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JE
 JE
(@je)
Advanced Member

I am trying to find a reliable daily local weather forecast site. I looked at a few sites this morning and their precipitation probability forecast are all over the place for the next three days. For example, here are the probabilities for the next three days from some sites:

Weather Underground (00820 Christiansted): 80%/40%/0%

Weather Underground (00851 Kingshill): 30%/40%/30%

The Weather Channel: 60%/50%/50%

Accuweather: 66%/18%/6% (might be interpreting this incorrectly)

Obviously one of the Weather Underground forecasts is way off as Christiansted and Kingshill are probably 5 miles apart.

I would guess that a local TV station weather forecast (not TWC) would be most accurate but I can't find a website for any of them. The daily temperature forecast is irrelevant but I would like to have a feel for what the chance of rain is for the next couple of days. For example, if there is a 60% chance of rain then I would probably not stain my deck railings that day but if it is 30% then I probably would, although I certainly realize that it could still rain that day. So what sites do you find most accurate for daily precipitation forecasts?

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Topic starter Posted : May 9, 2013 12:39 pm
Alana33
(@alana33)
Expert

It can rain on one side of the street here and not the other.
On Monday, while northside STT was having a deluge, my sister who lives up the hill from Gallows Pt. in STJ, had absolutely no rain.
She said it rained in Cruz Bay but they got not a drop, just a short distance away, outside of town.

I would wait to stain the decks until it is sunny again. Here on Northside STT, Inner and Outer Brass Islands have completely disappeared from sight. Maybe someone can tell you an accurate site to check weather.

For instance check out this Daily News piece: http://virginislandsdailynews.com/news/chance-of-rain-today-will-be-last-for-a-while-1.1485184
"Chance of rain today will be last for a while"
I guess you should not use the site they do for forecasts!

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Posted : May 9, 2013 12:54 pm
IslandHops
(@IslandHops)
Trusted Member

...I would guess that a local TV station weather forecast (not TWC) would be most accurate...

Too funny. You need to watch the local 7pm news broadcast on channel 8. I'm pretty sure their weather forecast graphic is a picture from an I-Phone. 8-).

You are better off listening to the weather forecast on the radio, or just going to NOAA's site (www.weather.gov). I also check windguru.com.

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Posted : May 9, 2013 2:00 pm
Alana33
(@alana33)
Expert

The www.weather.gov site looks good:

http://forecast.weather.gov/MapClick.php?site=sju&zmx=0.7916666666666666&zmy=0.7916666666666666&map.x=246&map.y=88

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Posted : May 9, 2013 2:29 pm
STXBob
(@STXBob)
Trusted Member

Those forecasts are all correct. They're all over the place, just like our weather!

I don't even understand the meaning of X% chance of rain. Does that mean rain anywhere in the region? In a specific spot? How much rain counts as "rain"? One drop? 0.10 inches? If the forecast says "50% chance of rain today" and it's raining now, why don't they update it to say "100% chance of rain today"?

I think George Carlin once joked that there's a 90% chance of rain today, but only a 50% chance of that.

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Posted : May 9, 2013 3:36 pm
OldTart
(@the-oldtart)
Expert

As Alana said, it can be raining on one side of the street and not the other. I can be deluged high up on the mountain where I live but a five minute drive down the mountain and it's dry as a bone. Unless there's a BIG system over us any given percentage merely indicates that it could rain that much in some spot somewhere in the islands. That's as good as it gets!

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Posted : May 9, 2013 4:07 pm
JAG
 JAG
(@JAG)
New Member
Exit Zero
(@exit-zero)
Trusted Member Registered

I use www.weathercarib.com produced in Tortola - If you are painting or varnishing look to the radar sites - and water vapor loops - it is easy to visualize the chances of rain and probably as good or better than predictive sites.

