Glenn’s Blog: Nate: Gulf Coast Hit, Rain For Us

8 photos
NOAA via Getty Images
A new storm in a familiar place.
Tropical Storm Nate sure doesn’t look like much on satellite loops. But the main reason for that may be that it’s actually centered OVER LAND! And there are plenty of high mountains affecting the circulation over Nicaragua and Honduras.
The green areas are low elevations-the brown areas are the mountains. Those mountains help add to the rainfall from any tropical system, and can be deadly. In fact, there have already been deaths from flooding in Central America due to Nate. And it’s barely a Tropical Storm, with 40 mph maximum winds.
nThe western part of the Caribbean Sea is the most favored area historically for tropical storms and hurricanes in October. The water temperatures sure haven’t gone down. This area has some of the deepest, warm waters anywhere on earth.
Tropical Storms and Hurricanes need water temperatures of at least 79 degrees Fahrenheit (26 degrees Celsius) to form and strengthen. The warmer the water is, the more “fuel” is added to development. The waters just off Nicaragua and Honduras are above 86 degrees F.
nBut it’s not just the water temperature at the surface that is important. The depth of the warm water can add even more fuel for development. We measure this in a term called “Ocean Heat Content”. This one shows just how extreme the water conditions are in the Western Caribbean. And we’re talking about October here-not August or September.
The actual numbers aren’t important here. Just look at how “red” the area is that Nate is about to travel over! And there are some reds in the Gulf of Mexico as well. That type of red area in the Gulf is similar to where Katrina tracked in 2005, when it strengthened rapidly. If everything else is equal, those red areas can lead to quick intensification. Those waters are even more impressive than the ones Harvey, Irma, and Maria tracked through.
nSometimes, the track of a hurricane is very uncertain. Not this time. When they have a significant “steering current” to guide them, computer models tend to agree on the future movement. That is the case with Nate. Look at some recent “Spaghetti Plots”, showing various model forecasts:
Not only is there terrific agreement on a track in the general direction of New Orleans, but there’s also high confidence in a track that ends up somewhere in our area early next week. That’s the reason for our rain forecasts for Monday into Tuesday. This time, we need the rain. We’ve gone 16 straight days without measurable rain in Philadelphia!
nWe were pretty confident that Irma and Maria were going to strengthen rapidly. Yes, the water temperatures were high. But the key factor was the lack of “wind shear”. Hurricanes can be surprisingly fragile, especially in the formation stages. The wind patterns above the developing storm have to be favorable, meaning very little wind shear. It was the lack of shear over Irma and Maria that led to their rapid intensification. And the lack of shear has been a significant feature throughout this hurricane season.
nHere is a good explanation of what shear is, and how it affects hurricanes:
The shear question is added to the question of how much land impacts Nate. The forecast tracks take it over the northeast corner of the Yucatan Peninsula. It’s very flat in that area, so it’s not enough to destroy Nate, but it can help delay or limit intensification.
nRight now, the National Hurricane Center is predicting Nate to become a Category 1 hurricane. But they do mention that rapid strengthening is not out of the question. That is going to be the main thing to watch over the next few days, and will determine whether or not the name Nate is added to the infamous list of 2017.
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