Do you know what a “flash” drought is? I always thought it was a rapidly developing severe drought that primarily affected agriculture. I was surprised to learn that there are actually two types of flash drought, as explained by John Feldt, the retired Hydrologist in Charge of the NWS Southeast River Forecast Center.
John now runs a private consulting firm and posts a blog called “Blue Water Outlook”. He provides information on climate and agriculture to groups across the US and posts information in a free newsletter which is available from https://ncga.com/weather. John recently posted this story about two kinds of flash drought and how the Southeast is currently experiencing the first, precipitation-deficit type of flash drought. Thanks to John for allowing me to share this in “On the CASE”.
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A flash drought is defined as the sudden onset of unusually high temperatures along with significant decreases in soil moisture. Flash droughts tend to intensify rapidly over the spring and summer months. There are two types of flash droughts.
A Type 1 flash drought is based on unusually high temperatures which results in increases in evapotranspiration and decreases in soil moisture.
- Only about 4-5% of total record.
- Occur in the eastern U.S. with maxima over the North Central U.S. and the Ohio River Basin (areas shaded in green above.)
- Primarily temperature driven.
- Can only occur in vegetation dense areas.
A Type 2 flash drought starts from the lack of spring rainfall. With a lack of rainfall, evapotranspiration decreases, setting the stage for possible (negative) feedback and resulting unusually hot temperatures.
- More common than Type 1, about 5-8% of total record.
- Maximum over the Southern Plains and the Gulf states (areas shaded in red above).
- Mainly precipitation driven – tends to occur during meteorological drought.
- Lack of precipitation will increase surface temperatures and result in heat waves.
Southeast U.S. Flash Drought
Could the severe to extreme drought (below-left) over parts of the Southeast be a Type 2 flash drought?
The onset of drought conditions was fairly quick with as much as a 3-class (Drought Monitor) degradation heading from spring to summer (below-right).
Remember that Type 2 flash drought typically originates from a lack of spring precipitation. This results in a decrease in soil moisture which leads towards an increase in temperature.
The following 4 images depict precipitation departures for the period of March 2016 – June 2016. Note the significant spring rainfall departures over northern Alabama and Georgia, mainly in April and May.
March April
May June
Thus, it appears that precipitation anomalies over northern sections of Alabama and Georgia since this past spring align with conditions associated with a Type 2 flash drought.
Potential Texas and Oklahoma Flash Drought
Not all flash droughts need to originate with a dry spring. Texas and parts of Oklahoma experienced an unusually wet spring with areas of flooding. At the current time, only scattered areas are classified “unusually dry” or within moderate drought.
Rainfall has been 25 percent or less than normal over parts of Texas and Southeast Oklahoma over the past 30 days (areas shaded in red).Temperatures have averaged well above normal during this same time.
If conditions do not improve, signs point towards the imminent development of a flash drought over parts of Texas and Oklahoma.
When Will Conditions Improve?
Persistent patterns of temperature and precipitation lead towards the formation of flash droughts. It will take a sustained and significant change in these patterns, or an unusual event such as an inland-moving tropical system, to begin to ease flash drought impacts.
The late summer and fall months are not usually times of active weather (other than tropical weather) so it is likely that drought impacts could persist over the Southeast U.S. and develop across parts of Oklahoma and Texas over the remainder of the summer.
Credits: NOAA CPC, NOAA Kingtse Mo CPC
