Hurricane Milton: RAP Generated Tornado Pathlength and Duration Predictions and Test Cases

 I collected data from Hurricane Milton in order to evaluate tornado pathlength and duration algorithms I'm currently testing for publication. Those pathlength/duration predictions generally increased in magnitude from morning into the afternoon as discrete pre-cyclone storms spread north across southern and eastern Florida, and as supercellular rainbands advanced inland across the state. Shear profiles were very favorable for rotating storms, and mid/upper-level flow was more typical of what would be observed across the Great Plains, Midwest, and Southeast during a standard tornadic ETC scenario. Another important aspect that likely enhanced tornado production was a dry midlevel airmass that Milton encountered as it advanced east toward Florida. The dry midlevel airmass allowed cloud breaks and resultant augmented surface heating to take place over southern/eastern Florida, which boosted temperatures into the 80s, led to steepening low-level lapse rates, and precluded substantial CIN and hurricane-related saturated low to midlevel thermodynamic profiles. In addition, dewpoints were well into the 70s as southeast surface winds caused rich moisture to spread inland across the state, which aided in the generation of a very deep buoyant environment.

The latest storm survey information from NWS Melbourne can be found here, and survey information from NWS Miami is here. The longest pathlength tornado surveyed in the Miami CWA was 48 km (29.9 miles) with a duration of 44 minutes that dissipated at 1811 UTC near Palmdale, FL, and in the Melbourne CWA, 33.9 km (21.2 miles), with a duration of 31 min, an EF rating of 3, and unfortunately a death toll of 6 occurring in the Vero Beach area. I captured a RAP sounding valid at 19 UTC, and located 81 km from the Palmdale event, and  ~30 km west of the surveyed tornado near Vero Beach. The data from the RAP sounding was used as input for the tornado pathlength/duration algorithms I'm testing, and yielded a predicted pathlength of 39.2 km, and a predicted duration of 41.9 min, which yields an error of ~ 9 km and 2 minutes for the Palmdale event, and ~6 km and ~10 min for the Vero Beach event. Algorithm output and other relevant data related to the tornado event are posted below.

 

RAP generated hodograph yielding a predicted tornado pathlength of 39.2 km and tornado duration of 41. 9 min. The sounding point was around 30 km west of an observed tornado with a surveyed pathlength of 33.9 km and duration of 31 min.

RAP sounding taken in proximity to an EF3 tornado. Besides the very favorable wind profile for supercells, dry air above the surface, favorably steep lapse rates, a warm surface airmass, and very moist boundary layer aided in creating a very buoyant airmass with minimal CIN that supported robust thunderstorm development contributing to the tornado threat.

Distance from the RAP sounding proximity hodograph point to the Palmdale tornado end point location.

RAP hodograph and sounding shown above were picked from the red point appended to the map. Distance to the Vero Beach tornado track is shown right of the red point.

Base reflectivity valid during the time of the EF3 tornado impacting Florida coastal communities.

Storm-relative velocity from the lowest radar scan valid during the time of an EF3 tornado impacting coastal Florida communities.

12Z 250 mb upper-air analysis showing upper-level flow over Florida that perhaps was more typical of a continental ETC tornado threat.

Midday surface observations over Florida ahead of Hurricane Milton. Note the pocket of relatively rain-free air over southern/eastern Florida that was associated with cloud breaks and enhanced surface heating. Also not the very rich moisture spreading inland via east-southeast surface winds.