U.S. Rainfall Needed to Neutralize Palmer Drought Severity Index

Fig. 1: Weekly rainfall required to end dry Palmer Drought Severity Index.

Discussion: During the past week, rainfall required to end dry Palmer Drought Severity Index (PDSI) increased in western Kansas, eastern Colorado, and southwest Texas while the remainder of the Great Plains drought area remain unchanged although shifting wetter in the Houston Area on the Texas Coast (Fig. 1). The Mid-Atlantic drought worsened. Rainfall needed to end drought in northern Virginia/Washington D.C. is now in the 12-15 in. range. The dryness represents a dangerous fire risk in the nation’s capital and vicinity and allows maximum temperature to overachieve if full sun or a down-sloping west wind. Southern Florida drought is easing but strong drought remains in-place for the remainder of the state into Georgia and Alabama where soil moisture dryness increased last week.

Fig. 2: SPI6 U.S. soil moisture outlook for end of July 2023.

Fig. 3: CAS U.S. soil moisture outlook for end of July 2023.

NOAA soil moisture forecasts for the end of July vary on the strength of Great Plains drought. The SPI6 end of July forecast maintains the Mid-Atlantic drought and introduces new drought risk in North Dakota (Fig. 2) while the CAS forecast eliminates the Mid-Atlantic drought (Fig. 3). The Great Plains drought outlook remains a low confidence forecast.

The short-term outlook indicates interesting and important differences between the GFS and ECM (models). The GS indicates the core of heavy rain in southeast Kansas/northeast Oklahoma this week (Fig. 4) while ECM is farther west with similar amount across the southwest Great Plains (Fig. 5). The best dynamic forcing supports the ECM outlook.

Fig. 4-5: GFS and ECM 120-hour rainfall forecast for the Great Plains.

Mega-cluster ensemble indicates a low confidence drier pattern in the Great Plains for the 6-10-day period (Fig. 6) followed by wetter risk for days 11-15 (Fig. 7).

Fig. 6-7: Mega-cluster ensemble “most likely” rainfall anomaly forecast for the medium-range.