Forecasting Tools for Tropical Cyclones

Let’s look at a real example of the types of forecast variables that tropical forecasters examine when forecasting tropical cyclones. First, to identify areas of low-level spin in the atmosphere that may mark the development of a tropical cyclone, forecasters look for centers of low-level cyclonic vorticity. As a tropical cyclone becomes more formidable, the center of low-level cyclonic vorticity becomes stronger. 

Here we have a really notable bullseye of cyclonic vorticity on this ECMWF 24-hour forecast prog for 850-mb heights, cyclonic vorticity, and wind. This happened to be Super Typhoon Sinlaku located over the western Pacific. So, we had a really formidable tropical cyclone here, with a center of cyclonic vorticity that was near the top of the color scale.

To get a sense for how strong tropical cyclones like this one will move in time, forecasters sometimes turn to 500-mb heights and winds as a proxy for the steering flow, which in reality is a bit more complex than that, but 500-mb can be a useful starting point. This forecast prog was initialized a day after the 850-mb prog we just looked at, but it’s also a 24-hour forecast. Discerning the steering flow can be a little tricky because if you look at the wind barbs right around the storm, they’re clearly impacted by the storm’s circulation itself. And, the strongest steering currents would be off to the north in the stronger westerly flow where the height gradients are larger. So, how can we get a sense for the actual steering flow impacting the storm? Well, for starters, we want to start looking at the winds several degrees latitude away from the center of the storm, and we want to use the overall pattern of 500-mb heights to help us.

Here, for example, there’s a 588 dm contour to the east of the storm, which encloses a center of high heights. So, we have an upper-level high, around which we would expect clockwise flow in the Northern Hemisphere.

On its western flank, we would expect flow from the south-southwest, much like what’s shown by the arrow, largely parallel to the height contour. So, the steering flow here would be steering the storm toward the north-northeast.

If we jump ahead two more days to the 72-hour forecast, we see that the model did move the storm to the north. Now, a deepening trough to the north is starting to bring the stronger westerly flow closer to the northern flank of the storm, but it’s still on the northwestern flank of the high to the southeast.

So, we would expect the storm to start to be steered more toward the northeast.

Jumping ahead another two days to the 120-hour forecast, we can see that the model did move the storm toward the northeast, and now it’s become embedded in the faster westerly flow, which should start to whisk it off to the east. 

Now, returning back to shorter-term 24-hour forecasts, let’s look at the factors that could impact intensity. Forecasters often assess vertical wind shear in a deep layer of the atmosphere, and the layer from 850 to 200 mb or 250 mb is common. This particular prog shows vertical wind shear between 850 and 200 mb expressed in knots, with arrows depicting the direction of the shear vector. It also shows the centers of surface lows, so that we can easily pick out our Super Typhoon. Strong vertical shear around 20 knots or more is often detrimental to tropical cyclones, so that roughly coincides to the greens, yellows, oranges, and reds on this prog. 

The first thing that probably jumps out at you is the belt of really strong wind shear from the west, which we would call westerly shear, north of the storm, associated with stronger westerly flow aloft that we saw on the 500 mb prog. But, our storm is south of that at this prog’s valid time.

And, on the southern flank of the storm, there’s a smaller belt of wind shear oriented from the east, which we would call easterly shear. In between the two, lies the center of our storm. But, also note that there are pockets of relatively strong shear embedded within the storm’s circulation. That’s something that forecasters must keep in mind when looking at shear forecasts, because the cyclonic circulation of winds around a tropical cyclone sometimes produces a narrow swath (or swaths) of stronger vertical wind shear within footprint the storm’s circulation itself. As a general rule, you should ignore these swaths and focus your attention on the overall pattern of the surrounding environmental vertical wind shear in which the tropical cyclone is embedded, because that’s what could really hinder the storm. Here, our tropical cyclone has found a pocket of weaker shear, which would be favorable for intensification, but as we already saw, the storm would be moving toward the north, so it would likely soon find itself in an environment with stronger westerly shear.

Forecasters also assess mid-level relative humidity to assess how favorable the environment is for sustaining organized convection. This particular prog shows average relative humidity in the layer from 700-300 mb along with the average winds in that layer in knots, though you may also find progs with other layers like 700-400 mb or even 700-500 mb. But, they’re all trying to assess the mid-level relative humidity.

Here, we see a pocket of very high mid-level relative humidity over the center of our storm – relative humidity values are well over 70 percent, and are even approaching 100 percent. Again, we have to be careful here, because we should expect to see a pocket of high relative humidity collocated with the storm itself. That's because the updrafts that sustain showers and thunderstorms promote cooling, lowering mid-level temperatures and increasing relative humidity there. 

Of greater importance here is that there’s quite a bit of dry air with much lower relative humidity – less than 30% in some areas – on the western and northern peripheries of the storm. Such low mid-level relative humidity can inhibit convection and weaken the storm, and with our storm moving north, and with dry air looking like it might be wrapping around the western side of the circulation a bit, we might expect this mid-level dry air to start contributing to some weakening going forward.

Finally, forecasters also look at sea-surface temperatures to identify areas where ocean temperatures are high enough to favor evaporation and moistening of the lower-troposphere, which favors deep convection. Sea-surface temperatures greater than 26 degrees Celsius are generally considered favorable for tropical cyclones, which corresponds to the yellow, orange, red, and purple shadings on this forecast. At the time this forecast was valid, the storm was predicted to be located over waters that were 27-28 degrees Celsius.

But, again, with the storm moving north and then northeast, it was going to soon run out of favorable real estate, and moving over the cooler waters can help stabilize the lower troposphere, and inhibit convection. So, our forecast overall is for a storm that may be able to maintain its strength or even intensify briefly, but beyond a day from our model initialization time, the environment was going to become increasingly hostile with stronger vertical wind shear, more dry air, and lower sea-surface temperatures.

So, let’s see what actually happened by looking at the actual storm track. 

Our model progs were initialized when the storm was about where the X is – when it had the intensity of a Category 5 storm on the Saffir-Simpson scale. The storm moved northwestward over the next 1-2 days, so the track forecast wasn’t perfect, but it did turn toward the north and then northeast as we expected. And, by the time the storm got to 20 degrees North latitude, it was a Category 2, and it continued weakening to a tropical storm as it continued turning northeastward. So, we were able to anticipate this general behavior by looking at model forecasts for key variables.