Forecast Discussion #15

[Anonymous]

From site homepage...just if anyone is interested. Decided to give the link to track graphic below rather than paste it because when pasted its too large and adds the dreaded horizontal scrollbar to the text.

IWIC Hurricane Isabel Forecast 15, 6 PM EDT September 12 2003

Note: This forecast is unofficial, and is NOT from the National Hurricane Center. See our disclaimer.

Isabel is still a category 5 hurricane. Dvorak T numbers from the Satellite Analysis Branch as of this afternoon remain at 7.0, which equates to 140 knots. Additionally, an aircraft reconnassiance plane found maximum flight level winds of 156 knots, which also corresponds to about 140 knots at the surface. The plane also measured a minimum central pressure at 920 millibars, which is slightly lower than the estimated pressure given earlier today. Based on this data, Isabel's initial intensity is kept at 140 knots for the 5 PM EDT National Hurricane Center advisory. This is not surprising, since Isabel has an impressive satellite appearance, no doubt about it. The eye is unusually large, yet well formed. The eyewall was earlier looking a little less stunning today, but is once again depicted as a symmetrical ring of deep convection around the eye. Isabel's upper level anticyclonic outflow remains excellent in all quadrants except the eastern.

The environment around and ahead of Isabel continues to look conducive. Sea surface temperatures are in the lower to mid 80s, plenty warm to sustain a major hurricane. CIMSS Saharan Air Layer analysis maps show a small amount of Saharan dust to the northeast of the system, but the quantity is not substantial enough to have any supressing influence. Hurricane Heat Potential ahead of Isabel is rather high, which also favors Isabel to strengthen or maintain its current intensity. Current shear profiles around the storm remain low, though some of the global models increase the shear towards the end of the forecast period. This could be the reason why SHIPS still weakens Isabel to a category 1 hurricane by the end of the forecast period. However, there are few problems with this. For one, SHIPS did the same with Fabian and probably many other storms in the past, and does not have the best intensity forecast record. Secondly, Isabel should be larger in size during the time due to a swelling of the upper anticyclone above it. Larger systems are less sensative to changes in the shearing environment. Since the increase in shear is expected to be relatively small anyways, I only expect Isabel to weaken at a slow pace. Bear in mind that even with slight weakening in the forecast, Isabel should remain a formidable hurricane through the 5 day period.

Currently, Isabel is moving on a general westward course. It did wobble some to the north earlier this afternoon, but has since resumed its westward motion. The storm is being steered by a strong ridge of high pressure to the north. However, this motion may change soon. On satellite imagery, Isabel's convection is already starting to align in a leftward diagonal fashion, which usually appears prior to an actual movement in that direction. More indicative is that the remnant low of Tropical Storm Henri remains intact offshore the United States mid Atlantic coastline. This is eroding the western prehiphery of the ridge to the north of Isabel, which in turn will cause Isabel to move more to the west-northwest as its satellite appearance suggests. The ridge will almost certainly build back, so Henri's remnants will not be sufficient enough to curve Isabel out to sea. Nonetheless, Isabel is a major hurricane; and major hurricanes have a taste for even the slighest weakness in ridges. Most model guidance suggests that there will still be a dent in this ridge even after Henri's remnants move out of the picture. It may only be a small weakness, but it should be enough to keep Isabel on a west-northwesterly movement through most of the forecast period.

What happens to Isabel beyond 5 days? Uncertainty still exists, but I have a little better idea then yesterday. At the very end of the period, this is the pattern I expect to be in place: Major Hurricane Isabel located just northeast of the northernmost Bahamas, a ridge of high pressure to the north, perhaps with less of a weakness, and then a strong mid latitude trough over the Central Plains. Given the expected west-northwest motion through the period, the trends with the latest tropical models, and the fact that the trough will be a little stronger than previously expected, it appears that a track into southern Florida is unlikely. While Florida is by no means yet out of the woods, especially northern Florida, the threat has diminished some since yesterday and Wednesday. Now a track out to sea still cannot be ruled out, especially if the trough moves into the picture quicker and stronger than anticipated. But even if Isabel were to avoid a direct hit on the east coast, it would come close enough to cause panic and media hype. Based on what I have seen with the model trends and the current steering pattern, the area at greatest risk of landfall is from the Florida-Georgia border to the Outer Banks of North Carolina. Keep in mind that it is still early in the game, and since any possible landfall would be in about a week, things can change easily. Indeed, a NOAA surviellance jet will investigate Isabel's surroundings this weekend to give us and the models a better idea on the future track. Therefore, all residents living along the ENTIRE east coast are expected to monitor Isabel's progress in case major changes in the forecast track occur.


Forecast Intensity:
00 hours: 140 knots
12 hours: 140 knots
24 hours: 135 knots
36 hours: 135 knots
48 hours: 130 knots
72 hours: 125 knots
96 hours: 120 knots
120 hours: 120 knots

Forecast Track Graphic

[Rainband]

Excellent discussion Thanks

[Anonymous]

Thanks band.

Btw...watch the latest movement closely. We'll probably see a few more wobbles as it keeps moving westward, but before too long this thing is going to start moving to the WNW. So when we see a northward wobble last longer than a wobble should, it's probably the start of the WNW motion. Given the convection alignment, it may be rather soon, but we will see.

[WeatherEmperor]

Good job.

[Rainband]

Thanks band.

Btw...watch the latest movement closely. We'll probably see a few more wobbles as it keeps moving westward, but before too long this thing is going to start moving to the WNW. So when we see a northward wobble last longer than a wobble should, it's probably the start of the WNW motion. Given the convection alignment, it may be rather soon, but we will see.

It will be interesting to see what the models say after the GS jet data is ingested