Tornadoes associated with tropical cyclones (TC) typically develop rapidly, with no classical descending mesocyclone. The rotational signatures may last for only 1 or 2 WSR-88D volume scans and require a quick warning decision. Situational awareness along with knowing the environment and how it is changing is critical in these environments. The environment on Tuesday, September 6, 2011 was in many ways consistent with many TC tornado events. These situations are often very challenging for forecasters who balance the need to issue timely and accurate tornado warnings with the large number of rotating thunderstorms that are potential tornado producers.
Tropical Storm Lee moved into the lower Mississippi Valley on Sunday and Monday and then into the southern Appalachians on Tuesday. A cold front dropped south into northern North Carolina on Tuesday morning as the remnant low pressure center moved into the North Carolina mountains with a trailing cold front extending south into South Carolina and eastern Georgia. Across central North Carolina, specifically near Wake County, a warm and very moist air mass was in place with surface dew points in the lower 70s. Surface based CAPE and mixed-layer CAPE values were analyzed around 500 J/Kg. Strong south-southeasterly surface winds veered to southerly at around 40kts at 700 and 500 hPa producing sufficient shear for organized convection and some supercells. Analyzed 0-1km SRH values were analyzed around 250 m2/s2 with the Sig Tor parameter approaching 0.5.
The NWS Raleigh issued more than a dozen tornado warnings on Monday and Tuesday with only three confirmed tornado touchdowns noted. Forecasters were frequently concerned about short lived rotational couplets that would quickly develop and then more often than not, dissipate without producing a tornado.
One particularly interesting cell moved across northeastern Wake County North Carolina near the town of Rolesville between 700 and 745 PM on Tuesday, September 6th. The animation below is a four panel loop of radar data from both the TRDU (TDWR) on the top row and the KRAX (WSR-88D) on the bottom row from 2315 UTC to 2335 UTC on September 6th. The TRDU 0.2 degree base reflectivity data is shown in the upper left and the 0.2 degrees base velocity imagery is in the upper right with both products available every minute. The KRAX 0.5 degree base reflectivity data is shown in the lower left and the 0.5 degrees base velocity imagery is in the lower right with both products available every 5 minutes. A similar loop of storm relative velocity imagery which does not highlight the circulation as well is shown at the bottom of the post.
The cell as viewed on the KRAX WSR-88D (bottom row of the animation) was rather innocuous with a modest vertical reflectivity structure and only limited amounts of CG lightning. A brief rotational couplet can be seen on the 2325 UTC volume scan just to the southwest of Rolesville with the weak circulation dissipating on the next scan. The TRDU TDWR with its unique VCP provides radar data on the lowest elevation scan every minute. This increased temporal resolution shows the circulation quickly developing and then dissipating. Inspection of the TRDU and KRAX storm relative velocity imagery at 2326/2325 UTC noted 30/27 kts of rotational velocity for the TRDU and KRAX respectively. While this storm did not produce a tornado, the NWS Raleigh received multiple reports of a funnel cloud near Rolesville. This storm was one of several dozen storms that forecasters were monitoring during the afternoon and evening and indicative of the difficult warning decisions that take place during these events.