Daytime heating, coupled with the lift provided by the cold front will allow for the development of thunderstorms later this afternoon. Hail to 1.5 inches and damaging winds will be the main threats with these storms. Isolated tornadoes cannot be entirely ruled out; there is more than enough shear for the development of tornadoes, however, the low level convergence maxima are somewhat isolated.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

An area of low pressure over northern portions of Michigan and Wisconsin will bring a chance for thunderstorms across portions of the Great Lakes this aftermoon and tonight. Small hail and damaging winds will be the main threats with these storms. The hail potential doesn’t look to be too impressive due to the lack of instability.

Across the Mid Atlantic and Southeast, relatively clear skies will allow for destabilization through this afternoon. Hail to 1.5″ and damaging winds will be the main threats, but isolated tornadoes cannot be ruled out.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

An upper level low over Northern Mexico will continue to move eastward through the day. Meanwhile, at the surface, there is a rich supply of moisture in place. The upper level low will provide ascent across Southern Texas, which will lead to thunderstorm development this afternoon tonight. Hail to 2″, damaging winds, and isolated tornadoes will be possible with these storms. The hail potential may be tempered somewhat in Southern Texas due to high precipitable water values, but damaging winds and isolated tornadoes will still be possible.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

Though ascent is rather weak, the environment across Southern Texas is still unstable and with the aid of daytime heating, thunderstorms will be possible through the afternoon. Hail to 1″ and damaging winds will be the main threats with these storms. In the Southeast, thunderstorms will also be possible ahead of a cold front. The main threat with these storms will be damaging winds. The potential for hail may be precluded somewhat due to high precipitable water values.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

The thunderstorm potential will be partially due to the subtropical jet that will advect moisture into portions of Southern Texas. This moisture, combined with further destabilization due to clear skies will allow for the potential for thunderstorms. CAPE values could easily exceed 2500 J/kg with ample daytime heating. Hail to 1.5″, damaging winds, and isolated tornadoes will be possible as thunderstorms develop. As the subtropical jet continues to intensify, bow echoes will also be possible, which will pose the largest damaging wind threat.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

If you’ve used RadarScope, you’re likely familiar with the types of products available in it. In this tutorial, we will look at how to interpret some of these products.

While looking at different radar sites within RadarScope, you’ll notice that some sites have more products than others. The reason for this is that as of this writing, the nationwide upgrade to dual-polarization radar is underway. Those radars that have already been upgraded will have some additional products that will be discussed later.

How radar works

Radar works by emitting radio waves into the atmosphere. When the radio waves that are emitted from the radar bounces off a particle (whether it be rain, hail, dust, etc.), energy from the particle is returned to the radar. The amount of energy returned to the radar is called reflectivity and is measured on a logarithmic scale. Doppler radar can also indicate the motion of an object toward or away from the radar (more on this later).

The update frequency of radar imagery depends on the mode that the radar is in. Often, in radar literature, you will see the term “full volume scan”. Depending on the mode of the radar, the imagery you see in RadarScope will update every 5-10 minutes, or after a volume scan is completed. A volume scan consists of a radar beam scanning an entire revolution of the radar, adjusting the beam elevation and doing another scan, and so on until the entire volume scan is finished.

Note: In the app, you will see various tilts for a single product. The tilt is referring to the angle that the beam makes with the horizon. The first tilt is usually .5°, but the angle of the subsequent tilt is dependent upon the mode of the radar.

Reflectivity

Assume that the radar beam detects a single drop. The power returned to the radar because of the drop is proportional to the sixth power of the diameter of the drop. In addition, a single drop can return as much power as a volume consisting of smaller drops.

Below is a look at a sample image:

In heavy rain, the radar beam will attenuate some as it travels through the area of heavy rain. In these situations, it is often helpful to observe the area with a radar to the east of the heavy rain.

Composite Reflectivity

The composite reflectivity image is similar to the base reflectivity image, but it has one critical distinction. The base reflectivity image is the reflectivity for a single elevation, but a composite reflectivity image uses the entire volume scan and identifies the strongest echoes in the volume scan.

Velocity Imagery
Doppler radar can determine the velocity of an object observed by radar. There are a few points that need to be kept in mind when analyzing velocity imagery.

