The Southeastern U.S. is poised for substantial amounts of rainfall in the coming days due to a deep moisture plume with a connection all the way to Central America. Figure 1 shows the SPoRT MODIS air mass RGB product centered on the Southeastern U.S. An upper low pressure area is slowly retrograding to the west, and is situated over western Missouri overnight on 3 July. The dry air wrapping around the southern/eastern edge of the upper low is evident by the red/orange shading in the air mass RGB over Arkansas and Missouri. At the same time, a Bermuda high pressure is becoming entrenched over the western Atlantic, with dry air wrapping around its west side, given by the orange plume to the east of Florida in Fig. 1. In between these two systems is a large plume of very moist air (given by the green shading in Fig. 1), accompanied by extensive cloud cover and precipitation. The deep southerly flow between these systems is advecting rich tropical moisture northward from the Gulf of Mexico into the eastern U.S.
The moisture plume can be traced back to Central America by examining CIRA’s total precipitable water (TPW) product. Figure 2 shows the CIRA TPW from the early morning hours of 3 July. Very high TPW values exceeding ~48 mm (~2+ inches) extend from the Yucatan peninsula of Mexico and western Caribbean Sea, across the eastern Gulf of Mexico and all the way north along the U.S. East Coast. This persistent moisture plume has already been responsible for substantial flooding in parts of the Northeastern states. The CIRA TPW anomaly (i.e., percent of normal TPW in Fig. 3) shows the very high anomalies in the Northeastern U.S., nearing 200% of normal values. TPW values are not nearly as anomalous in the eastern Gulf, as deep tropical moisture is more typical there for this time of year. However, the TPW values are still marginally above normal in a distinct corridor from the Gulf/Caribbean Sea into the Southeastern U.S. The persistence of this feature will continue to bring a prolonged period of heavy rainfall into the southern/eastern U.S. for the next several days.