E-portfolio #1: The Severe Weather Outbreak of May 13, 2009
During the late afternoon and evening May 13, 2009, the central US was hit by thunderstorms and tornadoes, including one that hit Kirksville, MO and caused several deaths. (The Kirksville tornado was extensively imaged, including rare footage from a direct hit).
The focus of this article will be the severe weather in northeastern Missouri. We will first look at the synoptic, or large scale, weather patterns that set the stage for this outbreak. We will turn to a mesoscale (intermediate-scale: 2 – 1000 km) analysis of the weather in this area to seek to connect it to the synoptic scale.
The Big Picture: The Synoptic Scale Setup
The May 13 storms that broke out over northern Missouri were associated with a surface cold front that approached the area from Nebraska and that extended towards Canada and westerly towards Colorado. That morning (12Z), the front lay across Nebraska.
By early afternoon (21Z), the big-picture had changed. The cold front had continued its march eastward and assumed a generally northeast-southwest orientation, while the warm front had moved in a northeasterly direction. The approaching synoptic-scale cold front provides a lifting mechanism which would help lift parcels to the LFC, encouraging deep moist convection (DMC).
The warm sector of a mid-latitude low is generally prime for development of severe thunderstorms. Northern Missouri fell into this sector by 21Z. We can see this further in an analysis of theta-e, or equivalent potential temperature. Theta-e serves as a proxy for low-level warmth and moisture. Let’s look to see how theta-e in the northern Missouri area developed through the day.
At 12Z, theta-e over northeast Missouri was below 328K.
A theta-e ridge can be seen to be developing as the day wears on from the Gulf up into northern Missouri. This is consistent with the approach of warm, moist air following the surface warm front that moved from the area in a northeasterly direction. The approaching cold front from the northwest, colliding into this air, primed the surface for DMC.
Turning to middle and upper troposphere, we begin by looking at 850mb, above the boundary layer. The 12Z and 21Z images both show a low-level jet stream drawing moist air north from the gulf into the central states, helping provide moisture to support DMC.
In the mid-troposphere (500mb be a representative level), we are looking to see if there is any mid-level divergence near the area of interest; mid-level divergence is associated with low-level convergence, and thus provides a lifting mechanism. At 12Z, we see a fairly potent short-wave trough over the Dakotas and extending into Wyoming and Nebraska.
While northern Missouri is still not squarely in the divergence-rich exit region of the jet streak, the multiple vort-maxes associated with the trough, as shown on the 18Z prog, does suggest that Iowa and northern Missouri will be in a region of at least some mild mid-level divergence.
As this trough continued to dip south and east in the hours after 12Z, the associated divergence-rich region would move toward northern Missouri, setting up for low-level convergence and associated lifting. The 300mb upper-air height, divergence and wind images are consistent with this interpretation. First, looking to 12Z at 300mb, we see the high-level jet streak setting up across the northern plains, with winds in excess of 100kts. However, northern Missouri is not located in either the entrance or exit region of this curved jet streak, and indeed is located quite some distance away.
The pink contours also show some relatively strong divergence occurring over northern Missouri at 12Z. However, when we turn to 21Z, we see that this is no longer the case.
By 21Z, the jet streak had tilted in a counterclockwise direction, and the divergence contours at 300mb do not highlight the northern Missouri area, though they do touch on it (in southern Iowa). However, applying the ‘Grenci methodology,’ I see no overriding reasons why the quadrant of the jet streak that northern Missouri falls into would not support DMC.
Finally, a review of wind speeds and direction at the various levels examined (surface, 850mb, 500mb, 300mb) indicates that there is significant wind shear in the troposphere, with southerly winds at the surface clocking to westerly at higher altitudes and wind speed increasing significantly with height. Strong, deep-level shear indicates the the mode of thunderstorm development favors supercells and tornados.
Turning from the synoptic scale to the smaller mesoscale, let’s look at the the main tools we have to examine mesoscale systems. First, the image below shows 21Z MLCAPE and CIN.
Let’s break down this acronym. ML means ‘mixed layer.’ It attempts to account for the fact that surface dewpoints make surface values of CAPE inaccurately high (hold for the definition of CAPE). To account for this, we use the mixed layer of the lowest 100mb of the atmosphere above ground. CAPE is convective available potential energy. It is a measure of the potential for strong updrafts. If a parcel is lifted and reaches the LFC in an area of high CAPE, it will accelerate upwards rapidly. CIN is the opposite – the convective inhibition, the force acting on an air parcel to prevent it from reaching LFC.
The image above shows that northern Missouri was in an area of reasonably high MLCAPE (varying between 1000 and nearly 3000 J/kg), with only minimal CIN in north-central Missouri (though a tongue of higher CIN in eastern Missouri). This indicates that the atmosphere over north-central Missouri was conductive to strong updrafts if a lifting mechanism – such as the approaching cold front from the west – were there to lift parcels to the LFC. As was discussed above, there was at least some moderate mid-and-upper level divergence present, enough to overcome the CIN.
We can also look at surface moisture convergence to look at both low-level convergence and moist advection.
The 21Z surface moisture convergence analysis shows (red contours) that northern Missouri was in an area of high moisture convergence, which correlates to initiating surface-based storms. Additionally, the green fill indicate mixing ratios of between 8 and 12g/kg, indicating moist air. This combination of moisture convergence and high mixing ratios indicates an area prime for development of surface-based storms.
NWS radar images showed the effects of these factors erupting into strong storms the evening of May 13, 2009. A regional radar mosaic shows the line of thunderstorms in the late afternoon.
This line marks the arrival of the leading edge of the cold front, which is indeed what the synoptic setup showed. These storms shown in the mosaic can be seen more clearly when turning to an individual site.
This base reflectivity image shows the line of storms and supercells associated with the oncoming cold front. I circled two supercells near Kirksville, which display the kidney-bean shape of a typical HP supercell. The heavy precipitation associated with these supercells may be concealing the mesocyclone’s hook echo.
The Storm Relative Velocity image reveals more.
Circled is a velocity couplet showing significant gate-to-gate shear. This is likely the rotation associated with the tornado that later hit Kirksville, though due to the distance from the radar site it cannot be definitively called a TVS (Tornado Vortex Signature). However, this velocity couplet is associated with the supercell shown northwest of Kirksville in the reflectivity image.
The setup on the afternoon of May 13, 2009, led to strong thunderstorms and tornadoes over northern Missouri. The synoptic pattern developed over the course of the day, bringing enough upper-level divergence into the area to break the relatively mild CIN present. ML CAPE indicated sufficient potential for strong updrafts. The warm, moist air drawn into the area from the direction of the Gulf left a low-level atmosphere prime for development. An oncoming cold front from the west provided the lifting mechanism needed to initiate DMC once the CIN was overcome. Upper-troposhereic divergence, while not particularly strong, was strong enough to help break the moderate levels of CIN that were present. Once this occurred, we had all the ingredients necessary for deep, moist convection at the mesoscale. This lead to microscale events such as the Kirkland tornado.