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CAP Talk  |  Operations  |  Aerospace Education  |  Topic: 0% precipitation with 50% chance of thunderstorms?
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Eclipse
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« on: May 31, 2016, 06:57:31 PM »

The weather is somewhat unstable by me right now, with heavy downpours followed by bright sun.
I'm trying to make a ride / no-ride decision for tonight.

My forecast app is saying "0% chance of precipitation, 50% chance of thunderstorms".

?

The answers Google brings are...unsatisfying.

Any explanation from weather people?
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PHall
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« Reply #1 on: May 31, 2016, 08:07:54 PM »

Ride if you have a death wish. Drive if you don't.
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JeffDG
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« Reply #2 on: May 31, 2016, 09:08:44 PM »

I've seen thunderstorms with plenty of lightning and no rain.

Would I fly anywhere near them?  Hell, no.  A thunderstorm's danger is not the rain but the convective activity.
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PHall
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« Reply #3 on: May 31, 2016, 10:35:02 PM »

Riding through hail on a bike does not sound fun...   [ouch]
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LSThiker
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« Reply #4 on: May 31, 2016, 10:55:37 PM »

My forecast app is saying "0% chance of precipitation, 50% chance of thunderstorms".

?

Probability of Precipitation is actually two items:  confidence and area.  Probability = C x A.  So if a forecaster is 80% certain that rain will fall, but it will only affect 20% of the area, then probability of rain =

.8 x .2 = .16 or 16% chance. 

So in this case, the confidence of rain and the area of rain is rather low.  Nevertheless, if it occurs, then there is a 50% chance it will be a thunderstorm, which makes sense for IL and those sudden popup afternoon thunderstorms. 

Perhaps a C = 10% and an A = 1%.  Probability of rain is .1 x .01.  Or 0.1%, which would be given as 0%.

Or as JeffDG has pointed out, in some cases, a thunderstorm will occur with no precipitation as it evaporates before hitting the ground.  This is known as a "Dry Thunderstorm".  Generally these are common in the American West, but can happen in other places.  The threshold for a dry thunderstorm is 0.1 inches of rain. 

Or what will also happen is the information is updated at different times and you just "get lucky" by accessing the data before it has been updated.  For example, I looked at the weather and it stated a 10% chance of rain, but it was already raining for my area.  It took about 15-30 minutes to get updated to a 100% chance. 
« Last Edit: June 01, 2016, 09:27:51 AM by LSThiker » Logged
Eclipse
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« Reply #5 on: May 31, 2016, 11:12:13 PM »

Riding through hail on a bike does not sound fun...   [ouch]

It isn't, though I have a full fairing and helmet, so that mitigates it somewhat.

As it turned out I drove. I don't have much issue with rain, but the streets were just wet enough and I
was wearing a white shirt for #1 son's graduation, so I took the roller skate I call a car,
it was a beautiful evening after all (at least I have a sunroof).
« Last Edit: May 31, 2016, 11:15:15 PM by Eclipse » Logged

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Eclipse
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« Reply #6 on: May 31, 2016, 11:13:53 PM »

Or as JeffDG has pointed out, in some cases, a thunderstorm will occur with no precipitation as it evaporates before hitting the ground.  This is known as a "Dry Thunderstorm".  Generally these are common in the American West, but can happen in other places.  The threshold for a dry thunderstorm is 0.1 inches of rain.  ce.

We get these on occasion, but today they were definitely wet storms.
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etodd
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« Reply #7 on: May 31, 2016, 11:51:41 PM »

You never know until you try. The weatherman called for lots of rain, yet you can easily skirt around it when its like this:

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etodd
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« Reply #8 on: May 31, 2016, 11:54:06 PM »

^^^^ A good example of why the weatherman is always correct for some people (in the rain) and wrong for those folks in the foreground.  LOL
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CAP Talk  |  Operations  |  Aerospace Education  |  Topic: 0% precipitation with 50% chance of thunderstorms?
 


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