Review of PDSI and EDDI as U.S. is Drying Out!

Fig. 1: Rainfall needed to neutralize negative Palmer Drought Severity index.

Discussion: U.S. drought concerns are increasing. The dry month of June combined with a lot of heat has caused Palmer Drought Severity Index (PDSI) to trend drier. Consequently, the rainfall needed to neutralize that dry soil moisture signature has increased DRAMATICALLY in late June (Fig. 1). Off the chart (>15 in.) values appear in the NOAA/CPC analysis in Washington, California, Texas, Georgia and North Carolina. As impressive is the rapidly developing deficits in the Midwest U.S. and most of the East. The Evaporative Drought Demand Index (EDDI) which combines soil moisture deficit and atmospheric available moisture identifies a large area of deficit across the southern 2/3 of the U.S. most dramatic in California and the Southwest U.S. to Texas and the Carolinas (Fig. 2). Indicated are areas susceptible to worsening drought conditions via a flash drought scenario. The 14-day change identifies the Midwest U.S. as rapidly drying (Fig. 3). The overnight GFS identifies the tendency widespread dry soils have on rainfall…unless directly from the tropics, areas of moisture tend to underperform. The overnight GFS identifies the southern states where direct exposure to the tropics/subtropics have wet risk and areas to the north, particularly the U.S. Corn Belt are dry (Fig. 4). The 24-hour forecast change reveals the wetter South/East scenario and drier forecast in the Midwest (Fig. 5). The wetter change near the U.S./Canada border is likely overstated. Bottom line? Significant summer of 2022 U.S. drought concern increasing rapidly!

Fig. 2: Latest U.S. Evaporative Drought Demand Index (EDDI) identifying areas “thirsting” for water. ED3/ED4 zones represent areas already in drought and potential for worsening flash drought.

Fig. 3: Now that we’re into meteorological summer, the EDDI short-term (14-day) change analysis becomes important. The Midwest U.S. is drying rapidly.

Fig. 4-5: Widespread regions of dry (or drying) soils have a tendency to resist synoptic (non-tropical) rainfall. Note that the overnight GFS is wet where tropical moisture is present (southern states) but to the north the forecast is drier.