On May 13, 2009, an outbreak of severe weather caused extensive damage across the Midwest, especially in Missouri. Three people were killed there by tornadoes, including two near Kirksville. In the image below, you can see the reports of tornadoes (red triangles) over the northern tier of the state. The post will review and analyze the factors that led up to this outbreak. First, we will examine the big picture or synoptic scale features that set the stage and primed the environment for an outbreak, then we'll look at the intermediate scale or mesoscale components that actually triggered and sustained the severe storms. Mesoscale features are generally between 2 km and 1000 km horizontally.

Figure 1: Storm Reports from May 13, 2009. Notice the trail of red triangles across Northern Missouri (circled in red) representing tornado reports. These tornadoes were associated with one long-lived supercell thunderstorm. The supercell and associated tornadoes were responsible for 3 fatalities. [Source: Storm Prediction Center]

THE BIG PICTURE

The air mass over Missouri changed significantly during the day and we'll examine the synoptic scale features responsible for those changes that ultimately set the table for the outbreak. We'll first review the environmental landscape at the surface at both 12Z and 21Z.

Surface Analysis

The surface map below shows several key features at 12Z. First is the southerly flow out of the Gulf over the Midwest. Surface temperatures and dewpoints are close to 70°F and 60°F respectively in the warm sector, below the warm front, which forms the boundary of this warm, moist air. At this time, our area of concern is north of the warm front.

Figure 2: Surface Map from 12Z May 13, 2009. Our area of concern (Northern Missouri) is north of the warm front at 12Z. [Source: Hydrometeorological Prediction Center (HPC)]

By 21Z, several things have changed; the approaching cold front is now into NW Missouri, and the warm front has lifted into Iowa. Northern Missouri should be experiencing surface warming and moistening, which can lessen Convective Inhibition (CIN) while enhancing the Convective Available Potential Energy (CAPE).

Figure 3: Surface Map from 21Z May 13, 2009. Notice the southerly flow out of the Gulf right up Red River Valley. Also note the warm sector between the approaching cold front and lifting warm front. [Source: Hydrometeorological Prediction Center (HPC)]

850-mb Analysis

The 21Z analysis at 850-mb shows a strong low level jet stream conveying warm, moist air into Missouri. Also of note is the confluence of winds with 20-25 kt winds out of the NW over Nebraska joining with 30 kt winds out of the SSW over Missouri. These two factors at 850-mb are priming the atmosphere for deep convection. This low-level jet stream moistened and warmed the low levels (enhancing CAPE, eroding CIN), while also bringing strong low level winds out of the SSW, setting the stage for enhanced bulk shear.

Figure 4: 21Z 850-mb Analysis. A low-level jet stream conveys warm, moist air over Missouri. The dark shade of green indicates dewpoint temperatures greater than 14°C. Note that the area of concern is generally shaded dark green and between the 16°C and 14°C isotherms at 850mb (red dashed lines), suggesting saturation. [Source: Storm Prediction Center]

500-mb Analysis

The dominate feature on the 500-mb chart is the curved jet streak over the northern plain states associated with a short wave trough. The key connection between a short wave trough with mesoscale features is upper level divergence east of the trough. Recall that divergence aloft supports convergence at the surface. This combination promotes upward motion and subsequently, cooling and steepening of the environmental lapse rate. So, as we look at this particular jet streak, we should expect divergence in the exit region.

Figure 5: 21Z 500 mb Analysis. The shaded regions indicate higher wind speeds at 500 mb, with the light aqua color representing winds over 80 kts and the Curved Jet Streak. Divergence aloft should be anticipated downstream of such a feature. [Source: Storm Prediction Center]

300-mb Analysis

A quick look at the 12Z 300-mb analysis shows a shortwave trough over upper mountain west, with a broad area of divergence east of the trough. Also of note is the relatively light winds around 25-30 kts over Missouri.

Figure 6: 12Z 300 mb Analysis. The divergence downstream of the trough promotes upward motion from the surface and steepening of lapse rates. [Source: Storm Prediction Center]

By 21Z, though, the trough has deepened and moved eastward, increasing our upper level wind speeds and maintaining an area of upper level divergence.

