Month: April 2016

I wanted to take a few minutes this morning to explain what happened with our forecasts for potentially very heavy rain on Friday night and Saturday, why we made the decisions we did, and then talk about what’s ahead for Houston in what will remain a wet pattern for a few more days.

FRIDAY’S MISS

For a couple of days Matt and I have been talking about the potential for very heavy rain in Houston this weekend, especially Friday night and Saturday morning. We weren’t alone, of course. The National Weather Service issued a flash flood watch. A lot of people were concerned. But I can only speak for our forecasts.

By early Friday afternoon we were pretty concerned, and wrote so here. Several high resolution forecast models were showing the potential for very heavy rains and severe storms later that evening and during the overnight hours.

These model forecasts were predicated on a capping inversion, an area of warmer air above the surface that prevents surface moisture from rising, breaking down. This didn’t seem like an unreasonable expectation, but unfortunately we don’t have good, timely data on the “cap.” The best information we have comes from “soundings,” essentially weather balloons sent into the upper atmosphere twice a day. But there are no Houston soundings. For Houston, the closest sounding locations are in Lake Charles, La. and Corpus Christi. A 7pm CT sounding from Lake Charles showed there was a stronger cap in place than the models were predicting.

Lake Charles forecast sounding at 7pm Friday evening. The stronger than expected cap is shown. (NWS)

Lake Charles forecast sounding at 7pm Friday evening. The stronger than expected cap is shown. (NWS)

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A steady line of storms has continued to make a westward progression toward Houston, bringing the threat of damaging winds and possibly an isolated tornado.

As the southern half of this storm system has weakened, the primary threat will remain largely to the north of Interstate 10, where a severe thunderstorm warning is in effect.

Area of severe thunderstorm warning as of 7:45am CT. (National Weather Service)

Area of severe thunderstorm warning as of 7:45am CT. (National Weather Service)

 

While these storms will unquestionably pack a punch as they move through, they are, in fact, continuing to move along. This will keep additional rainfall amounts this morning to 1-2 inches north of Interstate 10, and considerably less than that to the south. Also, with the rapid eastward movement, most of the Houston metro area will be in the clear by or before 10am CT this morning, leading to fairly benign conditions for most of the rest of the day. Further to the east, areas in Beaumont will see storm activity persist for a few hours more.

Posted at 7:50am CT Saturday

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 entire Houston metro area is under a severe thunderstorm watch through 11am today. (National Weather Service)

The Houston metro area is under a severe thunderstorm watch through 11am. (National Weather Service)

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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)

Radar as of 7:15pm CT on Friday night. Storms have so far developed north of Houston. (Intellicast)

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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)

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)

NAM model helicity forecast for 2am CT Saturday morning. (Weather Bell)

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