Documenting and explaining a heat burst
And an active Sunday from the Southern Plains into the Southeast
Good morning! The atmosphere provided an example this morning of what I personally find to be one of the more unusual phenomena it can produce very near me, so I want to take the opportunity to highlight it.
The above graphic is a meteogram from the Oklahoma Mesonet site at Chickasha, southwest of Oklahoma City. A meteogram is essentially a time series plot of meteorological parameters, in this case starting with temperature and dewpoint in the top graphic, then in descending order, windspeed and direction, pressure, rainfall, and solar radiation. I have highlighted the period between 4 and 6 am because something very unusual happens in that time frame.
You can see in the top graph that just after 4 am the temperature starts to dramatically increase while the dewpoint begins to dramatically decrease. By 5:15 am, the temperature peaks at 96F - an increase of 20 degrees - while the dewpoint bottoms out at 43F, yielding a relative humidity of 17%. Meanwhile, this is accompanied by strong winds as there is a peak wind gust of 59 mph from the southwest at 4:35 am, and winds remain gusty through the peak in temperature. The atmospheric pressure also falls significantly during this time, dropping about 5 millibars (about .15” of mercury).
Clearly, something very unusual is happening here. The temperature is not “supposed” to rise to 96F in the predawn hours of late May. To give you some idea of just how unusual this is, while I don’t have the historical data for the Chickasha mesonet site, looking at the climate history for the Chickasha Experimental Station, 96F would be the 4th warmest temperature ever recorded on May 25th - and it happened at 5:15 am, along with very low relative humidity, also unusual for the early morning hours.
We can also see that it was a localized phenomenon by looking at the maximum air temperatures so far today across the Mesonet. While a couple of nearby sites did also rise in the upper 80s, the 96F at Chickasha sticks out - to the point that it almost looks like bad data, but it is not.
A radar loop from the time does not look particularly impressive - but it does explain what was the cause of what transpired at Chickasha. A complex of thunderstorms that had started out in northwest Texas was moving up toward central Oklahoma and rapidly diminishing. Between 4:30 and 5:30 am the last remnants of radar reflectivity can be seen moving across the area around Chickasha.
What occurred in the area around Chickasha was a heat burst. The National Weather Service in Albuquerque has an excellent webpage about heat bursts that provides a great explanation of what causes them. Essentially, if a thunderstorm is dissipating in an atmosphere with dry air in mid levels of the atmosphere, rain falling into that dry air will evaporate, initially causing the air to cool which also causes it to start descending. As it descends, it continues to dry and as it continues to descend, it also starts to warm due to what meteorologists call adiabatic processes. This warming and drying process continues until the air reaches the surface as a heat burst which is what was observed at Chickasha at the time the last signs of the thunderstorm complex dissipate nearby on radar.
As the NWS Albuquerque webpage notes, heat bursts occur most commonly in the Plains regions in the late spring and summer months, and while they are certainly not commonplace they do happen from time to time. This example at Chickasha is one of the better ones I have seen, and given how well it was captured by the Mesonet and radar data, I wanted to share it here.
Turning our attention to today’s weather, another relatively active severe weather day is anticipated today after yesterday’s severe weather in the Plains and lower Mississippi Valley regions. The atmosphere is expected to become very unstable again across the Southern Plains to the south of a slow moving front, and a few complexes of severe storms are expected to evolve and move slowly east and southeast this afternoon into tonight. The most intense activity is expected across northwest Texas into Oklahoma, where storms capable of very large hail, wind gusts greater than 75 mph and tornadoes are possible and SPC has an enhanced (level 3 of 5) of severe weather in place. Severe storms capable of large hail and damaging winds will also be possible farther east into the Mid-South and Tennessee Valley regions where a slight risk (level 2 of 5) is forecast.
Flash flooding will also be an increasing risk today. Several swaths of heavy rain up to 5” fell in the last 24 hours across this region, and with multiple rounds of storms producing very heavy rain over the next 24 hours on already wet ground, some significant incidences of flash flooding are possible into tonight. The Weather Prediction Center has a moderate (level 3 of 4) risk of flash flooding for much of central and eastern Oklahoma, with a large slight (level 2 of 4) risk surrounding that.
The front will continue to push slowly south on Memorial Day along with the risk for severe storms and flash flooding.
North of the front, much below normal daytime temperatures are expected across much of the central US into the Mid-Atlantic, resulting an unusually cool Memorial Day for these areas.