Percent chance of Rain in the VIs

jmq · Post by **jmq** » Mon May 13, 2013 9:42 am

Looking at the weather forecasts for the VIs, the relatively high "percent chance of rain" that always seems to be in the forecast can worry newbies and frequent visitors alike. Here is something to consider when looking at that:

A chance of rain or chance of precipitation are terms used to describe the likelihood of a precipitation event. Technically, the percentages refer to the likelihood of an entire area receiving a measurable amount of precipitation over a particular time period. In easy terms, a measurable amount of rain usually refers to 1/100th of an inch (0.01 inches). If an area has a 50% chance of rain, that means that in 5 out of 10 cases over the forecast area, there will be a measurable amount of precipitation somewhere in the area. So, most of the time, the forecaster is expressing a combination of degree of confidence and area coverage. Clearly, the smaller the area, the more meaningful and accurate "chance of rain" is.

So "Chance of rain 50 percent" means there is a 50 percent chance that rain will occur at any given point over the forecast area, not that it will be raining 50% of the time over the entire area.

To repeat that: it is NOT the percent of TIME that precipitation will be observed over the forecast area, which actually may be the more common and/or intuitive way of thinking about it, which is why it can easily be misunderstood.

bdkane · Post by **bdkane** » Mon May 13, 2013 11:20 am

Can anyone provide credible information on average days of sunshine per year for STJ?

Just curious - I hear it all the time on House Hunters stating a certain location has "300"days of sunshine a year... I did a google search but cant seem to locate anything

piscesgirl0314 · Post by **piscesgirl0314** » Mon May 13, 2013 11:54 am

We've been traveling to St John since 2003 and after numerous trips realized we can't worry about something we have no control of...After all, a rainy day on the rock is better then a sunny day at home...

EagleRayLover · Post by **EagleRayLover** » Mon May 13, 2013 2:32 pm

bdkane wrote:Can anyone provide credible information on average days of sunshine per year for STJ?

Just curious - I hear it all the time on House Hunters stating a certain location has "300"days of sunshine a year... I did a google search but cant seem to locate anything

LOTS!!!

Jumper · Post by **Jumper** » Mon May 13, 2013 3:13 pm

piscesgirl0314 wrote:We've been traveling to St John since 2003 and after numerous trips realized we can't worry about something we have no control of...After all, a rainy day on the rock is better then a sunny day at home...

You nailed it!!!

adlnp · Post by **adlnp** » Tue May 14, 2013 12:41 pm

Just returned and can tell you this is the first trip down I was even thinking about possible bad weather. I was keeping a close eye on the forecast and was upset because every day it said 60-80% chance of precipitation. I was there for 9 days and can tell you we had one morning with cloud cover and on and off rain but around 11 am it was sunny and beautiful. One day we had mid afternoon rain for about 1 hour, hard rain. We grabbed some apps and a drink at a pool side bar and by the time we were done so was the rain! Other than that it was scattered clouds, 85 deg. and we all came home very very tan (even with 50 sunscreen.) Sooo no more worries you will have a wonderful time and if it does rain it will only be a little. Find a nice spot under some sea grape trees or just hang in the water. It will be over soon.

jaybfloyd · Post by **jaybfloyd** » Tue May 14, 2013 2:31 pm

Very well said everyone. I've only been there twice with my third trip coming up and I used to worry about the weather reports as well. I remember my wife pointing out once that the icons for each day on the reports were rain clouds. That definitely upset us at first. After all, you work your butt off all year stuck in an office with beautiful weather outside only to have your turn to enjoy that weather for 1 week a year and have it go to the birds. But as we quickly figured out after our second trip, it's exactly as you say. There is a chance of a passing shower almost every day. But the beauty part is that in that tropical environment, it's so fast that you really don't alter what you're doing. Just jump in the water for 10 mins and it's gone. It's not like in the states when you can have a stalled front just sit over top of you for 3 days and drench you. Doesn't seem to happen down there. It's also cool because you can usually see the shower coming in the distance. It allows you to plan accordingly. Man oh man, the more I talk about it, the more I need to get out of this place!!

jmq · Post by **jmq** » Tue May 14, 2013 4:42 pm

Of course what was noted in the original post and others have responded with here applies more to the typical patterns experienced, and do not apply on those relatively rare occasions when of those big tropical systems sits over the area for days at a time as noted in the "Weather?" thread by Kevin, copied here:

Coconuts wrote:CTBG: Traditional weather forecasts for the VI are not all that useful, and tend to spook people, since they typically call for showers (brief, somewhere) every day. The air temperature varies quite predictably over the course of the year, as does the average rainfall, so rather than look for day to day predictions, the following single graph has most of what you need to know. The ocean temperatures also vary very slowly, bottoming out in January at about 79 F, and peaking in August at 86 F. Once you are on island, satellite radar as per the web site I wrote is worth checking, especially if you are headed out on the water. I check the wind speed at the Two Brothers before I go sailing, but that is a password protected web site. Wave heights are important in the winter, and buoy 41043, 150 miles north of the VI, provides valuable warning of North Shore waves: http://www.ndbc.noaa.gov/show_plot.php? ... _label=AST
As Pia has noted, there is usually a 7 - 10 day stretch of back to back bad weather (rain, overcast) sometime every spring and fall, which can be a bummer if you arrive about the same time that it starts. Another "weather forecast" that can be worth monitoring from May to October predicts the levels of Sahara dust, which can make distant views hazy: http://forecast.uoa.gr/dustindx.php. And, of course, there is always the potential for a hurricane during that season (mostly August and September; see the second image below). Fortunately, both the eye and the island are not very big, so a direct hit or even a near miss is pretty unlikely. That is in contrast with the mainland coast, which is very long, and is pretty developed along its entire length. Hope this helps!

USVI weather.jpg

Hurricane frequency by month.jpg
All the best,

Kevin

usvi-on-line