Radar Tutorial
Chapter 2, Page 1: Velocity Radar Swirling Couplets of Magical Destruction

Base Velocity Image          Base Velocity Interpretation Image

Here is more information about Velocity Radar.

Animated Image

Velocity images are a new product of NEXRAD WSR-88D Radar systems. It is radically different in that its purpose is to detect wind speed. The image still shows reflective images of precipitation, but the color indicates what direction the wind is blowing. Red means the wind is blowing away from the radar system, green is wind blowing it, and yellow is blowing parallel to the radar station or weak winds. Purple shows what is called "range folding", which means that the wind direction is not detectable in those locations. The StormRAD product is "storm relative winds" -- where they subtract the storm motion from the wind return to get an idea of how the winds are moving within the cell.

In the image here, we see a straight line wind condition blowing roughly from SW to NE. The red passes over the NWS MPX Field Office in Chanhassen, MN and changes to green. Note however that the pattern is broken at a couple points where red and green are fairly close together. Look just north of the "n" in Mankato and watch it move east into Rice County. This is an area of rotation. See how in the last image of the loop in particular, dark red comes close to dark green. This is called a "couplet".Winds of 40 knots towards the radar system pass very close to winds of 30 knots away from the radar. As you can see the acuity of the image is limited, as is the clarity of dramatic wind shifts expected in a mesocyclone capable of spinning up a tornado. Also notice that this couplet is very close to the point where passes through the red/green transition as it passes the NWS MPX Field Office. This transition point can be a "blind spot" for a forecaster. However this image was sufficient for the NWS to warn the public with the confirmation of numerous reports: 

NATIONAL WEATHER SERVICE TWIN CITIES/CHANHASSEN MN
    750 PM CDT THU AUG 14 997
* TORNADO WARNING FOR
    EASTERN LE SUEUR COUNTY- SOUTH CENTRAL MINNESOTA
WESTERN RICE COUNTY - EAST CENTRAL MINNESOTA
* UNTIL 830 PM CDT
* SOME LOCATIONS AFFECTED
    KILKENNY
SHIELDSVILLE
    RURAL AREAS WEST OF FARIBAULT
    * AT 750 PM CDT DOPPLER RADAR DETECTED A POSSIBLE TORNADO NEAR
KILKENNY...OR 23 MILES NORTHEAST OF MANKATO. THE STORM WAS
    MOVING EAST AT 25 MPH.
    NUMEROUS REPORTS OF FUNNEL CLOUDS HAVE BEEN REPORTED WITH THIS
STORM IN EASTERN LE SUEUR COUNTY.
GO TO THE BASEMENT OR A SMALL INTERIOR ROOM ON THE LOWEST FLOOR!
MCBEATH PETERS

Notice also the similar pattern in the loop starting just NE of the "o" in Mankato and seemingly tighten up and increase in intensity in eastern Dakota County. This couplet didn't produce a warning during the life of the loop.


Animated Image

Notice now the corresponding reflectivity image above. You see a series of thunderstorms moving ENE from SW of Mankato to south of Hastings on the Wisconsin border.

The original tornadic mesocyclone is embedded in a line of thunderstorms. No obvious storm structure in the reflectivity suggests a tornadic event. Recent research suggests that this kind of mesocyclone that is basically a supercell embedded in line of thunderstorms is more common than the stand alone supercells from Skywarn training. In fact, it appears that tornadoes that develop from low level rotation difficult to detect by NEXRAD are the most common of all. Fortunately, these tornadic events tend to be weaker and of shorter duration.

The cell in Dakota County that appears to be developing a mesocyclone breaks off of the line as it moves east. It begins to take on the character of a supercell as it nears the Wisconsin border and the loop ends. As I recall, this cell made an abrupt turn to the right and formed several tornadoes just south of the Dakota County border.



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