Outer Banks tornadoes

An energetic shortwave traversing the Ohio River Valley took on a negative tilt and forced several storms on the North Carolina coast. As is somewhat common for the time of year, the kinematic parameters for this environment were very strong -- effective bulk shear ≥ 60 kts, meanwind 1-3km AGL ≥ 40 kts, and storm-relative helicity 0-1km AGL ~200 J/kg.

One storm in particular spawned two tornadoes: an EF0 at 19:06 UTC, and an EF2 from 19:10 to 19:16 UTC, with large sections of roofs removed from homes and powerlines down. The EF2 twister occurred on Emerald Isle.

ProbTornado (a product of NOAA/CIMSS ProbSevere) ramped up quickly to nearly 70% before the first tornado touchdown, but decreased  to 30% as reports came in. In the animation below (Figure 1), you can see the ProbSevere contours (inner contour) and the ProbTor contour (outer contour) colored using the same colorbar (top of image). Increasing rotation evident in MRMS azimuthal shear products, as well as an increase in ENI total lightning density in a very favorable environment led to the soaring ProbTor probabilities.

Fig. 1: ProbSevere contours with MRMS MergedReflectivity and NWS severe weather warnings.

A time series of this storm shows how the ProbWind, ProbHail, and ProbTor probabilities evolved (Figure 2). After a 70mph straight-line wind report at 19:50 UTC, ProbTor increased to 80% when a second tornado warning was issued, but there were no reports. The storm traveled over mostly water at this point, and was starting to reside in a region of high radar beam height, even at the lowest tilt, which potentially may have affected the azimuthal shears.

Nevertheless, this event shows how monitoring the ProbSevere products (namely ProbTor in this case), may help increase situational awareness, forecaster confidence, and ultimately lead-time to severe convective hazards.