Good morning. The very heavy rain stayed well to the north of Houston during the overnight hours, with 1 to 10 inches falling across east Texas, generally increasing from Conroe north through Shreveport (there’s a flash flood warning in effect for parts of San Jacinto County, East Central Montgomery County and Polk County). Areas in East Texas such as Henderson and Marshall were hit especially hard.
So far a capping inversion has held over Houston, to keep most of the metro area high and dry. However there’s one more round of heavy storms off to the west of Houston that will likely break this cap as it moves through later this morning. As a result the entire region is under a severe thunderstorm watch through 11 a.m.as this system moves toward Houston.
The Houston metro area is under a severe thunderstorm watch through 11am. (National Weather Service)
The concern about the potential for heavy rains and storms this evening appears to have been overblown as storms have remained well to the north of the Houston area. Although moisture levels are high, there just hasn’t been enough of an impetus to spur shower and storm development over southeast Texas.
TONIGHT
Given the quiet radar trends I think we can have some confidence that we won’t see the development of widespread rain showers over the Houston region through at least about midnight. That doesn’t mean we won’t see some isolated storms, including the potential for severe weather such as a tornado or two, but I don’t see the very heavy, widespread rain we were concerned about earlier. This is very good news because it cuts down on the potential for extremely high rainfall totals during this event.
Radar as of 7:15pm CT on Friday night. Storms have so far developed north of Houston. (Intellicast)
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.
SEVERE STORMS
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.
Severe weather outlook for Friday and Friday night. (NOAA)
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.
NAM model helicity forecast for 2am CT Saturday morning. (Weather Bell)
It looks like it’s going to be another one of “those” weekends here in Southeast Texas. We’ll have rain, storms, and potentially flooding to contend with, and all the while, we have a good deal of uncertainty still in the forecast. Here’s a short breakdown. I’ll follow it with an FAQ type section at bottom.
First, just a reminder: A Flash Flood Watch is posted for Houston and primarily the counties north and west of the city starting later today and going into Sunday morning.
TODAY
I don’t foresee a lot of issues this morning…if any at all. By mid-afternoon, storms should have started in Central Texas, along I-35 and points east. These will organize and spread east toward our area by late afternoon or evening. Storms this afternoon and evening will have the potential to be strong to severe. The main threats will be hail and damaging wind, and given how saturated the ground is, any sort of strong winds can be more problematic than usual. The tornado threat is low, though it isn’t zero (and as we sadly saw in Tomball the other morning, it doesn’t need to be a high risk or a strong tornado to be tragic, so please heed any warnings if issued). Obviously, our main focus will be flooding.
Simulated forecast radar for today. Storms fire along I-35 this afternoon & move toward Houston this evening. (Weather Bell)
TONIGHT
This is where confidence drops and our risks for flooding begin to go up. I expect we will see repeated rounds of showers and thunderstorms around the region. It’s still a bit difficult to say exactly who will see the most persistent storms, but anywhere in the area around Houston (north, west, or even south of the city) stands the potential to see this happen. It’s simply too tough to say where with any specificity. We will update you on this again later today. But the Friday night timeframe is the one we’re most concerned with, namely because we’ll have a substantial amount of atmospheric moisture to work with.
GFS Precipitable Water (PWAT) forecast this evening shows summer-like levels of available moisture. (Weather Bell)
Usually once you get over 2″, you’re talking considerable moisture, which means that any thunderstorms can produce torrential rain. The concern is that a boundary of storms could sit over one area and tap into this moisture. Hopefully this isn’t what occurs, but it’s something we have to watch.
