May 2, 2012

Meteorologic "false start?"

When does the dry season end?

This week’s drenching rains make a case for “now.”

The above chart shows "weekly interval" rain, i.e. each blue bar represents a week's worth of rain, for Big Cypress National Preserve for the last four dry seasons.  The vertically-oriented blue dotted line shows the traditional start of the summer wet season rains.  This year's dry season was book-ended with big rain events at the front end in October and now at the back end in April.  The result has been a much wetter winter in terms of the presence and longevity of surface water in the lower swamp areas.  Synthesis:  We call our winters dry seasons for a reason -- rain is relatively infrequent -- but each dry season is unique in its own way.

But as you can see from the dotted line on the above chart,

Historically speaking we are still in store for a few more dry weeks ahead.


Swamp waters traditionally bottom out in May.

Even when the summer rains arrive in earnest starting around Memorial Day it takes a good couple weeks, even the whole month of June, for them to soak into and fill up the swamp.

The above chart plots the median daily rainfall, as calculated from 1998 to 2009, for Big Cypress National Preserve.  This chart highlights the fact that the regular regime of summer afternoon showers doesn't traditionally start until the end of May.  Synthesis: Our recent week of drenching rain are still dry season rains -- not the wet season -- but that's splitting technical hairs.  Every drop of rain counts when it comes to deescalating wildfire severity which, without such meteorologic events, can often spike in April and May. 

It’s not uncommon to have a wet week of rain (or two) in April.

Just don’t confuse it with the real wet season.


I like to think of it as a meteorologic false start.

No comments: