LRandyB's Tropical Weather Discussion

Tropical Weather Discussion - August 27, 2012

By: LRandyB, 5:18 PM GMT on August 27, 2012

Good afternoon folks!

Tropical Storm Isaac is churning across the Gulf this afternoon headed for an eventual landfall somewhere on the LA coast. Despite what appeared to be conditions favorable for intensification, Isaac has stubbornly remained a tropical storm. We won't complain. I think the single biggest reason Isaac hasn't been able to reach hurricane status has been its shear size. A storm that covers as much ocean as Isaac takes time to consolidate all that energy to the center of the storm. And Isaac is trying to do that. The central pressure has steadily dropped but it's doing so at a slow pace. The winds haven't kept pace with the pressure, remaining steadily around 40-50mph. I'll be surprised to see Isaac not at least make a minimal hurricane it will struggle to make much more than that.

The track forecast has proven to be a challenge, as anyone following this storm can attest. It hasn't surprised me to see this storm track further west than was at first expected. The real question now is how far west. The models are usually pretty accurate within the 48-72 hour point. The single most significant factor at the moment for the track forecast is the development of the high pressure ridge over the SE US. The 11am storm discussion from the hurricane center mentions the fact that this high appears to be building more than forecast. The circulation around the high could push the track further west than the current forecast is indicating.

At the moment, I think the current model solution looks reasonable but I think we can expect the track forecast to be nudged a bit further west over the course of the next two days before landfall.




Elsewhere, we should keep our eyes on the eastern Atlantic. A new wave off the African coast has caught NHCs eye and they give it a low chance of becoming a tropical system in the next 48 hours. The GFS develops this wave fairly significantly in the central and western Atlantic by the end of the week. We'll keep an eye on that system.

Randy

Tropical

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Tropical Weather Discussion - Sunday, August 26, 2012

By: LRandyB, 5:07 PM GMT on August 26, 2012

All eyes are on Isaac this afternoon as it makes its way through the Florida Keys as a tropical storm. Current Key West radar shows the storm circulation very well though it still appears a bit ragged in terms of deep convection and eyewall development. But Isaac is now entering the Gulf which is quite warm. A ridge is building in over the SE US. Water vapor and visible loops show the ridge is beginning to effectively stall out the trough over the southern plains that might have a chance to pull Isaac north. As a result, the models have shifted west as has the NHC forecast track. I wouldn't be surprised to see the track shift a bit further west before landfall sometime Tuesday night.

At this point, the models are merging on the track forecast through 72 hours. There is some divergence in the models at the 72 hour point but that divergence is based on the differences in the way the models handle the trough. The intensity forecast is fairly straight forward. Water temps in the Gulf are more than warm enough to support development and wind shear is forecast to be fairly low for the entire track across the Gulf. The biggest issue that Isaac might have in intensifying is the lack of core organization at this point and the extent to which the storm is spread out. Interaction with the Keys and the SE FL peninsula will slow down immediate intensification. Systems spread out as broad as Isaac typically have a tough time getting organized quickly. The short distance (relatively) between Isaac's current position and i's forecast landfall position doesn't give the storm a lot of time to consolidate and build before it makes landfall. If Isaac follows the forecast path through my backyard, I don't think it'll have the chance to be much more than a cat 2 hurricane. If the track shifts much further west, that'll give the system a bit more time over warm water and it might be able to reach a weak cat 3.

Having said that, even a cat 2 is a storm to be reckoned with. Please take seriously all watches and warnings posted for your area.

I'll post again tonight after the 11pm EDT advisory.




Have a great day!

Randy

Tropical

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Tropical Weather Discussion - August 21, 2012

By: LRandyB, 5:01 PM GMT on August 21, 2012

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

Tropical

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Tropical Weather Discussion - August 7, 2012

By: LRandyB, 3:28 PM GMT on August 02, 2012

Things are heating up a bit! We have recently upgraded Hurricane Ernesto about to make landfall near Belize and a new invest in the eastern Atlantic.

Let's take a look.....

In the Gulf of Mexico ......



The weather is generally good across most of the Gulf as upper level ridging extends from the outflow of Hurricane Ernesto. Showers and thunderstorms along the FL peninsula were popping up with daytime heating aided by an upper trough over the eastern US. Hurricane Ernesto is expected to cross the Yucatan and emerge into the southern Gulf, where it will most likely gain some of its energy back before making a final landfall in Mexico.

In the Caribbean .....



Most of the Caribbean is dominated by an upper level ridge of high pressure extending from near Panama. This is producing generally fair skies over the region. A tropical wave is producing some showers and thunderstorms in the eastern Caribbean and we can expect to see that spread into the central Caribbean.

The big news here, of course, is Hurricane Ernesto approaching the coast of Belize. NHC just upgraded this system this evening. The forecast for Ernesto is pretty straight forward. We can expect some weakening of the system as it crosses the Yucatan tonight and tomorrow. Once Ernesto enters the southern Gulf tomorrow night, some restrengthening is expected to occur. Whether or not Ernesto regains hurricane status will depend to a great extent on how much energy it loses over the land crossing. Even if it does regain hurricane status, it will be a minimal hurricane at landfall in Mexico.

In the Atlantic ....



Our biggest area of interest right now in the Atlantic is invest 92L approaching 35W near 12N. On their latest TWO, NHC gives invest 92L a 30% chance of becoming a tropical cyclone in the next 48 hours. Given the warm SSTs and relatively low shear ahead of this system, it seems very likely to be our next tropical system. I would expect NHC to begin tasking the Hurricane Hunters on this system this weekend.


Have a great day!

Randy

Tropical

Updated: 3:19 AM GMT on August 08, 2012

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About LRandyB

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