Weather Spectrum

The image above is the Hydrometeorological Prediction Center's modeling of the total precipitation amount for the US over a 5-day period. This image is valid from yesterday evening until Monday evening. It is pretty intense-looking. Upwards of 6-8 inches of rainfall for the coastal areas of SE Texas, and then about 4-6 inches in the Houston area. Then 3 inches and below north of Houston. Depending on how fast this precipitation accumulates, places from San Antonio and the Central TX region east to the Louisiana border can expect to start seeing flood watches in place as you read this. Basically I-10 and areas south are most prone to the heaviest of rainfall from this system.

The culprit? An upper level low spinning across the southwest US/Mexico is tracking east towards Texas that is already beginning to generate showers in far west TX as of yesterday evening. Each radar site has a light greenish bluish ring around it, that's just clutter and false echoes, and the actual rain and thunderstorms are the areas of yellow and red just coming into far western TX.

Because I compose my posts the evening before, as you read this rainfall is beginning to affect central Texas, or the western portions of Central TX. By the late afternoon the rainfall will be approaching western portions of the southeast Texas area and Friday is shaping up to be a very gloomy day in southeast Texas.

Here is what the vorticity and heights look like at about 18,000 feet. I'll come back to vorticity momentarily.

See that cool-looking feature centered just south of Arizona and over western Mexico? The cool orange and yellow colors? Well that is the mid-level low pressure system that is generating all that rain that will eventually affect southeast TX on Friday. The colors denote areas of strong vorticity. What is vorticity??

Vorticity is a measure of the rotation of air in a horizontal plane. Positive vorticity (air rotating counter-clockwise or cyclonic) can be correlated with surface low development and vertical motion.

When the air spins counter-clockwise in the upper levels of the atmosphere it causes divergence or air to spread apart ahead of the rotation. Think about water for a second, if you try to spread water apart in a pool with your arms what happens? Water fills in the void from below. The same thing happens with air (they are both fluids; water & air). So when air separates far above the surface of the Earth as a product of rotating air (vorticity), air fills in from below, or from the surface. As air moves vertically, the pressure at the surface drops and a low pressure area forms.

In the image below, notice the rainfall (areas of green) is falling ahead of the vorticity areas on the prior map. Where the rain is falling is where the surface low is developing or has developed.

Knowing what we know now about vorticity and the formation of a surface low pressure and consequential rainfall we can look at the vorticity for the next few days and guess where the rain will fall.
By mid-day tomorrow, across central TX.

By Friday morning, across southeast Texas.

This is the precipitation estimates for the same time as the above image...

Note the heaviest rainfall is ahead of the upper low. This is because that is where the best upward motion is as a result of the air spreading out allowing air to rise. As unstable, moisture-filled, air rises it condenses to form clouds and when the air continues to rise quickly it builds into rainstorms and thunderstorms.

This upper level system is potent. Some severe storms are possible where the sun can shine for a bit to heat the ground which heats the air and increases instability in the lower atmosphere. If this happens, there is a small potential for a few tornadoes. However, if cloud cover hinders sunshine then it is less likely we will see tornadoes and more likely we will see some strong wind gusts from storms.

The rain will be consistent through the day on Friday and Friday night, especially along the coast before becoming a bit spotty in nature early morning on Saturday through mid-day. Scattered showers will be possible for the remainder of the weekend and you might have an umbrella handy for Mother's Day along the coast. However, the weather will be noticeably cooler as a result of the low pressure system moving east.
Next week it looks like highs in the mid 80's with light northerly winds, at least early in the week.

Enjoy this wet weekend. While it puts a damper on the outdoor plans, it is some much needed rain to make up for last year! BUT be safe and keep an eye out for water covering roads if you are out and about through the day Friday and Saturday morning.
I'll update again if possible Friday mid-day, and Saturday to recap the event. Thanks for reading.