What is Model Data?
Written by weatherTAP.com
Last updated 11/6/2017, 12:30:58 PM
This article is going to be a long one, so grab yourself a nice cup of coffee, relax and enjoy as we venture onto the next option weatherTAP has to offer: model data. Now, model data can seem intimidating, but it's really not that complicated. I will walk us through it in baby steps and by the end of the model lessons, you should have a good grasp on what model data is, where model data comes from, and how to interpret model data. ALWAYS, keep in mind that model data is not a substitution for an official forecast. Sometimes, even folks trained in meteorology rely all too heavily on the models and neglect to implement their meteorological training that should ALWAYS accompany the model data. The computers simply produce a forecast based only on numbers that are fed into them.
The data for the models is derived, at least in part, from weather balloon launches (radiosondes). Weather balloons are released twice a day at the same time around the country (7:00 a.m and 7:00 p.m). In the winter time it's 6:00 a.m. and p.m. due to daylight saving time. The balloon launches sample the atmosphere from the surface to over 30,000 feet up! They record temperature, pressure, and moisture as they ascend and radio back that info to the National Weather Service. Models can also take into consideration surface data that is fed into them from various, reliable weather stations across the country. The more data that can be fed into the computer, the more accurate the output forecast will be.
The first model that we come to is the RAP model. RAP is short for rapid, because this is a rapid refresh model. That means it updates rapidly compared to other models. In fact, this model updates hourly. It has a 13-km resolution. This model is practically a cousin to the HRRR (high-resolution rapid refresh) model that many of you have probably heard of. The main difference between the two is that the HRRR has a much higher resolution model at 3-km.
So, basically the RAP is a model that updates hourly and has a 13-km resolution (13-km~8 miles).
All of weatherTAP's model data has the same basic categories for choices, with some minor variations in options under these choices. Those choices are:
- Wind and height
- Temperature
- Relative humidity
- Dewpoint
- Severe indices
- Precipitation
The first option, wind and height, are important for understanding how wind changes with altitude.
Reading the Wind/Height Model Data
In meteorology we study altitude by pressure level (millibars), and each pressure level has an average height to it. Here on the surface of the Earth, we have an average pressure of about 1000 millibars (mb). If you go up to about 18,000 feet, you can find a pressure reading of about 500 mb. That varies slightly every day but I'll explain that later. I use 500-mb as an example because that is the level that we consider to be the middle of the atmosphere (half of the 1000 mb surface reading). Here are other pressures and their associated altitudes:
I highlighted the levels we are most concerned with.
As meteorologists, we divide the atmosphere into these different layers and analyze what is going on at each layer. That's necessary in order to understand how to make a forecast for each day.
We must understand the wind at various altitudes in order to even begin to forecast the weather. We need to know how fast a system is traveling, how likely the winds are to continue carrying the systems, and in what direction we can expect that system to move. If air rises, that can lead to precipitation. Remember, earlier we've talked about how rising air cools and condenses and can form clouds and precipitation. Air can be rising at one level but not at another. We need to understand all that in order to make an accurate forecast.
Now, when you click on, for example, the "850-mb wind and height" for the RAP model you should know that you have selected the rapid refresh model (RAP) and you are analyzing what the model is forecasting at the 850-mb level, or at about 5,000 feet up. You can choose to let the page display wind by coloring in the wind speeds for easy viewing, or by showing wind barbs, which show wind direction/speed (see https://goo.gl/jL6fKp). The lines you see are showing you the height of the 500-mb level. Add a zero to the end of the three-digit number and you get the height, in meters, of where you would find a pressure of 500-mb. Again, on average that height is about 5,000 feet up, but it varies slightly. Those slight variations tell us a lot about the weather.
Temperature
Temperature is a measure of energy available in the atmosphere. Just think about how intense summer thunderstorms can be when it is so incredibly hot outside. The storms have so much heat energy to work with, so they become quite strong. The same is true with hurricanes, forming and spinning in the hottest tropical regions of the world. So, yes, it is a fact that temperatures here on the surface are important, but it's also very true that we need to know what temperatures are doing as we go up into the atmosphere.
