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Weather Map Overlay

The new weather add-on allows Navigo Map users the ability to add current weather NEXRAD Doppler Radar, Weather Watch/Warning, and Satellite images to your display. These images are geo-referenced for display on your existing map and can be manually refreshed or updated automatically over the Internet. There are no ongoing costs after purchase of the add-on.

Radar images are updated from the network of NEXRAD Doppler Radar sites located across the country. Navigo users may select individual sites or regional views of the available data. A convenient drop down list sorted by state and city is provided for selection.

Since the data is presented as a transparent overlay it can be displayed along with your existing road and vehicle data. The Radar display will not disrupt your normal AVL or Navigo Map operation. Additional Terrain, boundary and road network map data is provided for viewing weather information outside of your existing map coverage area.

NEXRAD Data

NEXRAD Doppler Radar sites are operated by the National Weather Service. Data from radar is normally available on a 10-minute interval but is speed up to 5 minutes during periods of severe weather. The Navigo Weather Add-on will automatically refresh the data.

Available images include the following –

Reflectivity Images

These images are just as they sound as they paint a picture of the weather from the energy reflected back to the radar. There are two types available; Base (or ½° elevation) reflectivity and Composite reflectivity.

Base Reflectivity

Base Reflectivity is the default image. Taken from the lowest (½° elevation) slice, it is the primary image used to "see what's out there". There are two versions of Base Reflectivity image; the short-range version that extends out to 124 nm (about 143 miles) and the long-range version that extends out to 248 nm (about 286 miles).

Composite Reflectivity

Composite Reflectivity images utilize all elevation scans during each volume scan to create the image. It is composed of the greatest echo intensity (reflectivity) from any elevation angle seen from the radar. It is used to reveal the highest reflectivity in all echoes.

Another advantage of Composite Reflectivity is in mountainous regions. Often, the Base Reflectivity ½° elevation scan is not high enough to see over mountains. With the addition of higher elevations scans, weather information over mountain peaks can be seen.

Velocity Images

One of the best features on the 88d Doppler radar is its ability to detect motion. However, the only motion it can "see" is either directly toward or away from the radar. This is called radial velocity, as it is the component of the target's motion that is along the direction of the radar beam.

In all velocity images, red colors indicate wind moving away from the radar with green colors representing wind moving toward the radar. It is very important to know where the radar is located as that is your reference point for proper interpolation of the wind's motion.

Base Velocity

Base Velocity images provide a picture of the basic wind field from the ½° elevation scans. It is useful for determining areas of strong wind from downbursts or detecting the speed of cold fronts. However, since the radar only measures radial velocity, the strength of the wind will always be less than what is actually occurring unless the wind is moving directly toward or away from the radar.

Also, the surface winds are only for areas near the radar. As distance increases from the radar, the reported value will be for increasing heights above the earth's surface.

Storm Relative Motion

Storm Relative Motion images are very useful images to look for small-scale circulations (called mesocyclones) in thunderstorms. Often, these small-scale circulations are areas where tornadoes form.

What separates storm relative motion from base velocity is the motion of storms are "subtracted" from the overall flow of the wind. As storms move, their own motion can mask circulations within themselves. This motion is removed to make the view of the wind relative to the storm. In effect, what is seen is the wind's motion as if the storms were stationary.

Precipitation Images

There are two precipitation images made available: One-hour Precipitation and Storm Total Precipitation. The maximum range of these two images is 124 nm (about 143 miles) from the radar location. They will not display accumulated precipitation more distant than 124 nm, even though precipitation may be occurring at greater distances. To determine accumulated precipitation at greater distances you should add an adjacent radar.

Always check the time frame from which this image is created. There must be one hour without precipitation anywhere on the radar before the accumulation period begins again and, depending upon the weather patterns that may be up to several days.

One-hour Precipitation

One-hour Precipitation is an image of estimated one-hour precipitation accumulation. It is used to assess rainfall intensities for flash flood warnings, urban flood statements and special weather statements.

Storm Total Precipitation

Storm Total Precipitation image is of estimated accumulated rainfall, continuously updated, since the last one-hour break in precipitation. This product is used to locate flood potential over urban or rural areas, estimate total basin runoff and provide rainfall accumulations for the duration of the event.

Weather Warnings

If any portion of a county is affected by severe weather, the NWS issues a weather warning for the entire county. However, they actually refine the region affected by drawing the warnings in polygons to indicate the exact region they believe severe weather may occur.

Included with the radar images are graphics of severe weather warnings. The colors, red, yellow, green and blue represent the four types of warnings that will appear on NWS Doppler radar images.

Typical polygon warnings

•Red - Tornado Warning. Issued when a tornado is imminent or occurring. A Tornado Warning implies an immediate threat to life and property.
•Yellow - Severe Thunderstorm Warning. Issued when a severe thunderstorm is imminent or occurring. A severe thunderstorm is defined as hail ¾" or greater and/or a wind speed of 58 mph or greater.
•Green - Flash Flood Warning. Issued with flash flooding is imminent or occurring.
•Marine - Special Marine Warning. Issued for hazardous weather conditions (thunderstorms over water, thunderstorms that will move over water, cold air funnels over water, or waterspouts) usually of short duration (2 hours or less) and producing sustained winds or frequent gusts of 34 knots or more that is not covered by existing marine warnings.

Satellite Images

Images covering the eastern and western halves of the US are also available. These include visible cloud cover, infrared images, and water vapor. Satellite images are not available for overlay on your map display and are opened in a separate window.


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