Hurricane Harvey To End US Major Hurricane Drought, Catastrophic Impacts Probable

The last major hurricane to strike the US was Wilma in mid/late October 2005, making landfall in southern Florida as a Category 3 hurricane. If everything holds, that 12 year drought will come to an end in less than 48 hours. While Harvey has only been widely known the last couple days, it’s actually been around for a while, beginning in the Caribbean and crossing the Yucatan Peninsula before reemerging in the Gulf. The overall atmospheric pattern & conditions will allow this storm to strengthen before landfall, but then will cause it to stall & meander over SE Texas for several days. The models are spitting out ridiculous numbers for rainfall, and I don’t mean ridiculous as in unreasonable. Rainfall will likely be measured in feet, not inches, and combine that with winds over 100MPH, indicates a very serious and potentially catastrophic situation is probable. So let’s take a look at what’s going on.

The 5PM EDT advisory from the National Hurricane Center has Harvey as a 85 MPH Category 1 hurricane. The pressure has been steadily falling in the system, an indication the storm is strengthening. Why? Wind is driven by pressure differences, flowing from higher pressure to lower pressure. The greater the difference in pressure, the stronger the wind.

The storm will slowly move NNW over the next 24-36 hours before making landfall near Corpus Christi, Texas. The official NHC forecast has landfall as a Cat 3 system, but that could be underdone.

Could it be stronger?

Yes. Keeping everything simple, there are 3 things that can negatively impact hurricane development: Dry Air, Wind Shear, and Water Temperature. I’m only going to address the last two, as dry air really won’t be an issue with Harvey.

Wind Shear

For tropical systems, strong wind shear weakens/prevents development as it keeps the center of circulation and thunderstorm activity separate. If we look at the overall upper level winds, there is very little wind. This reduces wind shear, but is also the reason why Harvey will stall after landfall (it loses it’s steering currents).

Upper Level winds valid Thursday Afternoon. Notice the lack of any real wind (color fill) around Harvey. The jet-stream is positioned well to the north (typical of the summer months)

 

Gulf of Mexico Temperatures

The Gulf is incredibly warm, which is typical of late summer. Water temperatures in Harvey’s path are around 30°C or 86°F, which is optimal for development. In addition, the very warm water, is very deep, so as the storm mixes up the ocean, there is a greater supply of warmer water. Why is this important? Hurricanes derive their energy from what’s called Latent Heat, or the heat of moisture. When water vapor condenses in the atmosphere to form clouds/rain, it releases heat back into the atmosphere. Hurricanes thrive on this.
When water is warmer, it evaporates more easily. In addition, the air temperature over the ocean closely matches that of the ocean, so when the ocean is warmer, the air is warmer. Warmer air hold more moisture, and therefore, more moisture is available for the tropical system to feed on.

I wouldn’t be surprised to see this storm rapidly intensify overnight, and wake up Friday morning with the storm stronger than forecast, possibly already a Category 3.

Rainfall & Storm Surge

While winds over 100 MPH aren’t anything to joke about, the winds will probably not be the big story with Harvey. As I mentioned above, once Harvey makes landfall, it is essentially cut off from any upper level steering currents and will stall/meander over SE Texas for several days. Tropical systems already produce tremendous rainfall, but a stalled hurricane with access to a very warm large body of water is a VERY BAD combination. The models are simulating 15 – 36+ inches of rain over a widespread area.  It’s impossible to pinpoint the exactly where the highest rainfall totals will be, but it’s very possible someone in Texas could walk way with over 3 FEET of rain.

GFS Model rainfall forecast Widespread 15 – 25″ (dark yellow/brown)

And here’s the official NWS forecast as of Thu Evening.

Harvey rainfall forecast from NWS Weather Prediction Center

Not to mention, there is also the storm surge threat for the coastal areas, which will primarily precede landfall for the majority (but not everywhere) of the areas expected to get the incredible rainfall amounts.

Now you see why this is a probable catastrophic event. Widespread power outages are likely, and it will take days before that can begin to be restored with the system over the area for a prolonged period.

Looking Ahead…Where does Harvey Go?

Eventually Harvey will have to go somewhere. It will either dissipate over land, or, as many models are showing, it will loop back over the gulf (where restrengthening is possible) and attempt another landfall in Louisiana and tracking northeast. We’ll have to watch to see how it all pans out, but it appears Harvey isn’t going anywhere anytime soon.

Spaghetti models for Harvey. Storm could re-emerge over Gulf next week, strengthen, then move towards Louisiana?

One last note…

There are obviously lots of flaws to weather models. They often have trouble, when the upper level pattern becomes incredibly convoluted, or where upper level flow is weak. Models also have trouble picking up on rapid intensification. This is why there’s been uncertainty in the forecast prior to the last couple days, and why intensity forecasts can also be underdone.

Models are based on physics & chemistry, and use very complex equations to attempt to simulate the atmosphere. Many of the equations are from calculus & differential equations, branches of math that focus on how things change over time. If you’ve ever taken Calculus or Differential Equations, you know answers are no longer exact, but approximations in many cases.

Hope you enjoyed today’s little lesson in Meteorology. There will be an exam next week (just kidding).

Have a great evening, and let’s hope some of this changes for the better!

-Andrew