E-Portfolio #1 Meteo 361

E-portfolio #1: The Severe Weather Outbreak of May 13, 2009

Reports of Severe Weather on May 13, 2009. The tornado outbreak in northern Missouri is obvious, marked with red dots. Image Courtesy SPC.

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).

Image of the Kirksville tornado. Image courtesy of chasethestorms.com

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.

12Z Surface analysis showing cold front approaching from Iowa and warm front moving northeastwards. Dew points in Missouri and Illinois were in the 50s. Image courtesy HPC.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).

21Z surface analysis showing the progress of both fronts in the Missouri area. Dew points in nearby Illinois had risen to the mid-60s, indicating the arrival of much moister air.Image courtesy HPC.

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.

12Z Theta-e analysis. Areas of high theta (greater than 328 kelvin) are filled in varying shades of green, and theta-e advection is shown with red contours. Image courtesy SPC.As the day moved on, theta-e in the northern Missouri area increased fairly dramatically, as is shown in the 21Z image below.

21Z Theta-e image shows elevated levels of theta-e over northern Missouri, combined with strong positive theta-e advection. The warmer, moister air from the gulf had penetrated further north in the wake of the warm front. Image courtesy SPC.

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.

12Z 850mb analysis showing low-level jet drawing moisture into the central states. image courtesy SPC.

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.

500mb heights and winds at 12Z. Note the curved jet streak associated with the short-wave trough. There is also a closed low over southern Canada. However, this is too far away from northern Missouri to have the associated mid-level divergence affecting northern Missouri. Image courtesy SPC.However, later in the day, by 21Z, we can see that this short-wave trough has dipped significantly south and east.

500mb heights 9 hours later, at 21Z. Image courtesy SPC.

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.

The multiple vort-maxes (indicated by X) associated with the shortwave trough would have associated regions of rich divergence at 500mb to their east. This forecast is valid at 18Z. Image courtesy NCEP.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.

Image courtesy SPC.

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.

Contours of divergence on this 21Z 300mb analysis show that there is little divergence over Missouri, but just to the north, in Iowa, it was present. Image courtesy SPC.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.

Mesoscale Analysis:

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.

21Z ML CAPE and CIN, courtesy SPC. 

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.

Image courtesy SPC.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.

Base reflectivity from Kansas City, MO (KEAX) at 2248, about an hour after the mosaic image above. Regional composite radar imagery at 2155Z, as severe storms were passing across northern Missouri. The structure of the squall line was clearly evident. Image Courtesy NWSThis 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.

Base reflectivity at 2248 from the Kansas City, MO, radar, located to the southwest of Kirksvile by about 200 km. Image courtesy NWS.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.

The Storm Relative Velocity (SRV) image subtracts out storm motion, leaving just the velocity of the reflectors. It allows us to see rotation, such as that shown in the highlighted velocity couplet. Image courtesy NWS.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.

Conclusion

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.