August hurricane outlook, part II: steering currents

The tropical Atlantic is quiet and there are no threat areas to discuss today. Several of the models predict that some development could occur Monday off the North Carolina coast, at the tail end of an old cold front. Any such development would likely move northeastward out to sea, and would probably not have enough time to develop into a tropical depression. The UKMET model predicts formation of a tropical depression off the coast of Africa about 5-6 days from now, but no other models are predicting this.

August Hurricane Outlook Part II: Steering Currents
The hurricane steering pattern for all of July and the first week of August over the North Atlantic has predominantly acted to recurve hurricanes out to sea. The jet stream has been "stuck" in a standing wave pattern, where it dips southward over the East Coast of the U.S., creating a trough of low pressure capable of recurving tropical storms once they get north of the Caribbean Sea (20° latitude). Current long-range forecasts from the GFS and ECMWF models indicate that this pattern will last for at at least another week, and possibly longer. This pattern is in contrast to the steering pattern that set up in 2004 and 2005, when a ridge of high pressure set got stuck over the Eastern U.S. A ridge in this location does not allow hurricanes to recurve, and the U.S. took a terrific battering those years. There is currently no indication that a repeat of the 2004-2005 steering current pattern will occur in 2008.

One often hears about how the Bermuda High acts to steer hurricanes, but this semi-permanent surface low-pressure system is really a reflection of what is going on in the upper atmosphere, at the level of the jet stream winds. The tool I like to use to study steering currents is the 500 millibar (mb) upper-air forecast from the latest run of the GFS model. (One can also use the 500 mb forecast from the ECMWF model, but this forecast only goes out 10 days, compared to the 16 days of the GFS forecast). A pressure of 500 mb is found at about 18,000 feet altitude, where the jet stream is active. Plotted on these 500 mb maps are lines showing how high above sea level one finds a pressure of 500 mb. Where a U-shaped bend occurs, a trough of low pressure is present. Any tropical cyclones that get far enough north to "feel" the westerly winds of the jet stream embedded in the trough will recurve to the north and northeast. Conversely, an upside-down "U" in the 500 mb height lines reveals the presence of a ridge of high pressure. Ridges force tropical cyclones to move westward (in the Northern Hemisphere). A ridge of high pressure aloft always accompanies the surface Bermuda High, so one can talk about either one when discussing steering currents. As seen in Figure 1, a trough of low pressure was present this morning over the U.S. East Coast, with ridges of high pressure over the mid-Atlantic Ocean (over the surface Bermuda High), and over the Midwest U.S. Under this steering pattern, any hurricanes in the mid-Atlantic Ocean north of about 20° latitude would be recurved by the East Coast trough. This type of steering pattern was in place during Hurricane Bertha, which recurved out to sea very close to Bermuda. One can pull up a loop going out a full 16 days of the 500 mb forecast and watch the evolution of the trough/ridge pattern to see how the steering currents might change.


Figure 1. GFS model forecast of heights of the 500 mb surface above sea level (white lines) for 2pm EDT today. The colors show how much counter-clockwise spin is present (vorticity). High vorticity is associated with storms.

Uncertainty in the steering current forecast
The GFS model has been indicating for the past several days that the trough of low pressure entrenched over the Eastern U.S. will get unstuck 10-16 days from now, and replaced by a series of weak, fast-moving troughs and ridges. To get an idea of the uncertainty in this steering pattern forecast, a good tool to use is the Global Ensemble Forecast System (GEFS) charts. The GEFS charts show runs of the GFS model done using 20 slightly different initial conditions. This creates an "ensemble" of 20 possible forecasts. By examining how these 20 different forecasts diverge with time, one can get an idea of how confident one should be of major steering current changes forecast by the GFS model.

These 20 forecast solutions are plotted as a series of colored lines that trace out the height (in decameters, or tens of meters) above sea level where a certain pressure is found. It turns out that the southern edge of the jet stream is currently found at a 500 mb height of about 582 decameters (5820 meters). Go to the NOAA/NCEP Model Analyses and Forecasts web site, click on the link for the GEFS model, then select to plot up the "500mb 540/582 Hgt Contours". The loop takes a while to load, but gives one the best idea of how the steering currents might evolve. The 20 forecasts all lie close to each other the first few days of the forecast, then begin to diverge at later times. By the end of two weeks, you'll see why these are called "spaghetti plots" (Figure 2). You'll also see why I'm stretching a bit when I say we can forecast steering patterns out to two weeks. The plots of where the jet stream might be 16 days from now are all over the place. Still, one can often get the general picture of whether a dominant trough or ridge will set up over the U.S. East Coast 16 days from now.



Figure 2. Forecast of the location where the 500 mb pressure surface will be at a height of 582 decameters (5280 meters) above sea level. This height marks the approximate southern boundary of the jet stream. Top image: the forecast for 2pm EDT today. Bottom map: the forecast for 16 days from now. The 20 different lines correspond to 20 different runs of the GFS model with slightly different initial conditions. The runs were all initialized at 06 GMT (2am EDT) August 8. Image credit: NOAA/NCEP.

For this morning's GEFS run, we see that 16 days from now the 20 ensemble members cannot agree at all on whether there will be a trough of low pressure over the Eastern U.S. This is an indication that the GFS model expects the dominant East Coast trough to break down later in the forecast period, and be replaced by a series of weak troughs and ridges moving across the U.S. The exact timing of these fast-moving troughs and ridges is difficult to pinpoint 16 days in advance, resulting in a big spread in the "spaghetti". This pattern favors less recurvature of approaching tropical storms and hurricanes. How believable is this forecast? Well, I've found that the GFS model is often over-eager to break down a "stuck" jet stream pattern that has been entrenched for many weeks. The latest ECMWF model runs do not support the GFS prediction of a weaker trough over the Eastern U.S. ten days from now. I believe the jet stream will remain stuck in its current configuration for two more weeks. This pattern should act to recurve most of the dangerous "Cape Verdes" type hurricanes that form off the coast of Africa and penetrate north of 20° latitude. However, the GFS often has the right idea about a major pattern change, but jumps the gun a bit. I believe it is likely that after two weeks, the East Coast trough will weaken as the GFS is hinting at, and a steering pattern less favorable for recurvature will set up. It's too early to speculate what might happen in September during the peak of hurricane season.

Since I went on rather long about the steering current forecast, I'll save the discussion of wind shear and African dust for Saturday or Monday. I'll have an update Saturday morning.

Jeff Masters