Tropical Weather Discussion - August 21, 2012

By: Randy Bynon , 5:01 PM GMT on August 21, 2012

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The tropics are getting busier as we approach the statistical peak of the Atlantic hurricane season around Sept. 10th. NHC recently started advisories on TD #9 and they expect to upgrade that system to what would be Tropical Storm Isaac later today. Another system follows TD#9 in the Atlantic and it appears likely this system will become TD#10 and perhaps Joyce. A third area of disturbed weather is off the Gulf coast of Mexico but appears unlikely to develop before making landfall in Mexico today.

Let's take a look.....

In the Gulf of Mexico ......



A surface trough/front bisects the Gulf of Mexico from the FL Panhandle southwest into northern Mexico. There are numerous showers and thunderstorms along and southeast of this line in the southeastern and central Gulf and dryer, cooler air filtering in under the region of high pressure behind the front.

At the southern end of this front off the coast of Mexico, an area of disturbed weather is being monitored by NHC (invest 95L). This area is a combination of the remnants of TS Helene that made landfall in this area a few days ago and the southern end of the front that is stalling and washing out. It's not uncommon for tropical systems to develop in the Gulf on the tail end of the remains of frontal systems. NHC gives this system a 30% chance of development but even if it does develop, it is likely to move ashore rather quickly as the winds around the high pressure over the south central US push the system westward.

In the Caribbean .....



Most of the Caribbean is experiencing fair weather today. There are some showers and thunderstorms associated with a tropical wave approaching Honduras but otherwise, clear to partly cloudy skies persist across most of the region. This will change as TD#9 approaches the eastern Caribbean over the next day or so and progresses across the northern Caribbean.

In the Atlantic ....



A surface high pressure ridge dominates most of the open tropical Atlantic from 20N to 30N all the way into the Bahamas before we see the influence of the front off the US east coast.

TD#9 is currently near 14N 54W and is moving steadily west at 20 knots. The system has a lot of dry air north of it that is hindering development but the system has been slowly intensifying over the last 48 hours. Wind shear is expected to lessen and water temps will rise so we can expect to see TD#9 become Tropical Storm Isaac soon. In the short term, the models all agree on the westward motion of this system as it develops into a hurricane by about the 36-48 hour point (sometime Thursday). Beyond the 72 hour point, the models vary in their solution. A trough of low pressure in the western US is expected to dig southeast into the SE US by late in the week. The current solution that most of the models agree on is that this trough will dig far enough south to pull this system north. How far north and at what point the turn will occur varies by model with the current spread being anything from the GFS solution up along the west coast of FL to most of the other models that forecast a turn sooner bringing this system up through the Bahamas and up the eastern seaboard.

There are two major considerations in my mind for this forecast. First is the intensity forecast. If TD#9 doesn't become a deep, well defined system, it'll be less affected by the trough. A number of factors can influence the intensity forecast. One is the eastern Caribbean "hurricane graveyard". Low level speed divergence in the eastern Caribbean in the early part of the season is known to quell storm development in this area. However, this system is fairly well developed and we aren't really in the early part of the season anymore so this may or may not have an impact. Also, interaction with land along the Greater Antilles will have a significant impact on intensity.

The second consideration is the tendency of the global models to over-amplify high and low pressure trends in the mid-lattitudes in long range forecasts. It is not at all uncommon for the models to forecast a trough to dig too deep or a high pressure ridge to build too strongly in the long term. So I would take the long range projection for the trough digging too far into the Gulf with a grain of salt, especially this time of year.

So what would all this mean for the long term forecast for this storm? If the models are wrong about the trough digging so far south, or the storm doesn't develop as well and as deep as forecast, then the system won't turn north as forecast or at the rate that is forecast. That would mean a more westerly track. As usual, we'l have to wait and see with each new model run. But it won't surprise me to see future runs nudge the track forecast further and further west.

