From: Subject: Thursday Weather Topic in Greater Depth Date: Fri, 23 Oct 2009 10:14:14 -0700 MIME-Version: 1.0 Content-Type: text/html; charset="Windows-1252" Content-Transfer-Encoding: quoted-printable Content-Location: http://www.ametsoc.org/amsedu/dstreme/learn/r_sup.html X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2900.5579 Thursday Weather Topic in Greater Depth

SUPPLEMENTAL INFORMATION...IN GREATER DEPTH

To complement the Daily Summary for Thursday, 22 = October 2009=20

INTERPRETING RADAR SUMMARY CHARTS

In the DataStreme Atmosphere Project, current radar imagery is = available in several formats so we can better visualize where = precipitation is=20 falling across the nation. You have become acquainted with one such = product,=20 which is an overlay of the composite radar summary that is placed over = the=20 surface analysis that you retrieve from the current "Isobars, Fronts, = Radar=20 & Data" link on the DataStreme Atmosphere website. This = feature that=20 includes various color-coded regions on the surface analysis allows us = to see=20 the precipitation areas and intensity for the entire coterminous U.S. = Several=20 other charts are available that provide additional detail. A separate = radar=20 summary chart, with a link identified as "Radar", or a radar summary = with fronts=20 chart ("Radar & Fronts") can be similarly accessed from the Radar = portion of=20 the homepage.

COMPONENTS OF THE RADAR SUMMARY CHART

These various composite radar summary charts are prepared hourly at = 35=20 minutes past the hour from the data supplied by National Weather Service = radar=20 reports originating from the national network of primary weather radar = units.=20

The intent of the national radar summary chart is to graphically = depict the=20 large-scale distribution of precipitation. Precipitation intensities are = indicated on the chart by color codes. Some adjustment of the data may = be=20 needed, especially if precipitation echoes are reported by more than one = radar=20 site. When applicable, current severe thunderstorm and tornado watch = areas=20 ("watch boxes") are plotted on the radar summary chart.

ECHO INTENSITIES

The reflectivity or intensity of the reflected radar signal depends = upon=20 several factors to include the size, shape, density (the number of = targets per=20 unit volume), and state (rain, snow or mixture) of the hydrometeor = target.=20 Usually, the intensity of the reflected echo is assumed proportional to = the rain=20 drop density, thus indicating the precipitation rate or intensity.

Contours of echo reflectivity, also known as "VIP levels" (for=20 Video Integrator and = Processor), are=20 plotted on the radar charts as a color-coded display. The colors within = these=20 contours provide an indication of the precipitation intensity and the = size=20 depicts the areal extent of the detected precipitation. Based on = empirical=20 studies, six standard VIP levels are related to the rainfall rate for = the steady=20 "stratiform" precipitation typical of wintertime storms or warm frontal = systems=20 and for showery "convective" precipitation of representative summertime = storms=20 or cold front systems.

RADAR INTENSITY LEVELS
        COLOR     VIP       RAINFALL RATE       PRECIPITATION    =
INTENSITY  =20
                 LEVEL    [inches per hour]
                       STRATIFORM    CONVECTIVE
        Blue       1     < 0.1         < 0.2      Light            =
-
        Cyan       2   0.1 to 0.5    0.2 to 1.1   Moderate
        Green      3   0.5 to 1.0    1.1 to 2.2   Strong           +
        Yellow     4   1.0 to 2.0    2.2 to 4.5   Very Strong     ++
        Magenta    5   2.0 to 5.0    4.5 to 7.1   Intense          X
        Red        6     > 5.0         > 7.1      Extreme         =
XX
        White                                     Unknown

Typically, the blue colors indicate light rain or snow; the green = colors=20 identify light thunderstorms/moderate rain showers; yellows identify = moderate=20 thunderstorms, while the colors of magenta grading to red are reserved = for the=20 severe thunderstorms that can cause flooding rains. On a local scale = (not=20 possible on the national summary), an echo with red can also indicate = highly=20 reflective water-coated hail.

SEVERE WEATHER WATCH AREAS

The areas encompassed by an official severe weather watch issued by = the Storm=20 Prediction Center are plotted on the radar summary chart. These areas = appear=20 outlined by a rectangular "box", conforming to the map coordinates of = the=20 affected area. Weather watch numbers associated with each box are placed = in the=20 box.

The weather watch numbers are sequentially numbered throughout the = year and=20 have a letter prefix to identify the type of watch: a letter "S" = indicates=20 severe thunderstorm watch, while a "T" indicates tornado watch. As an = example:=20 

    T335   A tornado watch, #335.     S336   A severe =
thunderstorm watch, #336.

MAP INTERPRETATION

The radar summary chart helps fill in the precipitation regions = between=20 surface observation stations. Hence, the meteorologist can obtain an = additional=20 perspective of precipitation types, intensities and movements that are = not=20 provided by the conventional surface analysis. However, care must be = exercised=20 and surface observations should be inspected, because radar summaries = may=20 contain echoes of precipitation that does not reach the ground. This = condition,=20 called "virga", may exist because the air would be too dry and = evaporation of=20 the rain droplets has taken place. The inclusion of current severe = weather watch=20 boxes on the chart helps identify regions having weather conditions that = are=20 potentially conducive for severe weather.

Return to the Thursday = Daily=20 Summary

Return to DataStreme = Atmosphere=20 website

Prepared by Edward J. Hopkins, Ph.D., email hopkins@meteor.wisc.edu =
=A9=20 Copyright, 2009, The American Meteorological Society. =