An Atmospheric Bore from Oklahoma to Mississippi, April 30, 2010

   Philip Lutzak May 2010




  Atmospheric bores occur when the leading edge of one air mass intrudes into another of different density. This leading edge of denser air, the bore, forces the less dense air above it upward (the crest), but the force of gravity quickly pulls the dense air back down again (the trough), causing one or more waves to form ("gravity waves".) With enough moisture present, the rising air at the front of the bore-induced wave or waves produces a line or arc of clouds. The cloud line can be broken or continuous and the clouds within the line can be rough or smooth in appearance. 

  Among the most common causes of bores are cold fronts, surface troughs running out ahead of fronts, or outflow boundaries of rain-cooled air moving out ahead of a thunderstorm or thunderstorm cluster. Although these conditions produce surface to low level phenomena, bores can be present in any layer of the atmosphere up to jet stream level. They often stretch hundreds of kilometers in length but are usually quite shallow in depth, averaging 1-2 kilometers. Their intensity is mainly dependent upon the speed of the invading air current (gravity current) and the stability of the invaded air layer (gravity wave medium.) When the speed of the gravity current/bore is just slightly faster than the current it's invading, the waves produced are smooth and undular in appearance and the bore is called an undular bore. As the speed of the bore gets faster the waves become rough and the bore is called turbulent. Whether undular or turbulent, atmospheric bores often move  far out ahead of the feature that initiated it, and if they move into unstable air, their lifting force can produce further convection which in turn may lead to thunderstorms and severe weather. 

  In this report I'll analyze an atmospheric bore that formed in central Oklahoma and moved eastward on the morning of April 30, 2010. I'll describe the synoptic and mesoscale conditions at four specific locations that this wave passed through and show that it satisfied the conditions of an atmospheric bore. I'll also examine if it was undular or turbulent.

  Before it dissipated it showed the reasons why meteorologists watch them closely. All atmospheric bores, undular or otherwise, can trigger severe weather, as this one eventually did in central and northeastern Mississippi (Figure 1.)



Figure 1. Radar from 2010-04-30 0126 shows severe thunderstorms from central to northeastern Mississippi ignited by an intruding atmospheric bore. Courtesy College of DuPage.



  The 12Z surface chart in Figure 2a below shows a cold front stretching from Iowa in the north down into central Oklahoma, where a weak low and another front trailed southward into Texas. Figures 2a through 2d show that the low pressure and the section of the cold front in western OK initially moved quickly from western to central OK (Figure 2a and 2b) but then made little progress from 15Z through 21Z, while the section of the front to the north of there continued on more quickly. In cases like this we often see an initial "sideswipe" push to the southeast that dies out, but it is enough to send a wave of energy that continues onward independent of the initial pushing mechanism, the cold front. This wave of energy (bore) continued onward at a faster speed than the southern end of the front which had slowed down. In order to see this more clearly we can turn to the satellite and radar images. 


  Figure 2a. Surface analysis 2010-04-30 12Z. Courtesy HPC. Larger image.   Figure 2b. Surface analysis 2010 04-30 15Z. Courtesy HPC. Larger image.   Figure 2c. Surface analysis 2010-04-30 18Z. Courtesy HPC. Larger image.   Figure 2d. Surface analysis 2010-04-30 21Z. Courtesy HPC. Larger image.  



Satellite and Radar

  The visible satellite and radar images below in Figures 3 were chosen to correspond closely with the surface images above. I have also  annotated the position of the bore in the radar images (black line.) In the satellite image in Figure 3a we can first find cloud evidence of the bore in eastern Oklahoma at 13Z. Note the thin line of clear skies from southern Missouri and eastern Oklahoma into northeastern Texas. This line of clearing was due to subsiding air at the back edge of an atmospheric bore triggered by the cold front. In the Figure 3b satellite image we can see the bore denoted by the bright white line of clouds bisecting Arkansas from northeast to southwest. In Figure 4b the back edge of the bore is also easily visible. In the Figure 3c satellite image it is even easier to locate the bore, where an arc of bright white clouds stretches from northwestern Mississippi to northwestern Louisiana. At around the time of Figures 3d and 4d (23Z) the bore had gradually stopped moving due to a substantial decrease in push from the cold front and blocking from the large high to its east (Figure 2d.) Although a little hard to see due to the waning daylight, in Figure 3d we can see thunderstorms began erupting over central MS.

  Quite important to note in these images (especially visible in the radar) is how the distance between the bore and lagging cold front increases substantially over a few hours in time. Beside making it clear that the bore moved eastward much faster than the initiating cold front, it makes it much easier to separate the effects of the bore as opposed to those of the cold front. Note how the overcast and low clouds have been greatly reduced by the bore over all of Arkansas by 18Z (Figure 3c) and northwestern Mississippi by 23Z (Figure 3d), leaving only broken clouds. So the bore, by forcing the moist air forward but not very far upward, cleared out the low level stratus and allowed incoming solar radiation to heat and destabilize the lower atmosphere over Arkansas and Mississippi. This eventually allowed strong convection to develop.


