Focus on Nebraska/Iowa Rapid Drought Development Risk

Highlight: Focus on “rapidly developing drought” (NOAA) or “flash drought” (Climate Impact Company) for Nebraska/Iowa by mid-August.

Fig. 1-2: The end of June and end of July U.S. soil moisture ranking percentile.

U.S. discussion: According to NOAA, the U.S. soil moisture rankings have shifted from sharp dryness biased toward California and the Southwest U.S., Texas and the western Great Plains plus the Carolinas in late June (Fig. 1) to most of the U.S. in late July (Fig. 2). Although there have been streaks of very heavy rain in July (Fig. 3), they have been somewhat confined to very specific areas while the more overwhelming character of the mid-summer climate leading to the dryness is the steep evaporation rates inspired by widespread excessive heat (Fig. 4). The rapidly widening drought/soil moisture deficit areas are consistent for the month of July following 2 years of La Nina climate (Fig. 5-6). The climate pattern driving the expanding drought/dryness is difficult to break during mid-to-late summer as semi-permanent pressure systems become stagnant and high-pressure has a tendency to align with large soil moisture deficit areas. The latest GFS ENS 15-day forecast is very dry across the Great Plains and into the Midwest U.S. (Fig. 7). Wet weather is mostly to the east of this region in the 15-day outlook. All models are in general agreement with GFS the driest model and ECM slightly wetter for the Ohio Valley. The dry outlook coupled with dry acceleration of soil moisture conditions leads to a hot climate (Fig. 8) generating a feedback mechanism to the atmosphere which in-turn re-enforces the surface heating most effectively near Omaha, NE. Given the soil moisture conditions and trend and the 15-day forecast the chances of “rapidly developing drought” as stated by NOAA or “flash drought” as Climate Impact Company has mentioned is increasing. The target area appears to be Nebraska/Iowa.

Fig. 3-4: The NOAA U.S. 30-day percent of normal precipitation analysis and July (so far) temperature anomaly analysis.  

Fig. 5-6: Palmer Drought Severity Index for two analogs for months of July after 2 years of La Nina climate.

Fig. 7-8: GFS ENS 15-day percent of normal rainfall and temperature anomaly forecast for the U.S. agriculture belt.