Figure 7: 21Z 300-mb Analysis. Note the progression of the trough. Compare the wind speed and direction at 850-mb to those here. We should expect high bulk shear values. [Source: Storm Prediction Center]

Because the wind speed has significantly increased at a different vector than at 850-mb, we should also expect that the bulk shear values to be increasing. Bulk shear impacts the duration of storms by lessening the interference of downdrafts with rotating updrafts. The result is long-lived, rotating thunderstorms.

The conditions at 300-mb and 500-mb primed the area for deep convection, with upward motion and local cooling that eroded the CIN and enhanced the CAPE. Further, the location of the jet streak increased the velocity of winds at both levels, further enhancing the bulk shear.

MESOSCALE ANALYSIS

CAPE & CIN Analysis

Thus far, we have seen synoptic features that enhance CAPE, while eroding CIN. The chart below from the SPC shows the work of the synoptic scale, with a tongue of high CAPE values over the Midwest and CIN eroding eastward. At 21Z, notice that the CAPE at Kirksville is near 2000 J/kg, while the CIN is close to just 25 J/kg. Even with weak CIN still in place, convection requires that parcels get a mechanical boost to the LFC. The approaching strong cold front should have enough lift to get parcels to the LFC. Once there, the heightened CAPE suggests strong updrafts are in order.

Figure 8: 21Z SBCAPE & SBCIN Analysis. Note the tongue of high CAPE values consistent with the low-level jet stream. Also note the erosion of CIN from west to east. The synoptic features are at work priming the atmosphere for deep convection. [Source: Storm Prediction Center]

Moisture Convergence Analysis

Surface moisture convergence increases upward motion and contributes to thunderstorm development. Recall our low-level jet stream in place at 850-mb and its southerly flow. That rich, warm, moist air is converging with a dry, cold air mass along the cold front described in the surface analysis. The result is rising, warm, moist air that contributes to deep convection. The chart below shows areas of moisture convergence in red contours. Notice an area of moderate convergence over Northern Missouri and Iowa. Moisture convergence affects storm mode, and in this case, with organized but only moderate convergence, we should expect thunderstorms to develop linearly, with some discrete cells possible.

Figure 9: 21Z Moisture Convergence Analysis. Moisture convergence is organized along the frontal boundary. Compare this chart with the 850-mb analysis and it's clear how the low-level jet stream is driving this moisture convergence. [Source: Storm Prediction Center]

Radar Analysis

The regional reflectivity composite below shows the storms as they tracked across the state. While some of the storms formed as discrete cells, note how they formed linearly, which confirms the effect of moisture convergence.

Figure 10: 2155Z Composite Reflectivity. Notice how the storm mode reflects the moisture convergence. [Source: Storm Prediction Center]

The radar reflectivity image below from the KEAX site shows a more detailed view of this line of storms. The supercell that produced the tornadoes is identified on the image. However, due to the distance of the cell from the radar, the radar cannot inspect the lower levels of the storm and much of the classic storm shape is hidden.

Figure 11: 2148Z KEAX Reflectivity. A closer view shows the line of discrete cells impacting the area. Note the distance between the radar site (bottom left) and the tornadic thunderstorm (top right). [Source: National Weather Service]

In order to detect rotation, Doppler radar measures wind speed and direction of particles. In the velocity image below, particles moving toward the radar are negative velocities (green) and particles moving away from the radar are positive velocities (red). The close proximity of strong positive and negative velocities indicates a velocity couplet. To detect tornadoes, we look for couplets with gate-to-gate shear of 90 kts or greater. In the image below, we can clearly see a velocity couplet, indicating a mesocyclone, or rotating thunderstorm. But because the storm is beyond 100km, we cannot confirm a tornado or a Tornado Vortex Signature (TVS). In this case, the rotation was associated with the Kirksville tornado, but the radar was picking up rotation well above the surface, which is insufficient evidence alone.

Figure 12: 2148Z KEAX Storm Relative Velocity. A switch of modes shows a mesocyclone in the Kirksville thunderstorm, but a TVS cannot be verified at these distances. [Source: National Weather Service]

CONCLUSION

The tornadic supercell that caused three fatalities in Northern Missouri on May 13, 2009 had its roots in the upper air pattern in place at the time. Because the shortwave troughs at 300-mb and 500-mb caused divergence over the region, upward motion at the surface fueled a low level jetstream at 850-mb and caused local cooling and steepening of the lapse rates. Further, the strong westerly winds associated with the jet streaks contributed to the bulk shear, which enabled long track storms. The low-level jet stream helped to moisten and warm the low levels, while causing deep moisture convergence along the cold front. Ultimately, the cold front and moisture convergence trigger the storms in the environment set up by the synoptic features.

