Risk for U.S. hurricane landfall increasing

* Late week heat wave? Full Forecast | Hurricane Tracking Center *
* NatCast | Why weather matters | Warm nights fuel heat records *

As hurricane Igor weakens over the North Atlantic, subtle changes in the global weather patterns are now taking place that may well allow a tropical storm or hurricane strike in the U.S. before the month is over (or early in October).

Hopefully, the natural evolution towards a fall jet stream pattern, where stronger wind shear (turning and/or strengthening of the wind with height that tends to tear apart tropical storms) becomes a more effective inhibitor for tropical development, will limit the duration of this 'window of opportunity'.

For the past couple of months, the dominant upper-level pattern has featured a pair of high pressure systems (shaded in orange in the picture below) in the western hemisphere ; one centered over the Southern Plains of the United States, and one over the Eastern Atlantic.

Areas of high pressure colored in orange - one over the south central U.S. and the other over the central Atlantic -- have steered storms away from the U.S. Image courtesy NOAA.

As mentioned in my previous post, many of this year's tropical cyclones that have migrated westward across the tropical Atlantic have found a 'soft spot' in the upper flow pattern over the Atlantic, between these two systems. In this zone, between roughly 75W and 45W longitude, the winds aloft have seldom been directed toward North America. In addition, the high pressure center over the south central U.S. has pushed storms in the Caribbean towards Mexico disabling a turn north towards the U.S. coast.

The pattern shift we're beginning to see now is associated with a westward displacement of both high pressure centers which will change storm tracks.

One the one hand, this setup is not optimally suited for a cross-Atlantic transit by a Cape Verde storm (the name for storms that develop near the Cape Verde islands off the west coast of Africa). Fledgling tropical storm Lisa, for example, now west of the Cape Verde Islands, is more easily blocked by a headwind from the northwest (and west) on the eastern periphery of the Atlantic area of high pressure (as shown in the picture below). In fact, track guidance indicates Lisa will be deflected more to the north rather than heading west.

Winds from the northwest on the eastern side of the high pressure center over the central Atlantic typically deflect Cape Verde storms like Lisa. Image courtesy NOAA.

On the other hand, the westward shift of the two upper-level features projected over the next several days effectively moves the poleward (northward) flow on the western periphery of the Atlantic high pressure area closer to home, and coarsely outlines a path for a landfall from the Caribbean though the Gulf of Mexico. Should we soon see the development of another Caribbean storm, like Karl, for example, the steering currents might tug it northward rather than guiding it into Mexico. We will be watching this for you closely.