As anticipated, Alberto has arrived. The National Hurricane Center has decided that former Invest 93L has gained enough warm-core characteristics to be declared Tropical Storm Alberto. According to the NHC:
SUMMARY OF 500 PM EDT...2100 UTC...INFORMATION ---------------------------------------------- LOCATION...32.2N 77.7W ABOUT 140 MI...225 KM ESE OF CHARLESTON SOUTH CAROLINA ABOUT 120 MI...190 KM S OF CAPE FEAR NORTH CAROLINA MAXIMUM SUSTAINED WINDS...45 MPH...75 KM/H PRESENT MOVEMENT...SW OR 220 DEGREES AT 3 MPH...6 KM/H MINIMUM CENTRAL PRESSURE...1007 MB...29.74 INCHES
Alberto hasn’t changed much in terms of organization since the last blog post was written a few hours ago. There is still a tongue of dry air being entrained on the south side of the circulation:
This dry air is limiting the development of convection on the south side of the circulation. There are also signs that this dry air is causing the collapse of some of the deep convection over the center of the storm:
Notice in the visible satellite image above that there are bow shaped clouds on the eastern and southern side of the main convective mass. The bow shaped clouds are called outflow boundaries and form when thunderstorms quickly collapse, in turn shooting out cold air brought down from higher in the atmosphere. The cold air is denser than its surroundings so it sinks and spreads out horizontally near the ocean surface. Outflow boundaries are a classic sign that a storm is struggling with dry air.
The intensity forecast is very problematic as there are many factors that could disrupt the intensification of Alberto or even weaken the storm. First there is the dry air that is already described above. Second, Alberto is fighting against 20 knots of wind shear out of the SW according to a 21Z analysis from CIMSS. The direction of the wind shear may be assisting in injecting dry air into the cyclone. The third problem is the poor thermodynamic environment Alberto is situated in. While sea surface temperatures (SSTs) are currently conducive to intensification since Alberto is situated over the warm Gulfstream (27C), if Alberto moves closer to the coast SSTs drop off to a level that will most likely lead to weakening. There is also the possibility that Alberto may make landfall along the East Coast of the United States, which would of course also lead to weakening.
The statistical models are more bullish with intensification and bring Alberto to a strong tropical storm while the dynamical models are very unenthusiastic and keep Alberto a minimal tropical storm or even weaken it further:
The National Hurricane Center forecast is a blend of these two and keeps Alberto at 45 knots for the majority of the forecast package. Personally, I believe this is a very good forecast and think Alberto has too many factors going against it to allow it to strengthen much past 45 knots.
As I mentioned above, there is the possibility of landfall. The NHC has multiple states in the 5pm forecast track for Alberto:
It is important to note that first of all, the cone of error only depicts where the CENTER of the storm will be with the average forecast error 2/3 of the time. That means on average, the center of the storm is outside the cone 1/3 of the time. Also, there can be severe weather well away from the center of the storm. Currently, all of Alberto’s rain is on the NW side of the circulation. Unfortunately, this is the side of the storm that is closest to land. This means you can still receive very bad weather even if the center remains offshore!
The NHC is expecting that Alberto will drift to the SW over the next day as he is trapped by a surface a ridge to the NW. Since Alberto will be moving very slowly over the next two days, the storm has the potential to dump tremendous rainfall if it moves close enough to the coast. As a shortwave trough moves in from the west, it will erode the ridge and cause Alberto to begin to turn to the W, NW, then NE. As Alberto moves off to the NE, he will begin to accelerate. The National Hurricane Center did note that this track is of low confidence, so much can change.
I will update again as soon as conditions warrant. Stay tuned to the National Hurricane Center, your local forecast office, and emergency management for more information if you live anywhere from South Carolina up the Eastern seaboard.