Despite the work of the low-level jet stream and upper air divergence to erode the CIN, some CIN remained over the region, capping convection without lift. The cold front with significant moisture convergence lifted parcels to the LFC, where strong updrafts fueled severe thunderstorms. Without the bulk shear, a product of the intense upper level winds, these storms would not have been self sustaining. Thus, even after the storms initiated, the upper level environment played a role in sustaining the storms.

INTRODUCTION

This post examines the role of a sea breeze boundary triggering deep moisture convection and the favorable conditions at both the synoptic and mesoscales for their development. To guide us through this, we'll examine a case from June 4, 2006, when the Storm Prediction Center (SPC) issued Mesoscale Discussion (MD) 1064 concerning thunderstorm development along sea breeze boundaries.

Image 1: Mesoscale Discussion #1064. Note the forecaster was primarily concerned with convection along the east coast of Florida. (SOURCE: Storm Prediction Center)

The SPC issues Mesoscale Discussions when conditions are favorable for severe weather development. These provide meteorological explanations for what is currently happening and anticipated to happen at the mesoscale. This particular MD was issued at 1727Z (1:27 PM EDT) on June 4, 2006 concerning the possibility of severe storms developing along the sea breeze boundary on the Florida Peninsula, producing hail and gusty winds.

SEA BREEZE CIRCULATIONS

Before examining the conditions at both the synoptic and mesoscales, we'll review the concepts of the sea breeze circulation in order to relate to the conditions on June 4^th. Sea breeze circulations are thermally-forced, resulting from differential heating of land and water. As the land warms faster than adjacent water, the local pressure drops over land relative to areas over water. The result is cooler air moving onshore to fill the lower pressure. This cool air at the surface forms a boundary with the warmer air over land, resulting in forced lift and potentially convection. The surface air flow is generally perpendicular to the coast, and convection usually occurs just inland. Depending upon the synoptic flow, though, the sea breeze front may penetrate deep inland with a tailwind, or not form at all with a headwind. Convection is usually enhanced when these boundaries intersect another boundary or meet offshore synoptic flow.

Image 2: An idealized cross section of the Florida Peninsula shows the Gulf coast (west) and the Atlantic coast (east). Note the diurnal advance of the colder surface air off both the Gulf and Atlantic that triggers convection along the sea breeze front. (SOURCE: William Smith)

SYNOPTIC SCALE ANALYSIS

In order to understand the conditions at the mesoscale, it is first important to understand synoptic scale conditions present at the time. We'll first review the upper air at 12Z on June 4^th, 2006.

500 MB Analysis

The first impression at 500 mb is the weak wind speeds, with measurements between 15-20 kts over South Florida. While there is a weak trough at 500 mb over the region, vorticity and divergence downstream of the trough is weak in this area. The importance of the weak gradient is that sea breeze circulations form more readily in this environment. The winds are uniformly WNW at 500mb, which impacts the sea breezes on both the Atlantic and Gulf sides of the peninsula.

Image 3: 500 mb Analysis. Note the uniformity of the winds aloft and the rather weak gradient. Conditions like these are favorable for sea breeze formation. (SOURCE: Storm Prediction Center)

Also notice the temperatures at 500 mb associated with this trough are relatively colder than areas in the western Gulf of Mexico at -9°C and -10°C. Falling temperatures at upper levels steepens the environmental lapse rate, which enhances CAPE. The SPC forecaster mentions this feature in the MD:

"VIA PERIPHERY OF EASTERN STATES UPPER TROUGH...TEMPERATURES ALOFT
HAVE COOLED SLIGHTLY ACROSS THE FL PENINSULA SINCE YESTERDAY...WITH
-9.0 C AT TAMPA BAY/-10.0 C AT MIAMI AT 500 MB PER 12Z OBSERVED
RAOBS"

850 MB Analysis

A quick look at 850 MB shows winds are generally W to WNW at 10 knots. Conditions are favorable for a sea breeze with weak winds aloft, but note the apparent lack of shear from 500 to 850 MB.

