Leslie becomes a hurricane, and is a serious threat for Bermuda and the Canadian Maritimes, threat for New England has lessened.

Leslie has improved in structure overall the last few days, and has officially been designated as a hurricane. As I had said several days ago, Leslie would not begin to intensify until it was able to stack its circulations and mix out the dry air issues. By Leslie slowing down over the last few days, it has been able to establish more upper level divergence, and has been able to stack its circulations and begin to mix out the dry air to some extent. This has lead to appreciable strengthening, however, despite conditions improving over Leslie, there are still a few issues.

First of all, Leslie itself is a big cyclone, and will take time to consolidate, also with Leslie being so large that means it will have more air to spin up, and if the surrounding airmass is not favorable, intensification will be slowed. As is the case, the airmass surrounding Leslie is still somewhat dry due to the downward MJO pulse over the tropics, which has discouraged thunderstorm activity over the tropical atlantic, thus enhancing the dry air surrounding Leslie. Also with the downward MJO pulse, pressures have risen over the tropical atlantic, and that has encouraged ridging to develop. The problem with that is, generally areas just outside of the area of ridging, just north of the deep tropics is dominated by abnormally high wind shear, which has been where Leslie has been located. The issues with wind shear have complicated matters more for Leslie along with dry air, however, wind shear is starting to slowly drop and according to CIMSS, shear is down to near 15-20 knots, which is not all that unfavorable for strengthening, especially with a large system like Leslie. The upper level ridge to the southwest of Leslie has continued to limit outflow on the south and western quadrants of Leslie, and has been the main producer of Leslie’s wind shear, but as Leslie moves away form the upper level anticyclone, outflow should become better established in those regions of Leslie. Overall, conditions over Leslie have improved, and will continue to improve the next few days with dry diminishing as Leslie continues to mix out the dry air, and upper level outflow being better established and enhanced significantly as Leslie will be in an area near Bermuda where there will be 2 major outflow channels from two areas of upper level low pressure. One will be just east of Florida, another well northeast of Leslie, and with upper level ridging being enhanced over the system and warm SSTs and TCHP values, there’s no reason not to believe that Leslie will have a very good chance to become a major hurricane as it nears Bermuda.

As for the track of Leslie, it will be fairly erratic as is currently under very weak steering currents because of the ridge that has built up north of the system. These steering currents should become better established as a trough of low pressure now currently over the upper midwest dives southeastward, and picks Leslie up. Thankfully, it looks like any chance of a potential fujiwhara with the remnants of Isaac appear unlikely, and now that means Leslie will struggle to get too much west of 65, which is good news for the US, but it is very bad for the Canadian maritimes, as it now appears more likely Leslie will directly effect that area. I am now in pretty good agreement with the NHC as far as Leslie’s track goes, but the intensity is likely a little conservative given the potential with the environment that is in place. For those of you in Bermuda, Leslie will be a serious problem, and you will likely experience days of hurricane force winds and gusty winds, along with the potential for flooding rains. New England appears to be a little safer for the moment, and some fringe effects from Leslie are possible. Regardless, very high wave action will occur for the next several days all along the east coast, and it would be advised that you stay out of the water. I hope everyone enjoys the rest of their week, and I will have another update on Leslie by Friday evening.