1. The Nyquist velocity is the maximum observable velocity by the radar. If an observed velocity is higher than the Nyquist velocity, then an effect called range folding or aliasing occurs. For example, assume that a radar has a Nyquist velocity of 70 m/s. If the radar observes a velocity of 75 m/s toward the radar, because of the Nyquist velocity, it would be displayed as a velocity of 5 m/s away from the radar.
2. A positive velocity signifies a velocity away from the radar and a negative velocity signifies a velocity toward the radar.
3. The shorter the wavelength of the radar, the greater the Nyquist velocity, but the range of the radar is decreased as the wavelength is decreased.

Below is a look at a sample of what a velocity image might look like.

The greens and the hot pink indicate winds toward the radar and reds indicate winds away from the radar.

It is important to note that there are two different velocity products; base velocity and storm relative velocity. The base velocity is the average velocity of the targets in the beam whereas storm relative velocity is the velocity of the storm subtracted out. If you’ve taken fluid dynamics, think of storm relative velocity as an Eulerian concept (in other words, we assume that the storm is stationary and the storm relative velocity is the average velocity of everything else).

Rainfall Products

There are two main products for estimating the rainfall in an area; One-Hour Rainfall and Storm Total Rainfall. The One Hour Rainfall product estimates the amount of rainfall in the past hour whereas the Storm Total Rainfall is the total rainfall since a specific time. It is important to note that these products only pertain to rainfall and cannot be used for snowfall.

This is a Storm Total Rainfall image. A One Hour Rainfall image will look very similar to this image.

Vertically Integrated Liquid

The VIL is an estimate of the amount of water contained in a column of air. High values of VIL can indicate heavy rainfall or hail. There are several caveats that need to be kept in mind when using VIL values:

1. VIL is highly sensitive to beam blockages (such as in mountains).
2. Values of VIL for the potential for hail vary greatly dependent on season.
3. Though hail is often associated with high values of VIL, many studies (i.e. Amburn and Wolf (1997)) have shown that there is a poor correlation between VIL and hail. These studies also cite many cases in which severe hail (hail >1” in diameter) had been reported with low values of VIL.
4. A sudden decrease in VIL can indicate a possible downdraft.

 Below is a look at what the VIL product might look like.

Echo Tops

The echo tops product indicates maximum heights of precipitation echoes that are detected between 5,000 and 70,000 feet and have reflectivities greater than 18 dBZ. Echo tops can be used to identify strong updrafts and downdrafts. A sudden drop in echo tops can indicate a possible downdraft. For storms close to the radar, the radar beam cannot see the top of the storm so echo tops are usually underestimated.

This is an example of the echo tops product.

Clear Air Mode

When there is no precipitation in the area, the radar is usually in clear air mode. When the radar is in this mode, the radar scans more slowly. This allows the radar to become more sensitive to weaker echoes. This mode can be used to identify subtle boundaries such as cold fronts and drylines. Many times, in the case of light precipitation, the radar is often left in clear air mode so the radar can detect this light precipitation.

Dual Polarization Radar

As of this writing, all of the NEXRAD radars are being upgraded to dual polarization technology (Note: NEXRAD refers to the network of radars installed by the National Weather Service). Before we go any further, let’s explain what is meant by polarization. According to Rinehart (2004), polarization is defined as “the orientation of the electric field”.

Traditional weather radars send out horizontal pulses of energy and therefore, they are horizontally polarized. Dual polarization radars, on the other hand, send out pulses of energy that are both horizontally and vertically polarized. Because waves of both polarizations are transmitted, more detail about the orientation and shape of the hydrometeor that is detected by the radar can be obtained. Most dual polarization radars alternate polarizations between waves, but some can transmit waves of both polarizations simultaneously.

Some of the dual polarization products that are available are correlation coefficient, differential reflectivity, and specific differential phase. If these products are not available, then the particular radar site that you are using does not have dual polarization capabilities. All of the radars nationally should be upgraded by the spring of 2013.

The main variables that are measured Differential Reflectivity, Correlation Coefficient, Specific Differential Phase, and the Hydrometeor Class. These variables will be summarized below. It is important to note, however, that these variables are still being studied and there may be more applications for these variables that are not yet known.

Differential Reflectivity

Differential Reflectivity assesses the vertical and horizontal axes of a falling hydrometeor. Differential Reflectivity is related to the ratio of the power received from the horizontally polarized pulse to the power received from the vertically polarized pulse. Differential Reflectivity and this ratio are related by the following expression.