Image 4: 850 mb Analysis. Again, notice the similar pattern at 850 as at 500. General westerly winds and a weak gradient. (SOURCE: Storm Prediction Center)

Surface Analysis

Focusing your attention on Florida, you'll immediately notice a stationary front draped over the northern half of the peninsula. Consistent with conditions in the UA, there is a weak surface pressure gradient over Florida. Also at the surface, winds at 12Z are generally blowing offshore, with no synoptic-scale motion.

Image 5: Surface Map at 12Z June 4, 2006. The offshore flow suggests the presence of a nocturnal land breeze, with a favorable synoptic environment (weak gradient) in place. (SOURCE: Hydrometeorological Prediction Center)

Notice that at 18Z, little has changed with the surface pressure gradient. However, the surface winds have changed significantly reflecting the sea breeze.

Image 6: Surface Map at 00Z June 5th, 2006. The synoptic scale environment has remained generally consistent and favorable for sea breeze formation. (SOURCE: Hydrometeorological Prediction Center)

MESOSCALE ANALYSIS

Discussion In evaluating the mesoscale, we will look first at two major factors for enhanced convection: CAPE and Vertical Wind Shear. We'll then evaluate the forecast in the MD and discuss how and why the convection occurred.

ML CAPE

We evaluate the potential for strong updrafts by using CAPE, and in particular, we'll use the Mixed Layer CAPE (ML CAPE) in this analysis. The ML CAPE estimates CAPE by assuming a well-mixed layer in the lowest 100 mb. This is important, because on a sunny, warm June day on the Florida Peninsula, we can assume a well-mixed boundary layer through convective eddies.

Image 7: ML CAPE Analysis at 18Z on June 4th, 2006. ML CAPE values soared over Florida, while CIN remained low. The presence of CIN over the water is further indication of the differential heating between the land and adjacent water. (SOURCE: Storm Prediction Center)

At 18Z, you can see that much of the Peninsula had ML CAPE values in excess of 1000 J/kg, approaching 2000 J/kg. Also of note is the uniform lack of CIN over the land, but the presence of weak CIN over the water. If you recall the sea breeze model, cooler air resides over the water, suggesting a weak inversion of cold air at the surface. Thus, the high CAPE values and the relative lack of CIN is a favorable environment for strong updrafts to form.

Vertical Wind Shear

The MD was issued not because thunderstorms may form, but because they may produce hail and microbursts. In order for these storms to have the best chance to produce hail and microbursts, vertical wind shear is necessary to form rotating updrafts.

Image 8: Vertical Wind Shear at 18Z on June 4th, 2006. Very little shear is present over the region, consistent with the uniformity of wind vectors through the layers. However, a small area of wind shear along the east coast is enough to warrant concern for some rotating updrafts. This shear is the result of the sea breeze vector and the synoptic wind vector. (SOURCE: Storm Prediction Center)

The 18Z analysis of Bulk Shear shows relatively little shear over the peninsula, with weak values near 30 kts along the East Coast. If you recall the uniformity of the wind directions at 500 mb and 850 mb, this makes sense. The forecaster is generally concerned about the presence of this shear, albeit weak, combined with the large CAPE values, and a favorable synoptic environment for a sea breeze.

"MODERATE CAPE/MODEST LAPSE RATES IN PRESENCE OF WEAK
VERTICAL SHEAR MAY BE SUFFICIENT FOR ISOLATED/PULSE-TYPE
DOWNBURSTS/HAIL."

Atmospheric Soundings

Examining the sounding at Jacksonville at 00Z on June 5^th (evening of the 4^th), confirms several assumptions. The first is the presence of a sea breeze, which is well-depicted in the wind vectors at the low levels. Uniform westerly's dominate the layers above 850 MB, but easterly's are present at the lowest levels, indicative of a sea breeze circulation. The second is the weak shear present at 30 kts. This appears to be solely the work of the sea breeze. Also note the high ML CAPE over 2500 J/kg.

IMAGE 9: 00Z Sounding at JAX on June 5th, 2006. The sounding shows the large area of CAPE, the presence of the sea breeze below 850 mb and weak vertical wind shear. (SOURCE: Storm Prediction Center)

Storm Evolution

Below is a visible satellite image loop that shows the evolution of the sea breeze boundary, and the subsequent storms.

