Rainfall Deficits Increasing Midwest, Northeast and Southeast

Iowa Drying Out So Far in June
06/13/2021, 3:12 pm EDT
Western Heat/Drought Focus Continues, Expands to Northwest
06/17/2021, 12:27 pm EDT
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Rainfall required to neutralize dry Palmer Drought Severity Index (PDSI) has increased in the Midwest States, Northeast region and the Southeast particularly Florida. Western deficits maintain their severe dry signatures. The North Dakota deficits eased last week.

Discussion: In the “chart of the day” monitoring, the rainfall needed to suppress dry PDSI has increased over the Midwest U.S., the Northeast States and the Southeast including Florida during the past week (Fig. 1). This is a dangerous trend for June in regard to increasing anomalous heat risk for mid-summer which is 4-6 weeks from now. Southern Texas also turned sharply drier last week while the entire West U.S. drought stayed similarly intense.

Fig. 1: Rainfall needed to neutralize dry PDSI regimes.

Despite mixed reviews on forecast performance of the North Dakota rainfall last week, ECM is (nationally) the best forecast model compared to wetter biased GFS and drier biased CMC through 10 days. Right now, 11-15-day forecast should be viewed as extremely low confidence. The ECM 10-day percent of normal rainfall forecast (Fig. 2) indicates two wet zones: The Gulf Coast due to a tropical system and Iowa to Michigan due to interaction between a frontal system and tropical moisture. In the Iowa/Michigan stretch, 2-3 in. of rains is confidently expected.

Fig. 2: ECM 10-day percent of normal rainfall forecast for the U.S.

The mega-cluster ensemble is used for days 11-15 (Fig. 3). The model is most heavily influence by ECM. Indicated is wetter than normal climate for the Midwest to the Mid-Atlantic States. The model introduces a wet monsoon flow into the Southwest States. The forecast is generally supported by presence of the Madden Julian oscillation (MJO) forecast although that support is weak and could change. Marginally wet to wet weather is indicated for the U.S. Corn Belt to finish June.

Fig. 3: The mega-cluster ensemble day 11-15 most likely percent of normal rainfall forecast across the U.S.