Discussion: On Monday and Tuesday of this week New Orleans set daily records of 96 and 97. The onset of extreme heat is related to the rapid transition of drier soil moisture in the Mid-South U.S. during the month of May. Well known is the relationship between large areas of soil moisture deficit to increase anomalous heat risk during the summer season. Not as well-known is the ability for the same hot weather enhancement by a rapid drier soil moisture change.
As of May 15 there are many large regions of soil moisture deficit (i.e. drought) across the U.S. (Fig. 1). During springtime onset of dryness or wet soils can occur rapidly. Therefore the influence of soil moisture on summer climate is best evaluated as meteorological summer arrives (June 1).
The May soil moisture trend through mid-month has been markedly drier in the Gulf States and East-Central U.S. to the Carolinas (Fig. 2). Much of this region has experienced daily record hot temperatures at times so far this month.
The seasonal soil moisture change indicates the most profound dry trend has been across the southern Great Plains and Mid-South (Fig. 3). Interestingly, California trends much wetter in the seasonal chart reversing drier in May.
The next 2 weeks are CRITICAL for soil moisture as a predictor of summertime U.S. thermal climate. Will the current dry/wet zones hold? The best way to reverse a soil moisture trend as warm season arrives is with interaction of middle latitude weather patterns and the tropics (presence of Madden Julian oscillation or MJO). During the warm season the best way to reverse a drought is interaction with inland-moving tropical cyclones.
Currently, presence of MJO enhancing a wet weather pattern across Florida, the Southeast and East U.S. for much of the remainder of May will eliminate Florida, Southeast and Carolinas to Mid-Atlantic drought. The forecast is agreed upon by both the GFS ENS and ECM ENS (Fig. 4-5). Therefore the risk of anomalous dry heat producing daily records in May for much of these areas dissipates as meteorological summer arrives.
Additionally, a wetter than normal regime during the remainder of May eases northern Great Plains drought lowering anomalous heat risk into meteorological summer.
What about the dry zones?
The Southwest U.S., Texas and Mid-South look dry. Drought in this region expands and intensifies into early meteorological summer. The dryness and extreme heat risk promoted by this dry soil forecast also extends to Kansas. The Southwest U.S. to California and into the Great Basin can look forward to periods of record heat into June.
In the Northwest a recent dry trend appears to reverse wetter for the last half of May potentially suppressing heat risk for early summer. This forecast is made with low confidence.
Summary: Mid-to-late May and into early June is a CRITICAL time to assess U.S. soil moisture and potential implied influence on summertime thermal climate for the U.S. Currently, there are large drought areas causing an anomalous heat bias to parts of the U.S. and likely contributing to one of the warmest months of May on record. The ensemble forecasts indicates some of these dry zones, particularly in the East, Southeast and Florida will erode before June 1 shutting down anomalous heat risk in this region. Conversely dryness in Texas to Kansas to the Mid-South is likely to continue and maintain anomalous heat risk for this region during summer. California to the Southwest U.S. dryness also intensifies and extreme heat to follow is likely.
Fig. 1: Current U.S. soil moisture anomalies with dry areas highlighted due to their ability to produce a warmer thermal bias.
Fig. 2: Soil moisture trend in May. The dry trend up to May 15th has been in the Southeast and East-Central U.S.
Fig. 3: Soil moisture trend since Feb. 28, 2018 indicates a drier regime in the Central U.S.
Fig. 4-5: The 15-day percent of normal precipitation forecast by the GFS ENS and ECM ENS clearly indicates a wet regime in the East U.S. and Northwest quarter of the nation while dryness strengthens Southwest U.S. drought and Mid-South U.S. drought.