Movement of Weather Systems


Except for hurricanes, why do weather patterns (rain, cold fronts, etc.) seem to move from left to right across the USA?


The atmosphere is always in motion, and the primary direction of motion in the mid to upper latitudes (the range of latitudes from the Gulf of Mexico into central and northern Canada) is generally a west-to-east direction.  If you occasionally catch our WAFB AccuCast jet stream depictions, you will see that the jet stream often displays an undulating, "roller-coaster" pattern track (of ridges and troughs), but it is always moving in a general left-to-right (west-to-east) direction, even with these dips and rises.  These mid-latitude winds flowing from west-to-east are called the "westerlies."
But not ALL of the atmosphere moves in that direction.  As you noted, hurricanes often travel from east-to-west.  Why?  Because in the lower latitudes (from near the equator to the Gulf of Mexico), the atmosphere generally moves from east-to-west.  The winds located just north of the equator are called the Northeast Tradewinds ... winds just south of the equator are labeled the Southeast Tradewinds.
And close to the Earth's poles, the winds also tend to move east-to-west: these are called the Polar Easterlies. (Note that the flip-flop pattern of changing wind directions is the same for both the Northern and Southern Hemispheres of the Earth).
The result for us:  west-to-east flow (westerlies) over the U.S. is "sandwiched" by east-to-west flow well to our south and well to our north.
Now .. think about how that pattern of flow affects hurricanes.  Now you can explain the big "hook" we often see in the tracks of tropical systems.  When the storms are south of us, they are being steered mainly east-to-west in the "low latitude" flow of the tradewinds, so many head in our direction from the Atlantic Ocean.  But as these storms drift farther and farther northward, they come more and more under the influence of the west-to-east steering of the mid-latitude "westerlies."  The shift from the influence of the tradewinds to the influence of the westerlies produces the big hook we so often see in the hurricane tracks.
Hope this helps!
Jay Grymes
Chief Meteorologist
WAFB Storm Team