A rare meteorological phenomenon known as a “heat burst” struck the town of Julesburg in the far northeast Corner of Colorado last night, sending temperatures soaring from 78F to 97F in only 40 minutes shortly before 2:00am this morning. This type of event is uncommon but does happen on the Great Plains occasionally in the late spring or early summer, and typically happens in the middle of the night once temperatures have already initially cooled from afternoon highs.
The graph below shows the temperature graph over the previous 24 hours (ending at noon today, June 28). Notice the rapid spike that occurred in the middle of the night. Amazingly, the high temperature in Julesburg “only” peaked in the low 90s on the afternoon of the 27th, so the temperature of 97 that occurred at 1:42am was the 24-hour high temperature for the period ending at 7am this morning.
So what caused this bizarre weather event to occur? Essentially, heat bursts involve dying nocturnal high-based thunderstorms with a deep layer of extremely dry air at the low and mid levels of the atmosphere as well as very warm air aloft. Precipitation from these higher-based storms that are dissipating has a longer way to travel before reaching the ground than usual, and so it evaporates long before reaching the surface. When air evaporates, it becomes cooler and denser, which causes it to accelerate toward the surface – this is how downbursts occur, as the accelerating air means strong outflow winds once this air reaches the surface.
However, when what we think of as a typical downburst occurs, the air is still in its evaporative cooling phase, and so as a result you normally get strong winds along with much cooler temperatures. However, in the case of a heat burst, the dry layer is so deep that all of the water evaporates before reaching the surface. At this point, the air begins to warm due to compression – similar to how dry, sinking air in downslope Chinook winds off the Front Range lead to rapid warming near the base of the foothills.
By the time this warmer, drier, descending air reaches the surface, temperatures can quickly rise 15-20 degrees in a short amount of time. The other impact is that strong wind gusts occur (similar to what you would expect in a downburst or strong outflow winds), and dewpoints and relative humidity values plummet. In the case of last night’s storms – these storms formed in a drier atmosphere whereas low level moisture had increased farther east. Prior to the heat burst, the dewpoint in Julesburg was a sultry 67, before rapidly falling to 31 as the temperature rose from 78 to 97. The relative humidity obviously experienced a similar plummet, falling from 69% to 10% in a matter of 40 minutes.
The chart below illustrates when the heat burst occurred, as you can clearly see the upward and downward spikes where temperature rose and moisture dropped. Also, notice the spike in wind speeds with a gust of 49mph occurring as the temperature was rising.
And finally, the reason this was more likely to occur in the middle of the night was because temperatures had already cooled significantly. If this happened during the daytime hours, you wouldn’t have much of a temperature increase because daytime temperatures would already be hot.