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Posted : May 9, 2013 6:01 pm
JE
 JE
(@je)
Advanced Member

Thanks for the responses. I was just using the deck railing staining as an example of why I like to know the relative odds of precipitation - after the rain the past week my deck railing needs to dry out for a few days first anyway. Most days I usually do look at the San Juan Radar (which has been down for maintenance the past week) and the various satellite views for the Caribbean area on the National Hurricane Center website. That's one reason I can't understand how some of these sites arrive at their numbers when I can look at the satellite view and see we are in a pocket of dry air but the weather site says there is a 80% chance of rain. Statistical probability is just one tool in the toolbox. Even a 90% chance of something occurring means that 10% of the time it won't. Regarding what the probability factor means, I believe it means that there is an "X" chance of measurable precipitation for someplace within the forecast area within the forecast period.

I have lived on STX for 8 years and have owned my house for 11 years, so I am familiar with the hit and miss aspect of the rain showers here. A couple of days ago we had 1.9 inches of rain at my house over a 4 hour period in the morning. I went down to Home Depot, 3 miles away, and it did not look like it had rained at all that day there. But standing in the parking lot I could see my neighborhood in the distance and it was still raining there. But if you own a house here you always have a never ending list of both inside and outside projects. And if there is a valid 60% chance of precipitation on a given day, and an outdoor project is rain sensitive and non-critical, then I will probably play the odds and opt to do one of the many inside projects that day.

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Topic starter Posted : May 9, 2013 6:35 pm
Bombi
(@Bombi)
Trusted Member
Exit Zero
(@exit-zero)
Trusted Member Registered

Many of the National weather sites use the predictions for Pueto Rico for their VI weather probabilities since it is where the NWS is located but as we know here, 40 mi. and a much larger island and much more elevation makes for quite different weather in PR.

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Posted : May 10, 2013 1:34 am
East Ender
(@east-ender)
Expert

I asked Siri today if she thought it would rain. At the time I was in the middle of a deluge. Siri said she didn't think it would rain today. LOL

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Posted : May 10, 2013 1:45 am
vicanuck
(@vicanuck)
Expert

Why be worrying about the weather? Its rarely changes.

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Posted : May 10, 2013 11:38 am
Alana33
(@alana33)
Expert

I am greatful for the rain. Plants are refreshed and cisterns needed filling. It's been hot and dry for so long it's a nice change but I am ready for a bit of sunshine after almost a week of rain.

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Posted : May 10, 2013 11:53 am
Ca. Dreamers
(@ca-dreamers)
Advanced Member

But if you own a house here you always have a never ending list of both inside and outside projects.

It's not a job it's a Career!

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Posted : May 10, 2013 12:05 pm
STXBob
(@STXBob)
Trusted Member

I found the definition for "chance of rain", a.k.a. "Probability of Precipitation" or PoP. It means the chance of measurable rain at any given point in the area, as in: There's a 40% of rain at JE's house today. It does not mean there's a 40% chance of rain somewhere in the entire area. If so, the forecast would be 100% on most days, because it usually rains somewhere in the area, just not always at JE's house.

From http://www.srh.noaa.gov/ffc/?n=pop

"But, most of the time, the forecaster is expressing a combination of degree of confidence and areal coverage. If the forecaster is only 50% sure that precipitation will occur, and expects that, if it does occur, it will produce measurable rain over about 80 percent of the area, the PoP (chance of rain) is 40%. ( PoP = .5 x .8 which equals .4 or 40%. )

In either event, the correct way to interpret the forecast is: there is a 40 percent chance that rain will occur at any given point in the area."

So George Carlin had it almost right when he joked that there's a 90% chance of rain today, but only a 50% chance of that.

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Posted : May 10, 2013 2:56 pm
Matt T
(@Matt_T)
Advanced Member

If you really want to know whats going on, go to wunderground for your particular forecast area, and scroll to the bottom of the forecast and look for where it says "Scientific forecaster discussion"

This is composed by NWS meteorologists and gives a much better explanation of what is going on than trying to use % chance of rain.

http://classic.wunderground.com/DisplayDisc.asp?DiscussionCode=JSJ&StateCode=VI&SafeCityName=Christiansted

My .02

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Posted : May 10, 2013 4:37 pm
Dante
(@dante)
Advanced Member

I also used wunderground, however , it really did get a little frustrating. I decided some time ago not worry about it...now I listen for the tree frogs then I know rain soon come on my little hill :@)

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Posted : May 10, 2013 6:32 pm
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