This article takes you through a step-by-step process for finding and understanding hay drying windows using both the Probcast forecast charts and Weather Windows.
Using the forecasts
Step 1. Check the Total Precipitation Charts
Rain is the most important factor when drying and baling hay. Even small amounts of rain can reduce the quality of drying grass and increase costs if you need to use a dryer.
When looking at the Total Precipitation chart in Probcast, there are a few steps to follow:
First, check the blue and dark blue bars. These show the rainfall levels that the forecast suggests are most likely to be reached or exceeded. These are the periods you generally want to avoid.
Next, look at the yellow, red and grey bars.
These bars show the chance of rainfall reaching or exceeding the height of the bar.
The yellow bar shows the rainfall level that has around a 1-in-4 chance of being reached or exceeded. The red bar shows the rainfall level that has around a 1-in-10 chance of being reached or exceeded. The grey bar shows a lower-probability, higher-risk rainfall level - the kind of amount that only a small number of forecast simulations are showing.
These outcomes are less likely, but they help you understand the risk. Ideally, you would avoid any period where there is a chance of rain. In practice, that is not always possible. The aim is to understand the risk and make the best decision based on the value of the crop, the stage of the job, and the weather windows available.
How to use Probcast yellow, red, and gray bars to reduce risk?
There is rarely a perfect hay drying window, because rainfall can be volatile and changeable. But here are the things to consider to reduce your risk and avoid the most costly weather outcomes:
A. Consider the relative strength of the rain.
Not all rain risks are equal.
For example, you might be willing to accept a low chance of 0.3mm of rain, but avoid a low chance of 5mm or more. The probability might be similar, but the potential damage is very different.
This is especially important when you already have valuable grass drying in the field.
Top tip! When you have hourly bars stacked together, use the bar width selector to analyse the cumulative risk over a longer period.
Knowing there is a 1-in-10 chance of at least 2mm of rain over 12 hours is easier to understand than seeing lots of individual hourly bars and trying to add them up yourself.
B. Consider the cost if the rain does fall
The same rainfall risk can mean different things depending on where you are in the job.
If you already have a large amount of grass drying in the field, you may want to be more cautious about even a low-probability rain risk.
If you have only just started cutting, you may decide the risk is acceptable.
The key question is, what would it cost me if this rain actually happened?
C. Consider the opportunity cost
You also need to consider the cost of waiting.
If it is early in the season, you may choose to be more cautious and wait for a better window.
If you are under pressure to finish the job, you may decide to accept a small amount of risk.
D. Consider how far the forecast is ahead
Forecasts become more uncertain the further ahead they look.
For example, a 5-day forecast is usually much less certain than a 2-day forecast. This means the possible range of rainfall amounts is likely to be wider further into the forecast, and you may see more low-probability outcomes, such as 1-in-40 grey bar events.
Because of this, it is usually better to prioritise a good weather window sooner rather than relying on one further into the future, which is more likely to change.
Step 2. Check the weather variables that increase drying speed
Once you have checked the rainfall risk, the next step is to look at the other weather variables that affect drying.
High wind, high temperature and low humidity can all increase drying speed.
As a rough guide, good drying conditions often include:
Wind above around 10 mph
Temperature above around 22°C
Humidity below around 50%
These conditions can help grass dry faster, which may allow you to take slightly more risk with the forecast. Look for periods where drying conditions are strongest:
Once you understand both the rainfall risk and the drying conditions, you have the information you need to start planning your cutting, drying and baling windows.
Using the Weather Windows
You can also use the Weather Windows section of the platform.
Weather Windows gives you the same underlying forecast information as the forecast tabs, but it brings the relevant conditions together into clear working windows.
This removes some of the manual comparison needed when using the forecast charts alone.
Step 1. Make sure you have the right grass drying jobs set up
In the Job Conditions section, I would recommend setting up two different grass drying jobs.
Grass Drying
This job shows windows where rainfall is below 0.05mm in each hour.This helps you find periods where the forecast is effectively dry.
High Drying
For example, you could set this up to find windows where there is no rain forecast, and all forecast runs agree that at least one strong drying condition is met:Temperature is above 20°C
Wind speed is above 7 mph
Humidity is below 60%
These thresholds are set slightly below ideal drying conditions to allow for forecast uncertainty. For example, if your ideal drying wind speed is 10 mph, a window where all forecast runs show at least 8 mph is still a strong signal that wind conditions are likely to be useful for drying, and may well reach 10 mph in practice.
Drop us a message in the chat if you need help setting these up.
Step 2. Look at your High Drying Windows
Using Weather Windows!
The Weather Windows section only shows windows where all forecast simulations agree that the conditions are suitable.
In other words, these are the highest-confidence windows: periods where the forecast meets all the conditions you have set.
Outside of these windows, it does not necessarily mean the weather is unsuitable. It means there is less agreement in the forecast, or one or more conditions may not be fully met.
For these periods, we provide you the information needed to decide whether conditions are still good enough for drying grass.
Take a note of any windows which are shown as high drying.
You can click on a window to see why it has been selected in the window details:
You can also scroll down to see each variable chart. Periods highlighted in dark green show where all forecast simulations meet the threshold you have set:
Step 3. Look at the Grass Drying windows
The Grass Drying windows show periods where all forecast simulations expect less than 0.05mm of rain per hour.
These represent good drying opportunities.
Periods outside these windows are not automatically unsuitable. They simply carry more uncertainty or a slightly higher rainfall risk.
You can use the rainfall chart below the Weather Windows section to understand where that risk comes from. Then follow the same process described above: look at the yellow, red and grey bars, consider the amount of rain being shown, and decide whether the risk is acceptable for your job.
Weather Windows gives you a quick way to find the strongest drying opportunities.
The forecast charts then help you look more closely at the periods around those windows, so you can decide whether the risk is acceptable for your farm, your crop and the stage of the job.
By working through the steps above, you should have a clearer understanding of both the forecast and the risks around it. This should help you choose better cutting, drying and baling windows, reduce avoidable weather costs, and protect grass quality across the season.
If you have any questions, or would like help setting up your own grass drying conditions, just drop us a message in the chat or give us a call.