Monday, September 21, 2015

Tropical Storm Ida: September 20, Update A

A little late on the update alas, but I travelled Back From The Future today and am still recovering. The internet was a bit dodgy there, but there were flying cars, robots that talk, no illness, and lots of flowers and sequins and wine. :-)

Meanwhile, in the present, Tropical Storm Ida is still lurking out there at 18.9N, 47.0W, and currently heading NW at a very respectable 14mph. At first glance it looks like someone thought that they would use a dartboard to predict her track this week: 
But this drunken game of darts is actually somewhat plausible (for tomorrow at least - beyond that I don't know yet). There is a very good possibility that she will stall because of the surrounding atmospheric pressure fields.  

<Science Alert!> Why would a storm stall? You have heard of atmospheric high pressure and low pressure, right?  Imagine them as hills and valleys in the atmosphere, except made of air. Now imagine the storm is a ball on this undulating field. If the path ahead is low pressure, it's like a downhill gradient and the ball will happily keep on rolling forward. If the path in front is high pressure, it's an uphill gradient and the ball (storm) will slow down, or maybe move in a different direction until it finds an easier path downhill. But if the ball gets stuck in a 'dip', it is surrounded by high pressure on all sides, and it becomes difficult for the storm to move in any direction, so it will stop where it is until the pressure fields around it change (which they eventually will do). Of course, as pressure fields change, the longer range track forecast will also change, even though the storm may not have moved far. So not only do forecasters need to predict the track of the storm, by they need to predict the entire surrounding pressure field and how it will change before the storm moves again and that's where computer models really kick in. <End Science Alert!>

Ida should begin to slow down in forward speed soon because of the high pressure she is about to run into. It looks like the computer models are forecasting that this surrounding high pressure won't move for the next few days. I can see this possibility of stalling in the pressure data for tomorrow evening, but I don't have the data to see much beyond that (alas). If she does move after that, at the moment it looks (to me) like she will stay to the east of Bermuda.

By the way, if you happen to be sailing from the Caribbean to Bermuda this week, although she is quite far from you, you may have a slightly bumpy ride in a few days - especially if she stalls for a long time and gets stronger. 

Movie trivia #1: Marty McFly travelled from 1985 back 30 years to 1955 in the first Back To The Future movie. We are now 30 years in the future from 1985. #FeelingOldYet? ;-) 

Ida is still officially a relatively weak Tropical Storm with winds of 45mph, and a central pressure of 1005mb. I think this is too weak. I think she has winds closer to 60-65mph. She has strong circulation in the lower half of the troposphere, and in the last few hours the convection near her center has improved quite considerably: 
She isn't quite a hurricane yet because there still isn't a good circulation signal in the upper troposphere, but there isn't much to inhibit her from developing - wind shear is low, the sea surface temperatures are over 28 deg C with the upper 75m of the water column being warmer than 26 deg C, and she is moving into areas of higher water vapor in the atmosphere as you can see in this water vapour satellite movie: 
(She is the blob that has the blue bit in the middle).

I think she will continue to intensify for now. What will be interesting to see is how much she will grow once she stalls - if she does stay in the same place for a few days as forecast, she will begin to cool the water under her and may actually end up inhibiting her own growth! I'm staying in the same space/time zone this week (for a change), so I'll be watching with great interest. 

Movie trivia #2: October 21, 2015 is the date that Marty, Doc Brown, and Jennifer Parker travel to at the start of Back To The Future II. #FontOfUselessInformation #YouAreAGeekNowToo ;-) 

Night night for now!

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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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