VWP Hodographs

A hodograph is created by drawing a line between the tips of the wind vectors at many different altitudes. Altitude on this hodograph denoted by color and the numbered markers, which give height above radar level in kilometers. The colors are red, light green, dark green, purple, and cyan for the 0-3 km, 3-6 km, 6-9 km, 9-12 km, and 12+ km layers, respectively. The dashed portion is from the surface wind (if present) to the lowest VAD point, which is usually 100 m (and in some cases much higher) above radar level.

The data for these hodographs are created from a radar volume by many VAD retrievals, from which you can derive a root mean square error (RMSE) in the retrieval. The RMSE for each data point is given by the size of the shaded circles on the hodograph. Specifically, the radius is RMSE × √2. This is intended to be an upper bound on the error envelope (i.e. the true wind could be located anywhere in the circle), but in some cases the true wind probably lies outside the circle. I can provide the mathematical derivation for this on request.

There are several markers on the plot that may be of interest.

○RM	The Bunkers right mover vector (if available; see Parameter Descriptions)
○LM	The Bunkers left mover vector (if available; see Parameter Descriptions)
+SM	The user-specified storm motion (if one has been specified)
□MEAN	The 0-6 km mean wind (if available)
▽DTM	Deviant tornado motion (if available; see Parameter Descriptions)

Additionally, there is a thin cyan line from the storm motion (if available) to the lowest point on the hodograph (the surface wind, or if that is not available, the lowest VAD retrieval) and a thin magenta line from the lowest point to 500 m AGL. These lines outline the critical angle (see Parameter Descriptions).

Hodograph Options

To select a radar, click on a dot on the map on the top left. This will load all data for the last 45 minutes for that particular radar. If you would like this radar to load by default when you load the page, click on the "home" icon to the right of the radar name.

The row of buttons below the radar name controls where the origin of the hodgraph plot is placed. The default is to place the origin at the ground ("Ground"), but for severe storms nowcasting, it can be useful to place the origin at the storm by clicking "Storm". Because the bounds of the hodograph plot are computed from all loaded profiles, if any of the profiles do not have a storm motion, the "Storm" option will be unavailable. Selecting a storm motion manually (see below) will always make the "Storm" option available.

Below the origin control is the storm motion control. This allows you to select whether to use the 0-6 km mean wind ("MEAN") or one of the Bunkers supercell motion estimates ("BLM" for the left mover and "BRM" for the right mover) as the storm motion. See the Parameter Descriptions section for more information on the Bunkers estimates. Additionally, you can select a storm motion manually by clicking on the "DDD/SS" button and clicking on the hodograph plot where you would like to place the storm motion vector.

The QLCS motion control is similar to the storm motion control. The QLCS motion is assumed to be a vector perpendular to the line with a magnitude equal to the forward speed of the line. The default is "None" (for no QLCS) with the "DDD/SS" button to allow you to place a vector manually similar to placing a storm motion manually. The QLCS motion is plotted as a line perpendicular to the motion (i.e. along the line). When a QLCS motion is plotted, the 0-3 km shear vector will also be plotted in gray for reference in diagnosing QLCS tornado potential.

Next, the default surface wind is from a nearby ASOS site (the button reads "ASOS" when the page is loaded but will change to the 4-letter identifier of nearby the ASOS site after selecting a radar). Additionally, you can place a surface wind manually ("DDD/SS", similar to the storm motion and QLCS motion) or remove it ("None").

Finally, the "Auto-Update" is a toggle switch which controls whether to automatically update the profiles as they're available or to keep the profiles currently loaded. Auto-update is on by default.

Animation Controls

When profiles are loaded, dots representing the frames will appear to the left of the "▶▐▐" button. Click on one of these dots to jump to a particular frame. Start and stop the animation by clicking on the "▶▐▐" button. Increase or decrease the speed of the animation by clicking on the "+" or "-" buttons. Force a refresh of the data by clicking on the refresh button to the right of the "▶▐▐" button.

Keyboard Controls

SPACE	Start/stop animation
←	Move back one frame
→	Move forward one frame

Downloading Images

Clicking on the hodograph plot while not doing a manual selection will open the current frame as an image in another tab. Download this as you would any other image. To download the entire sequence as an animated gifPronounced "gif", click on the "GIF" button to the bottom right of the hodograph plot. This will open a new tab with the gif in it, after a few seconds to render the gif. Then download the gif as you would any other image.