Tuesday, July 26, 2011

Heat Burst

From the NWS in Indianapolis 7-5-2011

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.

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.

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