IMAGE 10: Visible Satellite from 14Z to 23Z on June 4th, 2006. Convection erupts linearly along the west coast of Florida and moves deep inland. Convection along the east coast is visible as well, although it remains more stationary. (SOURCE: Aviation Weather Center)

Note that the sea breeze front pushed well inland from the west coast and the storms developed mainly along the east coast. If you recall the mesoscale discussion, the forecaster predicted the convection would form generally along the east coast.

"GIVEN UNIFORM LOW TO MID LEVEL WESTERLY COMPONENT ACROSS MUCH OF THE PENINSULA...SEA BREEZE SHOULD LARGELY REMAIN NEAR THE EASTERN COAST."

IMAGE 11: Visible Satellite over Florida on June 4th, 2006 at 20Z. Note the sea breeze fronts pusing from west to east across the peninsula. Also visible is a sea breeze front near Jacksonville in NE Florida. Note the relative stable atmosphere behind these fronts. (SOURCE: Aviation Weather Center)

While the sea breeze developed along both coasts, the strong tail wind for the west coast component drove the convection much further inland. Further, the strong head wind for the east coast component enhanced the convergence along that boundary.

Image 12: An idealized cross-section of the Florida Peninsula showing the sea breeze front advances on both coasts. Note with the presence of the synoptic flow from the west, the western component moves deep inland. The eastern component is rather stationary, but with deeper convection. (SOURCE: William Smith)

CONCLUSION

The strong storms that developed along the Florida Peninsula on June 4^th, 2006 were indeed triggered by the sea breeze boundary, but that process was contingent upon a favorable synoptic environment. At the surface, the sea breeze was in full effect. Especially in late spring when water temperatures are still cool and land temperatures are rising, the sea breeze dynamics are in place. The rather weak pressure gradient aloft led to lighter winds and greater uniformity across the region. Further, the presence of the weak trough did introduce colder temperatures aloft, steepening the lapse rate, and thus, enhancing CAPE. Lastly, the weak-moderate synoptic flow west-east over Florida helped drive the western sea breeze component inland and enhanced convergence along the east coast. It also produced weak vertical wind shear as the westerly synoptic flow met the easterly mesoscale flow. The SPC forecaster recognized these features, and issued the MD as a result.

Destruction and Tragedy in Dixie: The April 27th Outbreak of 2011

I've just completed my second class at Penn State in their Weather Forecasting program. This class focused on severe weather and mesoscale forecasting. The main theme of the class was to associate the role of the synoptic scale with the mesoscale. For the class, we had to create blog entries on various subjects assigned to us by our instructor. I will post those that I made during the class, with some reflections and deeper insights for each. We had a fairly restrictive word count, so I had to aggressively edit my work.

This first entry was actually my last assignment. I chose to examine Tornado Watch #235 (PDS) issued on April 27th of this year. This was the WW issued across the areas hardest hit by the severe weather in the southeast. The Storm Prediction Center and the many NWS WFOs deserve much credit for the extremely accurate forecasts many days in advance of this storm system.

INTRODUCTION

On April 27^th, 2011 at 1845Z, the Storm Prediction Center (SPC) issued Tornado Watch #235 for much of Alabama and parts of Georgia, Mississippi and Tennessee until 03Z. The watch was labeled a Particularly Dangerous Situation (PDS), emphasizing the high confidence of the SPC forecasters for destructive tornadoes in the Tennessee River Valley. The severe weather event that transpired was of historic and unprecedented proportions.

Figure 1: Tornado Watch #235 at 1845Z on April 27th, 2011 (SOURCE: Storm Prediction Center).

This post analyzes the synoptic scale conditions in place that primed the region for this outbreak. We also review key mesoscale features that impacted the storm mode, longevity, and direction. To this end, we first review conditions at 500mb, 300mb, 850mb, and the surface. Then we review the ML CAPE, 0-6 km shear, and 0-1 km helicity. Because the watch was issued at 1845Z, much of the analysis begins at 18Z to mimic what the forecasters at the SPC were reviewing just prior to the issuance of the watch.

THE BIG PICTURE

Synoptic scale features set the stage for weather features at the mesoscale. This event is no different, as SPC forecasters were predicting this event in the experimental 6-day outlook the week prior using progs of synoptic features.

