Complex situation as model duel plays out

It has been an exceptional year in terms of how two of the world’s best global models have performed. I am speaking of the American generated GFS or Global Forecast System versus the European generated/supported ECMWF or European Centre for Medium-Range Weather Forecasting. Both models are highly sophisticated and provide a vast array of products for forecasters around the globe. However, both have had their share of issues over the years when it comes to forecasting tropical cyclone events.

Let’s take a look at TS Debby back in late June. The GFS was forecasting a turn to the northeast as a trough of low pressure dug in to the eastern portion of the U.S. On the other hand, the ECWMF was forecasting a ridge to build and push Debby back to the west. Here is a paragraph from the NHC’s discussion regarding Debby on June 24:

“THE TRACK FORECAST IS EVEN MORE COMPLEX. THE GFS INSISTS ON A TRACK TOWARD THE NORTHEAST AS DEBBY BECOMES EMBEDDED WITHIN A LARGE MID-LATITUDE TROUGH. HOWEVER…THE ECMWF AND THE HWRF BUILD A RIDGE TO THE NORTH OF DEBBY AND FORECAST A WESTWARD TRACK. GIVEN THE WESTWARD TURN INHERITED FROM THE PREVIOUS FORECAST…AS WELL AS THE HISTORICAL STRONG RECORD OF THE ECMWF…THE NEW OFFICIAL FORECAST MOVES DEBBY INITIALLY A LITTLE BIT TO THE NORTHEAST TO REFLECT CURRENT TRENDS BUT THEN TURNS THE CYCLONE BACK TOWARD THE WEST OR WEST-NORTHWEST IN 24 TO 36 HOURS. A MAJORITY OF THE GFS ENSEMBLE MEMBERS NOW ARE CONSISTENT WITH THE DETERMINISTIC RUN…WHICH WAS NOT THE CASE YESTERDAY…MAKING A STRONGER CASE FOR THE EASTWARD SOLUTION. WE MUST BE READY TO MAKE A CHANGE OF THE FORECAST TRACK AT ANY TIME.”

Note the reference to the ECMWF and how it has a “historical strong record”. Well, as it turned out, the ECWMF was wrong and Debby ended up turning north in to the Florida panhandle.

Forecast for Debby showing west track towards Louisiana

Forecast for Debby showing west track towards Louisiana

Later that day, the new forecast, which was the correct forecast, shows Debby headed north in to Florida panhandle

Later that day, the new forecast, which was the correct forecast, shows Debby headed north in to Florida panhandle

Now let’s fast-forward two months to Isaac in late August. Here again we had two significant differences between the GFS and the ECMWF. The GFS wanted to take Isaac north and in to a trough after entering the Gulf of Mexico, putting a substantial threat to the Florida panhandle and even the west coast.

Meanwhile, the ECMWF showed a much farther west track towards Louisiana. Here again is a quote from the NHC discussion on Isaac from August 22:

“HOWEVER…THE ECMWF CONTINUES TO SHOW A STRONGER RIDGE…AND THUS SHOWS A MORE WESTERLY MOTION THAN THE OTHER MODELS.”

As it turned out, the ECMWF was correct in a stronger ridge and Isaac ultimately made landfall in southeast Louisiana.

So what does this have to do with the current situation with Sandy? For the first few days of the forecast period, not much. Right now, the GFS and ECMWF are pretty close to each other since the pattern is fairly straight-forward right now. Sandy is forecast to move slowly northward and bend east of north and pass over or very close to Jamaica. Both models “agree” on this.

Then, both models have Sandy passing over eastern Cuba and in to the central Bahamas by around 72 hours. It’s after this time frame that the two models part ways and have two vastly different outcomes.

The GFS basically moves Sandy slowly to the northeast and out of the Bahamas. While the ECMWF takes Sandy more north by day four but also fairly slowly.

At day five, the GFS has Sandy roughly half-way between Florida and Bermuda while the ECMWF is quite a bit back to west, near 30N and 75W.

It seems that the GFS wants to hand off Sandy in to the Atlantic due to lower heights in the middle layers of the atmosphere because of a large ocean storm, partly the remnants of Rafael. This provides and escape route for Sandy to take, like a rock gradually rolling down a slope, not too steep, but just enough to build momentum and begin rolling. Sandy is the rock heading down the gentle slope.

