Why Strongest Wind Field Was North And West Of Delta At Landfall

Discussion: Category-2 Hurricane Delta made landfall at 6PM CDT Friday October 9^th south of Creole, LA. A record water rise (for this location) of 9+ feet was observed. Maximum wind speed is estimated at 100 mph just to the east of the center of the storm at landfall. What’s unusual about Delta at landfall is the extensive high wind speed profile north and west of the storm. Usually, the right-front quadrant contains the largest area of high wind associated with a hurricane. For a relatively short distance east of the Delta storm center that expected character was observed. However, wind gusts to almost 100 mph north and west of the storm at landfall was unusual.

When tropical cyclones recurve (turn north and northeast after departing the tropics) they do so by losing their steering currents provided by the subtropical ridge (“Bermuda High”) and increase forward movement (accelerate) if an approaching upper trough is to the west of the storm. These conditions were present Friday evening.

During the recurving process the most dangerous part of the tropical cyclone often shifts from the right-front quadrant to north and west of the storm. The reason for this shift is the interaction between the increasing baroclinicity north and west of the storm is caused by the dynamic forcing of an approaching cold core upper level low pressure trough and the remaining warm core of the tropical system. Meanwhile the difluence associated with subtropical ridge east and southeast of the storm breaks apart the previously intense right-front quadrant.

This process apparently started early with Hurricane Delta. Just west of Delta at landfall a southwest shear axis (which turned the storm to the north) was present and extended to the north of the storm. However, a short distance further north and west the jet stream axis was present and likely contributed to the unusually strong wind field north and west of Delta’s landfall.

Wind gusts of 90-94 mph were observed at Beaumont and Port Arthur, some 50-70 miles northwest of Delta’s landfall. Preliminary estimates of maximum rainfall (6-16 in.) also occurred mostly west, northwest and north of Delta’s landfall.

Delta was the 10^th land-falling tropical cyclone of 2020 which is a record. Delta is the 25^th tropical cyclone of 2020 which is 3 short of the record. Interestingly, the accumulated cyclone energy index for 2020 so far is 123.1 which ranks 23^rd highest in the 1950-2020 climatology. The tropical cyclone season still has about 7 weeks remaining.