Microburst Explainer.

Another day with spotty showers and storms, some of which have produced locally torrential rainfall while nearby locations have received very little.  Also, additional microbursts associated with those storms have produced localized strong and gusty winds but so far nothing severe.  Since we have received a number of questions recently regarding why there have been no warnings for those microbursts when they do cause damage, thought perhaps a brief explainer may be helpful. 

Thunderstorms are comprised of updrafts and downdrafts with the updrafts dominating during the formation and intensifying stage of development and the downdrafts dominating as the storm collapses.  Here is a chart  showing the storm during the downdraft stage in which the heavy precipitation loading within the storm has caused a strong downdraft to form.  Keep in mind that radar is a very useful tool but it does have its limitations.  One of which is that the winds we can detect with the Doppler feature are only detectable when the winds are blowing towards or away from the radar.  Therefore the winds in the downdraft will not be detectable by the radar beam as those winds are blowing straight down and not toward or away from the radar.  However, once that downdraft hits the ground, then the winds are forced to spread out horizontally and will then be detectable but that is after the fact and therefore little or no warning can be given in most instances.

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This photo shows that strong downdraft approaching the ground and basically looks like a huge raindrop about to strike the earth.  That is basically what is happening with what we refer to as a wet microburst which is what we have been dealing with lately.

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This next photo is of a wet microburst that occurred in Tulsa a number of years ago in which the heavy rains and strong downdraft winds have hit the surface and are now spreading out horizontally.  Notice at the leading edge of the rain column, the moisture is curling upward similar to Aladdin’s foot.  Basically, it is similar to what happens when you point a water hose at the ground and turn on the water full force.

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These next images show the Doppler velocities of the microburst that occurred in BA yesterday.  Keep in mind the position of the radar is just NW of Inola and is evident by the circular absence of any velocities adjacent to the radar site itself.  The first image shows no velocity of any consequence near BA, but the second one from just a few minutes later shows a strong downburst signature with a component toward the radar in green and away from the radar in red.  Notice how small it is.  That is not uncommon as these downbursts will often be very localized.  Keep in mind, these are straight line winds and are not tornadic but can still produce significant damage as occurred in Claremore last week.

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Granted, the above is very brief and simplified but hopefully helps to explain why these summertime storms can still turn out to be trouble makers, occasionally producing wind damage on a par with a weak tornado, yet at the same time can be more difficult to provide much, if any, warning.

The storms we had this afternoon/evening will also weaken and fall apart as we go through the night tonight.  However, a more significant system still looks to be impacting our weather for Wednesday with a better chance of showers and storms, some of which will be locally quite strong.  A stronger cool front will be pushing through the state during the day helping to set off those showers/storms.  It will also be followed by brisk northerly winds for Wed afternoon and through the day Thursday.  Those northerly winds will bring in milder and drier air giving us a nice, but brief, break in the heat and humidity of recent days.

As you can see on our forecast page, after tonight our overnight lows will be dropping into the 60s followed by daytime highs in the 80s.  Also, after Wednesday, our chances of additional showers/storms will be in the slim to none category.  However, a return to brisk southerly winds over the weekend will heat things back up and also return higher humidity levels once again.

That will continue to be the case going deeper into September as the 8-14 day outlook continues to suggest warmer than normal temperatures along with scattered showers/storms.  In spite of the trend to above normal temperatures over that time frame, triple digit air temperatures are still not foreseen although cannot rule out some heat index values approaching triple digits over the next two weeks.

So, stay tuned and check back for updates.

Dick Faurot