Steering current forecast

Thunderstorm activity in association with a westward-moving tropical wave has decreased over the Northwest Gulf of Mexico this morning. Wind shear is 15-20 knots over the wave, and is expected to remain at least 15 knots over the next two days. This is probably too high to allow tropical development to occur. Wind shear is also expected to be 20 knots or higher over the Caribbean for the next week, which should stifle any development there.

Two computer models, the GFS and ECMWF, develop a tropical storm off the coast of Africa by Monday. The African wave that would likely be the seed for this moved off the coast last night, but looks unimpressive this morning. Wind shear is 30 knots over the wave and the surrounding region of the Intertropical Convergence Zone (ITCZ), and is forecast to remain high in this region for several days. Any tropical storm development will probably have to wait until the wave gets at least 1000 miles from the coast. The Saharan Air Layer (SAL) is relatively weak and far to the north at present, so dry air and Saharan dust should not be a major impediment. The UKMET and NOPGAPS models do not develop anything over the coming week, and predict high wind shear over the region. My best guess is that the earliest we would see a tropical storm form between Africa and the Lesser Antilles would be Thursday August 2nd.

Steering current forecast
The hurricane steering pattern for the next two weeks over the North Atlantic should be near normal, with no areas at above-average risk for a hurricane strike. 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. Plotted on these 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 trough's presence will recurve to the north. 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.) As seen in Figure 1, a ridge of high pressure was present this morning over the U.S. East Coast, with troughs of low pressure over the mid-Atlantic and Great Lakes. Under this steering pattern, any hurricanes in the mid-Atlantic north of about 25 degrees latitude would be recurved by the mid-Atlantic trough, but storms closer to the U.S. would not get recurved until they came very close to the coast and began feeling the Great Lakes trough. 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 8am EDT today. The colors show how much counter-clockwise spin is present (vorticity). High vorticity is associated with storms.

To get an idea of the uncertainty in these steering pattern forecasts, 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 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 experimental model graphics web site, click on the latest "Charts" 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).



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 8am EDT today. Bottom map: the forecast for 26 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 (8am EDT) July 27. Image credit: NOAA/NCEP.

For this morning's GEFS run, we see that 16 days from now most of the 20 ensemble members are predicting a shallow trough of low pressure over the Eastern U.S., similar to what we've seen through most of June and July this year, and during the entire 2006 hurricane season. However, the trough is forecast to be not as strong as we saw in 2006, and thus will be less likely to recurve storms approaching the U.S. This trough is also forecast to be transient--GEFS runs ever past week are pointing to a near-normal jet stream pattern over the coming two weeks, bringing an alternating series of weak ridges and troughs across North America and the Atlantic. This will bring a near normal chance of landfalling tropical cyclones to all regions of the Atlantic. This is in contrast to the steering pattern of 2006, which saw the jet stream get "stuck" in place, with a strong trough of low pressure over the Eastern U.S. most of the season.

My next update will be Sunday, unless there's some major development to talk about.
Jeff Masters