Ceaseless Wind

Hurricanes really start to fire up this time of year.  The combination of warm ocean water with still summer-like weaker winds over the tropics makes this the best time for storms to form.

Cape Verde storms are very popular during this period and are named for the island cluster over the eastern Atlantic which sees easterly waves travel close by.

Easterly waves form over Africa and move toward the west over the Atlantic. These waves exist in the lower troposphere in the lowest 3 miles. They start to appear in April or May and last until October or November. They have a period of about 3 or 4 days and move west between 10 and 20 knots.

Shown below are typical storm frequency and paths for August, September and October.

The next 14 days seem fairly active with the long-range computer models showing various lows or even tropical storms developing.

Forecast Track for Earl

Cape Hatteras “sticks out” into the Atlantic.  For this reason, it catches the curve of many a tropical storm or hurricane.  Earl may be the next one!

Earl became a minimal hurricane early today.  It is on a path which takes it on the northern edge of the Leeward and Virgin Islands.

Earl is forecast to become a major hurricane by Tuesday with a borderline category 2/3 rating on the Safir-Simpson scale (below)

This storm is following a very typical track for late August hurricanes.  The included map shows the climatology of August storms including frequency and favored paths.

Radar Loop of Earl

August Hurricane Climatology

Air Flow in a Hurricane

As I recently informed my Introduction to Atmospheric Science students, well developed hurricanes have a feature you might not expect.

Everyone focuses (no pun intended) on the eye and eye wall and they should…it is awesome to watch and is the most powerful part of the storm.

Hurricanes spin cyclonically or counter-clockwise, a rotation initiated by the earth’s rotation. But if you look closely at the satellite loop of Danielle from Friday, the high, thin-looking clouds on the edges are actually arcing outward and have a slight anticyclonic or clockwise rotation, the opposite of what is happening below!

The reason for this behavior lies in the temperature profile of a developed hurricane. The massive amount of convection or thunderstorms in the center of these storms is releasing a tremendous amount of heat energy. This creates a warm bubble of air over the top and center of the storm. This in turn creates a relative area of high pressure which cause air to flow outward which is what is revealed in the cloud motion.  The whole system is what meteorologists call a warm core low.

With sustained winds at 135 mph, Hurricane Danielle is rated a 4 on the Saffir-Simpson scale.

Such strong storms are always great to watch especially when we know they are no great threat to any land areas. This storm is still presently forecast to move to the east of Bermuda.

The Samir-Simpson scale is a hurricane rating system from one to five. You can read more about the scale and how it was derived here.

The Atlantic is quite active presently with Tropical Storm Earl to the southeast of Danielle and another strong wave coming off of the coast of Africa.

This time of year favors storms that develop far out in the eastern Atlantic and are nicknamed “Cape Verde” storms after the island chain they past near or over while coming off of the African continent.  I blogged a bit about this several days ago if you would like to read more.

T.S. Earl

Tropical Storm “Earl” joins Hurricane “Danielle” in the tropical Atlantic.

Danielle is maintaining intensity and is forecast to become even stronger over the next three days. The NHC is suggesting a peak wind forecast of 95 knots but also a turning of the storm to the  north.  This track still puts Danielle east of Bermuda.

Meanwhile, tropical storm Earl is quickly developing in the far eastern Atlantic.  Forecasters believe Earl will be a hurricane as well in the next three days.

Unlike Danielle, Earl seems bent on traveling more westward and could in theory come closer to the eastern portion of the United States by the middle of next week.

"Wave" Off the African Continent

About this time of the year, forecasters begin to watch the area off of the east African coast for the next big hurricane.

“Cape Verde” storms are the result of “waves” or troughs of low pressure which leave Africa westward bound.  They often will spark disorganized thunderstorm activity.  As they encounter warmer water, more unstable air and less dry air from the Saharian Desert, they can develop a closed circulation, the first step to a tropical storm.  Assuming the wind shear isn’t too strong, these waves often do turn into something stronger.

What steers hurricanes?  Storms of the type I’ve been discussing are caught in the trade winds of the central Atlantic which blow westward off of a large high which sits north of their track.  A “wave” is an area of low pressure which forms in connection with the InterTropical Convergence Zone (ITCZ), which is where the trade winds of the northern and southern hemisphere converge.

A hurricane will travel westward or west-northwestward until it encounters a dip or distrurbance in the jet stream of the middle latitudes.  These winds, “the westerlies”, drive weather systems eastward.  If a storm travels into this zone, it will want to curve more to the north and eventually northeast.  If a storm is at a low latitude or never encounters any jet stream waves, it continues what it was doing and will likely slam into land in a zone from central America to the east coast.  The confidence in timing these hurricane landfalls with precision is higher because the winds are steady and there are fewer variables.

September Hurricane Frequency and Tracks

On the other hand, when storms encounter the jet stream, or large island landmasses or other surface pressure features, they become much more unpredictable.  Knowing when a storm will “do the curve” then is completely about the accuracy of the computer models in forecasting jet stream waves as they move eastward in the middle latitudes.  How accurate is this?  How accurate is the forecast five days or more into the future for us?  Sometimes o.k., sometimes the timing is way off!