Disorganized Leslie headed towards Bermuda
Tropical Storm Leslie continues to struggle with moderately high wind shear of 15 - 20 knots, due to strong upper-level winds out of the northwest. The shear is keeping heavy thunderstorms confined to the southeast quadrant of the storm. These thunderstorms are as far removed from the center as we've so far with Leslie, as seen on satellite loops. According to the latest SHIPS model forecast, the shear is expected to stay moderately high through Tuesday night, then drop to the low category, 5 - 10 knots, by Wednesday night. At that time, Leslie will be over warm ocean waters of 29°C, and the reduction in shear and warm waters should allow Leslie to intensify into at least a Category 1 hurricane by Friday, as predicted by most of the intensity forecast models. Intensification to a stronger storm may be hampered by its slow motion, which will cause Leslie to churn up cool water from the depths that will slow intensification. Once Leslie begins moving more quickly on Saturday, this effect will diminish, and Leslie could be at Category 2 strength on Saturday, as predicted by the HWRF and LGEM models. Steering currents for Leslie are expected to be weak on Tuesday - Friday, as Leslie gets stuck between two upper level lows. The latest guidance from our top computer continues to show Leslie making a very close pass by Bermuda on Saturday, and that island can expect a 3-day period of rough weather Friday through Sunday. Leslie will stay stuck in a weak steering current environment until a strong trough of low pressure approaches the U.S. East Coast on Saturday. This trough should be strong enough to pull Leslie quickly to the north on Saturday and Sunday, and Leslie may be close enough to the coast that the storm will make landfall in Canada on Monday, September 10. None of the reliable models have shown that a direct hit on New England will occur, but we can't rule that possibility out yet. The most likely long-term fate of Leslie will be for it to miss land entirely and brush by the coasts of Nova Scotia and Newfoundland, but any forecast of what a tropical cyclone might do a full seven days in advance is pretty speculative. Regardless, Leslie will bring an extended period of high waves to Bermuda, the U.S. East Coast, and Nova Scotia and Newfoundland this week. These waves will be capable of causing significant beach erosion and dangerous rip currents.
Figure 1. Morning satellite image of Tropical Storm Leslie. The low-level circulation center is fully exposed to view, thanks to strong northwest winds creating 15 - 20 knots of wind shear.
Invest 99L in the Central Atlantic
A small extratropical low pressure system that got cut off from the jet stream and is now spinning away in the Central Atlantic, near 26°N 42°W, (Invest 99L), is headed west at 10 mph, and has developed a small amount of heavy thunderstorm activity. This storm is not a threat to any land areas, and in their 8 am EDT Tropical Weather Outlook on Monday, NHC gave 99L a 30% chance of developing into a tropical cyclone by Wednesday morning.
Jeff Masters
Reader Comments
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What's gong on?
It appears that the steering currents are going to take the remnants of Isaac out into the east central GOM...
Most of the world isn't made up of Wundergrounders.
When people are deciding how to prepare for a storm, they don't consult the SLOSH models, or click refresh on the AHPS rainfall map. They turn on the television, and look at the graphic.
You're absolutely right that they reason based on past expectations. What you're missing is how attached most Americans are to the five categories. Storms are complicated; categories are simple. So if they've lived through a number of hurricanes, they'll think back to those storms. If they were fine through a few CAT 1s, got a little flooded in a CAT 2, and got hammered by a CAT 3, that will shape their expectations. They're likely to evacuate for a CAT 3, but not a CAT 1. And that holds true irrespective of the other information flooding in.
In the post-Isaac interviews I saw and read, I heard people say over and over: "We didn't expect this from a Category 1." But actually, the forecasts over the last 48 hours were spot-on - even, truth be told, a little worse than what most actually experienced. People focused on the number, not on the details. And - even more to the point - there's a widespread reaction on the Gulf Coast blaming the NHC for putting the storm intensity too low. Instead of thinking "Golly, there's more to storms than Saffir-Simpson Categories," a surprising percentage of the public is instead complaining, "Wow, that was nothing like previous Category 1s I've lived through, so it must have been a 2 or a 3."
If the NHC is going to put out a single number for a storm, and people are - foolishly, we all agree - going to base their decisions on that number, it should start using a number that's a whole lot more predictive of the experience on the ground.
Nice.
What in the world wide web are you trying to pull?
You are so far off base in your response, I don't even know where to begin. I did not say in any way, shape, or form that SS is the ultimate answer. What I did say, and will say again, is that *local* conditions are what ordinary people care about. Yes, the potential for a hit is greater if the storm is larger. Yes, there can be downstream flooding in some areas.
The problem with HSI is that it is yet another obscure, somewhat arbitrary, index that has no meaning for ordinary people. Just look at the words of Impact Weather in their overview of the HSI
Uses for HSI:
* Scheduling post-storm resources
* Objective guidance in implementation of a phased hurricane plan
* A way to quantify offshore wave development potential
This is stuff for professionals, not for convincing individuals to evacuate.
1, Start with 1015mb.
2, Find the approximate area covered by each full Millibar of pressure in square kilometers, but not overlapping stronger(lower) pressure values. i.e. the area of each amorphous "donut" of pressure lines around the CoC.
3, For each partition, subtract forecasted landfall intensity in pressure from 1015.
4, Square the result of the pressure subtraction for each partition and multiply by that partition's size.
5, Add all of the partitions.
6, Do the same for historical storms.
7, Compare the results of the forecast to actual values of other real hurricanes.
This would solve both the size and intensity issues by combining them in a multiplicative index directly related to size and pressure.
The initial values would be very large, but could be scaled down and formatted to manageable numbers by simply dropping the last N digits.
Then you would compare analog landfalls in the public broadcast.
This would give an exact, scientific value to the instantaneous severity of the forecast landfall, rather than "guesswork" on vague notions of size and wind speed.
In this system, a 940mb ring would be given 33% more points than a 930mb ring of the same size.
The score would be expressed in "scientific" units of (mb^2)*(km^2), which has an absolute meaning, rather than vague concepts, and is similar to the IKE value, except it represents an instantaneous severity.
ACE is truly useless for forecasting, and is only useful for ranking a storm after the fact.
Systems like HSI or the one I just made up on the spot, would be excellent as forecasting tools, because they can be predicted within certain margins of error, and they can be related to instantaneous destructive potential, rather than cumulative effects over long times (which truly are meaningless in most cases since most ACE is aquired out over the open water where it effects almost nothing..)
No chance at all.
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