2016 President's Day Tornado Event

2016 President's Day (February 15th)
Tornado/Severe Weather Event

Storms responsible for three tornadoes and numerous reports of wind damage affected the NWS Mobile County Warning Area on the afternoon of February 15, 2016. One tornado in Century, FL was later rated EF-3 on the Enhanced Fujita (EF) scale by NWS Mobile. This tornado is the first EF-3 to be documented in Escambia County, FL since the National Weather Service began officially using the Fujita scale in 1973.

Figure 1: Preliminary Storm Reports 2/15/2016

Figure 2: Surveyed Tornado Tracks

Details on individual storm surveys can be found on the initial event page at weather.gov/mob/20160215_tornado.

Meteorological Details

The atmosphere was primed for severe weather as the afternoon hours arrived on February 15th. In the mid and upper levels of the atmosphere there was a deep trough of low pressure that progressed eastward as the morning and afternoon wore on. This trough was characterized by a few features which served to set the stage for severe weather and tornadoes at the surface. Winds in excess of 50 knots in the mid-levels encroached upon the region as the day progressed and there was also pronounced spreading of the height contours signifying diffluence of winds over the region. This “spreading out” at mid and upper levels is favorable for rising motion in the lower levels. Low pressure troughs are also characterized by colder than normal temperatures aloft, which when combined with warm and moist conditions near the surface, the atmosphere becomes unstable; a key ingredient in thunderstorm formation.

Figure 3: 500mb Height, Temperature, and Winds

At about 5000 feet above ground level (850 millibars) there was what meteorologists call a low-level jet. This fast moving corridor of air serves to transport deep Gulf moisture northward and increase another essential ingredient for severe thunderstorms; wind shear. On February 15th winds at this level steadily increased through the day with nearly 60 knots of southerly flow over the region by 4pm.

Figure 4: 850mb Height, Temperature, and Winds

The mid and upper-level trough of low pressure caused the formation of a wave of surface low pressure that deepened and moved eastward in concert with the mid and upper level features. The area of lowered surface pressure intensified from 1010mb at 9am to around 1004mb by 4pm. In response to the deepening pressure field, surface winds increased and took on a southeasterly component. The strengthening and “backing” of the surface winds increased moisture transport off the Gulf and contributed to increased wind shear in the atmospheric column over the region.

Figure 5: Surface Pressure and Wind at noon 02/15/2016

Thunderstorm Ingredients

Moisture is rarely in short supply along the north-central Gulf Coast, even in February. Surface dew points were in excess of 60⁰ F within about 100 miles of the coast on February 15th. Another measure of atmospheric moisture is precipitable water (PW). In this case the PW was analyzed to be in excess of 1.3 inches, a value nearly twice the average for the middle of February. Relatively deep low level moisture is essential for strong, long-lived thunderstorm updrafts and severe weather.

Moisture is one essential ingredient that contributes to atmospheric instability. The other half of the instability equation comes from changes in temperature with height. Warm and moist air near the surface overlaid by cooler and drier air aloft results in an unstable air column. The aforementioned pressure trough supplied the cold temperatures aloft and the surface wind pattern was already transporting Gulf moisture northward into the region. In the cool season, the limiting factor in most severe weather events is a lack of instability as low sun angle and pervasive cloud cover serve to limit the heating of the surface layer. Of note on this day was a pronounced 2-3 hours of clear skies after sunrise that contributed to greatly increased surface heating and more instability than is usually found in the cool season along the Gulf Coast.

Figure 6: Visible Satellite showing clear skies on the morning of Feb 15th

Warm surface temperatures with cooler air aloft resulted in an unstable air column. There are a few ways that meteorologists quantify the degree of instability, but most are variations of Convective Available Potential Energy (CAPE). On this day surfaced-based CAPE was on the order of 1000 J/Kg near the coast with about 250 J/Kg further to the north. If there is sufficient forcing for ascent provided by upper level features, 250-500 J/Kg of CAPE is more than enough instability for severe weather, provided the last ingredient for severe convection is present; wind shear. 

Figure 7: Effective Shear

Wind shear, or winds that increase in speed and/or change direction with height, is essential for long-lived, severe convection. Generally, 40-50 knots of shear is sufficient for rotating supercell thunderstorms, which are responsible for nearly all strong to violent tornadoes. On February 15 we had pockets of 50+ knots of effective wind shear by the afternoon hours. Another measure of wind shear, storm relative helicity (SRH) in the 0-1km layer, was in excess of 300 m2/s2 by 3PM, a value very favorable for tornadoes which only increased as the afternoon wore on, especially east of the I-65 corridor, where all the tornadoes formed.

Figure 8: 0-1 Km Storm Relative Helicity

One way to easily view all of these ingredients together is through visualization of data obtained by launching weather balloons. We do not launch balloons here in Mobile so we must rely on sites around our area that do, one such location is the NWS New Orleans office located to our west in Slidell, LA. On most days these balloons are launched at 00 and 12 UTC (6PM and 6AM CST) but on special occasions extra launches are warranted. February 15 was one such occasion with Slidell sending up a special balloon at 18 UTC (noon CST).  It is reasonable to assume that the air column sampled in Slidell at noon was relatively unchanged and near or over our region at the time of the severe weather.

Figure 9: Slidell 18 UTC Feb 15, 2016 Upper Air Sounding

One can see all the ingredients required for severe weather in Figure 9. Copious low level moisture and warm surface temperatures are represented by the green and red lines on the left panel, respectively. Resulting instability is shown in the text portion below. Wind shear is quantified both graphically (by the hodograph on the upper right) and in the text portion below the temperature and moisture traces. All of these parameters together illustrate the risk for strong to severe storms with the distinct potential for tornadoes, which did in fact come to fruition on the afternoon of February 15, 2016. 

Figure 10: Radar reflectivity and Doppler velocity for the Century, FL EF-3 tornado

Figure 11: Radar reflectivity and Doppler velocity for the Johnsonville, AL EF-2 tornado

Figure 12: Radar reflectivity and Doppler velocity for the Red Level, AL EF-1 tornado

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Acknowledgements: Page created by Ryan Rogers (forecaster) and updated by Morgan Barry (forecaster). 

LAST UPDATED: February 2017