The ECMWF does not allow Sandy to feel the slope, so to speak, and keeps it farther back to the west, much closer to the Southeast U.S. coast. Then, all “you know what” breaks loose with that model.

By day six, a significant deep trough of low pressure digs in to the west of Sandy and causes the flow to turn more from the south out over the western Atlantic. Instead of pushing Sandy out like a broom sweeping across the floor, the trough instead scoops Sandy up and swings it back to the northwest and eventually in to New England as a powerful hybrid storm. Basically, the trough “captures” Sandy and pulls it in, not allowing it to turn east and out to sea like the GFS shows. The result is a remarkable storm for the mid-Atlantic and New England; the so-called “storm for the ages” that I alluded to yesterday.

Which solution will be right? I have no idea. I can see why both models show what they show but I also know that the GFS tends to have a bias in handing off tropical cyclones in to lower height fields more often than not. In other words, the GFS is taking the easy way out and sending Sandy in to the Atlantic. The ECMWF does not do this and instead keeps Sandy in the western Atlantic long enough for the big trough to dig in and capture it, creating a large ocean storm of epic size and strength.

Day seven of ECMWF showing powerful storm as Sandy gets drawn in to trough in the East

Day seven of ECMWF showing powerful storm as Sandy gets drawn in to trough in the East

There is a lot of talk about this situation in the weather blogosphere. Opinions about both scenarios abound and some folks simply refuse to believe such a wild scenario with the ECMWF. However, considering its track record in the long term, it has to be considered and thus, once Sandy is in the Bahamas, the story will begin to unfold as to what ultimately ends up being the true course that it will take.

In the end, one of these two powerhouse global models will end up being correct. Which one that is will have enormous implications on the weather for millions of people along the East Coast of the U.S. towards the very end of the month. Stay tuned, this has only just begun….

 

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A storm for the ages? Perhaps. First, it is a Caribbean concern

99L about to become a tropical depression in the Caribbean Sea

99L about to become a tropical depression in the Caribbean Sea

There are very few instances when I have posted a headline like the one in this blog. Hype is not a tool I use to get the attention of my audience unless it is warranted and I feel it could help to save lives and property. What I am seeing in some of the global models is worthy of getting your attention and if it’s hype, then all the better in the long run.

There is a storm brewing in the Caribbean that will soon get a name: Sandy. Right now we know it as 99L, an area of investigation with potential to develop. Within the next 10 days, we may remember it as one of the great ocean storms of recent memory. Before all of that, it will be a problem for the Caribbean and that much is certain.

So far, development of 99L has been slow. It now looks as though things are coming together in the western Caribbean, a few hundred miles south of Jamaica. Water temps are as warm as could be and upper level winds are becoming more and more favorable. The NHC gives the area an 90% chance of becoming a tropical cyclone within 48 hours. I think it’s as good as done at this point and we’ll soon have TS Sandy to track.

72 hour ECMWF showing "Sandy" over Jamaica

72 hour ECMWF showing "Sandy" over Jamaica

People with interests in Jamaica need to be watching this system closely. It will bring periods of heavy rain and an increase in wind as it moves slowly towards the northeast with time. This slow forward motion is going to be a problem as the tropical rains will have a lot of time to fall over the same area for a couple of days or more. I would also not be surprised to see this become a hurricane before it reaches Jamaica. Intensity prediction is very poor even in this day of high-end computer models. Hopefully we’re only talking about a weak tropical storm in a few days but remember that hope is not a good planning tool. Being prepared is far better.

After Jamaica, the threat shifts to Cuba, Hispaniola and the Bahamas. Here too, the main issue will be excessive rain from what is surely to be “Sandy” by mid to late week. How strong it is depends on how quickly it can develop a solid inner core. Interests in the region should be paying close attention to what happens as this feature grows. We are not talking about an “in and out” system that gets kicked in to the Atlantic in a day or so. I think the big story will be the heavy rain even if this does become a hurricane in the Caribbean.

It’s what happens after the Caribbean that has the potential to make this storm one that people talk about for a long, long time.

