Tropical Storm Watch Issued for North Carolina

The Tropical Atlantic has become very active with 2 Tropical Depressions developing in the last day close to the Southeast Coastline, one near Florida and the other threatening North Carolina.  Hurricane Gaston also quickly intensified into a category 3 Hurricane but remains no threat to land.  NOAA aircraft are scheduled to investigate both Tropical Depressions today.

Major Hurricane Gaston

Gaston remains a well organized hurricane and current satellite images indicate that the eye remains quite distinct with deep convection around it.  The upper-level outflow is well  established both to the west and the east of the system providing good outflow at the top of the system.  Maximum sustained winds are 120 MPH and the minimum central pressure has dropped to 957 millibars as Gaston continues to strengthen slightly.

Gaston has not moved very little during the last several hours and should remain generally stationary overnight and Monday. Gaston remains in weak steering currents caused by a blocking mid-level ridge to its northwest.  A trough that is currently over eastern Canada is expected to dampen by the time it nears Gaston, but it should be strong enough to erode the ridge and allow the hurricane to become embedded in the mid-latitude westerlies. This pattern change should result in Gaston’s turning east-northeastward continuing in that direction through the remainder of the forecast period.

The atmospheric conditions suggest that Gaston could maintain its strength for the next day or so, however, given the expected slow motion of the cyclone there is some chance that cold water upwelling would counteract that.  Beyond that time, the hurricane is likely to encounter an environment of increasing shear, drier air, and cooler water. Given these expected conditions, Gaston should begin to weaken on Monday.

Tropical Depression 8, Tropical Storm risk for North Carolina

Tropical Storm Watches have been issued for the coast of North Carolina from Cape Lookout to Oregon Inlet.

Satellite imagery shows that Tropical Depression Eight is currently comprised of a swirl of low-level clouds accompanied by minimal shower activity.  This structure is due to the impacts of 20-25 kt
of southeasterly vertical wind shear and abundant mid- to upper-level dry air seen in water vapor imagery. Maximum sustained winds are currently 35 MPH and the minimum central pressure is 1010 millibars.
The initial motion is West to Northwest at 10 MPH.  For the next 48 hours, the depression is expected to move west-northwestward to northwestward toward a weakness in the subtropical ridge near the North Carolina coast.  After that time, a mid-latitude shortwave trough moving through the northeastern United States is forecast to erode the ridge and cause the cyclone to recurve Northeastward into the westerlies.  The track guidance is in good agreement with this scenario, and the new track forecast lies near the consensus models through 48 hours which would bring the storm within 35 nautical miles of Cape Hatteras.

Wind shear is expected to decrease during the next 48 hours and  depression 8 is expected to move into a more moist environment.  Based on this, the intensity guidance is showing
strengthening as the system approaches the coast of North Carolina. The intensity forecast also shows some strengthening, but it is on the low side of the guidance envelope due to uncertainty about
whether the environment will become as favorable as the models are suggesting.   Depression 8 is expected to recurve but with such a small distance between its expected location and the coast landfall as a tropical system is certainly not out of the question.

Tropical Depression 9

Flight-level wind data from an earlier NOAA reconnaissance mission along with WSR-88D Doppler radar data from Key West indicate that the depression had been moving southwestward.  However, the most recent radar data and nearby surface observations suggest that the cyclone has now turned toward the west. The last reliable wind data from the NOAA WP-3 recon aircraft supported an intensity of 35 MPH, and that intensity is being maintained for this advisory given that the radar and satellite signatures haven’t improved. The central pressure of 1007 mb is based on a reliable observation from ship WMKN, located just north of the center.

The initial motion estimate is to the West at 9 MPH. Now that deep convection has waned, the system has turned westward and this motion is expected to continue for the next 24 hours or so. This short term motion is supported by NOAA recon dropsonde data, which indicated that 500 mb heights were 10-20 meters higher over the southeastern Gulf of Mexico than what the global models have been forecasting. After that time, the global and regional models are in surprisingly good agreement on the cyclone slowing down and turning toward the west-northwest and then northward in the 36- to 48-hour periods as the depression moves around the western periphery of a narrow subtropical ridge that is expected to be located over South Florida. By 72 hours and beyond, the tropical cyclone is forecast to lift out and accelerate to the northeast towards Western Florida coast.  The current forecast Track brings the system on shore North of Tampa.

Strong vertical shear that has been inhibiting this system for the past week is expected to gradually subside to less than 15 MPH in 18-24 hours, which should allow for more organized convection
to develop. However, the southerly low-level inflow will still be disrupted by the terrain of western Cuba.  By 36 hours and beyond, the depression will moving over SSTs greater than 30C and the light vertical wind shear is expected to back around from a northerly to a southwesterly direction, which usually favors more significant intensification. However, there is  lot of dry air in the region north of Key West and this will play a factor in preventing rapid intesnfication of this system.

Robert Millette

Staff Meteorologist

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