Climatological Study of Cold Season Tornadoes along Alabama Gulf Coast

Forecasting the occurrence of tornadoes can be challenging along Alabama’s coast and surrounding areas during the cold season (defined as November through February). Though tornado season is defined as March through May (National Weather Service Birmingham Forecast Office, n.d.), cold-season tornadoes are not unlikely for the coast. Medlin (personal communication) performed research which found that 37% of all tornadoes occurred between November and February from 1950 to 2018 across the central Gulf Coast. During the cold season, the amount of vertical wind shear and thermodynamic instability available for tornado occurrence is critical. Guyer et. al. (2006) found cold-season Gulf Coast significant tornado events (F2+) were characterized by very strong shear, especially below 850 hPa, and concentrated high values of low-level instability, as measured between the surface and 3 km. Similar findings regarding strong low-level shear and tornado occurrence during the cold season are also supported by a local study performed by Medlin and Bunkers (2008), which reconstructed hodographs within 120 km (60 mi) of Mobile, Alabama.

The present research study is a restart of a COMET Partners project originally conducted between the University of South Alabama and the Mobile National Weather Service Forecast Office from December 2019 into early March 2020. We have compiled a climatology of historical tornado event days from 2003 to 2023 for Alabama Climatic Zone 8 (NCEI CONUS Climate Divisions), which consists of Mobile and Baldwin Counties. For the purposes of this research, we have extended the cold season to be defined as mid-October into mid-March. This was done to include tornado outbreaks in Mobile and Baldwin Counties that occurred at the end of October and early March, which brought the total number of tornadoes from 13 to 35, allowing us more available data upon which to build composite analyses. We defined a tornado event day as a 24-hour period on the same calendar day when tornadoes were observed within our defined study area and documented in the NCEI Storm Events Database. Characteristics such as intensity, distance, time of occurrence, and location were tabulated for 35 individual tornadoes over the 21-year study period. We utilized the 3-hourly gridded data from the North American Regional Reanalysis (NARR; Mesinger et. al. 2006) residing on the NOAA/ESRL website to build composite analyses using the closest 3-h analysis time preceding the first tornado occurrence on an event day. Composite analyses were created by analyzing each case day’s 500 hPa and 1000 hPa charts. The 500 hPa level is used for analysis of major troughs and ridges, while the 1000 hPa level is used to determine where closed low-pressure and/or high-pressure systems are located in relation to the defined study area. After analyzing each level for the 13 case days, two subtypes were identified and composited. The first subtype shows a low-pressure system just to the west of the study area. Six out of the 13 case days are associated with this subtype. The second subtype shows a low-pressure system to the northwest of the study area. Four of the 13 case days are associated with this subtype. The other three case days do not have similar features and therefore were not able to be composited. Analyzing the composites of these two subtypes will help forecasters recognize the synoptic patterns associated with potential tornado-genesis in this region.

References

Guyer, J.L., D.A. Imy, A. Kis, and K. Venable, 2006: Cool Season Significant (F2-F5) Tornadoes in the Gulf Coast States. Preprints, 23rd Conf. Severe Local Storms, St. Louis MO.

Medlin, J.M., and M. Bunkers, 2008: An Examination of North Central Gulf Coast Cold Season Pre-Tornadic Vertical Wind Shear Environments since 1996. 33rd National Weather Association Annual Student Meeting, Louisville, KY.

Mesinger, F., and Coauthors, 2006: North American regional reanalysis, Bull. Amer. Meteor. Soc. 87 (3), 343-360, https://doi.org/10.1175/BAMS-87-3-343

National Weather Service, Birmingham, AL, n.d.: Severe Weather Awareness Week. Retrieved on 21 August 2024 from https://www.weather.gov/bmx/outreach_swaw#:~:text=Alabama%20is%20no%20stranger%20to,spring%20(March%20through%20May).