The faster temperature decreases with height, the more unstable we consider the atmosphere. Warm air rises and the faster it rises the stronger the updrafts will be in the clouds. This is why summer storms get so intense. The hotter the temperature is here at the ground, the faster temps will decrease with height, leading to air that rises more quickly. The rising air lead to stronger updrafts. The stronger the updrafts are the stronger the storms will be. We call the rate at with temperature decreases with height the "lapse rate". Lapse rates are important to know anytime precipitation is expected to occur. A parcel of air can only rise if the air around it is cooler than itself (again, warm air rises).
WeatherTAP's model data allows you to see what the temperature is expected to be at various heights in the atmosphere. We start at the surface and work our way up. You may see that the models are forecasting surface temperatures in the 70s for your area. However, when you look at the 500-mb map (18,000 feet up) you notice a very cold pocket of air (purple colors) expected over your location at the same time you are expecting surface temps in the 70s. This means you have steep lapse rates and may be looking at some rainfall or thunderstorms in your area. Cold air over the top of warmer air encourages rising motion because lapse rates become steeper, which encourages clouds and precipitation. I should note that temperatures generally decrease with height every day, but when we either make that cold air colder or make the surface air warmer, we cause the difference to be even greater than it is on a "normal" day, which is when we get clouds and rainfall.
You'll also notice that you can add wind data to the maps, which allows you to see the speed at which the different air masses with different temperatures are moving. It also helps to see the direction that they're moving. You can also see the isobar lines, lines of equal pressure. The closer these lines are to each other the faster the winds are moving (more on isobars later on).
Precipitable Water
The first item we come to under the precipitation tab in model data is precipitable water. In the last lesson we talked about how important temperature data was. Equally important is moisture, because moisture is the fuel that drives the storms. Basically, warm air rises, as we discussed last time, and is the transportation for water vapor to get lofted into the atmosphere. Rising warm, dry air will not lead to cloud development (think desert). Warm, moist air rising has a good chance of forming clouds (think jungle). Since moisture is the fuel for storms, the higher moisture content in the air means a better chance of storms. Moisture is to storms what gasoline is to your car. In weather, the "octane" level of the fuel depends on the moisture content and the degree of warmness of the air transporting the moisture upward. In addition, the warmer the air is the more moisture it can hold. Again, the more moisture you have the more intense your rainfall or storms can be.
Precipitable water is important because it can give us an idea of how much moisture is available in the atmosphere. Precipitable water is calculated by taking a column of air from the ground up and condensing all the water vapor out of it. The liquid content that results from this is called your precipitable water. Just imagine standing outside and looking straight up. If all the moisture straight up above you were brought down to the ground, precipitable water is the amount of water you would be standing in. Higher precipitable water values indicate that there is a lot of available moisture in the atmosphere. Values over one inch are typically considered high, though that can change based on your geographic area (ie. desert vs coast).
It is important to remember that precipitable water does not determine how much it will rain. This is a rather common misconception. If the precipitable water value is 1.25" that does not mean it will rain 1.25". That's not to say it won't rain 1.25" but other factors can influence the final rainfall tally, including wind shear. For instance, if storms are moving northeast but the surface winds are blowing from the southeast, a constant and fresh supply of moisture-rich air being channeled into the storms. This can significantly enhance rainfall amounts and produce amounts that may far exceed the 1.25" found in the precipitable water calculation.
Next week we'll discuss the precipitation accumulation tool and how you can use that to get an idea of how much it's going to rain!
Since we're getting close to be finished with the gist of reading model data, I'm working on some projects that will test your ability to use model data to figure out what the weather might do! I think you'll really enjoy this!
More Precipitation Model Data
The next selection after precipitable water is precipitation accumulation. Sounds pretty straightforward, right? The one thing you have to keep in mind is that the different models do this calculation in different time intervals. For instance, the RAP model does precipitation accumulation in one hour intervals, the NAM in three, and the GFS in six. In other words, if you are looking at the RAP model that is valid for 2:00 p.m., it will be showing you the precipitation it expects will have accumulated from 1:00 to 2:00 p.m. Just be sure you keep that in mind when analyzing the model data.