Day five of the ECMWF showing "Sandy" after moving through the Bahamas

Day five of the ECMWF showing "Sandy" after moving through the Bahamas

To put it in simple terms, some of the global models are indicating that this storm will get caught in the southwest Atlantic and grow in to a hybrid mix of a hurricane and a Nor’easter the likes we have not seen since as far back as 1991 with the “Perfect Storm”. The closest event I can recall is “Nor-Ida” in 2009 which took the tropical leftovers of hurricane Ida from the Gulf of Mexico and transitioned it in to an epic ocean storm that blasted the North Carolina coast and points north. I was in that one and will never forget it. This storm could make that one look like a day in the park.

It all seems to have begun yesterday when some of the global models began to change their track for 99L from an out to sea event to one that may affect people from Florida to Maine. The Canadian was one of the first to show it. Then the American based GFS and finally, the very reliable ECMWF or Euro. People began talking about it within the weather blogs as if sniffing out something that movies are made out. It was incredible to read what people were saying could happen if this came to pass. Surely it was a one time fluke in the models and things would return to normal a mere 12 hours later. Not so much.

The overnight run of the Canadian global model shows pretty much the same scenario as yesterday. It takes what would be Sandy and turns it in to this enormous ocean storm that would cause coastal flooding, high winds and heavy rains for a large portion of the U.S. East Coast.

Day 8 of the ECMWF which shows a powerful coastal storm impacting the Mid-Atlantic region

Day 8 of the ECMWF which shows a powerful coastal storm impacting the Mid-Atlantic region

Looking at the latest Euro run, it too continues its forecast of developing a very large and powerful storm as the tropical energy from what would be Sandy gets pulled in to a deep trough digging in. This entrainment and phasing is rare but when it happens, it can lead to very powerful hybrid storms that have both tropical and non-tropical characteristics. The Perfect Storm in 1991 was just such an example. The overnight run of the Euro is jaw-dropping, there is no other way to describe it.

On the other hand, the GFS has all but abandoned this idea and simply sheds off the energy from the tropics in to a separate ocean storm way out in the open Atlantic. The result is….nothing. No big storm once 99L/Sandy leaves the Caribbean. It is remarkable to see such vast differences in the models and goes to show how complex the situation is. We are talking about an event that is forecast by some of the models to take place more than a week away. I debated whether or not I should even discuss it since it’s so far out in time. But I figured that rational people who read my blog would understand and appreciate the heads up if this in fact comes to pass….the bad storm that is. Maybe it’s all just a fantasy by the models that show it and the only concern, albeit a very legitimate one, will be for the Caribbean and the Bahamas. If not, and this storm happens the way the Euro shows it and the GFS showed it yesterday for a time, then we will be talking about this well past the hurricane season.

For now, we wait and go with the short term which is that we see what is a developing tropical depression in the western Caribbean. The first impacts will be felt in Jamaica and eastern Cuba. From there, we will just have to see how things turn out. The next several days could be very interesting if not very important in shaping how the hurricane season comes to an end.

I’ll post another short update this evening to go over the latest on the situation in the Caribbean. I’ll also have the video blog posted to our iPhone app by early this afternoon and it will incorporate the overnight model runs as well as the early morning or 12Z model runs to compare. If you don’t have our app, you’re missing out on a great tool in the daily video blog. It brings this discussion to life with numerous graphics, satellite shots and an in-depth explanation of what’s going on in the tropics now and what’s forecast several days out.

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Good news and not so good news concerning Debby

I have some good news to share tonight about Debby. I also have some not so good news. First, the good….

TS Debby

TS Debby

The deep convection with Debby is really falling off. This means that the strong showers and thunderstorms that drive the heat engine are not functioning too well. You can easily see the void of deep convection in the graphic. The green circle indicates the area where the center is located. Without deep thunderstorm activity near the center, a tropical cyclone cannot thrive. This is important because it means that the storm is not strengthening and may be on a weakening trend. Now I cannot possibly know for sure, but seeing the collapse of the deep convection that was definitely there last night and for a good deal of today is a good sign. Perhaps the GFS’ idea of a sheared, weaker, pulsing convection type storm is really what will pan out. The ECMWF forecast of a deeper, stronger system seems to be fading quickly.

