Combining Deep & Shallow Soil Moisture Deficits to Produce Drought Outlook

Combining Deep & Shallow Soil Moisture Deficits to Produce Drought Outlook

Discussion: In a recent Journal of Climate published by the American Meteorological Society (“Potential Re-emergence of Seasonal Soil Moisture Anomalies in North America” by Kumar, Newman, Wang and Livneh; May 15, 2019) research combining deep and shallow soil moisture to project most likely drought areas was reviewed. Long-term dry climate regimes are generally responsible for deep layer dryness while shallow layer dryness is more representative of short-term weather. However, presence of shallow layer dryness combined with deep layer dryness increases the risk dramatically of developing drought and sometimes flash drought.

To make an assessment of the potential for drought heading into meteorological autumn (SEP/OCT/NOV) in the U.S., Canada and Mexico we take a look at current soil moisture anomalies at 0-10 CM depth (shallow layer) and 10-200 CM depth (deep layer) simply combining the dry areas to identify ongoing or potential drought. Finally, we’ll add the 15-day precipitation outlook by the GFS ENS to further adjust where the most likely drought areas heading toward meteorological autumn are located.

According to NCDC/PSD the 0-10 CM soil moisture deficits from the past 30 days are located across northern Mexico to New Mexico and western Texas and north to Wyoming (Fig. 1). The Southeast U.S. is in a shallow layer soil moisture deficit regime. Finally, vividly displayed soil moisture deficits stretch from Wisconsin across Ontario and into Quebec.

Deep layer (10-200 CM) soil moisture anomalies from the past 30 days are located in northern Mexico, the Southeast U.S. and centered on Ontario and vicinity (Fig. 2). Note that deep layer soil moisture is still wet in the Mid-South U.S. and vicinity.

Fig. 1: Zero to 10 CM depth soil moisture anomalies based on NCDC/PSD analysis for the 30-day period ending August 8, 2019.

Fig. 2: 10 to 200 CM depth soil moisture anomalies based on NCDC/PSD analysis for the 30-day period ending August 8, 2019.

Combining both the shallow and deep layer soil moisture deficit regions produces a most likely drought presence for the Upper Midwest U.S. northeastward across Quebec and the Southeast U.S. (Fig. 3). Greatest risk is over southeast Ontario and Mississippi/Alabama based on NCDC/PSD data.

Fig. 3: Combining regions where BOTH deep and shallow layer soil moisture deficits are present is an indicator of where drought is most likely to occur into meteorological autumn.

Fig. 4: The GFS ensemble 15-day percent of normal rainfall forecast across North America with dry zones highlighted.

The 15-day GFS ensemble percent of normal rainfall outlook carries us almost through the end of meteorological summer (JUN/JUL/AUG). The outlook indicates persistent dryness across Texas and California. Due to deep layer wet soils in Texas a dry-to-drought scenario is likely due to the accelerating influence of late season anomalous heat. In California the lack of important heat is slowing what would otherwise be a fast evolving dry-to-drought scenario. Drought over the San Juaquin Valley certainly accelerates.

Combining the shallow and deep layer soil moisture anomalies to project dry-to-drought regions into meteorological spring most clearly implicates the Southeast U.S. and Upper Midwest to Quebec. The GFS ensemble favors more dry weather the next 15 days from Ontario to Quebec increasing forecast confidence for that region. In the Southeast U.S. we’re entering the time of year when tropical systems can effect that region and very rapidly erase soil moisture deficits. Aside from tropical features much of the Southeast U.S. dryness should increase. The GFS ensemble indicates wet risk for the north-central Gulf Coast across southern Georgia.

Summary: Recent research has shown that projecting future drought risk is more confidently forecast by combining the influence of long-term climate measured by 10-200 CM soil moisture deficits and the effects of recent weather patterns implied in 0-10 CM soil moisture anomalies. Combining soil moisture anomalies for both layers from the past 30 days and the 15-day rainfall anomaly forecast by the GFS ensemble indicates areas most susceptible to drought entering meteorological autumn are the Upper Midwest U.S. to Ontario to Quebec and the Southeast U.S. (although tropical cyclones could change the Southeast soil moisture regime quickly).