The large number of radial flight legs over two days at the same flight level (850 hPa) made it possible to examine the changes in storm structure that occurred as Elena intensified from a disorganized category 2 to an intense category 3 hurricane. In the plots below, changes in Elena’s symmetric storm structure are examined by constructing radius-time Hovmollers of azimuthally averaged quantities collected during the reconnaissance missions.

Azimuthal averages were calculated by dividing the time period of study into four-hour bins and averaging all radial flight legs that occurred in each bin. The average number of flights that occurred in each four-hour window was seven, so that each quadrant of the storm was sampled at least once. The four hour azimuthal averages were smoothed with a 9 point Bartlett filter which removes features with wavelengths less than ~5 km. The averages are shown from the eye out to 150 km and missing data always appears as dark blue in the Hovmollers below.

As a reminder, Elena was slowly intensifying between 00 and 18 UTC 31 August and rapidly intensifying until 00 UTC 2 September, after which the storm filled slowly until landfall according to the best track data set.

The height of the 850-hPa surface remains steady through 01 UTC 1 September when rapid height falls begin in the inner 25 km. Outside the inner core, the height gradient increases throughout the time period. The plot shows that Elena reached its maximum intensity in terms of minimum 850 hPa height ~20 UTC 1 September.

The maximum tangential wind speed moves radially inward with time from 70 to 25 km. The wind speed increases rapidly starting 07 UTC 1 September as does the gradient of tangential wind within 25 km of the eye. Peak intensity at 850 hPa is seen between 03 and 07 2 September, well after the best track peak intensity at 18 UTC 1 September.