September 7: Forecast Update for Hurricane Leslie

     As most of you have heard by now, the tropical system named Leslie has been churning in the Atlantic for over a week. She has been slow-moving, only moving a couple hundred of miles northward since Monday morning. We will be taking a look at what is in store for the future of this system.

Infrared image of Leslie at 7:45 EDT September 8 courtesy of CIMSS

     As of 2 PM AST, Leslie was downgraded to tropical storm status by the National Hurricane Center, with sustained winds at 65 mph. Leslie has been struggling with organization over the past week. Vertical wind shear aloft over the last few days has been preventing strong convective development. Now, slow propagation northward due to little or no steering currents has caused upwelling of cold waters; this has lead to the weakening of the system in the past 24 hours.

     Over the next 36 hours, Leslie should move northward over waters with ample warmth allowing it to restrengthen in an environment with little vertical shear. The latest runs of the GFS, ECMWF, HWRF, and GFDL models have all suggested that the storm will reach wind speeds of up to 92 mph in that time, insisting category 2 strength. The extensive mid-level trough currently moving eastward across the United States will accelerate the steering winds south of Leslie, pushing it north-northeastward. 

ECMWF model output graphics courtesy of WeatherBELL Models

     As of now, models are having the storm sparing Bermuda from direct impact, passing just east of the island. As it moves northeast into higher latitudes at an increasing rate, cooler waters and wind shear will weaken Leslie considerably. Models 96 hours out then suggest an interaction with Hurricane Michael, illustrated by the 12Z run of the ECMWF model (pictured left). This interaction would lead Leslie to deflect into a more northerly path. This northerly track would put Leslie in line for landfall in Newfoundland, impacting cities such as St. John's and Saint-Pierre. The 18Z model ensemble on the tracking of Leslie is noted below. The majority of the models are hinting landfall near St. John's, Newfoundland.

Model ensembles courtesy of http://derecho.math.uwm.edu/models/

     For us at Stratustatus, Leslie is forecast to become a weak category 2 hurricane before being carried away by the southerly flow of the incoming mid-level trough. The system will weaken gradually as it moves northeastward past Bermuda into the higher midlatitudes. Michael, which will likely be a tropical storm, will interact with Leslie, causing Leslie to take a northern trajectory towards the eastern portion of Newfoundland. Leslie will potentially make landfall near St. John's, likely as a tropical storm by Wednesday, September 12th. This forecast will be updated when necessary.

See our previous forecast here: http://stratustatus.blogspot.com/2012/08/forecast-for-tropical-storm-leslie.html