Why weather forecasts not more accurate?

| 31/07/2019

With all the equipment and manpower that Cayman has, why can’t someone produce an accurate weather prediction? Miami can pinpoint a shower to the exact minute. Why can’t we at least get the days correct?

Ask Auntie, CNS Local Life, Caymanian status

Auntie’s answer: It seems to me that weather forecasters have always been among the most-maligned professions. If someone predicts a 70% chance of rain, for example, which may be an accurate estimate, that still leaves a fairly healthy 30% probability of no precipitation. Yet, there they are every day setting themselves up for failure.

That is not to say your concern is not valid; it just means forecasting is always going to be a bit tricky so I have to admire all those weather people who put themselves up for public scorn on a daily basis.

On the other hand, what do I know about meteorology? In a word, nothing, so I contacted the Cayman Islands National Weather Service (CINWS) to see what the experts had to say.

There are several things to consider when looking at the accuracy of predictions, a CINWS official explained. The first issue is one of adequate staffing. Though the service has access to weather radar and an advanced satellite system, there are only four “operational weather forecasters, who are responsible for aviation, public (and) marine forecasting” in addition to having to deal with other matters.

Larger cities like Miami, which you reference, “would have multiple forecasters working on each of those matters separately”, the official said, adding that the CINWS does not operate 24/7 “as it should rightfully be”, but that situation is being worked on.

However, during tropical cyclone threats, the hours of operation are expanded with staff working longer.

The next point is technical and I will do my best to simplify the explanation. The CINWS, like other weather services, uses Global Numerical Weather Prediction (NWP) models, which process current meteorological data to determine a weather forecast. The mathematical equations employed in this exercise attempt to calculate the effects of such factors as the movement and speed of air masses which are “too small, brief, complex or poorly understood to be explicitly represented”, the official explained.

In addition, most tropical weather forecasts are determined using this method, which means that by definition, “Almost every step in NWP includes omissions, estimations, approximations and compromises.”

One more thing to note, which makes predictions difficult for small places like Cayman, is that, at best, the resulting forecast will typically cover an area of from 10km-25km, so our little group of islands is “just a speck on this scale”, making it difficult to isolate Cayman’s expected weather.

For anyone still with me (and I hope there are many), I will address the reader’s question of why forecasts don’t seem able to accurately specify days for expected weather. “Part of the problem lies in the public misunderstanding about what we mean when say 30%, 60% or any percentage of showers,” the official said. The percentage is determined by several factors relating to rain, including moisture and temperature in the “vertical profile” of the atmosphere, where the rain will fall and how long the showers will last.

“So when we forecast X% chance of showers, those showers could fall anywhere in that area and at any time within the forecast period. The other part of the problem is that most critics of weather forecast are making comments based upon their observation at one point and do not consider other points on the forecast area,” the CINWS representative said.

As an example, if isolated showers are expected across the Cayman area, this might be forecast as a 30% chance of rain. “Now suppose we consider two individuals, one in George Town and the other in East End. The isolated showers may be over the George Town area and the perception of that individual is that the chance of showers should be higher while the perception of the East End individual is that the chance of showers should be lower. But if the showers are isolated, then 30% chance of showers is accurate.”

Looking at the models used for forecasting, these are better able to predict larger systems compared to small scale shower activity. And that means that there will always be a problem in calculating the exact location of expected showers for our little islands.

It might just be that in this age of technology overload the best way to check on the weather is to look up in the sky or call a friend to do the same if he is in the part of the island where you are heading.

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Comments (7)

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  1. Anonymous says:

    Here in Cayman, they puts little pieces of paper in a box and shakes it up and each day they pull out a little piecee and reads it and thats your weather for the day.

  2. Anonymous says:

    For clarity, if they say 30% chance of rain, they mean 30% of the land in the forecast area will get rain and the other 70% will not. The forecast area is bigger than the actual land, so some of those offshore squalls count in the 30% even though they never touch land.

    • Ti m says:

      Anon 0944 thanks for your explanation but the forecasters also has to take time into consideration.

  3. Anonymous says:

    been here 20 years and the weather serive has always sucked…..the amount of times torrential flooding has not been warned of is unbelievable…

  4. Anonymous says:

    why does cayman national weather service sat image show the time in a different time zone than cayman?

    • Anonymous says:

      Currently the app is not responding, the technicians are trying to repair it to work it properly ASAP, I’m an intern there at the NWS and I told the director about it and they all know about the problem, it is being worked on to be fixed for the public to use once again.

  5. Anonymous says:

    You need a weather rock.

    This rock is the perfect weather indicator, it
    never fails. It is more accurate than your local
    weather person. This rock is the living word and
    is 100% correct. This is how it works:

    a dry rock means fair weather.
    a wet rock means it’s raining.
    a dusty rock means a dust storm.
    a swaying rock means it’s windy.
    a shadow under the rock means it’s sunny.
    a white rock means it’s snowing.
    if the rock is jumping up and down, an
    earthquake is upon us.
    if the bottom of the rock is under water it’s a

    most beautiful of all, though the rock is not
    attached to its’ existence, it doesn’t mind which
    of the above is happening.