Category: Weather whys

Why are thunderstorms so common during spring months?

Posted by Braniff Davis at 11:59 AM

As most Southerners know, the arrival of spring also means the arrival of severe weather season. On Wednesday morning, many Houstonians heard the first rumblings of serious thunder for 2018, and if history tells us anything, this is just the beginning of a potential spring full of thunderstorms, flooding, lightning and tornadoes. What is it about the springtime that initiates this activity?

Hot, Hot Heat

Thunderstorm formation depends on atmospheric stability. We’ll forego talk about lapse rates and latent heat for now, and describe stability in simple terms. During the wintertime, our atmosphere is generally very stable. This means that conditions in the air prohibit any vertical motion. For thunderstorms to develop, they need warm air to rise from the surface, then to cool and condense into a tall, vertical column of clouds. In a stable atmosphere, this is nearly impossible.

As winter turns to spring, two things occur that make the atmosphere much more unstable. First, the air around us, especially if that air comes in from the Gulf, becomes much warmer and much more humid. Yesterday, February 20th, Bush Intercontinental Airport recorded a high of 80°F, with relative humidity values staying above 70%. This warm, moist air mass created a perfect environment of instability around us.

This diagram shows how air, heated at the surface, rises in the atmosphere until it condenses into a cloud. (Ahrens)

Second, as days become longer and the sun rises higher in the sky, the sun heats the ground, which, in turn, heats the air directly above the ground. This warm air will eventually rise in an unstable atmosphere, pushing cold air out of the way, as the diagram above shows. As it goes up, the air begins to cool, and as it cools, the water vapor in the air will condense–into clouds! If the atmosphere remains unstable, and these pockets of warm, condensing air rise rapidly, we have the recipe for a thunderstorm.

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La Niña and Houston’s Winter Weather Outlook

Posted by Braniff Davis at 9:36 AM

Now that “Fall Day” has ushered in autumn, and we hopefully won’t see 90 degrees again until 2018, the time has come to look ahead to winter. Specifically, we want to look at the impact a potential La Niña could have on Texas. The Climate Prediction Center recently issued a La Niña watch, giving the Northern Hemisphere a 55 to 65 percent chance of experiencing cooler than average waters in the eastern Pacific Ocean. Let’s take a step back however, and discuss what La Niña is, and how it could influence our weather in the coming months.

La Niña (and El Niño)

El Niño and La Niña are the warming and cooling phases of water in the eastern Pacific Ocean, off the coast of equatorial South America. El Niño occurs when the water is warmer than usual, La Niña when the water is cooler than usual. The changes aren’t radical—just 0.5-4°C either way. However, that type of change, over such a wide swath of ocean, has a massive impact on the global climate. It’s a classic example of the butterfly effect. A seemingly small change in another part of the world results in big impacts everywhere else.

As of the last update, surface ocean temperatures off the coast of Peru are between 0.5°C and 3°C below normal. The longer this pattern persists, the more likely La Niña will impact the globe.

Sea surface temperature anomalies for October 2017 (Climate Prediction Center)

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This afternoon’s post will focus on now-Tropical Storm Harvey’s inland track. Where the center goes from here is going to make a big difference in terms of whether Houston sees widespread flooding—or not. Unfortunately, the track forecast has remained something of a mess.

But before that discussion, just a quick update on the state of rains in Houston. The heavy band we were concerned about this morning has slowly moved north-northeast of the central Houston area, and this has allowed the city and its bayous to dry out a bit. This is good, because we expect more rain bands to move into the area later today and especially tonight. For now travel is mostly safe, but try to stay off the roads unless you really need to go out, and complete any trips before dark. Also, if you’re wandering around outside, be careful of wildlife. It, too, is looking for shelter from rising waters.

Look what we found in League City, near Clear Creek, this afternoon. (Space City Weather)

Harvey’s track

The big question is where Harvey’s center (and its outer rain bands) will go over the next few days. While 10 to 15 inches over three or four days is a lot of rain, for most of Houston it is manageable. However, 20 inches or more will cause widespread problems.

What I think I know today is that there’s a chance the Houston region escapes catastrophic flooding. I feel better than I did yesterday at this time, when the forecast was almost completely grim. Let me explain why.

Both the GFS and European model runs have Harvey wobbling around south Texas, and the central-Texas coast today through Monday, and then they bring the center back out into the Gulf of Mexico near the point where it made landfall late Friday night. But neither model keeps it offshore for long, and at this time I don’t think significant re-intensification is a major threat. No, the long-term worry for Houston, the Texas coast, and much of southern and southeast Texas remains rainfall.

GFS ensemble model forecasts for Thursday morning. (Weather Bell/Space City Weather)

By Wednesday night and Thursday morning the GFS model ensemble solutions (shown above) start to diverge, with a slight majority dragging Harvey toward the Rio Grande and ultimately toward lower pressures over northern Mexico. This is a good scenario (Dying in the Rio) for Houston because it ultimately limits rainfall potential over the area. The remainder bring the storm north toward Houston (The Wandering 59 special). This results in more rainfall for Houston during the middle of next week, when we certainly will already be waterlogged.

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When inland flooding comes, rainfall rate is the key

Posted by Braniff Davis at 12:49 PM

By now, we’ve all been inundated (pardon the pun) with model graphics such as the one Eric shared earlier this morning:

Rain accumulation forecast for now through Friday morning, Sept. 1. (NOAA)

As he mentioned, it is indeed a realistic portrayal of the rain our area could get over the next week. Twenty-plus inches of precipitation in such a short period of time is daunting, especially when you consider Houston averages around 50 inches a year.

We’ve had a number of questions about how the area’s bayous will manage the rain. Surely, they could handle four inches day, for five days, right? Unfortunately, for tropical systems like Hurricane Harvey, it isn’t the amount of rainfall that becomes a problem, as much as the rainfall rate. What is rainfall rate, and how will it and other factors influence how bad our flooding will be this week?

What is rainfall rate?

Rainfall rate describes how much rain falls over a period of time, and is measured in inches of rain per hour. A rainfall rate of 0.5″ per hour is considered heavy, while anything above 2.0″ per hour is intense. For context, Harris County experienced a maximum rate of 4.7″ per hour during the 2016 Tax Day Floods in a few isolated locations. It’s the difference between your sink faucet dripping for a week (a low rate), and your faucet breaking off for a half hour (a high rate). The same amount of water may come out of your pipes, but one will flood your kitchen much faster than the other.

Ditches and storm drains in Houston and Harris County can generally handle a rate of 1″ to 2″ per hour, for a few hours. Anything higher than that, over a longer period of time, leads to excessive runoff that can flood roads and fill the bayous faster than they can handle it.

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