The 2018 Atlantic Hurricane season officially just began on June 1st, but the tropical action started a bit early this year as Subtropical Storm Alberto spun to life on May 25th near Mexico’s Yucatan Peninsula. While May storms are nothing new in the Atlantic, with 50 of them having been observed since 1851, Alberto now makes it a 5-of-the-past-7-year run in which the Atlantic season has gotten off to an early start (only 2013 and 2014 have not had a pre-June 1 storm in the years since 2012). Fortunately, Alberto’s wind and storm surge impacts were minimal, but it was a prodigious rain producer for a large portion of the southeastern United States. Here, we present our preliminary rainfall analysis of Alberto while also discussing some of the storm’s more unusual characteristics.
Firstly, what makes a storm “Subtropical” as opposed to tropical? From the time of its formation until it was well inland, Alberto was actually classified as a subtropical storm, meaning that it had the warm inner core characteristic of a tropical storm, but also had asymmetries in temperature and thunderstorm coverage like a non-tropical or midlatitude storm. The asymmetries are clearly evident in Fig. 1, where the swirl associated with the low-level center of circulation is clearly visible, with most of the convection (deep, dense clouds) offset to the storm’s west side. In Alberto’s case, there was also a strong moist conveyor belt offset well to the east of the center of circulation, oriented over Cuba and Florida at the time of the image above. This had significant consequences, as much of the very heavy rain occurred well to the east of the storm center (more on this later).
While Alberto spent a significant portion of its life as a subtropical system, it did eventually make a transition to fully tropical — while over western Tennessee, of all places! As Figure 2 indicates, the system was subtropical throughout its development in the Gulf of Mexico and as it traversed the Florida panhandle and much of Alabama (subtropical status denoted by squares), but it became gradually more symmetrical and tightly organized as it tracked inland. While highly unusual, the reason for this transition was very simple: the atmosphere above the central Mississippi and lower Ohio River valleys as Alberto tracked inland proved extremely favorable for maintaining a tropical cyclone, as described by Levi Cowan via Twitter (see below). In fact, as the track in Figure 2 indicates, Alberto was not declared fully post-tropical until over northern Michigan, remarkably far north for any tropical system.
The maintenance of #Alberto over #Indiana is pretty neat. Model analysis shows a vertical profile with moist, daytime-heated boundary layer, CAPE, deep moisture, and uniform zero-shear flow throughout the troposphere. This environment can actually sustain a tropical cyclone. pic.twitter.com/YB8WxuC8aw
— Levi Cowan (@TropicalTidbits) May 30, 2018
All the unusual aspects of Alberto’s life cycle and track notwithstanding, it typified early-season tropical cyclones in one key way: rainfall was by far the dominant impact. Thanks in part to subtropical structure, Alberto was responsible for spreading heavy rainfall over an extremely wide area, ranging from South Florida to North Carolina and from the Alabama Gulf coast through western Tennessee and into Michigan. Storm total rainfall during Alberto’s trek across the southeast is depicted in Figure 3, which is our preliminary MetStorm rainfall analysis. Alberto’s main path from the Florida panhandle NNW through Alabama and Tennessee is apparent given the broad pockets of heavy rain along that axis, mainly as a result of the rain and storms around the core of the system. However, the heaviest rainfall associated with Alberto actually occurred well to the east, with southeast-facing slopes in the Appalachians of northern Georgia, extreme northwestern South Carolina, and a broad swath of western North Carolina receiving between 4″ and 22″ of rain from Alberto.
Of course, such dramatic rainfall in hilly and mountainous terrain can have dire consequences, as residents downstream of the Lake Tahoma Dam near Marion, NC, can attest. After nearly 7″ of rain in less than 24 hours in areas surrounding the dam, a landslide near the main embankment raised concerns among engineers that the dam may become unstable. A Flash Flood Emergency was issued early in the morning on May 30th, declaring that residents of Marion and surrounding areas may be in immediate danger and must evacuate immediately. Fortunately, the dam held, and an inspection by engineers later that morning determined that the dam was safe. While Alberto contributed the bulk of the rain for the month of May across the southeast, it hadn’t exactly been a dry month of May leading up to its arrival, as shown in Figure 4. Total monthly precipitation anomalies for May were greater than 300% in many parts of FL, GA, southern AL, northwest SC, and western NC, so in some places Alberto’s rains fell on already-saturated soils.
Despite the hot start to the season, most agencies and research groups that predict season tropical storm activity are not necessarily expecting a repeat of last year’s very active and destructive season, as described in this Weather Channel article. Most groups are anticipating storm activity to be near or slightly below average, although a lot of spread exists in this year’s forecasts. As always, we here at MetStat will be monitoring the tropics and the rainfall that tropical systems bring to parts of the U.S., so be sure to check back with us for more storm analyses when tropical cyclones strike.