One of the nicer things about the RAP model being more short term is that it can also do precipitation rate. That can come in handy when you need to know the intensity that precipitation will fall. Our RAP model is the only model with this particular feature.
Composite reflectivity shows you were the model thinks there will be precipitation. Just use the scale at the bottom to see at what intensity the precipitation is expected to fall. Just keep in mind that this is a model and there may be certain factors the model is not considering, such as dry air at the surface that may keep precipitation from falling to the ground before evaporating. Only the RAP and NAM have this particular feature.
Well, that wraps up the precipitation model data! Now I'll explain the significance of the relative humidity and wind features of our model data. The wind data gets us into talking about jet streams. With winds that can exceed 200 mph, you can bet they can have a very profound effect on the weather!
Relative Humidity in the Model Data
We've already talked a bit about the importance of moisture in weather forecasting. Moisture is the fuel for big storm systems. What you may notice with the model data is that we can look at relative humidity at multiple levels of the atmosphere. This is because, as I've mentioned before, the atmosphere has various levels to it and each one is important for making a weather forecast.
We all know how it feels outside before it rains or storms. It usually feels damp because you can feel the surface moisture that will eventually lead to precipitation falling from the sky. As we move up into the atmosphere, we must check the moisture at the various levels that we encounter. If we have moist here at the surface but we have dry air at a level above us, that can affect the precipitation that falls here at the surface.
If you recall, I shared with you the millibars and their relative elevations earlier.
As meteorologist, we always look at the 700-mb level during severe weather times. Dry air at that level may "cap" the atmosphere. Imagine a clear, sunny day that is hot and humid. Normally, you see cumulus clouds go up. However, if there is dry air at the 700-mb level, the clouds can't go up. Heat and humidity continue to build here at the surface. It's like having a lid on a pot of boiling water. If something happens to cause the heat and humidity to suddenly be able to overcome the dry air at the 700-mb level, we say the cap breaks and thunderstorms explosively develop. It's like taking the lid off your pot of boiling water. The steam will punch upward! This is a dangerous scenario because you can go from clear skies to tornadic thunderstorms in minutes. You'll hear me talk about this more during severe weather season.
Dry or moist air at any level can affect the weather forecast, and we must be as familiar as we can be with the moisture conditions at each level. If we don't pay attention to each level of the atmosphere, we risk a busted forecast. And no one wants that!
Wind in the Model Data
In my opinion, wind is as important as moisture in weather forecasting. One thing to keep in mind is that wind is ALWAYS the result of an imbalance in pressure. Wind is nature's way of correcting a pressure imbalance. In fact, the very reason we have weather is because things get imbalanced on this planet. Nature is constantly working to keep everything in balance. The more out of balance things become, the harder nature has to work to bring back equilibrium. I always used to tell my students that, in Nature's eyes, it's too cold at the Poles and too hot at the Equator. Nature is constantly trying to balance that difference out and she uses weather to try and accomplish that goal.
Since temperature and pressure are directly related, one might also say that wind corrects imbalances in temperature. So, before moving forward let me give you a quick lesson on pressure.
Pressure is the force put upon you (or anything else) by the atmosphere. It's a measure of how much the atmosphere is pushing down on your shoulders. When the pressure is high, air is sinking. Think about it this way, in life we sometimes say we feel like we have the weight of the world on our shoulders. In meteorology we would say you have high pressure. You feel like you have the weight of the whole atmosphere pushing down on your shoulders.
High pressure = atmosphere pushing down (sinking air).
On the other hand, when air is rising we have low pressure. The air is rising and therefore you have less of the atmosphere pushing down on your shoulders. You literally weigh less when the pressure is low. Rising air leads to cloud development and stormy weather.
Low pressure = rising air and less atmosphere pushing down
Also, remember that wind always blows from high to low pressure. If I'm outside and the wind is hitting my face I can know that I'm facing the higher pressure and the low pressure is behind me. Wind always blows from high to low.
