Jacksonville bridge closings? Not likely, but...

By John Gaughan, Chief meteorologist, jgaughan@wjxt.com
Published On: Jan 28 2014 02:56:43 PM EST
Updated On: Jan 28 2014 03:10:00 PM EST

People took rare advantage of bridge closings on Dec. 24, 1989.

JACKSONVILLE, Fla. -

The last time that I can recall when the bridges in Jacksonville were closed due to winter weather was during the great pre-Christmas sleet/snow of 1989.

Aaahhh...  so, nice.  But, so bad. 

Bridges were shut down, some for days, as freezing rain, then sleet, then snow worked them over. The ice accumulation was so bad that there were folks actually sliding/sledding down the Main Street Bridge.

And it all started as temperatures were above freezing!  Huh?  Well, bridges are fully exposed to air and as such the don't retain heat once clouds roll in, or at night.  By daybreak they are basically as cold as the air temperature.  Overnight, the air temperatures in the Downtown/Arlington/Jacksonville area should still be above freezing.  Whew!  Right?! Bridges stay open!

Not-so-fast...

The arctic air rushing in, is both cold and dry.  The dry portion is what can allow a bridge to freeze before temperatures actually drop below freezing.  Why? Evaporational cooling.  Stick your finger in your mouth, get it wet and then blow on it.  Feel the cool air?  Well, this becomes a problem for bridges when dew point temperatures (a measure of wet & dry) are below freezing, they can allow wet surfaces to freeze.  The further below freezing the dew point is the more likely freezing can occur.

But wait!  It's compounded by another issue, wind.  The more you blow on your wet finger, the cooler it feels.  Overnight, winds will be up to 15 mph.  This will also give us wind chill temperatures around 25°.  Brrr...

Bottom line:  It will be a close call.  So, stay tuned to Channel 4 and here at News4jax.com, if it happens we will make huge announcements, typically, it will be on a bridge by bridge basis.

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