A slowly-moving storm was migrating northeastward along the East Coast of the United States on 25 September 2015. The GOES-13 water vapor animation, above, shows the evolution of this system between 22 and 25 September 2015. The center of circulation in the water vapor imagery has wobbled around the state of Georgia for the past three days as a strong jet along the United States-Canada border has has maintained a series of surface High Pressure systems northeast of the storm, effectively blocking the Low Pressure’s exit from the southeast US.
Moisture available to this storm is depicted in the animation of MIMIC Total Precipitable Water, below. Moisture that moves southwestward towards the southeast US along the Gulf Stream is being joined by moisture moving northward from the tropical Atlantic Ocean.
This system has generated flooding rains over South Carolina. The GOES-13 Visible animation from 24 September 2015, below, shows persistent inflow off the Atlantic Ocean. Click here for a media report; Columbia SC reported 2.84″ of rain on 24 September, a record for the date.The thunderstorms associated with this system also caused a brief EF-2 tornado near Johns Island in Charleston County, SC (SPC Storm Reports). An animation of auto-detected Overshooting Tops, below, captures an overshooting top sequence persisting near Charleston at about the time of the tornado (0445 UTC). The toggle between Terra MODIS (0315 UTC) and Suomi NPP VIIRS (0618 UTC) 11 µm infrared data shows the general northwestward motion of the thunderstorm that produced the tornado (GOES-13 Infrared animation). The coldest cloud-top IR brightness temperature from MODIS was -75º C, with -76º C detected by VIIRS. The pressure gradient between the storm over the southeast US and the High Pressure over the Canadian Maritimes had caused long-fetch onshore winds over much of the mid-Atlantic coast. This (and a Full “Super” Moon on Sunday) presages a weekend of coastal flooding. The animation of ASCAT (from METOP-B) winds below, shows 20-25 knot winds over a large region of the Atlantic Ocean between the Gulf Stream and North America. That long fetch will help generate large waves. The surface circulation off the coast of Georgia is also apparent in the 0220 UTC image.