The weather pattern — we’ve named it ‘Government’ because it moves so slowly — that has plagued the plains, midwest , and southeast for several weeks with tornadoes, hail, flooding rain, and lightning, has reached a crescendo with historic flooding in Texas and Oklahoma.
Following more potential flooding today in Texas, Arkansas, Louisiana, and southern Missouri, Government’s effects will ease off later this week, followed by a period of respite.
Forecasts indicate that this might be just a recess, rather than a permanent adjournment.
As of Sunday, Oklahoma City had over 18 inches of rain in May, easily surpassing the record for May and the less than 15 inches that was the previous single-month record set in June of 1989. Observations go back to 1890.
Getting Through Memorial Day
Weather Pattern Government amended its agenda slightly over the weekend, producing flash floods as copious moisture from the Gulf of Mexico at low- and mid- levels of the atmosphere conspired with the lift associated with a curl in the jet stream.
With the rains moving east, Texas and Oklahoma could see some sunshine today. This would increase the risk of afternoon severe thunderstorms and the possibility of tornadoes.
What Is A Flash Flood And How Does It Happen?
Under normal weather conditions, water is well-behaved. The hydrologic cycle begins with evaporation from surfaces of land and water. The atmosphere can hold only a limited amount of water vapor, and when saturated air is lifted, the water vapor condenses and returns to the surface of the earth as precipitation. Most of the precipitation eventually drains back into the sea.
Natural and manmade drainage systems, rivers and storm drains, routinely hold all the water that falls.
If the rain is very heavy compared to the average, the drainage systems can be overwhelmed, and the water can end up in places where it is unwelcome, such as city streets and people’s houses.
The ground can absorb some precipitation, but when rain falls over a period of weeks, the ground becomes saturated and any additional rain runs off immediately.
That is what has happened in this flood event. In many parts of Texas and Oklahoma, the rains accumulated to levels never seen in May, and in some cases, never seen in any month.
The Danger Of Flash Floods
When water overflows a riverbank or overwhelms a storm drain, the water level can rise rapidly, often inundating roadways. About half of all flooding fatalities take place in motor vehicles. A car can be swept away by water only a couple of feet deep, because the vehicle is buoyant until it fills with water, which can take several minutes. Drivers routinely assume that gravity will keep the car anchored to the road, but that is not the case.
The National Weather Service has the slogan “Turn around, don’t drown,” which refers to any occasion on which the depth of the water is unknown.
Where houses have been built in low-lying areas, water can rise to the rooftops in a matter of minutes. In the current flood event, many residents were saved by helicopter from the roofs of their houses.
Government’s Warm Side
What goes up must come down, and this is true of undulations in the jet stream. Weather Pattern Government is characterized by a deep trough (dip) in the western United States. The opposite phase of this wave is a ridge (bulge) in the east. Under this ridge, a heat wave is likely to occur. It is a modest one by midsummer standards, since the summer maximum of temperature is over a month away, but temperatures will reach 90 degrees in Washington on Tuesday, and continue to run ten to fifteen degrees above normal in the southeast and mid-Atlantic through the week.
The Jet Stream Pattern Flattens Out
By midweek, the jet stream trough will have weakened, the rains decreased, and generally things will have quieted down.
The flattening of the jet stream means less severe weather, though afternoon heating will still produce seasonal thunderstorms and the possibility of a tornado. The forecast position is farther north than usual, so to the south, the warm air will remain in place.
Is It Climate Change?
No single event will prove or disprove that the climate is changing. But an accumulation of anecdotal evidence leaves less doubt all the time.
This flooding in Texas occurs where there has been a drought for several years. One of the hallmarks of global warming/climate change appears to be persistent weather — what you see is what you get for a long time. Weather Patterns like Government — unchanging for weeks, months, or years — have been increasingly common in the 21st century.
A slow, steady change of climate is relatively easy to deal with. Lengthy bouts of extreme weather like the flooding in the plains this week: not so easy.