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Weather Blog: Rain for some

by Gene Norman / KHOU 11 News

khou.com

Posted on June 21, 2011 at 7:12 PM

Updated Wednesday, Oct 30 at 8:12 AM

What about ME?  That's what a lot of you were saying today as you hoped for rain.  After all, that was what was forecast, right?  There were a few showers that drifted across the area starting in the mid-morning hours continuing through the early afternoon.  There were even a few rumbles in Liberty county which drifted up toward Polk county.  Then, the atmosphere closed up shop and any signs it had rained evaporated in the steamy heat left behind.

There's often a lot of confusion and annoyance about the "percent chance" forecast, especially in the summer time.  Let me weigh in on this in hopes of clarifying (though I doubt it).  On Monday, I advertised a 30 percent coverage of showers.  The problem with even this forecast is that it doesn't address where or when the rain will occur.  I might argue that the rain east of I-45 as shown on the map above reflects a 30% coverage of our 13 county viewing area.  Measurable rain fell in parts of Galveston, Chambers, Liberty, Polk and Trinity counties, or 38% of the counties where KHOU 11 can be seen. 

If the rain missed you, you might feel that the forecast was worthless and if you got rain - you feel differently.  The problem is that our ability to absolutely predict where and when rain will occur due to summer convection isn't an exact process.  Computer models and even experience identify the processes that create clouds to build into showers, but the complicated nature of rising, humid air makes it very difficult to predict in time.  The best we can do is say how confident we are that the "ingredients" for showers or storms exist and perhaps how many will form.  Duration and coverage, however, can't be more than estimates.

So what are you, the viewer and consumer to do?  There's a certain amount of interpretation to be sure, but if a percent chance of rain is advertised, you could expect rain.  Often, when I am confident that we'll see rain, I will say that I expect it to cover a certain percentage of the viewing area. 

Hopefully, you'll be in that coverage area tomorrow, when we'll see our best chance this week of getting wet.  The combination of a stalled front to the north and very humid air swept in from the Gulf should allow showers and storms to form again.  But this time, I expect them to be more widespread and to last a bit longer than today.

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