Learn About Weather with Meteorologist Mark



Model

 
U.S. Current Temperatures

If you click on Forecast, the very first option is U.S. Current Temperatures. Have you ever wondered where those temperatures come from? Those temps come from METAR sites, which is basically a fancy way of saying that they come from airports. A METAR is a Meteorological Terminal Air Report. By the way, meteorology is famous for acronyms! (ha)

METARs are reports that pilots make good use of. I'll have more on METARs when we get to the aviation stuff under the aviation tab. Just an FYI, the correct way to take temperature is within a white box, protected by wind but with sufficient ventilation, about 4 feet off the ground. I have a picture below of one. This is a typical METAR temperature-taking device.

Notice I said it has to be four feet off the ground. This is why we can get frost when the temp is in the mid 30s. On clear, calm nights the ground cools off faster than the air above it. It has to be 32 degrees for water to freeze. The air right at the ground may be 32, while the air four feet above is 35 degrees.

 
Predictive Radar and Satellite

The EarthCast global model is run twice a day (7:00a.m / 7:00p.m.), producing a 36-hour forecast. It is a full Earth System Model, which means it takes into account the atmosphere, land, and oceans and not just the atmosphere alone. The model is run with cloud-resolving physics, something that no one else is doing.

The model goes out to 36 hours from the run time. You can select if you want just satellite, just radar, or a combination of the two. The farther in time one goes out from the model run, the less accurate the model is liable to be. For instance, if the model is run at 7:00 a.m. it will be most reliable within the first few hours after 7:00, and much less so by the afternoon.

At the bottom left-hand corner of the model image you'll see a valid time. That is the time the model is depicting. Below that you'll see a Model Run time, which is the time the model was run. The model cannot tell you exactly where precipitation will fall, but if you see blotches of green across your area, you know that the model is predicting scattered precipitation, some of which could fall on your area of concern.

That's your basic run-down of predictive radar and satellite! Let me know if you have any questions!

 
National Radar - Radar Mosaic

Have you ever wondered what a mosaic is? That's just simply the combination of the data of multiple radars. It makes it look like the country is covered by one giant radar, when it fact it's covered by 159 NEXRAD radars. (Remember, the TDWR radars only cover the airports and the area closest to them.)

Once on the map, you can click on it to zoom in. The first map is national, the second regional, and the most zoomed-in one is state-level radar, which also outlines the counties. Radar basically sends out a signal. If that signal strikes something (ie. Rain drop, hailstone) it returns to the radar. The radar receiver detects this return and uses that information to determine how large the struck object was. For instance, if the radar strikes a hailstone, the radar return will be much stronger (show up brighter on your radar) than if the radar hit a raindrop (that would show up as a lighter color on your radar).

Radar sites can only be found near National Weather Service offices. This can mean that some areas have very poor coverage. Notice the gaps in coverage on the map below.

Complete radar scans are done every 4.5 to 10 minutes, depending on the setting. If there's nothing going on, it will be set to scan every 10 minutes. Notice that even on its best day it scans every 4.5 minutes. This is why some chasers get caught off guard. A LOT can happen in 4.5 minutes.

So, is there level I data? Yes! Level one is the raw data that comes straight from the radar with no quality control done to it. It's a mess!

Level II is much different. With this data, each pixel size is the same, no matter how far away from the radar it is. This is also called super resolution data. Data has been filtered to get rid of buildings, mountains, and other obstructions that are always there. This data can be "smoothed" to look better to users.

Level III is even more processed but you only get 4 scans of a storm. These four scans are of the lowest levels of the storm. Now, while there are fewer scans with level III, you get a LOT of information from these few scans. You get your precip estimates, vertically integrated liquid, TVS signatures, mesocyclones, hail indicators, storm relative velocity, etc etc from just these four, data-heavy, scans. You may notice some of these products on RadarLab HD+, but we'll talk about them in more detail when we get to that point.

That is all for now! We'll talk about echo tops next time.

 
Echo Tops

Echo tops is a radar product offered by weatherTAP that gives you some valuable information on a storm's vertical development. Always keep in mind that echo tops do not tell you how tall a t-storm is. Many people have that misconception.

Echo tops tell you the top of an area of precipitation, as detected by radar. It is the highest level of a storm that the radar detects "bounce-back" energy of at least 16 dbz. Radar measures energy in dbz units. That's the measure of the amount of energy that returns to the radar after it is bounced back, whether from hitting a rain droplet, hail stone, snowflake, etc. Light rain returns a signal of about 16 dbz. The radar really cannot detect anything less than that. So, radar will tell us that the highest cloud top is, say, 35,000 feet because at 35,000 feet it detected a "16-dbz's" worth of precipitation. However, the cloud will go even higher than that because the radar can't detect the mist that makes up the cloud (clouds would return a dbz much lower than 16 dbz).

