As Ron Burgundy would say, “Boy, that escalated quickly.” Things got crazy last night in the skies over Houston catching many people off guard, especially if they did not watch the 10 pm news. Most people either woke up this morning and had a “wait what happened?” moment or they woke up in the middle of the night to the now all too familiar roar of heavy rain. So what happened?
The origin of last night’s rain goes back to the storm system that brought us heavy rain early yesterday afternoon. That storm system created a small area of low pressure called a Mesoscale Convective Vortex, or MCV, which hung around even after the storms died off yesterday afternoon. This little spin in the atmosphere started showing up fairly distinctly on radar yesterday evening as it sat over the College Station area.
By mid evening the local weather community (#wxtwitter as it is affectionately called) and emergency management began buzzing about this MCV and its potential impacts overnight. The worry was that this low pressure area would drift closer to the Houston area – where the atmosphere was already juiced up – and begin drawing even more moisture in off of the Gulf. By 10 or 11 o’clock last night it became rather apparent that his how things were going to play out. The low provided the lifting mechanism or trigger to turn all that moisture into rain. At times the system took on the appearance of a tropical system, albeit without the wind, with bands of rain rotating around a center of circulation. This is a long loop of the radar last night into this morning which shows how things developed around this MCV.
You can clearly see how the low begins drawing moisture and storms inland off the Gulf as it rotates towards the east. Eventually the overnight storms worked over the atmosphere enough that the rain began to die out by sunrise, but only after dumping between 2 and 7 inches over the area. However, at the end of this loop you can still clearly see the center of this circulation is still sitting just north of Houston. You can also see it spinning pretty clearly in the below satellite image from this afternoon. Because this low is still hanging around we will continue to have a chance for rain for the rest of the day and probably into tonight.
So was the forecast for last night a huge miss? Yes and no. For the past couple of days a possibility of rain was in the forecast for last night, however the extent to which it rained was not really predicted until a few hours out. The overall atmospheric pattern was conducive for rain, but small scale features like this MCV, which can be very efficient rain makers, are notoriously difficult to forecast more than a few hours out. Because they are so small and often arise out of storms preceding them they are not picked up very well by longer range computer models which predict weather on a much larger scale. Although there may not have not been much warning about this event the truth is old fashioned human forecasting is the main reason the potential for flooding made it into the 10 pm weather segment before many went to bed. Forecasters saw the ingredients coming together early in the evening for a potential flooding event and did their best to get the word out before most people went to bed.
Follow me on twitter @stephenuzick