An early April eastern US coast cyclone exhibited rapid intensification as it advanced north-northeast off the coast of North Carolina during the evening of 02 April 2019.
A 24-hr GOES-East water vapor loop showed a shortwave trough quickly advance across the southeast US during the previous evening and interact with a low situated off the Georgia coast by the afternoon. The features organized into a single much larger storm that accelerated to the north-northeast just off the east US coast. Rapid strengthening of the storm is evident as dry air expands on the southwest descending side of the low, and cool air expands and clouds develop on the northeast ascending side of the low, wrapping around the center of the low in a counterclockwise manner. Convection is also obvious near the center of the low, as well as along a cold front extending south. MSAS MSLP analysis contours overlaid on the water vapor imagery help depict the organization and deepening of the surface low.
1-min imagery was available over the storm to aid forecasters in analyzing its evolution, particularly convective elements during the day. Figure 2 combines 120 minutes of 1-min visible imagery with 1-min Flash Extent Density (FED) 5-min accumulation. The imagery reveals persistent convective activity on the northwest side of the low. Towards sunset, convection develops quickly southeast of this area of convection, as evidenced by a rapid increase in FED. 1-min IR imagery also shows rapid cooling of cloud tops in this area, indicating updraft acceleration, with consistently cold tops in the broad area of convection to the northwest.
Bill Line, NWS
Adding to Bill’s post, my colleagues and I at the NWS Ocean Prediction Center were looking at this impressive storm through the Air Mass RGB and 1-min visible imagery as we issued hurricane-force wind warnings for the Offshore Zones.
The above animation shows the evolution of a meso-low on the southwest flank of a supercell-like thunderstorm, which becomes the new surface low pressure. Vorticity increases and the pressure falls (~13 mb in 3 hours!).
A snapshot around 2014 UTC on 04/02/19 shows the main thunderstorm updraft with surface low pressure nearly coincident. The arrows point to the cold conveyor belt (north of low), the dry, descending air scouring out low-level clouds on the south side of the convection (see Air Mass imagery below), and a wavy coastal front that will aid in the development of the bent-back front. It appears in the animation that some extreme winds are pushing out the southwest flank of the supercell with a haze near the cloud edge. Could this be precipitation being pushed out similar to a rear-flank downdraft? Or is it sea spray? Regardless, at the time of this image, winds were estimated over 65 kt (75 mph) and possible gusts approaching 90 kt (105 mph)!
Finally, these two images showcase the Air Mass RGB utility in a rapidly intensifying extratropical cyclone. The GLM is highlighting the lightning flash density near the supercell mentioned above and the convection that is lined up along a pre-frontal trough and eventually a fast moving cold front. The dry, descending air (red coloring) is indicative of stratospheric air that is descending rapidly and transporting potential vorticity (PV) towards the developing coastal storm. Note how the upper-level feature moving through the southeast U.S. loses organization as the energy jumps to the coast storm and associated strong convection.
We will try to update this blog post as necessary with new findings, but as of 11 pm EDT (0300 UTC), winds are subsiding on the Outer Banks of NC after gusts of 60-70 mph were reported. This leads me to believe that stronger gusts existed with the nearly closed, eye-like feature observed on radar tonight.
Thanks for reading!
~Michael Folmer, NWS/OPC