ZDR ~ 10log(ph/pv)

(Scharfenberg)

If the Differential Reflectivity is positive, then the hydrometeor that is observed by the radar is horizontally oriented; if it’s negative, then the hydrometeor is vertically oriented. If the two dimensions are approximately equal, then the ratio is approximately 1, which means that the Differential Reflectivity is approximately 0 and therefore, the hydrometeor is approximately spherical.

In the case where the Differential Reflectivity is approximately zero over an area, the area is referred to as a “ZDR Hole” and is usually an indication of where hail is falling.

This is what a Differential Reflectivity image may look like.

Correlation Coefficient

If you’ve taken a course in statistics, you’re likely familiar with the concept of the Correlation Coefficient. In the context of dual polarization radar, it has a very specific interpretation; this parameter describes how similar the backscatter characteristics of the horizontally polarized and vertically polarized waves are.

For meteorological echoes, the correlation coefficient is typically greater than .7, however, it is possible to have very large hail in areas where the correlation coefficient is .5 or lower. It is recommended that Differential Reflectivity be used in conjunction with Correlation Coefficient to discriminate between meteorological and nonmeteorological echoes. Some articles have even suggested that the correlation coefficient may be used to identify possible tornadoes on the ground by looking for low correlation coefficient values in the vicinity of a hook echo.

Below is a sample of what the Correlation Coefficient product would look like.

Specific Differential Phase

The differential phase is defined as the phase difference between the two pulses at a given range along the propagation path (Scharfenberg). The specific differential phase is the range derivative of the differential phase (how differential phase changes with respect to the range from the radar). Values greater than zero indicate horizontally oriented hydrometeors, while values less than zero indicate spherically oriented hydrometeors. Values near zero indicate nearly spherical hydrometeors.

This is a sample of what the Specific Differential Phase product would look like.

Hydrometeor Classification Algorithm

The Hydrometeor Classification Algorithm uses a fuzzy logic scheme to assign types. Often, the use of only one polarimetric variable is not sufficient to discriminate between types. The algorithm evaluates all of the polarimetric variables and then assigns a classification based on specified criteria.

These algorithms are always under development, but as of now, there is some uncertainty as to whether the algorithms are always accurate. Some algorithms have a “warm” and “cold” mode that can be selected depending on the season.

This is a sample of what the Hydrometeor Classification product would look like.

References
Amburn, Steven A., Peter L. Wolf, 1997: VIL Density as a Hail Indicator. Wea. Forecasting, 12, 473–478. doi: http://dx.doi.org/10.1175/1520-0434(1997)012<0473:VDAAHI>2.0.CO;2
NIDS Product Suite, http://okfirst.mesonet.org/train/nids/nidsproductsuite.html
Rinehart, R. E., 2004: Radar for Meteorologists. Rinehart Publishing, 482 pp.
Scharfenberg, K, Polarimetric Radar Case Studies, http://www.cimms.ou.edu/~kscharf/pol/

The thunderstorm potential is associated with a cold front across portions of the Plains. This front will be the focus for thunderstorms later this afternoon and tonight. As these storms develop, hail to 1.5 inches, damaging winds, and isolated tornadoes will be possible. It will be possible to have a few storms that produce hail larger than 1.5″ inches, but those storms will be rather isolated. The best chance for tornadoes, albeit isolated, will be across portions of the Ohio River Valley where the shear is more organized and there will be some low level convergence.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

There is already some activity across the Northern Plains and this  will continue, as well as expand eastward, through the day. Large hail to 2″, damaging winds, and isolated tornadoes will be possible with these storms. Ahead of the storms where surface heating is occurring, this will be where the highest chance for damaging winds later this afternoon and evening as the lower levels of the atmosphere both warm and dry. Though the amount of shear is high, widespread tornadoes are not anticipated due to the lack of low level convergence.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.

There is currently a broad area of northeasterly flow across the Central Plains, which is bringing moisture to portions of the Plains and Great Lakes. With ample surface heating, thunderstorms will continue to develop across the Northern Plains and Great Lakes. Hail to 2″, damaging winds, and isolated tornadoes will be possible with these storms. The threat for damaging winds will primarily be tonight when the lower levels of the atmosphere will be drier and the storms begin to weaken.

Please stay tuned to ChaserTV for further updates and live streaming video of weather events.