Satellite Images - Courtesy NOAA SAT Archive.

  Figure 3a. Visible satellite image 2010-04-30 13Z. KMKO, Muskogee, OK is circled. Larger image.   Figure 3b. Visible satellite image 2010-04-30 15Z. KFSM, Fort Smith, AR is circled.  Larger image.   Figure 3c. Visible satellite image 2010-04-30 18Z. KLIT, Little Rock, AR is circled. Larger image.   Figure 3d. Visible satellite image 2010-04-30 23Z. KTUP, Tupelo, MS is circled. Larger image.  


Radar Images - Courtesy SPC Archive National Sector.

  Figure 4a. Radar reflectivity image 2010-04-30 13Z.   Figure 4b. Radar reflectivity image 2010-04-30 15Z.   Figure 4c. Radar reflectivity image 2010-04-30 18Z.   Figure 4d. Radar reflectivity image 2010-04-30 23Z.  



  Now we'll look at the data for the same four locations noted in the satellite images from Figure 3 above. We'll also look at the bore strength, which is the height of the inversion after being lifted (h1) divided by the inversion height before it got lifted (h0). This value must be between 1 and 2 for an undular bore. Values higher than 2 indicate a non-undular, or turbulent bore.


NOTE: All of the skewT diagrams used here, although state-of-the-art interpolations from NASA Larc, are approximations from RUC data rather than from real soundings.


Surface Indicators for Atmospheric Bores

1. Wind shift, almost always a veering, and usually from southeast to southwest, however slight. With undular bores we often see a more noticeable increase in wind speed and change in direction.

2. Temperature remains steady or increases. NO temperature decrease.

3. No change in dewpoint is a positive indicator. An increase or a decrease is neutral.


Upper Level Indicators for Atmospheric Bores

1. Increase in height of the stable layer (the inversion was lifted by the bore.)

2. Wind maximum in a thin layer above the surface. (Winds above and below this layer will be less.)

3. Wind max is always below the top of the inversion during and after the lifting.




  Muskogee, OK is the first location where the cloud and radar evidence indicated a bore passage. All of the signs we look for were there: Figure 5a shows a pressure rise at 09Z and a brief wind shift from southeast to south at 11-12Z with no appreciable temperature or dewpoint change. Figure 5b shows the inversion was lifted to 775mb and the wind maximum of 31m/s (nose of the bore) occurred at about 850mb, below the top of the inversion. From 06 to 09Z the inversion was lifted by the bore from 1630 meters to 1880 meters. 1880/1630 = 1.15. While this indicates an undular bore it indicates a very weak one.

Figure 5a. Meteogram for KMKO, Muskogee, OK to 2010-05-01 06Z. Pressure rise at 09Z indicates approximate time of bore passage. Note also a slight wind shift from the SE to S at 10Z. Courtesy University of Wyoming.   Figure 5b. SkewT KMKO 2010-04-30 09Z, same time as circle in Figure 5a. Note the southwest wind max of 31m/s at 850mb and the inversion layer elevated above it to about 775mb. (see the lift from 06-09Z.) Courtesy NASA Larc RUC plot.



 At location KFSM we have more evidence of the bore passage. Figure 6a shows a pressure rise at 11Z and a wind shift from south to southwest at 12Z with no appreciable temperature or dewpoint change. Figure 6b shows the inversion was lifted to 700mb and the wind maximum of 26m/s occurred at 800-850mb, below the top of the inversion. From 09 to 12Z the inversion was lifted due to passage of the bore from 1900 meters to 2430 meters. 2430/1900 = 1.27. This also indicates an undular bore.

Figure 6a. Meteogram for KFSM, Fort Smith, AR to 2010-05-01 06Z. Pressure rise at 11Z indicates approximate time of bore passage. Note also a slight wind shift from S to SW at 12Z. Courtesy University of Wyoming.   Figure 6b. SkewT KFSM 2010-04-30 12Z, one hour after circle in Figure 6a. Note the southwest wind max of 26m/s at 850-800mb and the inversion layer elevated above it at 700mb. (see the lift from 09-12Z.) Courtesy NASA Larc RUC plot.



  Little Rock, AR also shows solid evidence of a bore passage. Figure 7a shows a pressure rise at around 13Z with a wind shift from southeast to south-southwest at 14-15Z and no appreciable temperature or dewpoint change. Figure 7b shows the inversion was lifted to roughly 720mb and the wind maximum of 27m/s occurred just below it about 750mb. From 09 to 15Z the inversion was lifted by the bore passage from 1930 meters to 2470 meters. 2470/1930 = 1.28, indicating an undular bore.

