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Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Friday June 16, 2023 to Thursday June 22, 2023

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT June 08 2023

Synopsis: Mid-level high pressure is forecast across the south-central and southeastern contiguous U.S. during week-2 favoring increased chances of excessive heat over parts of the Central and Southern Great Plains, Middle and Lower Mississippi Valley, and Southeast, with drought expansion a concern across the Lower Mississippi Valley. Some areas of enhanced rainfall are possible across portions of the Midwest and extending into parts of the East throughout the period as potential shortwaves propagate overtop the mid-level high pressure to the south. Snowmelt season continues for portions of the West after exceptional winter accumulations, leading to the continued potential for flooding.

Hazards

• Moderate risk of excessive heat across parts of the Southern Plains and Lower Mississippi Valley, Fri-Mon, Jun 16-19.
• Slight risk of excessive heat across parts of the Central and Southern Plains, Middle and Lower Mississippi and Tennessee Valleys, and the Southeast, Fri-Thu, Jun 16-22.
• Possible flooding for portions of California, Nevada, Utah, Wyoming, and Idaho.
• Possible flooding along portions of the James River in South Dakota.
• Rapid onset drought risk across portions of the Lower Mississippi Valley.

Detailed Summary

For Sunday June 11 - Thursday June 15: WPC Days 3-7 U.S. Hazards

For Friday June 16 - Thursday June 22: The GEFS, ECMWF, and Canadian ensembles remain in good agreement depicting a low-amplitude ridge across the south-central and southeastern CONUS during week-2. While above-normal temperatures are increasingly likely over much of the central and eastern CONUS, excessive heat concerns are mainly focused across the southern tier. The hottest actual temperatures are predicted across Texas and the Lower Mississippi Valley where the National Blend of Models indicates daily record high temperatures (upper 90s to low 100s deg F). The skill-weighted GEFS/ECMWF excessive heat tool also indicates at least a 40 percent chance that heat indices exceed 105 deg F (110 deg F in some areas). Therefore, a moderate risk of excessive heat continues across parts of the Southern Plains and Lower Mississippi Valley for Jun 16-19. While the GEFS and ECMWF reforecast tools depict at least a 20 percent chance maximum temperatures exceed 90 deg F across the Midwest and Ohio Valley, humidity levels are forecast to be lower than normal over these areas, suggesting heat indices will be less likely to reach hazards thresholds. Occasional precipitation may also result in relatively cooler temperatures. Therefore, the slight risk of excessive heat is only extended northward through the Central Plains and Middle Mississippi Valley where there is a stronger signal for heat indices above 100 deg F based on the excessive heat guidance, and is valid for all of week-2.

Given the excessive heat forecast along with antecedent dry conditions, drought expansion appears possible across portions of the Lower Mississippi Valley. Therefore a rapid onset drought risk has been added to this region where soil moisture values have dropped below the 30th climatological percentile in some areas. The largest source of uncertainty is the increasing potential for tropical moisture to move across the region later in week-2 and into week-3. However, individual models are inconsistent, and reforecast guidance from the GEFS and ECMWF leans toward near to slightly below-normal precipitation for week-2 as a whole. This area will likely need to re-evaluated early next week should models come into better agreement.

There is a potential for weak shortwaves to propagate overtop the ridge axis to the south, bringing intermittent enhanced precipitation chances to portions of the Midwest through the eastern CONUS. The uncalibrated GEFS, ECMWF, and Canadian ensembles generally show an elevated signal for enhanced precipitation over the eastern CONUS starting late week-1 and persisting into the outset of week-2. Another system is indicated around the middle of week-2 across the central CONUS. The ECMWF reforecast tool is particularly robust with this second feature, indicating some areas of the Middle Mississippi Valley having at least a 20 percent chance of 3-day precipitation exceeding the 85th climatological percentile and 1-inch during the day 10-12 (Jun 18-20) period. Due to the model uncertainty and marginal precipitation totals, no related precipitation hazards are posted. However, due to the increased signals for slightly above-normal precipitation for week-2 as a whole, the rapid onset drought risk is removed across the Midwest and Ohio Valley.

Snowmelt season continues across the western CONUS after accumulating a very large and in some cases record-breaking snowpack, inducing river flooding especially across higher elevations for some areas in the West, where a few sites still report over 40 inches of snow water content. As snowmelt continues in parts of the Sierra Nevada, Great Basin, and Northern Rockies, flooding may continue to occur along many rivers, and in areas adjacent to the higher elevations. Possible flooding also remains across northern parts of the James River in South Dakota as river levels are receding slower due to saturated ground conditions and relatively flat topography.

Mid-level troughing is favored for the Aleutians and Alaska Mainland, resulting in increased chances for near to below normal temperatures and slightly above-normal precipitation across much of the state. No areas of hazardous weather are indicated across Alaska.

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