Hurricane Delta Discussion – Wed

Hurricane Delta made landfall early this morning on the Yucatan Peninsula of Mexico. Per surface and aircraft recon observations, the interaction with land has weakened the storm to 100 mph, after what had been one of the most rapid intensifications of a hurricanes ever in the Carribbean/Gulf, peaking around 140 mph yesterday. This time of year waters are very warm down there, and we’ve seen these rapid October intensifications before with Hurricanes Opal in 1995 and Michael in 2018.

It is still moving WNW around a ridge of high pressure over Florida. But, the main natural purpose of hurricanes is to move heat from the tropics to the midlatitudes and polar regions, so they tend to find a path of least resistance to move north. That opening will likely start to show up tomorrow, as a trough of low pressure over Texas, and the storm moving away from the ridge over Florida, and allows the storm to turn northward.

The computer models are in amazing agreement right now, rarely seen, showing a landfall along the Louisiana Coast, east of Lake Charles, on Friday afternoon.

Hurricanes get their intensity from heat and humidity in the center. Warm, humid air is lighter than cool, dry air, so the pressure near the surface is lower underneath the warm, humid air in a hurricane. Thunderstorms release latent heat of condensation, and some of the warm water in the ocean, released in spray by waves, makes it way into the storm. The image below shows a cross-section through a hurricane with equivalent potential temperature…a good measure of temperature and humid normalized by height. Note the extremely high values near the center, producing the low pressure underneath.

From Hawkins and Imbembo (1976)

Given the warm waters in the central Gulf, a deep humid atmosphere, and low wind shear, Delta is expected to re-intensify today and tonight into a Cat 3 or Cat 4 storm. As it approaches the coast on Friday, the shallow waters are cooler from recent cold fronts (and shorter days). And, the storm will likely encounter a bit of wind shear, tilting the warm core over to the side a little bit and spreading out the effects of low pressure at the surface out a bit. Both of these factors could weaken the storm a bit before landfall. But, it is still expected to come in as a Category 3 hurricane with winds of 120 mph.

With a large area of hurricane-force winds over the Gulf, a large area of storm surge will affect the coast from Houston all the way to Biloxi (and maybe even farther east). Storm Surge Watches have been issued along the Gulf Coast by NHC, from High Island, TX to Pascagoula, MS, along with Hurricane Watches. Given the wind direction, their could be a significant storm surge in Lake Ponchartrain near New Orleans. Wind damage is likely in Louisiana and even parts of southern Mississippi, with tropical storm force winds occurring as far inland as Shreveport, Monroe, and Jackson. The city at most risk for wind damage is probably Alexandria, LA.

Also, as the decaying storm moves inland Friday night and Saturday, tornadoes are possible over parts of Alabama, Mississippi, and NW Florida, even as far north as Tupelo, Birmingham, and Huntsville.

Dr. Tim Coleman

Consulting Meteorologist

Coleman and Knupp, LLC

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