Nov 27, 2012

Chancy rain season

Did you know 7 percent of south Florida's Big Rain Days occur in the months of November and December?

That isn't much of a chance, but it's better than none.

The bar chart above shows the monthly distribution of south Florida wide Big Rain Days, as tabulated from daily rainfall data from 1992 to present made available by the South Florida Water Management District. On average, June (at 20 percent) accounts for the highest proportion of Big Rain Days, but July (at 2 percent) is surprisingly low.  Why is that?  July is a core rainy season month to be sure (local pop up showers are the norm), but the replacement of lingering spring instability in the upper atmosphere with the homogeneous Bermuda High results in a drop off in the chance of regionally-significant large rain events. Also unusual at first glance is the number of Big Rain Days that occur in the atmospheric dry season.  That's the result of moisture-laden continental fronts.  They are rare, aperiodic at best, but when they break through they can deliver drenching rains across a wide regional area.

Even more:

South Florida has only registered 2 Big Rain Days all year.

The long term average is between 5-6.

Are we desperate and long overdue?

No and yes. Water level in the swamp has dropped down about a half foot since its early October peak. View chart  That puts water out of the pines (and now shallowing in the marl prairies) but still a foot and a half deep (i.e. up high on your shin) in the middle of the pond apple forests.  That means there is still plenty of water out there ... but we could use a new infusion, too.

This years two rain days fell on April 21st (on which an average of 1.12 inches of rain was recorded south Florida wide) and August 27th (Isaac's 3.87 inches)

What is a Big Rain Day you may ask?

A formal definition will follow at 2 pm.

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