We can get a lot of information from echo tops by knowing the highest altitude that radar estimates precipitation to be. Generally, the higher the echo tops, the stronger the storm. This is due to the fact that stronger updrafts should lead to precipitation being hoisted higher into the atmosphere. We'll talk more about dbz a bit later on, especially when we get to RadarLab HD+ material.

Below is an example of how I have used the echo top product this morning. I'm focused on that line of storms approaching the Mississippi River. Notice how the echo tops correspond to some of the heaviest rainfall? We can use the echo tops product and feel confident that cloud tops are exceeding 40,000 feet, giving us an indication that these storms are strong enough to produce very heavy rainfall, strong winds, and dangerous lightning. It's always helpful to be able to utilize more than one product to determine a storm's strength. That helps cut down on false-alarm warnings. Basically, the radar product shows us what kind of precipitation we can expect near the surface, while the echo tops product shows how high up the radar detects precipitation.

Next week we'll talk about the lightning data product!

 
Infrared Satellite (IR)

You may have seen weatherTAP's satellite products and wondered what the difference was between the satellite products. Today, we'll talk a little bit about IR and visible satellite imagery. The next product on our list is the radar/satellite overlay under the "Forecast" tab. This particular overlay alternates between using IR and visible satellite. The visible is used by this product during daylight, and IR is used at night, though IR can be used during day or night.

The IR sensor detects the amount of thermal (heat) radiation being emitted into space from the earth and its atmosphere. All objects radiate IR light because all objects radiate some type of thermal energy. IR satellite sensors can detect the temperature of an object by the character of the IR light it emits. The warmer the temperature, the grayer it appears on satellite. The colder the temperature, the whiter something appears on the satellite image. So, clouds should appear whiter than the surface, since they're colder. Some problems from IR satellite imagery include clouds that are really low to the ground and nearly the same temperature as the ground (ie fog). The low clouds will show up as nearly the same color grey as the ground and are very difficult to see. IR satellite works any time, day or night, which is helpful in tracking storm systems at night.

IR's counterpart, visible satellite, is only useful during the daytime, because it relies on sunlight reflected up to the satellite. Visible satellite has a much better resolution than IR and can be used to detect smoke, fronts, and other subtle surface boundaries.

You may have also noticed on some IR satellites that the highest clouds tops were in various shades of yellows and reds. That's called enhanced IR satellite. That is simply an algorithm developed to determine which cloud tops are the absolute coldest, since a regular IR satellite image only shows shades of grays and whites. Since colder cloud tops typically indicated stronger storms, this can be useful in identifying storm intensity. The colder cloud tops are colored red, while the warmer cloud tops are greys and blues. Notice how much better the colder cloud tops show up on the hurricane in the enhanced IR image.

Next time, we'll talk more about what GOES East and West satellites are and how GOES-16 is going to forever change satellite science.

 
What is GOES?

The next choice we have on our list is GOES East or GOES West visible satellite. We've already talked about the difference between visible and infrared satellite, but what exactly is GOES?

First of all, GOES stands for Geostationary Operational Environmental Satellite. The satellite is a joint effort between NOAA and NASA that began in 1966. GOES's operational lifetime extends through December of 2036. There are two GOES satellites, one positioned at 75 degrees west longitude (GOES-East) and the other at 135 degrees west longitude (GOES-West) (see pic below). There is also one in the middle for backup, just in case something happens to one of the other two satellites.

As the name implies, these satellites are fixed at certain points above the earth, so we get a continuous picture of the earth below them. And, of course, GOES East gives you a view of the eastern side of North America, while GOES West gives you a view of the western side of North America.

Now, we have GOES-16. GOES-16 is providing incredible images of storms faster than any other satellite data we've ever known. GOES-16 can scan in 15 minute, 5 minute, or 30-60 second intervals, all at the same time! This allows for incredible evaluation of storms in real-time. The satellite also has 4 times greater resolution, while being five times faster than ever. In addition, hurricane tracking, warning lead-times for severe thunderstorms, lightning detection, and solar flare warnings will all be improved with the addition of GOES-16. The addition of GOES-16 will allow us to see the weather like never before. GOES-16 will take the place of GOES East, while another launched satellite will take the place of GOES West next year (2018).

 
The National Lightning Detection System

Your next weatherTAP option is national lightning, where you can actually view where lightning strikes have occurred. A legend below the screen tells you how old the strike is, with white being the most recent. You can choose to just watch lightning strikes or you can add either infrared satellite or visible satellite to the image.

I should note that there is already a tutorial in place for this lightning data. Just select "tutorial" on the National Lightning screen and you'll get a good review of how the lightning data is gathered and much more!

Lightning can give you a good idea of how robust or severe a t-storm is. All thunderstorms produce lightning. In fact, lightning is what makes a storm a storm! Without lightning it just isn't a storm.

So how does lightning even form? Well, we do know that lightning results from a difference in charges. The greater the difference between positive and negative charges, the greater the chance for a lightning strike. We also know that water carries a negative charge and ice carries a positive charge. The bottom of a cloud is mostly liquid, heated in part by rising air that feeds into the updraft that actually builds the cloud in the first place. As we go higher and higher, the air gets colder and ice crystals are allowed to form. This creates difference of negative charges at the base of the cloud, and positive charges at the top of the cloud. Nature doesn't like imbalance, so when this imbalance gets too great.....BOOM... a lightning strike. As soon as that strike takes place, the charges balance out. As soon as the strike is over, the charge differences start building up again. How soon those charges imbalance again determines how long it is until your next strike.

Now, it's rather easy for lightning to travel through that cloud. After all, it's made up of water droplets that conduct electricity rather well. That is why you don't put electrical things in the water! This is one reason why we think there are at least five times more intracloud strikes than cloud-to-ground (CG) strikes. In order for a strike to travel from a cloud to the ground, it has to make it through the air that is in between. Air is not a good conductor of electricity. So, the charge imbalance has to really build up before the strike can happen.

Increased lightning activity can indicate a storm that is intensifying. Some studies have found that a sudden increase in lightning can indicate a developing tornado. The most important thing is to make sure you don't find yourself on the other end of lightning's strike! If you're close enough to hear it thunder, you're close enough to be struck.

 
Surface Map Analysis

Being able to analyze a surface map is crucial to understanding the weather. I have attached a recent surface map to help you follow along. You'll immediately notice the blue 'H's and red 'L's. These are high and low pressure areas, respectively. Air rises in low pressure. Since temperatures decreases with height, this rising air cools and condenses into clouds, thus, the weather around low pressure is cloudy/stormy. Air sinks in high pressure. Sinking air compresses, warms and dries out. Therefore, the weather around high pressure is often cloudless.

In addition, air blows toward low pressure (convergence) and away from high pressure (divergence). Air also circulates counterclockwise around low pressure and clockwise around high pressure. Since each L or H describes the center of the lowest or highest pressure, you can tell which way the wind is blowing by knowing where these centers are located.

Also, wind always blows from high to low pressure. In fact, wind is the direct result of pressure imbalances. Wind is nature's way of balancing the pressure fields. The more imbalanced pressure becomes between two areas, the stronger the winds will be between these two areas. For example, the air pressure inside a tornado is really, really low. That makes the pressure incredibly imbalanced between the location of the tornado (low pressure) and locations surrounding the storm, creating severe winds that work to try to balance the pressure differences.

The white lines you see on the map are called isobars. These are lines of equal pressure, connecting places that have the same pressure readings. The closer the lines are to each, the more quickly pressure changes with distance. This pressure imbalance will cause stronger winds. Notice the isobars are so close to each other across the Southeast. This is Tropical Storm Irma on September 11, 2017. Sustained winds at the time of this image were 60 mph.

I should also take a minute to explain pressure. Pressure is measured by the weight of the atmosphere pressing down on an object. If the air is rising (low pressure) the weight of the atmosphere is less. If the air is allowed to sink (high pressure), the weight of the atmosphere would be greater, and you'll have higher pressure readings.

There are several ways to get air to rise. Mountains are one way. When wind encounters a mountain it has two options. It can either go around the mountain or up and over the mountain. When it goes up and over the mountain the air is forced to rise.

Fronts do the same thing. When air converges at a front it is forced to rise. Fronts represent the boundaries between different air masses with different temperature and moisture characteristics.

Air rises quicker along cold fronts, so you often see your stormiest weather along them. They are the line with the triangles. The triangles point in the direction the front is moving. There is one stretching from Canada into Minnesota on the map. If it is moving at less than 5 mph, it is called a stationary front. A stationary front has triangles and half circles alternating along it. You can see one stretched across South Dakota on the map. Air rises more gently along a warm front. This usually leads to more widespread, gentle rainfall. Warm fronts are the lines with half circles along them. The half circles point in the direction the front is moving. There is one extending eastward from Irma on the map. You'll also notice the dashed lines. These are areas of lower pressure that we call troughs. They're basically weak disturbances that don't have a great enough temperature or moisture difference around them to be a front, but they do represent an area of lower pressure.

