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Old 09-25-2012, 02:50 PM
nei nei started this thread nei won $500 in our forum's Most Engaging Poster Contest - Thirteenth Edition (Jan-Feb 2015). 

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Location: Western Massachusetts
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from NOAA in northern NH:

Sunday A chance of rain. Partly sunny, with a high near 64. Chance of precipitation is 30%.
Sunday Night A chance of rain. Mostly cloudy, with a low around 44. Chance of precipitation is 30%.
Monday A chance of rain. Mostly sunny, with a high near 64. Chance of precipitation is 30%.

Would you guess it might be a dry day most of the time with a few sprinkles? Obviously won't rain when it's mostly sunny, might it be cloudy a few hours with rain? Or does it mean NOAA doesn't know very well and just giving us probabilities? Would looking at computer models help?

My experience if I see partly / mostly sunny and a low chance of rain is either:

1) The weather most likely will be good (good to me means some sunshine and little to no rain) but NOAA isn't completely sure
2) It will rain for a short part of the day and then the sun will come out
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Old 09-25-2012, 05:13 PM
 
Location: Near the Coast SWCT
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I broke it down by day to say what I think.

Sunday A chance of rain. You should see the sun but clouds will also be overhead so the sun will be in and out all day and there is a 30% chance that convection will develop and rain overhead.

Sunday Night A chance of rain. With the front coming through clouds will be around more than the stars so mostly cloudy night with a 30% chance that it can rain whether its drizzle or rain.

Monday A chance of rain again but should be a mostly sunny day. Even though the sun is out, there's still a chance that rain can develop due to the moisture in atmosphere and ingrediants in place. Chance of rain is low at 30% but it can happen.

How did I do?
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Old 09-25-2012, 06:52 PM
 
Location: Perth, Western Australia
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I'd interpret it as giving the highest probability of rain Sunday afternoon to early Monday morning, though any rain observed would likely be just a brief passing shower.
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Old 09-25-2012, 07:22 PM
 
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Sunday: convection might develop and cause rain showers

Sunday night: Convection has developed and there is a chance of rain. the convection has increased the cloud cover greatly.

Monday: the convection is moving away but you might still get a very showers. It spotty at best since its not a front moving through, but a luck of the draw event.
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Old 09-26-2012, 06:24 AM
 
Location: Laurentia
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A 30% chance of rain at those temperatures means to me that there will be some scattered showers in the area. The scattered nature of it makes it unlikely that any random location will be impacted with rain or cloudiness, hence the low rain chance. Outside of the showers it will be sunny, mostly cloudy, or whatever.
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Old 09-29-2012, 04:28 PM
 
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Quote:
Originally Posted by Patricius Maximus View Post
A 30% chance of rain at those temperatures means to me that there will be some scattered showers in the area. The scattered nature of it makes it unlikely that any random location will be impacted with rain or cloudiness, hence the low rain chance. Outside of the showers it will be sunny, mostly cloudy, or whatever.
The 30% is just the probability that any locale will see rain, not necessarily that 30% of the area will be see rain and remainder of the area won't.

Usually if scattered showers are expected without a doubt, the NWS would simply put a specific wording in the grid for scattered precipitation versus chance of precipitation.

The mostly sunny, partly sunny, mostly cloudy, etc. portion is just what the weather will be otherwise if it doesn't rain.
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Old 09-29-2012, 04:32 PM
nei nei started this thread nei won $500 in our forum's Most Engaging Poster Contest - Thirteenth Edition (Jan-Feb 2015). 

Over $104,000 in prizes has already been given out to active posters on our forum and additional contests are planned
 
Location: Western Massachusetts
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Here's what the forecast is now:

Sunday Showers likely. Cloudy, with a high near 61. Calm wind. Chance of precipitation is 60%. New precipitation amounts between a tenth and quarter of an inch possible.

Sunday Night A chance of showers. Cloudy, with a low around 46. South wind 3 to 5 mph. Chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.

Monday A chance of showers. Mostly cloudy, with a high near 63. West wind 6 to 9 mph. Chance of precipitation is 50%. New precipitation amounts of less than a tenth of an inch possible.
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Old 09-29-2012, 08:48 PM
 
Location: Laurentia
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Quote:
Originally Posted by 313Weather View Post
The 30% is just the probability that any locale will see rain, not necessarily that 30% of the area will be see rain and remainder of the area won't.
I know that. However, if the rain was widespread and not scattered, or widespread but some distance away, then the probabilities would usually be higher than 30%. A rain that produces a huge glob on the radar that's heading towards a location never is forecast at 30%; in my experience it's at least 60%, usually 70%.
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