500 mb Analysis

The first analysis is at 500 mb, which at 18Z shows a deep trough across the middle portion of the country. This trough has a tight gradient of height lines nudging into the region, encouraging cooling aloft. The trough encourages divergence aloft downstream, which is directly over our area of concern.

Figure 2: 500mb Analysis at 18Z. Note the deep trough over the central US, with the area of concern downstream. This trough encouraged steepening lapse rates by cooling aloft. (SOURCE: Storm Prediction Center)

As the trough approaches, upper level pockets of cold air steepen the environmental lapse rates, thereby enhancing CAPE. This will be apparent as we analyze the mesoscale.

A loop of 500 mb shows the evolution of the trough through the forecast period, advancing through the area of concern.

Figure 3: 500 mb loop from 16Z to 05Z (SOURCE: Storm Prediction Center)

300 mb Analysis

At 300 mb, we can see a well-defined jet streak present over the southern plains and curving up slightly towards our area of concern. Jet streaks at 300 mb, especially curved ones, can prime areas in the right exit region for deep moist convection by amplifying divergence aloft. Note in the image below, that the right exit region is directly over the area of concern (dotted area).

Figure 4: 300 mb Analysis at 18Z. Note the curved nature of the jet streak and the right exit region over the area of concern. (SOURCE: Storm Prediction Center)

This jet streak certainly intensifies as the event unfolds. By 23Z, during the height of the outbreak, the jet streak is sharply curved and promoting upward motion with high levels of divergence.

Figure 5: 300 mb analysis at 23Z. Note the sharp curve and the divergence present downstream.

850 mb Analysis

At 850 mb we look for low level jet streams conveying warm, rich air inland from the Gulf of Mexico. At 18Z, a low level jet stream is well organized over the Southern Mississippi Valley, helping to warm and moisten the boundary layer. The timing also intersects with key diurnal heating periods. Winds are out of the SSW, which contrast starkly with the winds at 500 mb, which were out of the W and WSW.

Figure 6: 850 mb Analysis at 18Z. Note the conveyer belt of warm moist air from the Gulf, priming the environment for DMC. (SOURCE: Storm Prediction Center)

Surface Analysis

At the surface, we can see a low centered over Arkansas with a trailing cold front and a dry line along a pre-frontal trough. Initially, the dry line will provide the synoptic lifting over the region, although later the cold front over takes the dry line.

Figure 7: Surface Analysis at 18Z. Note the presence of the dry line along the pre-frontal trough. (SOURCE: Hydrometeorological Prediction Center)

Surface Temperatures & Dewpoints

At 18Z, notice the tongue of warm, moist air from the Gulf, with dewpoint temperatures over 70°F in our area of concern. This is a broad area that could support long-lasting convection, meaning storms could develop and potentially have a steady supply of warm, moist air. Also of note is the dew point gradient representing the dry line through Louisiana.

Figure 8: Surface Temperatures and Dewpoints at 18Z (SOURCE: Storm Prediction Center)

MESOSCALE ANALYSIS

Now that we have set the table with the synoptic scale, we can evaluate the features at the mesoscale. For this analysis, we review four elements: ML CAPE, 0-6 km shear, 0-1 km helicity, and storm motion as determined by the Rasmussen Technique.

100 mb CAPE & CIN

CAPE is Convective Available Potential Energy and measures the positive area between the level of free convection (LFC) and the equilibrium level (EL), which translates into energy measured in J/kg. What this essentially tells forecasters is the potential for strong updrafts if parcels reach the LFC. Mixed Layer CAPE, or 100 mb CAPE, is determine by using the mean potential temperature and the mean mixing ratio of the lowest 100 mb of the troposphere. This considers the effect of mixing eddies in the lowest levels that affect the LFC height. ML CIN, on the other hand, is convective inhibition, and measures the energy required to lift a parcel to the ML LFC. Generally, forecasters look for areas with high ML CAPE and low ML CIN to forecast strong updrafts and potentially severe weather.

The image below shows the ML CAPE and ML CIN at 18Z. Note the broad area of ML CAPE values >2000 J/kg and absence of ML CIN over our area of concern. Parcels of air would require minimal forcing to reach the LFC and when they did, they would erupt in strong updrafts to the EL.

