Tag Archive for: Tropical Storm Allison

Different Factors Affect Hurricane Strength, Rainfall

The factors that create hurricane strength may not be the same factors that create intense tropical rainfall. According to NOAA, warm sea surface temperatures can increase storm intensity. Meanwhile, the absence of steering currents and wind sheer can cause even weak storms to stall over an area and dump huge amounts of rainfall.

Two things happened this week to bring these factors into focus.

First, sea surface temperatures in June have already reached those not usually observed until late July or August in Galveston.

Second, this week marks the anniversary of Tropical Storm Allison, which set rainfall records for its era and caused all the flood maps to be redrawn (until Harvey). That prompted more research into meteorological factors that affect hurricanes, their formation, and their destructiveness.

Record Heat Tied to Higher than Usual Sea Surface Temperatures

Jeff Lindner, Harris County’s meteorologist, released a report this morning that said, recently all along the Texas coast, the nighttime lows have reached near record highs. Galveston, for instance, has failed to fall below 83 degrees for the last 72 hours and the low yesterday was only 84 degrees which is 1 degree shy of the all-time high “record low” of 85 from last summer.

These extremely high “low temps,” says Lindner, are more typical of August than June and directly tied to the nearshore water temperature which is already 83-86 degrees along the Texas coast.

28-30 degrees Celsius translates to 83-86 degrees Fahrenheit.Source: NOAA.

That raised two questions for me:

  • Are sea surface temperatures warmer than normal?
  • If so, how does that affect hurricane formation?

Sea Surface Temperatures Much Higher than Normal

I first researched sea surface temperature anomalies. You can see from the map below that the entire tropical Atlantic, Caribbean and Gulf of Mexico show higher-than-normal temperatures. How much higher?

Anomalies are departures from normal. This map shows anomalies for today. Source: NOAA.

Most of the upper Gulf Coast is 1-2 degrees Celsius above normal. That translates to about 2-4 degrees Fahrenheit.

Eighty-six degrees Fahrenheit is the normal average for August in Galveston. And we’re already experiencing that in June!

Relationship Between Sea Surface Temps and Hurricanes

So how will that affect hurricanes? The short answer: it will likely make them more intense, according to NOAA. Here’s how.

In order for a hurricane to form, two things must be present: a weather disturbance, such as a thunderstorm, that pulls in warm surface air from all directions and water at the ocean’s surface that is at least 80° Fahrenheit (27° Celsius).

Because warm air and warm seawater spawn these storms, they form over tropical oceans where seawater is hot enough to give the storms strength and the rotation of the Earth makes them spin.

Hurricanes start simply with the evaporation of warm seawater, which pumps water into the lower atmosphere.


Converging winds then collide and turn upwards, where water vapor starts to condense. That releases heat that warms the surrounding air, causing it to rise as well. That causes even more warm, moist air to spiral in to replace it.

As long as the base of this weather system remains over warm water and its top is not sheared apart by high-altitude winds, it will strengthen and grow. More and more heat and water will pump into the air. The pressure at its core will drop further and further, sucking in wind at ever-increasing speeds.

Eventually, hurricanes turn toward mid-latitudes, i.e., Texas. When they move over cold water or land, they lose touch with the hot water that powers them. The hurricane then weakens and breaks apart.

Recent studies have shown a link between ocean surface temperatures and tropical storm intensity – warmer waters fuel more energetic storms.


Other Factors Correlate with Higher Rainfall

Energy and intensity, however, do not correlate directly with rainfall. Other factors play larger roles in creating monster rainfall rates.

A slow moving storm that meanders or stalls can dump more rain than fast moving storms that blow through areas quickly. Tropical Storm Allison makes an excellent example.

This week is the anniversary of Tropical Storm Allison (June 5-10, 2001). NOAA has a special web page that tells the story of Allison and its destructive rains. Before Harvey, Allison set records for much, but not all, of the Houston Region. Greens Bayou at Mount Houston Parkway, for instance, received 38.78 inches of rain.

Allison lingered around the Houston area for days, went up to Lufkin, and then backtracked over already saturated ground before moving east.

The absence of strong steering currents allowed Allison to stall and dump huge rainfall amounts on Houston.

“The devastating flooding from Allison is a stark reminder that rainfall from tropical cyclones does not depend upon the strength of the system.”


The Hydrometeorological Prediction Center found six factors that impact the rainfall potential of landfalling tropical cyclones:

  • Storm track (or movement)
  • Time of day
  • Storm size
  • Topography
  • Wind shear
  • Nearby weather features

Between June 5th and the 9th, the two major factors leading to heavy rainfall over Southeast Texas turned out to be Allison’s slow movement and the time of day. These were aided by an abundance of available Gulf moisture.

Graphic showing rainfall totals for Harris County, Texas for June 5 - 9 2001 during Tropical Storm Allison. The highest recorded rainfall was 38.8 inches. Image courtesy of Tropical Storm Allison Recovery Project.
Tropical Storm Allison 5-day rainfall totals in 2001 related primarily to the storms track and slowness, caused by the absence of steering currents and wind sheer.

Time of day deserves more explanation. On Day 4 of Allison, the sun cleared over much of Houston. That increased daytime heating. And the heat caused feeder bands to intensify over areas that previously flooded. No one died during the first three days of the storm. But 22 died during the last two as rainfall from those bands reformed over areas already badly flooded.

Give Your Kids a Science Assignment for the Summer

Weather is one of nature’s biggest puzzles. I find it endlessly fascinating. If your kids are bored already by the summer’s heat, give them a science assignment. Have them research NOAA’s website to learn more about hurricanes and the heat. Hint: ask them how that bright red area in the northern Pacific (in the SST anomalies map). Then ask them how that’s related to drought, trade winds, wind-sheer, and predictions for an above-average hurricane season.

Posted by Bob Rehak on 6/9/22 based on information from NOAA.

1745 Days since Hurricane Harvey