-50 percent probabilities for maximum temperatures to exceed the 90th climatological percentile. While the ECMWF PET initially focuses on higher probabilities across the Southeast, both PETs consolidate larger probabilities across the Lower Mississippi Valley and Southern Plains by the middle of the period. A large contribution from increased humidity is expected, with a strong signal emerging in calibrated skill-weighted GEFS-ECMWF heat risk guidance. Some areas have at least a 50 percent chance the maximum temperature or heat index value exceeds the 95th climatological percentile, with heat index values potentially exceeding 110 deg F. In addition, the National Blend of Models (NBM) also depicts a number of locations with nighttime lows nearing and/or breaking daily records, where little overnight relief to daytime heat is expected. The moderate risk for extreme heat is now extended through Aug 21 and now includes portions of the Central Plains and Middle Mississippi Valley to account for a greater expansion of the ridge axis and increased potential for heat index values to exceed 105 deg F. A slight risk surrounds the moderate risk area and continues for all of week-2. The GEFS is more robust in depicting increasing heat expanding into the Northern Plains later in week-2. Since the ECMWF and Canadian ensembles keep the heat across the Central Plains and south, no northward expansion is indicated in today's outlook but trends will continue to be monitored.

Troughing initially near the West Coast of the CONUS favors an influx of moisture across much of the western U.S., and expanding through the Midwest and Eastern Seaboard as evidenced by anomalous precipitable water values in the 0z ECMWF and GEFS ensembles. This is also tied to broad troughing over eastern Canada leading to enhanced westerly to northwesterly mid-level flow across the Midwest into the East, and a general progression of frontal waves or Mesoscale Convective Systems (MCSs) overtop the ridge axis situated across the south-central and southeastern states. Therefore, a slight risk of heavy precipitation is posted extending from the Four Corners, through portions of the Northern and Central Plains, and into the Midwest, Tennessee Valley, Appalachians, Northeast, and Mid-Atlantic, Aug 17-20. The GEFS and ECMWF PETs continue to depict at least a 20 percent chance 3-day precipitation totals exceed the 85th climatological percentile and a half-inch across the Four Corners. This region is still prone to flash flooding even with relatively small precipitation accumulations especially along dry creek beds (arroyos) and in canyons. Caution should always be exercised in such areas should thunderstorm activity initiate nearby. Pattern recognition supports the extension of the slight risk into portions of the central and eastern U.S.

Toward the middle of week-2, troughing along the West Coast is favored to retreat with rising heights across the West tied to expansion of the ridging over the south-central CONUS. This evolution would decrease the moisture advection into the West, and also promote an increase in above-normal temperatures and possibly extreme heat across portions of the Interior West. However, probabilities in the PETs for maximum temperatures exceeding the 85th climatological percentile are for the most part under 20 percent, and no related extreme heat hazards are posted across the West at this time.

A Rapid Onset Drought (ROD) risk remains designated and includes parts of Oklahoma, northeastern Texas eastward through northern Mississippi. This ROD area continues to register 30-day precipitation deficits greater than 2 inches, with little rainfall forecast during the next two weeks. Much of this ROD area is also included within the moderate risk of extreme heat as the potential for increased evapotranspiration rates also looks to reinforce this risk later in August.

Across the eastern Pacific, the National Hurricane Center (NHC) is monitoring three areas located near 140 deg W, 125 deg W, and 115 deg W, with 90 percent, 60 percent, and 10 percent chances of tropical cyclone (TC) development, respectively, during the next seven days (as of 11am PDT). For the western watch area near 140 deg W, both dynamical model and Google AI guidance are indicating a clustering of tracks near or just to the south of the Hawaiian Islands, although potential impacts are shifting more into the late week-1 timeframe. However, the GEFS and ECMWF PETs continue to depict at least a 20 percent chance 3-day precipitation totals exceed the 85th climatological percentile early in week-2 across most of the state, and interests across Hawaii are advised to monitor the latest forecasts from NHC (https://www.nhc.noaa.gov/).

Forecaster: Thomas Collow

$$ Please consult local NWS Forecast Offices for short range forecasts and region-specific information.

Resources

Week-2 Probabilistic Extremes Tool

GFS Ensemble Forecasts