Houston faces a significant rain threat later today and during the overnight hours, and the recent forecast model trends are concerning. I think there are two main threats we must consider: the potential for severe weather, and, separately, very heavy rains.
Much of the Houston region falls into the “slight” risk of severe weather for tonight from NOAA’s Storm Prediction Center. I believe this may undersell the risk a little bit.
One of the variables forecasters look at when trying to gauge the possibility of severe weather is helicity, which is essentially a measurement of rotation in the upward drafts of a storm system. The greater this value, the higher the threat of tornadoes and damaging winds.
For example, the latest run of the NAM model shows some very high helicity values over much of the northern and northeastern part of the Houston metro area for late tonight. Note that values above 400 allow for the possibility of tornado formation.
For this reason I think we need be at least moderately concerned about the threats of hail, damaging winds and tornadoes from about 4 or 5pm CT today through Saturday morning. The threat is probably highest for northern parts of the metro area, running up to Huntsville. The threat of high winds is doubly concerning given the already wet soils, which will make it easier for any strong winds to knock down trees with shallow roots of power poles.
The other primary threat, of course, is heavy rain. The forecast models remain all over the place, but some of the better mesoscale models are showing the potential for very heavy rainfall tonight.
With ample moisture it is possible we will see rainfall rates in excess of 2 inches per hour, and this could lead to high rainfall totals where storms stall. Matt and I have discussed this, and believe it’s reasonable to expect that some parts of the greater Houston metro region may see rain bullseyes of 5 to 10 inches between later this afternoon and Saturday morning. If this occurs over the wrong part of town, such as western or northwestern Harris County, we could see significant flooding once again.
I am not guaranteeing this. There’s too much uncertainty for that. But we can confidently say there is the potential for a major rainstorm across the Houston area during the next 24 hours, with widespread rain accumulations of 2 to 5 inches, and more locally. Perhaps quite a bit more.
Stay tuned to us here, as we’ll keep you as updated as possible.
Posted at 2:25pm CT Friday