The other news in the Atlantic is Invest 96L in the far east Atlantic. This system is a bit further south than TD#9 was at the start and it gets to ride along in the moisture trail of TD#9 so I think it's a pretty safe bet that this system will develop. The model forecasts are iffy at the moment since the models are often not too accurate until the systems become well developed so I wouldn't speculate just yet on the long range forecast for this system but for the moment it will stay on a westward track around the southern edge of the high pressure ridge dominating the Atlantic. Warm water, low shear, and a fairly moist environment all point toward development of this system.

I'll post again tomorrow as these systems develop.

Have a great day!

Randy

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7. Randy Bynon , Dropsonde SysOp/AVAPS PM
4:10 PM GMT on August 24, 2012
Quoting AllyBama:
Hey Randy! where are you these days? Still down here on the coast?


Hi Ally!! Yup, I live on the MS Coast. Waiting to see what Isaac is going to do.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
6. AllyBama
2:12 AM GMT on August 24, 2012
Hey Randy! where are you these days? Still down here on the coast?
Member Since: August 3, 2006 Posts: 133 Comments: 20639
5. Randy Bynon , Dropsonde SysOp/AVAPS PM
11:50 AM GMT on August 23, 2012
Quoting JPCSTORM:
Hello Randy.

Thank you for the update. I'm also watching this storm for South Florida implications.

Seems the GFS has had this storm pegged on South Florida for some time. That would be amazing accuracy for GFS from so far out, if Issaac were to follow the current path.

Thank you again for your forecast.

John


Well Florida does kinda stick out there into the tropics! :-)

I don't really forecast! There are lots of people out there doing that and some are much smarter than I am! What I try to do is break down the current situation into layman's terms and then talk about the factors that could affect the future trend of storms.

In short, I try to help the reader understand the weather.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
4. JPCSTORM
12:01 PM GMT on August 22, 2012
Hello Randy.

Thank you for the update. I'm also watching this storm for South Florida implications.

Seems the GFS has had this storm pegged on South Florida for some time. That would be amazing accuracy for GFS from so far out, if Issaac were to follow the current path.

Thank you again for your forecast.

John
Member Since: July 12, 2005 Posts: 0 Comments: 67
3. BahaHurican
6:01 AM GMT on August 22, 2012
Thanks for the update, Randy...

Stay safe!
Member Since: October 25, 2005 Posts: 19 Comments: 22733
2. Randy Bynon , Dropsonde SysOp/AVAPS PM
2:15 AM GMT on August 22, 2012
Quoting BrowardJeff:
Thanks, Randy.  Watching this one closely in South Florida!

I'm trying to balance warning my friends that something is out there, at the same time telling them it's too soon to know if it's coming here.
For some reason, that's hard for some people to understand.

We should be getting some better data to initialize the models, now that the HHs are active.  What's your feeling for when we'll have a better handle on the trough, and it's impact? 

-Jeff



From my experience, the most accurate range for the models is within about 72-96 hours. Outside that, they tend to be over amplified. Having said that, the models should have a better handle on that trough by about 48 hours from now, by which time the storm will be approaching Hispaniola.

The models this evening are indeed shifting further west. And NHC has upgraded TD#9 to TS Isaac.
Member Since: July 17, 2001 Posts: 190 Comments: 2012
1. BrowardJeff
9:13 PM GMT on August 21, 2012
Thanks, Randy.  Watching this one closely in South Florida!

I'm trying to balance warning my friends that something is out there, at the same time telling them it's too soon to know if it's coming here.
For some reason, that's hard for some people to understand.

We should be getting some better data to initialize the models, now that the HHs are active.  What's your feeling for when we'll have a better handle on the trough, and it's impact? 

-Jeff

Member Since: August 30, 2008 Posts: 5 Comments: 940

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I was an AF aviation weather forecaster for 12 years, then 15 years as a dropsonde systems operator with the AF Reserve Hurricane Hunters.

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