Alert-paired  Product The  WSR-88D  is  capable  of  producing  alert messages,  accompanied  by  audible  alarms.  Such alarms are sounded at each PUP site when prescribed conditions occur. Alert criteria are usually based upon severe weather parameters. All alert conditions are user-definable and should be established jointly since they affect each user of an associated RDA. Once criteria are established at the UCP, selected products, known  as  alert-paired  products,  will  be  transmitted automatically  with  each  alert.  These  products  aid  in severe weather evaluation. Each PUP site receives both the alert message and any alert-paired products. This eliminates the need for one-time requests and allows forecasters to assess the situation rapidly. Alert messages  and  alert-paired  products  are  given  the highest priority by the system. Significant weather events may be missed under certain circumstances. Thus, alert-paired products reduce the potential of operator oversight resulting from poor product selection. Alert-paired products might mean the difference between ample warning of severe weather or no warning at all. For example, if 50-knot winds will trigger an alert, a base velocity product would most likely confirm the phenomena and assist  the  forecaster  in  evaluating  the  circumstances surrounding that event. This saves time and provides forecasters with the best product for validating the alert occurrence. BASE PRODUCTS Recall that the WSR-88D creates two types of products: base and derived. Base products provide a broad overview of the meteorological situation and are the next best thing to viewing electromagnetic energy in its raw state. They graphically illustrate returned energy as reflectivity, velocity, or spectrum width data. Base Reflectivity (REF) Product As previously discussed, a radar measures the amount of electromagnetic energy returned to the antenna. The strength of this energy defines a target’s intensity. With this in mind, it stands to reason that stronger intensities ofprecipitation from severe storms will   contrast   sharply   from   that   of   surrounding precipitation. In this respect, the WSR-88D truly pays off  by  transforming  backscattered  energy  into  useful, colorful,  and  interpretable  displays. The practice of displaying reflectivity values from backscattered energy is nothing new. However the process   and   methods   used   are   becoming   more sophisticated. The increased accuracy of the WSR- 88D along with the addition of color, makes older radars   obsolete.   Color-coded   targets   placed   on geographical   background   maps   increase   the effectiveness of radar interpretation by providing a clear  and  informative  visual  presentation.  Operators can quickly compare target strengths and distinguish severe cells at a glance. Figure 2-32 is an example of the Base Reflectivity (REF) product. Reflectivity data levels are indicated by the color scales found along the product’s right margin. With base reflectivity, the data levels refer to target  reflectivity  intensity  (dBZ).  Rainfall  rates  can easily be estimated from REF. Reflectivities from light rain average around 20 to 25 dBZ, while reflectivities from thunderstorms average around 45 to 55 dBZ. Normally, it is difficult to distinguish precipitation type  based  on  reflectivity  alone.  For  example,  snow and  light  drizzle  both  produce  nearly  the  same reflectivity  values.  Very  high  reflectivity  values  are usually   associated   with   hail.   Although   base reflectivity has a maximum range of 248 nmi, its best resolution (.54 nmi grid size) is limited to 124 nmi from the RDA. All range folded areas are displayed as purple  shading. Base  reflectivity  provides  a  weather  snapshot  of the entire radar coverage area. Highly reflective storm cores and embedded thunderstorms appear quite nicely in color. These same features were difficult to observe on  conventional  radarscopes. Using  base  reflectivity,  an  operator  can  also identify distinct radar signatures and correlate targets to  their  geographical  location.  Radar  signatures  are visible patterns commonly associated with certain phenomena.  Meteorologists  have,  over  many  years, linked a number of weather events with the signatures they produce. Classic radar signatures are often the key to  identifying  severe  weather  potential  before  it occurs. Some of the more important signatures are briefly  discussed  here. HOOK ECHO.—The hook is a pendant-shaped echo,  resembling  the  figure  6.  It  is  produced  by precipitation   being   wrapped   around   a   vortex. Therefore,   hooks   are   typically   associated   with tornadoes. The hook is not the actual tornado; it is merely an indicator. The right/rear quadrant of a storm (with respect to storm movement) is the best place to monitor development,    since  tornadoes  commonly 2-34