Figure 7a. Meteogram for KLIT, Little Rock, AR to 2010-05-01 06Z. Pressure rise around 13Z indicates approximate time of bore passage. Note also a slight wind shift from SE to SSW at 15Z. Courtesy University of Wyoming.   Figure 7b. SkewT KLZK 2010-04-30 15Z, two hours after circle in Figure 7a. Note the southwest wind max of 27m/s at 750mb and the inversion layer elevated above it at roughly 725mb. (see the lift from 09Z-15Z.) Courtesy NASA Larc RUC plot.


  The above meteograms show a distinct pressure rise and a slight veering of the wind without a notable decrease in temperature or humidity at the first three locations. This is a classic surface signature of a bore. From the skewT diagrams we can see that the inversion was lifted, and there is a wind maximum from the same direction over a relatively thin layer of the atmosphere below the top of the layer that has been lifted. This is a classic upper air signature of a bore. Finally, from the calculations shown, the bore strength numbers, albeit weak, indicated an undular bore at all of these locations.





  Looking at this last location, Tupelo, MS, in northeastern Mississippi, it appears that as the bore came to a halt and weakened it still helped enhance thunderstorm development there. We need to look at the meteogram in Figure 8 at right and the sequence of skewT images below in Figure 9 to understand what happened. In the meteogram there are almost no surface clues of a bore arrival - there are some minor wind shifts at 21Z but there is no pressure rise that we would normally associate with a bore's presence at the surface.

  The bore had now slowed to an almost complete stop due to a loss of push from the now far lagging cold front and resistance from a strong high pressure cell to its east. Looking at the sequence of skewTs in Figure 9 below, we can see the inversion was lifted, presumably by the bore, between 15 and 17Z. Note also how immediately after this the lapse rates above the cap increased from 17Z to 01Z, when thunderstorms commenced; especially noticeable from 19Z to 01Z there is a rapid increase in the moisture content above 900m. The result was a quickly saturated air mass above the cap, with lapse rates that were clearly unstable. The end result was an outbreak of elevated thunderstorms in northeastern Mississippi.


For comparison, look at the development of the skewT diagrams for Greenville, MS, in the east central part of the state. Note how the inversion was lifted but the mid-levels remained very dry. Although both areas initially had a setup for hail and high winds due to dry air at mid-levels, the atmosphere at Tupelo eventually became so saturated that it didn't happen there. But there were areas between the Greenville and Tupelo areas that did get severe weather with hail and high winds.


Figure 8. Meteogram for KTUP, Tupelo, MS to 2010-05-01 06Z. There is no concrete surface evidence that the bore arrived or passed this location. However it is clear that thunderstorms developed just after 00Z. Courtesy University of Wyoming.


Figure 9. SkewT diagrams for KTUP, Tupelo, MS from 2010-04-30 15Z through 2010-05-01 at 02Z.  The inversion is still evident in these analyses, and after an initial lift from the bore it slowly weakened. What is more evident is that the air parcels above the bore have gradually become saturated. Elevated thunderstorms were occurring at Tupelo by 01Z. Larger version.





  The evidence indicates that this was a significant atmospheric bore, and it was weakly undular. Though the cloud evidence was not as impressive as in other undular bores I've studied, there was certainly visual evidence of a single leading wave at 13Z, followed by a set of waves at 15Z which were slightly undular in appearance. This fits with the low bore strength values of 1.15 to 1.28, i.e. it was a weakly undular bore.

  As is common with many atmospheric bores, this one was initiated by a fairly strong, uniform southeastward push from a cold front. But even as the cold front slowed down considerably, the bore, because of its initial momentum, continued forward at a faster clip than the feature that initiated it. Due to its momentum, it continued on at a fairly fast clip, not slowing down until it entered eastern and central Mississippi, where in addition to losing momentum it experienced blocking to the east. For most of its journey this particular bore pushed the stable layer of very moist air ahead of it forward, but not very far upward, because the air layer it moved through was a stable inversion, and by definition bores duct their energy forward through a stable layer with very little of its energy getting lost in upward motion. (It is the layer of conditionally unstable layer above the inversion that gets lifted into a thin cloud line above it that shows us the bore's cloud signature.) But as the bore slowed down to a stop over central Mississippi, it appears that, due to blocking to the east, it no longer could lift the stable layer, and its remaining momentum was translated upward. The evidence bears this out: it appears the humid air it contained was forced upward above the stable inversion that existed below 900-950mb, causing numerous thunderstorms over northeastern Mississippi, some of which were severe.

  Although an indirect cause, but in some ways more significant, the bore appears to have cleared out the lower level stratus and fog over Arkansas, allowing considerable incoming solar radiation and a large increase in CAPE. The resulting thunderstorms triggered by the ensuing cold front were catastrophic, resulting in deadly, tornadic thunderstorms that resulted in considerable deaths and damage over Arkansas.