During the warm season months, you may notice a dry line front moving across Texas and Oklahoma. These fronts separate dry, desert air to the west from warm, moist air in the east. These fronts move from west to east through the day and can cause severe thunderstorms. The front is drawn like a warm front, with the half circles pointing in the direction of movement, but unlike a warm front, the half circles touch each other all along the front.

Also, on this map you see the L for low pressure over the California/Arizona border. That represents a thermal (heat) low in that desert area. Warm air rises and air in the Desert Southwest gets REALLY warm. That rising air creates a flow of moisture that will move in from the Pacific and the Gulf of California, leading to rainfall. We call this the monsoon, which is just a seasonal pattern of wind that develops.

Anytime air converges, or comes together, it is forced to rise. As air comes across the Pacific Ocean it doesn't encounter anything to slow it down until it reaches the West Coast of the US. Then, the friction of the land slows that wind down and causes air to converge along the coast. Since converging air must rise, an area of low pressure forms along the coastline. This can also lead to rain showers.

Hopefully, this has been a good crash course in basic weather map reading!

 
Temperatures

Under the "Forecast" tab you'll see an option for "U.S. Current Temperatures". These temperatures are reported primarily by airports that have sophisticated temperature-measuring equipment. Temperature is technically the measurement of molecular activity in the air. The standard height for a thermometer is about four feet off the ground.

It is important that you know that temperatures are taken four feet off the ground in order to understand frost. You may have noticed forecasts like this, "Expect clear skies and calm winds tonight. Temperatures will drop into the middle 30s, so expect areas of frost to develop after midnight."

If water freezes at 32 degrees how can you get frost if the temperature is above freezing? I'll try to explain. On a clear, calm night the heat radiates from the earth's surface very efficiently. The ground cools very quickly and, thus, cools the air in contact with it very quickly. Air is heated by longwave radiation from the ground. The ground is heated by shortwave radiation from the sun. So, the sun doesn't heat the air because the air is heated by the ground. Air cannot efficiently absorb the shortwaves from the sun, but it can efficiently absorb the longwave radiation emitted by the ground.

Since the ground cools more quickly than air, the air in contact with the ground ends up being much colder than the air up off the ground. This is called an inversion, when temps get warmer farther up from the ground. This means that by the time the air gets to your thermometer, which is four feet off the ground, the temp is closer to 35 or 36 degrees. You must have calm winds because wind will mix all the air up and prevent frost from forming on the ground. Clouds will prevent heat from radiating away from the earth efficiently, as they will act as a blanket and radiate the heat back to earth, therefore it must be a clear night.

Other terminology you may hear with respect to temperature involves averages. You may hear your local weather forecaster say that the temperature is below average, average, or above average. Averages are calculated based on the 30-year average high/low temperature for that day. If you take all the high temperatures that have occurred on this day over the past 30 years and that equals 65, and today's high is 67, today's high is two degrees above the average high for the day. Sometimes the word "normal" is substituted for the word "average."

Surface temperatures can tell us a lot about the atmosphere. For instance, warmer temps can mean more available energy for storms. Temperatures that are around freezing (32℉) can tell us what the risk is for ice and snow. Temperatures can also tell us where fronts are located. If the temperatures in Kentucky are in the 70s, for example, and the temperatures in Missouri are in the 40s, we can be very confident that there is a cold front between Missouri and Kentucky. We can study how temperatures change across the landscapes to identify many different types of fronts. This is helpful since fronts tend to produce precipitation.

Next week, we'll explore predictive radar and how that works!

 
Predictive Radar and Satellite (revisited)

This is the second time we have come upon the predictive satellite and radar product, but it's worth a revisit. Predictive Satellite and Radar is a product offered by weatherTAP that lets you see the precipitation and cloud cover that the North American Model (NAM). The forecast extends out to 36 hours from the time of the model run. The model run images are in 3-hour time steps.

You can always tell what time the model data was constructed by looking in the lower left corner of the image. The run time and the period of time the model is valid are both shown here. Here is an example:

The NAM model is a major weather model used for short-term weather forecasting.

With this weatherTAP product you can always choose to turn off the satellite or the radar by unchecking those options on the left hand side of your screen.

Revisiting predictive satellite and radar is a good stepping stone into what we'll be discussion next week, the model data page offered by weatherTAP. Don't worry, we'll take things in baby steps. Model can be difficult to explain and interpret but I think you'll understand it a lot better after our model lessons! As always, if you have any questions at all just let me know!

 
Model Data

Now, we venture to the next tab over on the homepage to 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:

  1. Wind and height
  2. Temperature
  3. Relative humidity
  4. Dewpoint
  5. Severe indices
  6. Precipitation

The first option, wind and height, are important for understanding how wind changes with altitude. We'll dive into how to read this very important map next week!