Figure 9: ML CAPE/CIN Analysis at 18Z showing a broad area of high CAPE (>200 J/kg) and minimal CIN. This area is prime for strong updrafts. (SOURCE: Storm Prediction Center)

0-6 km Shear

Vertical wind shear is the change in direction and speed of wind with height. The 0-6 km analysis measures this change through the layer from 0 to 6 km AGL. For forecasters, wind shear helps define storm mode, longevity and evolution. Wind shear values in excess of 35 kts are capable of supporting supercells with rotating updrafts. Wind shear helps maintain discrete updraft and downdrafts so that they do not interfere and rob the storm of warm moist air. The forward flank downdraft is typically found NE of the updraft region, as the upper level winds carry the precipitation downstream. This allows the updraft to maintain a source of warm air.

Figure 10: 0-6 km Shear at 18Z. Shear values greater than 35 kts will support supercells. The area of concern has values over 60 kts and upwards of 80 kts. (SOURCE: Storm Prediction Center)

0-1km Helicity

Helicity is a combination of low level shear and storm relative winds. This measures how fast an updraft intakes streamwise vorticity at the lowest levels (measured in meters² seconds^-2), which in this case is the lowest 1000m. Forecasters look at values greater than 300m²s^-2 for an increased threat of tornadoes. You can see in the analysis below that at 18Z, values of helicity are well above 300m²s^-2 for much of the area of concern. Certainly, storms in this region had the potential to inject streamwise vorticity at exceptional rates.

Figure 11: 0-1km Storm Relative Helicity. This shows how fast storms inject streamwise vorticity, which forecasters use to predict supercells and tornadoes.

Determining Storm Motion: Rasmussen Technique

Using the Rasmussen Technique, storm motion was determined to be out of the WSW (260°) at 35 kts. This is a bit off from the motion the SPC forecasters listed in the WW, which was 250° at 40 kts. The calculations of the Rasmussen Technique and sounding from the Birmingham WFO at 18Z are shown below.

Figure 12: Rasmussen Technique used to calculate storm motion (direction and speed). This method was slightly off that listed by the SPC in the WW. (SOURCE: William Smith)

Figure 13: 18Z sounding from BMX shows a curved hodograph and high low level shear. (SOURCE: Storm Prediction Center)

Radar Reflectivity from BMX from 18Z - 23Z

The Birmingham radar shows the impressive tornadic supercells racing across the state. Note the key features of the echoes:

1. Well-defined hook echoes in the SW quadrant of the storm

2. Well-defined v-notches indicating strong updrafts that divert the upper level winds left and right downstream

3. Forward flank downdrafts in the NE region, well away from the updraft region

Figure 14: BMX Reflectivity from 18Z to 23Z shows well-defined supercell thunderstorms.

Figure 15: The regional composite reflectivity shows a broader view, and you can get a sense of the longevity of these supercells. The supercell that struck Tuscaloosa and Birmingham continued through Georgia and Tennessee before dissipating. (SOURCE: UCAR)

CONCLUSION

The outbreak of April 27^th, 2011 is one of historic proportions. As mentioned earlier, this event came as no surprise to forecasters, as it was included in every Convective Outlook for a week prior. Those forecasters were looking specifically at synoptic scale features that would prime the environment for this event. The deep trough over the middle of the country would increase divergence aloft, increase cold air aloft, and bring strong mid level winds that led to a highly sheared environment. An upper level jet at 300 mb curved and placed its right exit region directly over an area primed for convection. That convection was primed due to an active 850 mb low level jet stream that conveyed warm humid air into the Lower Mississippi Valley. At the surface, the large scale forcing from the cold front arrived at the height of the instability and easily forced parcels to the LFC. The highly sheared environment, a result of the contrasting winds at 500 mb and 850 mb, determined the storm mode and sustained long-track tornadic supercells.

As this post is written, damage assessments and field surveys are ongoing. This will likely be one of the worst modern day tornado outbreaks in US history. Over 300 people died, and thousands more were injured. Early field surveys indicate multiple EF-5 tornadoes, and many at EF-4 and 3. The image below shows the tornado tracks from the entire event. Overlooking the number of tornadoes, the most impressive feature is the length of unbroken paths.

Figure 16: Preliminary Tornado Paths from April 27th, 2011 (SOURCE: NWS)