Friday, August 25, 2017

Looking at Harvey

This is a blog focused on mountain weather, but sometimes our attention does wander, today to Hurricane Harvey and its impacts on the Gulf Coast.

First a little context.  It has been 4323 days since the last major (category 3 or higher) hurricane landfall on the Gulf or East Coast of the US.  This is by far the longest major hurricane "drought" since the mid 1800s.  It has been a good run, but one that may be ending today or tomorrow.

Hurricane Harvey is currently lurking off the Texas Gulf Coast.  Some perspective on the scale and significance of the storm is provided by a continental US sateltite and radar perspective.  Precipitation associated with Harvey essentially spans the entire Texas Gulf Coast, with associated cirrus clouds covering a broader region that is, well, approximately the size of Texas.

Zooming in, one can see Harvey's eye quite well, with a classic spiraling rainband pattern.

The latest advisory issued by the National Hurricane Center at 7 AM CDT (6 AM MDT) this morning reports that Harvey has maximum sustained winds of about 110 mph.  Doppler velocity imagery, which maps the wind speed toward (i.e., "inbound", cool colors) and away (i.e., "outbound", warm colors) from the Corpus Christi radar site, shows a clear "couplet" of inbound and outbound velocity associated with the hurricane circulation (due to the slope of the radar beam, these winds are sampled above the surface).

Note that the scale of that couplet is relatively small compared to the coverage of clouds and precipitation.  The strongest winds are confined to near the eyewall and the coverage of hurricane force winds is much smaller than the cloud and precipitation region.  From a wind perspective, life near that eyewall is far more terrifying than in the tropical storm force wind area farther from the low center.

Currently, Harvey is moving northwestward at 10 mph, but is expected to slow and strengthen prior to landfall.   The National Hurricane Center issues a "cone" of uncertainty meant to illustrate the probable path of the low center.  I put the cone in quotes because the remarkably slow movement of Harvey basically leads to a circle of uncertainty due to the unclear storm track from days 2-3 (solid white) and 4-5 (stippled).

Water is the agent that is responsible for the majority of hurricane deaths and damage.  Hurricane categories are based on maximum sustained winds, not the severity of storm surge or flooding.  The winds are certainly a concern for Harvey, but the potential for storm surge and flooding damage is very serious regardless of Harvey's category at landfall.  In the case of rain-related flooding, current forecasts from the Weather Prediction Center call for more than 15 inches of rain across a significant portion of the central Gulf Coast of Texas from 1200 UTC Today (Friday) through 1200 UTC Monday, with a maximum of nearly 25 inches.

As much as 35 inches is expected through Wednesday, which is expected to cause "devastating and life-threatening flooding."

And, just to add insult to injury, the Storm Prediction Center convective outlook includes a possibility of tornadoes and locally damaging winds today and tonight northeast of the Harvey low center.

The bottom line is that this is a very serious storm and a significant threat to anyone near the Texas (and possibly Louisiana) Gulf Coast.  It also represents a significant stress test for individual, local, state, and federal emergency management and response given the long major hurricane drought in this region.  I am hoping that the storm surge, precipitation, and flooding are not as bad as anticipated.


  1. Looks like it may make landfall as a Cat 4- I imagine that is significantly more rare than even the cat 3?

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  3. It seems we need to have a new way to rate tropical cyclones considering storm surge and rain induced flooding are the #1 and #2 causes of loss of life and damage in these types of storms whereas wind which is what the ratings are based on comes in at #3.

    Good luck to the people of the gulf coast. Let's hope this fizzles as much as possible.

    1. Brooks:

      You are right. The meteorological community is well aware of this issue. Significant effort has been ongoing for Harvey to ensure that the public is well aware the flooding threat is very serious and could be ongoing for an extended period. How to adjust or "replace" the current categorization system is a tough question. The current system does have advantages (maximum winds are important and, despite observation issues, quantifiable). On the other hand, it can distract people from other threats and peoples response to warnings is very much dependent on category, even though a weak hurricane can be deadly.