Another thing to keep in mind is that high pressure rotates clockwise and low pressure rotates counterclockwise. Please see map below:
As with all meteorology data, we have to analyze the wind from the surface to at least 30,000 feet up. We divide the atmosphere into several layers, as we've discussed before, and we must know what the wind is doing at each layer before making a forecast we can have confidence in.
In order to make an accurate forecast, there must be a good understanding of the wind throughout the atmosphere. We must not only understand what is occurring here at the surface, but we must also understand how the wind is behaving throughout various levels of the atmosphere.
Wind data tells us if storms will be able to organize and whether or not the threat from storms is large hail, tornadoes, flooding or any combination of these. We judge the volatility of an environment, in part, based on wind shear. There are two kinds of wind shear; directional shear and speed shear. Directional wind shear is the change in wind direction with height. Speed shear is the change of wind speed with height.
Shear is important for storm strength. If a storm is moving north and the surface winds are blowing into the storm from the east, those surface winds are supplying warm, moist air to the storm and it can continue to grow and intensify. If, on the other hand, the storm is moving north and the surface winds are blowing from the south, rain-cooled air from the storm will be blown right back into the storm, causing it to be weaker than it would otherwise be.
Shear can also give a storm?s updraft a rotational component. If winds are changing direction and speed with height, the storm's updraft will be able to rotate. This could lead to tornadoes.
If the jet stream is over the top of your location, you will have incredible speed shear in the highest levels of the atmosphere. This increased wind flow aloft will encourage the air down here at the surface to rise and fill the void of the air that is being swept away. Rising air leads to cloud development, which may then lead to precipitation. We watch for the jet stream, as well as areas of higher winds within the jet stream, called jetstreaks, throughout the year. The faster the winds aloft are, the faster the air here at the surface may rise.
WeatherTAP's model data displays the wind as a wind barb. An example of a wind barb is shown below. The flag represents winds in knots, which are really close to mph. One knot is 1.15 mph. If you want to keep it simple, just remember the flag represents winds of nearly 60 mph, while the long bar is ~10 mph and the short is ~5 mph. The barb always points in the direction the wind is coming from.
We'll discuss more of the wind products when we get to our "jet stream analysis" products.
The Severe Indices
The next option with model data after wind/height is severe indices. Before I dive too much into these, I would like to explain why the 500-mb height level can rise and fall, if even slightly. When mb levels rise, that signifies warming. For example, if south winds begin to blow here at the surface, transporting in warmer air, the 850-mb level (at 5,000 feet) will rise slightly in elevation. It's like the atmosphere swells as it warms. So, when we see mb levels rise, we know warming is taking place below that level.
Warming can take place at different levels. For instance, if we have a cold air mass in place but a warm front from the Gulf starts lifting northward, that warm (and much lighter) air will move up and over the cold air that is at the surface. That can create a layer of warmer air on top of a layer of cold air that is trapped here at the surface. That is how we get ice storms, when rain in the warm layer falls into the cold layer here at the ground.
You'll notice that the option for surface wind and mean sea level pressure is also available to analyze. It is crucial that we know where the surface low pressure center is located. Severe thunderstorms are most likely to the east (and especially southeast) of the surface low, where the greater amounts of warm, moist air are located. Wintry precipitation is more common north and west of the low center, where cold air is most likely to be. Knowing the tracks of lows is crucial for knowing what kind of weather you are most likely to have, whether it be rain, storms, ice, or snow.
Now, the severe indices may mean nothing to you at this point. That is about to change! The first one we come to is CAPE. That stands for convective available potential energy. Whew! That's a mouthful, right? Without bogging you down with too many details, CAPE is basically the amount of energy that is available for thunderstorms. Generally, you need at least 1000 J/kg of CAPE for severe weather, though CAPE as low as 500 J/kg can give you storms, especially in the winter time. I usually don't get too excited about severe wx possibilities if CAPE is under 1,000 J/kg. The amount of energy present can tell us how strong updrafts will be. The stronger the updraft, the stronger the storm.
CAPE is probably one of the most widely used severe indices there are. We can know a lot about the environment by knowing how much energy is within the environment. Energy basically comes from heat and moisture. The more of each you have, the more powerful storms can become. Think about how powerful hurricanes become. All they have fueling them is hot, moist air.
