Tropical Storm Ana has finally developed. Tropical Depression Two, which was declassified Thursday evening, had sufficiently redeveloped a closed circulation and plenty of convection late last night. Shortly after midnight, the National Hurricane Center reclassified the depression and, with the 5AM EDT advisory package this morning, upgraded to a tropical storm. Currently, the National Hurricane Center is forecasting Ana to threaten the southeast United States coast as a strong tropical storm. But, this is a tricky forecast.
Not since 1992 has the first named system of the year come so late. That storm, Category 5 Hurricane Andrew, became one of the costliest hurricanes to strike the United States. Ana is not forecast to develop that strong. So, what are the factors playing into Ana’s future?
Upper-Level
Ana is located in an area lacking deep general moisture. This, however, should gradually improve over the next couple of days. More important to Ana’s future will be wind shear. Currently, Ana is fighting a little bit of shear on the north side. However, for the most part, the system is in a good position. It is expected to remain the case throughout the typical 5-day forecast period. Low wind shear will allow the convection to build deeper into the atmosphere. However, with little moisture around, Ana will have difficulties sustaining any bursts of development, which is what we are seeing now.
Tropical Storm Ana mid-level wind shear
SST’s
Sea surface temperature (SST) maps show Ana has some decent warm waters in front of it. However, sst’s are not sufficent to support any rapid intensification. If Ana is well organized towards the end of the 5-day period there is a likelihood for additional strengthening as the system approaches even warmer waters. It should be noted now intensity forecasts are very difficult and all will depend on Ana’s organization – it cannot strengthen if it’s not organized, i.e., vertically-stacked, symmetrical, good outflow, etc.
Organization
Infrared satellite imagery shows Ana is not quite as organized as it was overnight. NHC positions estimates put the center of the storms circulation on the northern side of the heaviest convection. In order for Ana to strengthen it must become more symmetrical. It is likely the current disorganization is also causing a vertical tilt of the storms center which will also slow down additional strengthening. This is typical of young-blood tropical storms. It is a good sign for the storms health to see cirrus clouds fanning from the spiral bands. This is a clear indication of outflow over the storm system and also indicative that shear is not playing an impactful role in the life of this storm. Should Ana find a way to stack its center in one location and not redevelop new centers (also typical of young cyclones) then convection should build over the center of circulation. Only then would the tropical storm have a chance of threatening a hurricane classification.
Tropical Storm Ana IR Satellite Image courtesy NRL Monterey
NHC Forecast
The National Hurricane Center has forecasted a strong Tropical Storm Ana to be off the Florida coast late Wednesday night into Thursday morning. It should be assumed that if the track forecast were to hold true any potential landfall could be as early as 24 hours prior due to traditional forecasting errors; therefore, potential landfall could be as early as Tuesday night.
The question becomes how accurate is the forecast track. It is my personal opinion that the NHC has a good handle on this. The trick is how Ana will develop. A weaker system will stray a bit further south (remember, the original forecast for TD Two was for recurvature into the Atlantic). A stronger system, such as what the NHC is predicting, will take the forecasted track. This is because the system is deeper into the atmosphere and controlled by a different atmospheric conditions. I’ve written about this before (see Hurricane Ike computer models underestimating storm’s potential).
Furthermore, Ana is being steered currently by a high pressure ridge to the north. Some computer forecast models are anticipating an approaching low pressure trough to break down the high, which could cause more of a northerly track. Though such an occurence would still not completely eliminate the likelihood of a US-landfalling tropical storm or hurricane.
As of right now, all residents should be prepared for either a strong tropical storm or weak hurricane to make landfall in the southeast United States. Unless there is a considerable slow in Ana’s forward progress and the trough does weaken the ridge, there is no reason to believe Ana will turn to sea prior to landfall.
Tropical Storm Ana Forecast Aug 15, 2009 0900 UTC
