During the overnight hours of March 19-20, 2017, an amplifying upper level shortwave moved off the Mid Atlantic coast and led to the rapid development of a mesoscale oceanic cyclone across the Gulf Stream east and southeast of Cape Hatteras. The upper level feature moved south and southeast along the backside of a deep upper level long wave trough near 68W. The global models including the GFS and ECMWF were not well initialized with the upper level shortwave and consistently, over the previous several model runs, were only indicating a weak trough would develop at the surface. Conversely, the 4km NAM and HRRR were each showing surface low development and significantly higher associated surface winds than shown by the coarser global models. OPC forecasters had been carrying storm warnings across a few offshore zones through 00 UTC March 20, 2017.
The GOES-16 water vapor imagery, including the 6.9 um mid-level and 6.2 um upper-level, suggested that the mid/upper shortwave was more amplified than initialized by the global models. The feature was also apparent in the GOES-16 7.3 um lower-level water vapor imagery, indicating it may be vertically stacked or at least extend through the lower levels. The three water vapor channels alone indicated there was likely adequate forcing through the upper and mid levels, and even into the lower levels, to support the development of a surface low. However, the low level circulation analyzed in the GOES-16 3.9 um shortwave infrared imagery confirmed the presence of the surface low. In addition, the enhanced baroclinicity the system encountered as it tracked across the Gulf Stream likely played a big role in the storm’s intensification. The sea surface temperature (SST) gradient along the north wall of the Gulf Stream can be seen in the GOES-16 3.9 um shortwave infrared animation.
Upon reviewing the GOES-16 imagery and evaluating the most recent model guidance, the overnight OPC forecaster extended the storm warning through the night period, and also expanded the warning to include the outer mid Atlantic offshore waters. The significantly improved temporal and spatial resolution of the GOES-16 imagery, along with the additional water vapor channels, allowed forecasters to better diagnose the strength of the upper level shortwave and also, the presence of the surface low, which then gave forecasters more confidence in amending the warnings. Even as the both the upper level feature and the surface low appear to shear and weaken in the three GOES-16 water vapor channels and 3.9 um shortwave infrared band around 06 UTC, there was a ship which reported gale force winds (35 kt) at 06 UTC well southwest of the surface low.
Thanks for reading!
James Clark (OPC) and Michael Folmer (CICS)
“The GOES-16 data posted on this page are preliminary, non-operational data and are undergoing testing. Users bear all responsibility for inspecting the data prior to use and for the manner in which the data are utilized.”