Ophelia re-strengthening

Ophelia weakened last night, as strong upper-level winds out of the south continued to shear the storm's southeast side. The NOAA high-altitude jet measured a thin layer of southerly winds of 20 - 30 knots at the top of the storm, which is a significant amount of shear. Ophelia had been able to slowly strengthen the past two days in the presence of 10 - 15 knots of shearing winds, but 20 - 30 knots of shear almost always causes weakening, as we observed.

Indications are now that the shear has relaxed significantly, and Ophelia is intensifying once more. The 8am EDT hurricane hunter flight found a center pressure of 983 mb, Ophelia's lowest pressure yet. This is a drop of eight mb in just four hours, the fastest rate of intensification we've seen with this storm so far. The 10am hurricane hunter report shows the pressure has stopped dropping, though, with a 984 mb reading. Satellite imagery shows a well-organized and expanding area of outflow, but restricted on the southeast side where the shear is. The storm is growing in size, and is transitioning from a small storm to a medium-sized one.

Ophelias's surface winds are below hurricane force, but with the large fall in pressure seen by the hurricane hunters, it is only a matter of 12 hours or so until the winds increase to hurricane force once more. The maximum observed winds at 8am were only 57 knots (65 mph) at 5,000 feet altitude, but had increased to 67 knots (77 mph) on the 10am hurricane hunter eye report. The maximum temperature in the eye was 2C warmer than just outside the eye, which is not very impressive. This difference was 5C yesterday. Doppler radar wind estimates also indicate maximum winds are below hurricane force.


Figure 1. Wind speed estimates from the Melbourne Doppler radar. The red colors on the south side of the eyewall indicate strong winds blowing away from the radar. Because the radar uses the Doppler effect to measure wind speed, it cannot tell what the wind speed is when the winds don't have a component of motion towards or away from the radar. Thus, winds on the east and west eyewall are coded white (a speed of zero towards the radar) and the winds on the north eyewall are coded blue (blowing towards the radar).

Ophelia's signature on long range radar out of Melbourne is not too impressive; only half an eyewall is apparent. Very little convection exists on the east side of the storm. The radar also indicates a slow north-norhteast motion away from Florida. Looking at this radar signature, it is surprising to me that the hurricane hunters measured such a large drop in pressure.

OK, now the "where will Ophelia go?" game. We are still at least four days from a possible landfall, and steering currents are weak, so the track forecast is highly uncertain. Data from the NOAA jet's high-altitude mission last night were used to help initialize the computer models today, and they are much more tightly clustered. This gives credence to the idea of a landfall in Georgia or South Carolina sometime in the Tuesday - Thursday timeframe. The NHC did not buy this initially, prefering to see another run of the models before committing to this idea. Now, however, they have come on-board and are also forecasting a landfall in South Carolina four days from now. Certainly, residents of Florida and North Carolina cannot breathe easy yet, until the models portray a more consistent picture of Ophelia's future track.

The intensity forecast, as usual, is highly speculative and low-confidence. The models all show Ophelia strengthening to a Category 1 to Category 3 hurricane by five days from now. The shear that has dogged Ophelia its entire life is forecast to remain, at least in the short term. A weak trough pushing off of the East Coast Saturday may also generate some shear the next few days, as could a large upper-level low over Puerto Rico approaching from the east. Dry air on the north side of the storm may also be a problem for Ophelia. Clearly, there are many hurdles for Ophelia to overcome. That being said, this storm has shown the ability to intensify in the presence of some modest shear, and as she expands in size, will be able to shrug off the shearing winds surrounding her more easily. Ophelia is over warm Gulf Stream waters and will remain so for the next five days. All these factors considered, and given the fact that hurricanes during this unprecedented hurricane season of 2005 have shown an uncanny ability to become intense hurricanes, it would be no surprise if Ophelia grows to a Category 2 or 3 hurricane by early next week.

Elsewhere in the tropics
Maria and Nate are both weakening tropical storms with just a day or two left to live as they move northeast over cold waters. The rest of the tropics are quiet.

Jeff Masters