Around August 10, another tropical wave moved off the coast of Africa. Initially, atmospheric conditions did not favor development and the system did not produce much thunderstorm activity as it traversed the tropical Atlantic. It was only upon approach to the Lesser Antilles around August 16 that it showed signs of organization. The wave produced some rain for the islands as it passed into the Caribbean the next day, but its fast forward speed kept these rains short-lived. A broad low pressure center formed in association with the wave on the 18th. As the system neared the southwestern edge of the subtropical ridge, its forward motion slowed and allowed some more consolidation. Very deep convection blossomed late on August 19 and Tropical Depression Fourteen formed the next morning.
The depression had a curious structure that day, featuring a warm spot on satellite imagery near what appeared to be a mid-level circulation even though surface and aircraft reconnaissance data indicated a surface low further south. The storm did not intensify over the next day, thanks in part to land interaction with Honduras, which Fourteen passed very close to overnight. Feeling the influence of a trough over the Gulf of Mexico, the depression turned rather sharply toward the north and slowed down during the morning of August 21. Outflow from the same trough lay some sinking air over Fourteen, limiting convective activity.
Interestingly, the storm began to build a central dense overcast that evening "from scratch," building an expanding core over the next day. Late in the evening (early on the 22nd UTC time), it strengthened into Tropical Storm Marco. This once again marked a new record for earliest "M" storm, this time surpassing a tie between Maria of 2005 and Lee of 2011, which both formed on September 2. Note that in the second case, though Lee wasn't technically an "M" storm, it was still the thirteenth tropical storm-strength cyclone of the season, in light of the unnamed tropical storm that was identified in post-season analysis. Marco sat atop the highest ocean heat content in the Atlantic, and tapped into it on the 22nd, strengthening rapidly into a strong tropical storm.
Defying forecasts, Marco traveled north-northwest most of the day. Instead of hitting the Yucatan Peninsula, it passed east, in between Cuba and Mexico. In fact, of the two landmasses, only the western tip of Cuba received tropical storm conditions as the storm passed. The storm entered the Gulf of Mexico, where very warm ocean waters and high relative humidity prevailed. As Marco moved north-northwest closer to the trough mentioned earlier though, wind shear steadily increased out of the southwest. As a result, the satellite presentation looked a little "squashed" in the southwest quadrant. The storm fought it off on August 23 enough to strengthen to a category 1 hurricane, reaching its peak intensity with 75 mph winds and a pressure of 991 mb.
But the shear was relentless and increased to over 30 knots that evening, causing the storm to begin to decouple. This began a period of rapid weakening and Marco lost its brief hurricane status. All convection was displaced from the center overnight, so that even though the center was headed toward the Louisiana coastline, that state received almost no rain! The heavy rain that did occur was from the displaced thunderstorms over the Florida panhandle and coastal Alabama. By the afternoon of the 24th, Marco was a weak tropical storm. The center of circulation turned abruptly westward and made landfall near the mouth of the Mississippi river that evening. The weakening vortex paralleled the Louisiana coastline overnight as Marco weakened to a tropical depression, and then a remnant low early on August 25. This low dissipated within a day as Hurricane Laura approached from the southwest.
The above image shows Marco near peak intensity. Notice the straight line cloud formations to the north and the strong winds pushing them toward the northeast. These upper-level winds are what caused Marco to rapidly weaken before landfall.
Despite forming in the Caribbean, Marco took a path that resulted in relatively few land impacts.
No comments:
Post a Comment