Severe weather analysis

The map above says a lot about why and where severe weather will occur today.  Notice the big temperature change between Hoover (70, south of front), and Trussville (55, north of front).   You can see that cyclonic flow has already developed along the front in the map below, showing isobars and winds.

When boundaries like this are present, they add to the tornado possibility two ways.  First of all, the counterclockwise flow along it adds vertically-oriented spin (same as tornadoes) for storms to tap into.  Secondly, the warm air south of the front rises, and the cold air north of the front sinks, especially if the sun comes out south of the front and throws the flow more out of balance.  This rising and sinking creates spin along a horizontal axis, that storms can tilt upward (like a football spiral getting tipped at the line of scrimmage). 

Also, the surface low over Oklahoma and Texas will move eastward this afternoon, causing the flow at low-levels to back around, increasing the wind shear further.  Right now, there is not a lot of wind shear for tornadoes, but that could all change as the low moves eastward.  And the boundary will provide its own spin anyway.

The other thing is instability.  North of the front, it’s cloudy and raining in a lot of spots, so the main severe weather threat should stay south of a line from Hamilton to Cullman to Gadsden.  Dewpoints will rise into the mid 60s with south winds in these areas, and with occasional sun, temperatures will rise also, probably to near 80 as far north as Clanton.

Storms could start any time, but the main threat will be from mid-afternoon to late evening, as it warms up and the low approaches.  Have a tornado plan in place, and have a source of weather information handy.

Category: Alabama's Weather