NE Gulf Heat Content Map

NE Gulf Heat Content Map

The other issue is heat content. Hurricanes get their energy from the latent heat which is stored in vast quantities in the worlds’ oceans. The deeper the warm water (about 80 degrees F) extends, the more heat content (also called upper ocean heat content) is available. Shallower water tends to hold less heat content and the shallow shelf waters off the eastern parts of the Florida panhandle are notorious hurricane killers due to this lack of energy (see the graphic to the right- the deep blue is low ocean heat content region). I think that the slow movement of Debby is helping to churn up this shallower water, exhausting what little heat content there is; further diminishing Debby’s ability to maintain deep convection.

All of this adds up to the prospect of Debby weakening and not having much of a chance of recovering. This would keep the wind and surge issues to a minimum but the rain is another problem. This is the not so good news part.

Tropical cyclones = rain. That is how they release the heat stored in the oceans. Condensation is a warming process and the release of all that rain also releases heat. This is the very nature of what makes a tropical cyclone tick, so to speak. Unfortunately, too much rain will lead to problems and it looks like the potential exists for a lot of rain for portions of the Florida panhandle and the peninsula. There is no way to know how much rain will fall and where. This all depends on the convection that I mentioned earlier. If rain bands develop, they will drop heavy rain. But those bands can fall apart very quickly too. So the timing and areal coverage of the rain is difficult to forecast. This is why it is important to keep up with what Debby is doing several times per day. The storm is dynamic, it changes. You need to use more than a few resources to keep up with what’s going on. Whether it’s the NWS site (weather.gov) or a commercial outfit such as your local TV station or other website (like us), you will want to know what the rainfall situation is even between the major advisories issued by the NHC. One great resource is the HPC site linked here where you can get detailed precipitation forecasts. The bottom line is that you need to be aware of the fairly quick changes that Debby could bring your specific area. There are numerous ways to do that.

I’ll have more updates throughout the day tomorrow. And for our Client Services subscribers, do not forget, we have a LIVE broadcast each weekday at 2pm ET where I go over detailed graphics LIVE. We also have exclusive use of Stormpulse maps, live chat and other great features that allow you to gain even more info that can help you stay informed.

 

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Will predicted MJO pulse bring chance for first development of the season?

GFS MJO Forecast

GFS MJO Forecast

Even though we had tropical storms Alberto and Beryl already, they were both outside of the hurricane season. Since then, things have been nice and quiet. There are, however, indications that this may change. Let’s take a look at why.

There is an atmospheric phenomenon called the MJO or Madden-Jullian Oscillation that can sometimes lead to increased tropical cyclone formation when it is in its favorable mode. Think of it as a period of fertility in the tropics. A time when rising motion in the atmosphere is more prevalent, allowing deep tropical convection to form. This is what we call the “wet phase” of the MJO pulse and it typically adds a lot more showers and thunderstorms to the tropical regions of the globe where it circulates through.

On the other hand, the “dry phase” is easy to spot due to the sinking motion of the atmosphere, suppressing tropical convection. While development can take place during an MJO dry phase, it’s a lot more common to have development during the wet phase.

There are several computer models, the global models, that predict the MJO pulse and we can track that on various sites. The graphic I have linked to for this blog shows the operational GFS and its ensemble mean- the average of the various other runs of the GFS with slightly different variables. This model, along with others such as the reliable ECMWF, point to an increase in the wet phase of the MJO for the eastern Pacific and eventually the Atlantic Basin. When the MJO is within regions 8 and 1, we should begin to look for an increase in tropical convection. So, going by the graphic, we will need to pay closer attention to the tropics after about the mid part of the month as the wet phase moves through.

You’ll notice first, as it moves through the western Pacific, that we’ll see a typhoon or two develop followed by likely development in the southeast Pacific off the coast of Mexico and Central America. After that time, it is possible that a window of opportunity will open for the western Caribbean. It’s just another piece of the puzzle or a clue that we can use to know when the tropics are perhaps a little more ready for development than at other times. I’ll keep up with it and post more here as we get closer to the predicted date of the favorable MJO pulse.

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