As of Thursday, October 8, 2020, Hurricane Delta was located in the Gulf of Mexico. The storm was classified as a Category 2 hurricane due to its maximum sustained wind speeds of 100 mph. The 5-day outlook published by the National Hurricane Center Thursday morning at 7 am CDT indicated the storm strengthening into a major hurricane by 1 am CDT Friday, then weakening back to a Category 2 storm before making landfall on the Louisiana coast Friday afternoon just after 1 pm CDT. Most of the Louisiana coastline was under a hurricane warning, while a tropical storm warning spread along the Texas coast to the far eastern portion of the Louisiana coast. After the storm was projected to make landfall, it was due to weaken into a tropical storm by early Saturday morning, and then further weaken to a tropical depression by Saturday afternoon. We will look at some of the atmospheric dynamics present that helped to determine the strength and track of the forecasted hurricane.
The following map shows the 300-200 hPa potential vorticity (shown in the gray contours), irrotational wind vectors (shown in the black vector arrows), the 600-400-hPa ascent (shown in the red contours), the 250 hPa jet (shown in the blue-pink fill pattern), and the precipitable water (shown in the green-orange-pink fill pattern). Let’s first look at the irrotational wind vectors. These illustrate the magnitude of the irrotational wind, or the wind without any vorticity taken into account. Irrotational wind also shows whether the air is converging or diverging in the upper troposphere. In the still image shown below, at 06 UTC on October 8, there is slight divergence over the center of Hurricane Delta. However, if we progress through the forecast period, the divergence greatly increases over the center of the storm as it approaches landfall. This indicates air evacuating the center of the storm, which corresponds to a lowering of the surface pressure and thus a strengthening of the hurricane. Then, right before the storm approaches the Louisiana coastline, the magnitude of the irrotational wind vectors decreases, indicating the storm weakening once again. The regions of vertical ascent, shown in the red contours, also indicate upward vertical motion. The strongest example of ascent associated with Hurricane Delta is shown later in the forecast period, after the storm has made landfall. There appears to be a frontal system associated with the remnants of the storm, enhanced in part by this upward vertical motion. The jet stream helps to show the direction of motion of the hurricane, especially following its landfall. The jet stream is oriented meridionally, stretching from the Rocky Mountains in Canada to the Atlantic Coast of the United States. As the forecast period progresses, the jet stream becomes even more horizontal. The remnants of the storm become ingested in the lowest reaches of the jet stream, and this helps to steer the storm off the Atlantic Coast. One final ingredient the storm needs to maintain its strength is moisture, and this map shows that there’s plenty of it. Over the Gulf of Mexico, the values of precipitable water are very large. This will feed the storm and allow it to bring high rain totals to the Louisiana coast. After the storm makes landfall, this moisture source will be cut off, so this will contribute to its continual weakening as it moves father from the Gulf of Mexico.
The following map shows 500 hPa geopotential height (shown in the black contours), temperature contours (shown in the red contours), ascent (shown in the blue contours), cyclonic relative vorticity (shown in the warm-colored fill pattern), and wind barbs. Let’s take a look at the temperature contours. There is a concentration of red contours around the center of Hurricane Delta, indicating that this is a warm core system. This gradient becomes slightly stronger while the hurricane is sitting over the Gulf of Mexico. At this point, the storm is absorbing the moisture from the warm ocean below. This will enhance the gradient of the temperature at the core of the storm. The cyclonic relative vorticity contours show the rotational nature of the hurricane. In the Northern Hemisphere, storms rotate counterclockwise due to the directional influence of the Coriolis force. The darker red shades of the fill at the time of the map indicates the intense rotation of Hurricane Delta. The cyclonic relative vorticity increases as the storm moves northward, and right before landfall, the intensity of the fill pattern decreases, indicating the storm’s weakening. After the storm makes landfall, there is some cyclonic relative vorticity that appears around the remnants. This frontal-like structure also has rotation associated with it. This front may also be enhanced by jet stream dynamics. Finally, the 500 hPa geopotential height contours indicate the general direction of the storm. Though they don’t illustrate the movement as clearly as the jet stream, the remnants of the hurricane will follow along the bottom of the mid-level trough sitting over the Great Lakes/Northeast region.