The Severe Indices (continued)
The next two severe indices that we come to are CIN and Lifted Index. CIN is Convective Inhibition. CIN basically calculates the amount of negative buoyancy that is present. In other words, it measures the amount of energy that would discourage air from rising. Air that rises forms clouds, so if air isn't rising clouds are not forming. On the weatherTAP scale, you don't need to pay attention to the numbers as much as you do the colors. The bluer colors indicate low CIN, while the redder colors indicate higher CIN. The higher CIN, the less likely we are to see severe weather.
The next indice is Lifted Index. To better understand this index, you need to understand that in meteorology we have what we call air "parcels". Imagine you are out in the middle of a big parking lot in the middle of the summer. It's really, really hot. So, "bubbles" (or parcels) of hot air will rise from the parking lot like balloons. In meteorology, we imagine that these parcels of air stay in a bubble as they rise (like air in a balloon). As long as the air around the balloon is cooler than the air within the balloon, the balloon will continue to rise (warm air rises, right?). Now, the difference in temperature between the air in the balloon and the air outside of the balloon is called the Lifted Index. The greater the temperature difference, the faster the balloon will rise. So, on your Lifted Index model, the greener colors represent a slowly rising balloon, while the brighter colors represent a faster rising one. The faster the air rises, the more likely you are to get thunderstorms.
You may notice that weatherTAP offers two Lifted Index products. One is surface based and the other is called Best Lifted Index. The Surface Lifted Index is best used when gauging a severe weather risk because it takes the parcel (balloon) from the surface up to about 18,000 feet (500mb level). In other words, the temperature of the parcel at the surface is compared to the temperature of the parcel after it is lifted 18,000 feet. The greater the difference between the two temperatures, the more unstable the atmosphere is.
The Best Lifted Index measures the difference in temperature between the air inside and outside of the balloon at various heights from the surface to 850 mb (5,000 feet). This is most useful when there's a shallow layer of cold, stable air at the surface but warmer, more unstable air above that layer. This can happen quite easily when warm fronts are moving in and the colder, heavier air is reluctant to leave, while the warmer, lighter air glides up and over the top of it. Sometimes the warmer air above the colder air can become unstable enough to produce thunderstorms. That's where the Best Lifted Index comes in handy! In other words, unlike the Surface Lifted Index, the temperature of the parcel at 18,000 feet is compared with various levels from the surface up to 5,000 feet and not just from the surface alone.
Tropopause Pressure
The next product offered in the weatherTAP model data is the tropopause pressure data. The pressure at the tropopause can give us important information about the temperature profile of the atmosphere.
First of all, the tropopause is the highest limit of the atmosphere that we expect weather to occur in. The height of the tropopause varies from about 11 miles up at the equator, to only 5.5 miles up at the poles. Understanding why there is such a height variation between the equator and the poles with the tropopause is key to understanding why we are concerned with it.
As the atmosphere warms during the day, it expands. As it cools at night, the atmosphere contracts. Warm air heats the atmosphere and expands it, pushing the tropopause up higher into the atmosphere. This is why the tropopause is so much higher up at the equator. There is so much heat pushing it upward; a consequence of a warming and expanding atmosphere.
At the poles, the earth is very cold and there is not much heat to expand the atmosphere or push the tropopause level upward. Therefore, the tropopause height is only about 5.5 miles up.
Incidentally, climate change studies focus on the tropopause to study atmospheric warming. If the climate is warming due to the Greenhouse Effect, we should first see the warming in the atmosphere, where the gases responsible for this effect reside. If the atmosphere warms, then the tropopause level would rise, and that's exactly what atmospheric scientists are interested in studying.
Pressure decreases as you go up into the atmosphere. Therefore, the higher the tropopause level is, the lower its pressure will be. Consequently, the higher up into the atmosphere the tropopause level is, the warmer your surface temps will generally be. Keep in mind, the warmer surface temperatures are probably what caused the atmosphere to heat and expand in the first place.
Identifying areas of lower or higher tropopause pressures can give us a better understanding of how the weather at the surface is affecting the atmosphere. This all goes back to the philosophy in meteorology that in order to understand the weather, you must understand what is taking place at each level of the atmosphere from the surface to the tropopause.
