NHC showing likely development of “something” as we head in to the weekend

NHC 5-day outlook map showing the high chance of development for invest area 91L off the Southeast coast

NHC 5-day outlook map showing the high chance of development for invest area 91L off the Southeast coast

The area of interest off the Southeast coast, now known as “invest 91L”, is slowly getting better organized. The NHC has increased the chances of development in to the high category as we head in to the big holiday weekend.  But development in to what, exactly? That remains to be seen.

According to the latest statement put out this morning, a tropical or subtropical storm could form from the system as it approaches the coast this weekend. Let’s take a look at the differences between the two possible scenarios:

A tropical storm is what we are most used to hearing about. Winds are more or less concentrated around a well defined area of convection or thunderstorm activity close to the center.

A subtropical storm is more like a hybrid storm that has some tropical characteristics while also displaying some non-tropical signs as well – such as having winds and energy spread out over a larger area and more loosely defined convection. In other words, a subtropical storm hasn’t quite bundled all of its energy around a distinct, warm-core center like we are used to seeing with purely tropical systems, especially hurricanes. Subtropical storms usually transition completely in to classical tropical storms if they remain over warm water long enough.

In the case of 91L, right now, it remains spread out and not very concentrated, therefore, development has been slow. As long as this continues, we won’t see much more than a nuisance rain maker for the Carolina coast this weekend. However, water temps in the Gulf Stream, which is still to the west of the developing storm, are quite warm and it is possible that we will see a pure tropical storm form which would mean more wind, rain and rough surf conditions for the coastal areas that it impacts.

The good news is that none of the model guidance suggests anything too strong coming from this. After all, it is only late May, not September. That being said, we should never ignore a festering tropical feature that is so close to land. If you have plans along the beaches from Georgia to Cape Hatteras, keep them, but be aware of this feature and the potential for heavy rain and some gusty winds. The other hazard that would concern me is rough surf. Water temps along the beaches are still sub-80F but this will not keep people out of the water this weekend. Watch for local conditions to change including the chance of increased rip currents. Remember, tropical storms have the potential to be deadly if people don’t understand the local impacts. Keep an eye on the kids if they plan on heading in to the ocean in the affected area during the long holiday weekend.

I will post a video discussion concerning 91L later this afternoon followed by a blog post update here this evening.

M. Sudduth 9:50 AM ET May 26

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Erin Now a Depression, Gulf System Disorganized

As of 11:00AM this morning, most of the deep thunderstorm activity with what is now Tropical Depression Erin has vanished.  The culprit is dry air sitting over the majority of the tropical Atlantic, which has essentially put a lid on convective development – much like throwing a glass of cold water in a boiling pot of water.  Here’s a water vapor image, showing what is left of Erin and the stable environment out in front of the former tropical storm:

Tropical Storm Erin and the Atlantic

Tropical Depression Erin (pink arrow) and the dry environment ahead as seen in Water Vapor imagery

 

Meanwhile, closer to home, a low level swirl has emerged into the southwest Gulf of Mexico.  This is what is left of the tropical system we’ve been watching over the last few days.  Again, dry air and strong upper level winds have created an unfavorable environment for development in the Gulf, even though a distinct low-level swirl can be seen in visible satellite imagery:

Visible image of 92L in the Gulf

Tropical disturbance in the Gulf of Mexico as of Friday morning. Note the low level swirl, free of thunderstorm activity.

 

As long as the environment continues to be hostile like this, it will be very difficult for either system to get better organized.  However, since the Gulf system is close to land – it will continue to be monitored for signs of development over the next few days, and recon is standing by to fly into the system if necessary.

Looking forward a few days, the large-scale environment is about to change across the tropical Atlantic.  A large-scale atmospheric wave called the MJO (or, Madden-Julian Oscillation), is expected to move into a phase which will enhance upward motion in the Atlantic next week.  Generally speaking, this creates an environment with less dry-air, lower surface pressures and more shower and thunderstorm activity across the deep tropics.

As noted in the latest update from Colorado State University, it is possible we will see one or more systems develop in the second half of August as we quickly approach the peak of the Atlantic hurricane season.

M. Watkins – Friday, 12:31PM EDT 8/16/2013

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East Coast surfers get ready, Leslie will be a wave maker as we watch to see if it does in fact recurve

As the remnants of Isaac dump rain across a good deal of the mid-Mississippi Valley, causing even more problems along the Mississippi River downstream, we now begin to focus more on TS Leslie and its future over the next week to 10 days.

The NHC track has indicated a turn to the north near 60 degrees west longitude; just west of there to be exact. However, I think this is going to change in the coming days as indications are that there will be stronger Atlantic ridging to keep Leslie moving more west than north. How far west and for how long remains to be seen and it is entirely possible that a trough of low pressure in the steering currents of the atmosphere will come in and dig east and south just enough to scoop Leslie up and turn it out to sea.

