Record Low Pressure!

4:00 pm in Did You Know?, Extreme Weather, In the News by Ted Keller

Very Deep Low 10/26/10 18z

Not since the Superstorm of 1993 has a low pressure system over the United States achieved such a low central barometric pressure!

From the National Weather Service:

“New record set today for the lowest pressure in a non-tropical storm in the mainland U.S. The massive storm system barreling across the central U.S. had a minimum central pressure of 28.24″ or 956 mb (equivalent to the minimum pressure of a Category 3 hurricane). This breaks the old record of 28.28″ (958 mb), set on …Jan. 26, 1978, during the Blizzard of 1978 (aka the Cleveland Superbomb). This is also lower than the March 1993 Superstorm (aka “The Storm of the Century”), or the “Witch of November” storm that sank the Edmund Fitzgerald in 1975, or even the Columbus Day Storm of Oct. 1962.”

Editors note: it looks like 28.21″ is the official lowest pressure of this storm.

Development of "bomb" low! Thanks NSSL!

Such intense lows at the surface of the earth are brought about in this case by a powerful jet stream wind pattern.  For lows at the surface of the earth to deepen, air must be evacuated out of the top of the low faster than it can be replaced from below.  When this is true, air molecules are removed from the column of air leading to less air pressure as measured by a barometer.  This is called hydrostatic pressure or the total weight of the air on the barometer.

Strong jet stream winds high in the atmosphere can set up a path for air to take away from the deepening low leading to a drop in pressure.

Wind at the Jet Stream Level (300mb) 12z 10/26/10

Wind and pressure are intimately related.  The deeper the low, the larger the difference in pressure or pressure gradient observed.  The stronger the pressure gradient, the faster the wind will blow.

This storm had widespread 40-50 mph wind gusts connected to it over areas of Iowa, Illinois, Minnesota and Wisconsin eariler today.  Some of the gusts near shower and thunderstorm bands reached 60-70+ mph.

Also, the storm system has spawed many tornadoes in parts of Illinois (EF2 near Peotone), Indiana and Kentucky.  At  this writing, many tornado warnings were in effect for counties in Tennesee, Kentucky and Alabama.

Link to a great write-up by the Duluth National Weather Service.