On July 23, a large tropical wave exited Africa. It was perhaps the first of the season to really punch a hole in the dry Saharan air layer (SAL) to its north. The disturbance passed south of the Cape Verde Islands and pushed rapidly westward. Within a few days, the wave had a huge area of vorticity associated with it, but was highly disorganized. It's often the case that larger systems take longer to consolidate, and this was no exception. Though it developed a broad low pressure center, no well-defined center of circulation appeared for several days. At its fast clip, the outer rain bands of the tropical wave began to affect Barbados by July 28. By that time, tropical storm force winds were already occurring to the north of the center.
Overnight, the low passed over the Windward Islands at a remarkable 25 mph clip, bringing strong wind and heavy rains to nearly all the Lesser Antilles during that period due to its large size. As far as internal dynamics were concerned, the system was still struggling. Instead of a well-defined center of circulation, there was only a sharp trough axis oriented southwest to northeast, with competing mid-level vortices and convection clusters at each end. The large-scale cyclonic spin helped to turn the axis toward south to north on the 29th, but the system was still a mess on satellite imagery. It was only that evening that a new central area of thunderstorm activity formed atop a clear center and Tropical Storm Isaias was born, already packing 50 mph sustained winds. Not only did Isaias become the new earliest "I" storm (previous record-holder: Irene of 2005, which was named on August 7), but it also brought the number of named storms in July 2020 to 5, also tying an Atlantic record held by 2005.
Beginning later that day, the storm brought significant flooding and mudslides to Puerto Rico, the Dominican Republic, and some of the surrounding isles. The low-level center soon made landfall along the southern coast of the Dominican Republic, but the overall circulation was so large that the land encounter did not significantly disrupt the system. In fact, at the same time, a mid-level circulation moving east to west parallel to the northern coast became better defined. Soon, the surface center reformed under the nascent vortex and strengthening resumed. This is not to say interaction with Hispaniola did not affect Isaias at all: the central overcast became a bit separated from the eastern and southern portions of the circulation by a few tongues of drier air. Nevertheless, the detached bands pulled an immense amount of tropical moisture northward into the storm that fueled intense thunderstorms as well as the system's further development.
Overnight, Isaias moved northwestward into the Bahamas and strengthened into a category 1 hurricane. Some struggles with dry air caused the storm to level off in intensity on the 31st. Overnight, it peaked in intensity with 85 mph winds. The minimum pressure was 987 mb. Meanwhile, it continued northwestward at a fairly quick speed under the continuing influence of the neighboring subtropical ridge. During the morning of August 1, Isaias's center passed directly over Andros Island; around the same time, dry air, pushed into the circulation by upper-level winds out of the southwest, completely overcame the storm. A totally exposed center of circulation moved west-northwestward off of the island and nearly all convection was left on the other side of a north-south wall of low-humidity air. This caused Isaias to weaken back to a tropical storm that afternoon.
The storm continued to battle back, however, and convection blossomed again that evening. This sort of fluctuation continued, though less extreme, through the next few days. The shear persisted throughout and Isaias was not quick vertically stacked even thunderstorm activity was on the upswing: the mid-level center remained east of the low-level one. Fortunately for Florida, the system moved north-northwestward paralleling the coastline, and shear meant that the west half of the circulation was almost dry. Impacts to the state were confirmed to a few bad squalls. Late on August 2, the storm began to accelerate northward as it felt a cold front approach the U.S. east coast.
During the day of August 3, the interaction of the front with Isaias changed the shape of the storm: it took on the characteristic comma shape of a cyclone entering the mid-latitudes and the heaviest rainfall now fell northwest of the center. The outer bands of Isaias swept across South Carolina through the evening as the center approached. The inner core also recovered a bit from the latest intrusion of dry air and managed to build a more complete eyewall on radar. This resulted in the storm regaining hurricane strength.
Baroclinic forces (which are the mechanism by which non-tropical systems strengthen) also contributed to the intensification of Isaias: an anomalous jet stream out of the south fed unusually strong winds into the cyclone's eastern side a few thousand feet above ground. This phenomenon slowed the storm's weakening during and after landfall. Moreover, some of the winds mixed down to ground level, bumping up Isaias's intensity and matching its peak of 85 mph winds and a pressure of 988 mb. It maintained this through landfall just north of the Carolina border just before midnight. Isaias joined Bertha, Cristobal, Fay, and Hanna to become the earliest fifth named storm landfall in the United States on record.
By this time, the storm had begun to accelerate in earnest. On August 4, its forward speed exceeded 30 mph toward the north-northeast and it weakened back to a tropical storm. Nevertheless, rainfall totals of 3-6 inches (with isolated higher totals) were common along the storm track through the Carolinas and up the mid-Atlantic though Virginia, inland Maryland, and eastern Pennsylvania. Sporadic tornadoes broke out on the system's east side nearer to the coast. Isaias moved over the interior northeast and became post-tropical that evening. The remnant system brought rain and wind as far north as southern Quebec before dissipating.
The above image shows Isaias after achieving hurricane status for the first time on July 31st.
After moving across the tropical Atlantic as a tropical wave for nearly a week, Isaias was finally named in the Caribbean.
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