GFS 500mb Chart Showing Leslie SW of Bermuda in 8 Days

GFS 500mb Chart Showing Leslie SW of Bermuda in 8 Days

If we look at the 6Z GFS model at 500mb, about the middle steering layer, we see Leslie very well as that dark red ball to the southwest of Bermuda and that 588 “oval”. That is a height line which is kind of like a contour line on a topographic map. This shows that the Atlantic ridge is quite strong and won’t allow Leslie to turn north in to. Leslie is akin to a small water balloon while the ridge is akin to a huge, more dense water balloon. The storm has to go around the ridge and look for a weakness. We heard this a lot about Isaac and when it might turn north. The ridge turned out to be stronger than originally forecast by the global models and Isaac moved much farther west in to the Gulf as we all know now.

Since Leslie is forecast to be a hurricane, and likely a strong one too, its proximity to the United States around day 8 and beyond means an increase in long period swells is coming. For surfers this is great news. For swimmers, especially those with little experience in the ocean, this can mean dangerous surf conditions are in store. We’ll address that as needed down the road. Right now, the upcoming holiday weekend will be JUST FINE and there are no worries about Leslie anywhere. But later next week, we will need to watch and see how much that Bermuda High expands and thus how much of a correction west the models indicate.

Elsewhere, Kirk is a solid hurricane but is of no concern to land areas at this time.

In the eastern Pacific, hurricane Ileana will turn west under a strong ridge of high pressure and move away from the Mexican coastline while we monitor another large area of disturbed weather farther south and east of Ileana for possible development.

I am currently heading home from our Isaac mission and will have regular blog posts throughout the weekend ahead.

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MJO pulse starting to show up as development potential increases in southeast Pacific

MJO Chart

MJO Chart

Right on cue, the MJO or Madden-Julian Oscillation, is moving out of the west Pacific and in to the eastern portions of the Pacific. The yellow shading in the graphic indicates the forecast for the MJO from the GFS model over the next several days. The enhanced upward motion associated with the MJO is likely to give birth to a tropical cyclone off the coast of Central America in the southeast Pacific.

The NHC is currently highlighting an area of showers and thunderstorms not too far off of Costa Rica. It has a very pronounced curl to it which indicates to me that development is likely. Water temps are plenty warm and upper level winds will probably just improve over the next few days.

A look at computer models suggests that the system will move rather close to the Pacific coast as it steadily develops. Interests in the region from Costa Rica northwestward to the coast of Mexico should be paying close attention to this feature. At the very least, heavy rain and squally weather will likely impact the immediate coast along the Pacific side of Central America over the next few days. I’ll post more info on this developing system daily with additional updates on our Twitter and Facebook pages as well.

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East Pac and Atlantic Basin trying to get ahead of schedule

The east Pacific hurricane season does not officially begin until May 15 and the Atlantic season not until June 1. However, that does not mean that the tropics will wait until those dates to start producing interesting weather.

Many of the global computer models are hinting at tropical cyclone development in the southeast Pacific and possibly in the western Caribbean over the next week to 10 days. Water temps are warm enough, no question about that. But will upper level winds and other factors such as dry air be pro or con for development? Let’s take a look.

May 10-20 Points of Origin Map (graphic 1)

May 10-20 Points of Origin Map (graphic 1)

First, how about climatology. If we look at the points of origin for tropical cyclones from May 10-20 (graphic 1) we see that both basins have almost the same number of developments 9 for the Pacific, 8 for the Atlantic. So it is possible to see development in both regions during this upcoming time period. This also makes me wonder why the east Pacific starts on May 15 and the Atlantic does not. They both seem to have the same chances for development from this date forward. Interesting, but not for me to decide.

Caribbean Sea Vertical Instability Graph

Caribbean Sea Vertical Instability Graph

Next, let’s look at the vertical instability for the Caribbean right now. Granted this is the current pattern, but it will give us an idea of whether or not the mid-levels of the atmosphere are too dry. As we can see, the vertical instability is currently running just a tad above climatological levels, meaning that the air is nice and buoyant or unstable. This is important because a stable, drier environment, like we saw for almost the entire hurricane season last year in this region, will not allow for tropical cyclone formation or at least keep it weak and disorganized. This is a marked change from last season and something we’ll need to monitor throughout the upcoming season.

GFS 168 Hour 200mb Forecast

GFS 168 Hour 200mb Forecast

Next, upper level winds. The GFS forecasts a fairly nice area of high pressure at the 200mb level in a week. I won’t look beyond a week since too many variables come in to play. But right now, the latest operational run of the GFS shows an area of favorable upper level winds developing over the western Caribbean Sea, extending from the southeast Pacific actually. All we need now is a disturbance of lowering of the surface pressures to set off tropical convection. Will that happen? The various models suggest that it might. I would bet more on the east Pacific than the Caribbean right now, but do not discount entirely the chance of a named storm in the east Pacific followed by the chance of one forming in the western Caribbean Sea before June 1. It’s rare, but it does happen and the region we’ll be watching, according to the climo map in graphic 1, shows that we’re looking in the right place.

So for now, just something to monitor and nothing to be concerned about, not in the least. As things develop, or not, I’ll post more info here and via our Twitter and Facebook pages. Not following us there yet? Click either of the two icons above to join the social media side of HurricaneTrack.

 

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