Heat Burst Occurs in the Indianapolis Area

In addition to the impressive natural fireworks show, gusty winds, small hail, and very heavy rainfall from a strong thunderstorm complex across central Indiana, an interesting and fairly rare atmospheric phenomenon occurred in the Indianapolis area during the very early morning hours of July 3 - a heat burst.

Heat bursts are not completely understood, partially owing to the fact that their small spatial scales and short duration make observing them difficult.  However, several researchers over the past few decades have offered hypotheses based on observational studies of heat bursts occurring in areas with an established mesonetwork (small scale network of observation sites).

In general, these studies indicate that heat bursts are the result of dry downbursts (strong, small scale downdrafts from thunderstorms) occurring from decaying nocturnal thunderstorms, perhaps with a contribution from a descending rear inflow jet into the decaying storms.  Typically, these downbursts originate from fairly high in an atmosphere with significant dry air aloft.  These downbursts descend quickly, with the air drying and warming as moisture in the air evaporates and increasing pressure compresses and warms the air.  As with the downbursts that typically cause damaging winds in a severe thunderstorm, this downburst strikes the ground and spreads out horizontally, resulting in a rapid increase in wind speed and gusts.  In addition to increasing temperature, decreasing dewpoint, and increasing winds, rapid pressure drops are often observed as well.

Some heat bursts can be particularly severe, with damaging winds of 80-100 mph, and temperature increases to above 100 degrees occurring in the middle of the night.  A recent case in Wichita, KS resulted in temperatures rising to 102 degrees with damaging wind gusts.  A handful of historical cases have occurred where temperatures associated with heat bursts have briefly approached 130+ degrees.

Fortunately, the heat burst that occurred in the Indianapolis area on July 3 was not nearly this severe.  Observations around 1:30 AM EDT in the area indicate the temperature rose and the dewpoint dropped nearly 15 degrees in a less than an hour, causing the relative humidity to drop nearly 40-50%.  Winds increased rapidly, with gusts to near 50 mph.  One NWS Indianapolis employee reported that his neighbor's patio furniture ended up in his backyard.  It is likely that at least a few small limbs were downed by these winds as well.

The observation site at Eagle Creek Airpark (KEYE) best observed the temperature, dewpoint, and pressure changes.  The site at Indianapolis International Airport (KIND) observed the strongest wind gusts associated with the heat burst.  Here are some observational data from various platforms.

 

  • Radar reflectivity image from 0300 UTC (11:00 PM EDT).  Note the initial surge of outflow (thin blue line marked with white line) from the thunderstorms arriving at Indianapolis.

KIND 03Z RADAR

 

  • Radar velocity image from 0600 UTC (2:00 AM EDT).  Note the area of higher velocities just a few hundred feet above ground level (AGL) in Marion county.

KIND 06Z RADAR

 

  • Temperature and dewpoint meteogram for KEYE.  Notice the rapid changes between 0500 UTC (1:00 AM EDT) and 0600 UTC (2:00 AM EDT).  The temperature rose from 72 to 86 degrees, and the dewpoint dropped from 63 to 50 degrees.

Temp Dewpoint Trace KEYE

 

  • Relative humidity meteogram for KEYE.  Note the sharp temporary drop in relative humidity associated with the rapidly increasing temperature/dewpoint spread. 

RH Trace KEYE

 

  • Wind meteogram for KEYE.  Winds gusted to 30 mph at KEYE.  Wind gusts at KIND were as high as 48 mph!

Wind Trace KEYE

 

  • Pressure meteogram for KEYE.  The steady drop during the afternoon was associated with an approaching weak cold front.  The initial rise between 0100 and 0300 UTC was associated with the arrival of an outflow boundary surging out from the storms as they were over north central Indiana.  The sharp drop thereafter occurred with the heat burst

Pressure Trace KEYE

 

  • Local gridded observations for central Indiana from 0500 to 0700 UTC (1:00 AM EDT to 3:00 AM EDT).  Note bullseyes of warmer temperatures over Hendricks and Madison counties, in addition to Marion county.  These suggest that smaller, less intense heat bursts may have occurred nearby as well.

Gridded Obs 05Z

Gridded Obs 06Z

Gridded Obs 07Z

 

  • Local model sounding near KEYE at 0200 UTC (9:00 PM EDT).  Note ample dry air aloft above hot and humid conditions at the surface.  The solid green lines indicate how temperature and dewpoint change with height.  The dashed green line is how the conditions of a parcel of air lifted from the surface would be in relation to the environment.  When the the dashed green line is left of the solid green line, the parcel is stable (cooler than the environment) and downward motion is preferred.  When the dashed green line is right of the solid green line, the parcel is unstable (warmer than the environment) and upward motion is preferred.

EYE LAPS Sounding 02Z

 

  • The same sounding now at 0500 UTC (1:00 AM EDT).  Note the atmosphere has moistened significantly due to the thunderstorms, but that a significant amount of dry air remains in the lower levels, and the sounding is stable below about 600 millibars, or about 15,000 feet.  This explains both why the thunderstorms were dissipating over Indianapolis, and how descending air from the decaying thunderstorms was able to gather so much momentum.

EYE LAPS Sounding 05Z

 

It is always good to remember that even weak and decaying thunderstorms pose a threat, and that no place outside is safe during a thunderstorm.  While thunderstorms can be very interesting, especially when an unusual event occurs, it is best to observe them from inside a safe shelter a safe distance from windows.



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