Snow Depth
This comes in quite handy in the winter time! This product is only offered by the NAM and the RAP and is not available with the GFS data.
Each model has a different temporal resolution for the snowfall prediction. The RAP model forecasts in one-hour increments, up to 21 hours out in time. The NAM forecasts in three-hour increments, up to 84 hours out. Just keep in mind that each model is forecasting in those increments and not for a specific hour. For instance, if I look at the RAP model for 2:00 pm and it shows 1 inch of snow, that means the model is forecasting one inch of snow to fall sometime between 1:00 and 2:00 pm.
As with any model data, make sure you know when the model was run, as well as for what time the model is forecasting for. Both of these can be found in the lower left hand corner of the screen. The run time tells you what time the model was initialized (or started). You may need to refresh the page to ensure you're getting the most updated model run. The valid time is what time the model is forecasting for.
The RAP conducts a model run every hour. Remember, this is the RAPid refresh model. The NAM model conducts a new model run every six hours. There is a tab under the heading "Model Runs" that lets you select which model run you're looking at. This can be handy if you want to see what model run-to-run consistency looks like. If, for example, the three consecutive model runs of the RAP model predicts 5 inches of snow for you, you can have a higher confidence in that forecast. On the other hand, if the last three consecutive model runs forecast 2 inches during one run, 10 inches in another, and 5 inches during another, you're confidence in that model is low. Model consistency is key to a high confident forecast!
Each model has its own scale, as well, which can be found in the lower right hand portion of the screen. Always make sure you understand the scale, so that you understand how much snow is being forecast. Both scales are in inches.
Dew Point
The next topic we will address takes us back to the importance of moisture in the atmosphere. Keep in mind, moisture is the fuel for storms. The more moisture that is available in the atmosphere, the more fuel that is available for the storms to build and intensify.
Knowing how much moisture is in the air is key to knowing how strong storms could potentially become.
Dewpoint is one of the best measures of moisture we have. Therefore, it is absolutely vital that we know what that dewpoint temperature is throughout the atmosphere. The dewpoint is the temperature to which the air must be cooled to in order to reach saturation. In other words, dewpoint is the temperature that must be reached in order for the relative humidity to be 100% (fully saturated air).
Every air mass has a certain temperature at which saturation will occur. Saturation is ALWAYS reached by cooling the air. You NEVER warm the air to its dewpoint, you always cool the air to its dewpoint. This is one of the very rare times when we can say the words "always" and "never" in meteorology. A desert airmass may have a temperature of 90 degrees and a dewpoint of 20 degrees. While a coastal location, such as New Orleans, may have an air temperature of 90 degrees and a dewpoint of 80 degrees.
So, what does that mean?
If the air temperature is 90 degrees and the dewpoint is 20 degrees, then we know that the air can only be completely saturated at 20 degrees. Since we are 70 degrees away from that saturation temperature, we can say the air is very dry. That's what you expect in the desert, right?
On the other hand, if New Orleans has an air temp of 90 and a dewpoint of 80, we would know we are only 10 degrees away from saturation and the air is very humid. That's what we might expect in a coastal city like New Orleans, right? Generally, as long as the air temperature is within 10 degrees of the dewpoint, we can say that the air is moist enough for precipitation. Again, the closer the air temperature is to the dewpoint, the more moist the air is. If your air temperature and dewpoint are both 70 degrees, the relative humidity is 100% and the air is fully saturated and it is raining outside.
Since you know that the air temperature will never go below the dewpoint, you can use the dewpoint as a rough guide for how cold your overnight low will be. If the dewpoint is 40 and the afternoon high was 60, you know that your overnight low can't go below 40 degrees. You would only be able to go below 40 degrees that night if your dewpoint dropped further.
It is worth noting that the air temperature is much more easily changed than the dewpoint. You'll see only small changes with dewpoints over time (usually), but the air temperature can change much more dramatically in a short period of time.
I hope this helps you understand dewpoint and that this will help you when using our model data's forecast dewpoint temperatures!