Tropical Storm Arthur - 11am Tuesday Discussion

By: National Hurricane Center
By: National Hurricane Center

1100 AM EDT TUE JUL 01 2014

Radar and satellite imagery indicate that the convective
organization of the cyclone has improved since the previous
advisory, and the cyclone is being upgraded based on a sustained
wind report of 33 kt from Settlement Point (SPGF1) on Grand Bahama
Island earlier this morning that was outside of the deep convection.

After remaining nearly stationary earlier this morning, Arthur
appears to to be drifting northwestward now with an uncertain motion
of 315/02 kt. Otherwise, there is no significant change to the
previous forecast track. The latest model guidance has continued the
trend of a pronounced mid-tropospheric trough digging southeastward from the upper-midwest into the northeastern and mid-Atlantic region of the United States by 72 hours. The 500 mb flow pattern is almost identical in the GFS and ECMWF models, which increases the confidence in this evolving pattern. As a result, a steady increase in southwesterly steering flow over the southeastern United States is expected to gradually turn the tropical cyclone northward over
the next 24-36 hours, and then accelerate the system faster toward
the northeast on Thursday and Friday. By Days 4 and 5, Arthur is
forecast to move over the far north Atlantic as an extratropical
cyclone. The NHC track forecast is just an update of the previous
advisory track, and lies down the middle of the tightly packed
guidance envelope close to the consensus model TVCA.

Northwesterly vertical wind shear is forecast by the models to
gradually subside over the next 48 hours, which should allow the
cyclone to develop its own upper-level outflow pattern. In fact,
latest visible and water vapor imagery indicates that cirrus
outflow has been expanding on the north side of the system during
the past few hours, suggesting that the shear conditions could
already be subsiding. The low shear conditions and warm
sea-surface temperatures should allow for at least steady
strengthening, and the cyclone is expected to become a hurricane by
72 hours. The official intensity forecast is similar to the
latest intensity model consensus IVCN through 36 hours, and then
slightly higher after that.

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