Composite Map Day 8-14

Experimental Probabilistic Outlooks Temperature Hazards Precipitation Hazards Wind Hazards Snow Hazards

Valid Friday September 20, 2024 to Thursday September 26, 2024

US Hazards Outlook
NWS Climate Prediction Center College Park MD
300 PM EDT September 12 2024

Synopsis: One or two low pressure systems may develop near the Southeast or Mid-Atlantic coast during the next week to ten days. Mid-level low pressure over the West is forecast to result in surface low development across the Central High Plains by the beginning of week-2. This low pressure system is then expected to track northeastward to the Upper Mississippi Valley. On September 20 and 21, there is an increased chance of a frost or light freeze for portions of the Great Basin. Mid-level low pressure is expected to persist over Alaska with enhanced, onshore flow affecting southeastern Alaska.

Hazards

• Slight risk of heavy precipitation for portions of the Mid-Atlantic and Southeast, Fri-Tue, Sep 20-24.
• Slight risk of high winds for the coastal Mid-Atlantic and Southeast, Fri-Sun, Sep 20-22.
• Slight risk of heavy precipitation for portions of the Great Plains and Mississippi Valley, Fri-Sat, Sep 20-21.
• Slight risk of high winds for much of the Rockies and Great Plains, Fri-Sat, Sep 20-21.
• Slight risk of much below normal temperatures for parts of the Great Basin and Southwest, Fri-Sat, Sep 20-21.
• Slight risk of heavy precipitation for southeastern Alaska, Fri-Tue, Sep 20-24.

Detailed Summary

For Sunday September 15 - Thursday September 19: WPC Days 3-7 U.S. Hazards

For Friday September 20 - Thursday September 26: A mid-level trough may result in the development of more than one low pressure system near the Southeast coast during mid to late September. As of 2pm EDT on September 12, the National Hurricane Center states that there is a 30 percent chance of a non-tropical low to form and become a tropical cyclone across the western Atlantic. This initial low pressure system would likely track inland well before the start of week-2. However, for the second consecutive day, an increasing number of GFS and ECMWF ensemble members favor a second low forming near the Southeast coast early in week-2. The uncalibrated ECMWF model along with the GEFS and ECMWF Probabilistic Extremes Tool (PET) support a 20 to 40 percent chance of heavy precipitation for portions of the Southeast and Mid-Atlantic. Since this low pressure system would move very slowly, the slight risk of heavy precipitation is valid from September 20 to 24. Regardless of any surface low development, enhanced onshore flow leads to a slight risk of high winds (September 22-24) from coastal northeastern Florida to the Chesapeake Bay. Beyond September 24, daily ensemble forecasts indicate that the onshore flow should ease.

The GEFS and ECMWF ensemble means are in good agreement and remain consistent indicating an amplified 500-hPa trough progressing eastward from the West. This longwave trough along with leeside surface cyclogenesis across the Central High Plains supports a broad area highlighted with a slight risk of high winds for much of the Rockies and Great Plains through September 21. Based on the ECMWF PET and its uncalibrated model output, a slight risk of heavy precipitation (locally more than 1 inch) is posted for parts of the Great Plains and Mississippi Valley for the same period. Given the enhanced low-level moisture and instability, the cold front could initiate thunderstorms within the specified heavy precipitation hazard.

The amplified 500-hPa trough is also forecast to result in cooler-than-normal temperatures across the interior West early in week-2. As surface high pressure becomes centered across the Great Basin and Southwest, there is an elevated chance of a frost or freeze. A slight risk of much below normal temperatures is posted for parts of the Great Basin and Southwest where the GEFS and ECMWF PETs show at least a 20 percent chance of minimum temperatures falling below the 15th percentile and 32F. The passage of the mid-level trough could lead to accumulating snow in the highest elevations of the Northern to Central Rockies, but is not expected to reach hazardous criteria.

An anomalous 500-hPa trough is likely to promote enhanced, onshore flow across southeastern Alaska. The GEFS and ECMWF ensemble means depict multiple low pressure systems tracking from the Bering Sea to the northern Gulf of Alaska through much of week-2. A slight risk of heavy precipitation (valid September 20-24) is posted for southeastern Alaska where the GEFS and ECMWF PETs depict greater than a 20 percent chance of 3-day precipitation amounts exceeding the 85th percentile and near 3 inches. A high wind hazard is not posted at this time since the consensus among the GEFS and ECMWF ensemble members is for these low pressure systems to be of moderate strength.

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