Harvey’s two-step across Texas: The peculiar meteorology behind a US rain record – Washington Post

 In U.S.

Over the past few days, a staggering amount of water fell across Southeast Texas and Louisiana. All this water disgorged from one of the greatest meteorological events in U.S. history: an intense Category 4 hurricane named Harvey. This analysis attempts to explain how this extreme storm evolved — from landfall to eventual exit from Texas, with emphasis on the rain generation.

The rain accumulation totals

Our strategy here is to look at the behavior of this weather system day by day, from Aug. 25-30. But first, let’s examine a map of the preliminary rain amounts across the greater Texas region (above), over this multiday period from Aug. 23-30.

These values are all subject to continued verification. The deepest water accumulations encompass Houston, Beaumont, Galveston, Pasadena and Port Arthur.

To describe the history of this storm, I have created two key figures, mosaics that show snapshots of the storm, every day for six days. Both of these figures (shown below) can be enlarged by clicking on them.

The first figure displays a surface weather map, with isobars of pressure and the pattern and intensity of rain (colored regions). I have added some details illustrating the movement of the storm (black arrows) and also the position of frontal boundaries (red heavy line = warm front, alternating blue/red line = stationary front). Surface airflow vectors are also shown by tiny orange arrows.

In the next mosaic, I focus on the radar and satellite structure of the storm. The satellite presentation becomes more important after Day 3, and I present a view called “color-enhanced water vapor.” Shown is the water vapor concentration in the middle and upper atmosphere, using colors that clearly delineate saturated, rainy regions from dry areas of airflow.

Day 1. Friday, Aug. 25: Landfall and onset of heavy rain

As a marginal Category 4 hurricane, Harvey blasted ashore across the Central Texas coast. At the same time, its outer rain bands (called feeders) began raking across the upper Texas coast, including Houston. Rain rates also began to pick up near the landfall point, as Harvey’s double eyewall crossed the shoreline.

Harvey created a counterclockwise flow of warm and humid air off the Gulf of Mexico, forming a front along the coast to the east of the storm’s center. This stationary front separated the warm, Gulf air mass from a slightly cooler and drier air mass over land. This setup would factor significantly in the generation of widespread, heavy rain in the days to come.

Already, several processes were creating heavy rains along the upper Texas coast:

  • Feeder bands on the periphery of the storm, which were popping up along the coast, due to “frictional convergence” i.e. the sudden deceleration of gulf air as it encountering land, forcing the air to rise.
  • Air forced upward along the newly created frontal boundary, a process termed “isentropic upglide.”
  • An unstable air mass originating over the gulf, which enabled air to ascend as strong convective cloud turrets producing torrential rain.
  • Concentrations of convective cells over the same locations within the feeder bands (link to Halverson’s first Harvey story here).
  • The exceptionally slow forward speed of the storm.

Day 2. Saturday, Aug. 26: A persistent corridor of heavy rain sets up

Once 100 miles inland, the storm rapidly weakened (dropping from Category 4 to tropical storm), but it also slowed to a crawl. The storm stalled because the “steering winds” (the deep air current in which the storm was embedded) were extremely sluggish. The winds were weak, in turn, because the westerly jet stream was well north of the storm.

From the two figures above, note that the isobars became spread apart and fewer in number (the signature of weakening) and the eyewall’s intense ring of rain diminished significantly. At the same time, the system developed a striking asymmetry: A prominent, intense rainband that set up over about 80 to 100 miles north and east of the storm’s center, over Houston. There this band would remain parked, for a very long time.

This stout band was rooted in a corridor of unstable, very moist (dew points in mid-upper 70s) air streaming off warm gulf water. Additionally, the cyclonic inflow became strongly convergent east of the center, creating a type of “warm sector” more akin to everyday cyclones of the mid latitudes. Deep convective rain cells continuously erupted within this plume, racing north across the warm front, where they received additional uplift. The parade of cells trained constantly over the greater Houston area, with rain rates in the 2 to 3 inches per hour range, and up to 5 to 6 inches in extreme cases.

More than any other reason, this moisture conveyor dumped prodigious quantities of water on the upper Texas coastline. Understanding how it formed, and why it persisted, are key to unlocking the mechanism behind this weather disaster.

Within the warm sector, a strong jet of moist wind from the south, approaching 90 mph, howled along at 5,000 feet above ground — enabling Tropical Storm Harvey to freely slurp Gulf moisture through a gigantic pipe. And the deeper air mass feeding into this pipe was incredibly moist, with precipitable water values (PWV) — how much rain would fall in a 1-meter-square column, measured from the ground to the top of the atmosphere in the 2.5-to-2.8 inch range. At times, PWV surged in excess of 3 inches: We have at least one weather balloon measurement of 3.26 inches, close to the theoretical maximum for this location and time of year.

Day 3. Sunday, Aug, 27: Harvey becomes lopsided

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