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ADDS - Icing Help Page (4 of 4) Back to Icing Page or Icing help page 3 |
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FIP Overview The Forecast Icing Product (FIP) is an automatically-generated forecast of icing probability, severity, and SLD potential. FIP examines numerical weather prediction model output (from the Rapid Update Cycle, RUC) to calculate these fields. The algorithm analyzes the model output from a vertical column, determines the cloud top and base heights, checks for embedded cloud layers, and identifies a precipitation type. Once the likely locations of clouds and precipitation are found, the physical icing situation is determined, and a fuzzy logic method is used to determine the icing conditions. The fuzzy logic interest maps are based on clues from the model output, reflecting relevance to the presence of icing. Numerous fields from the model are used to determine the icing conditions at each model grid box, horizontally and vertically. Because FIP is a forecast it cannot use observations like CIP. Surrogates for these observations are derived from model data when possible so that FIP can use the same logic and equations as CIP in most cases. For example, CIP uses radar data to determine the areal coverage of precipitation and to boost or deplete icing severity, depending on the situation. FIP uses a precipitation field that is derived from the model QPF and microphysics to do these same things. The FIP icing potential has been calibrated to an icing probability and now has the same meaning as the CIP icing probability. However, the maximum probability decreases as the forecast length increases, due to uncertainties introduced at longer forecast times. Generally lower probabilities of icing with longer forecast times must not be interpreted as a lower likelihood of icing; instead, this represents greater uncertainty in the predicted location of icing in the future. Cooler colors represent lower icing probabilities while warmer colors represent higher values. The forecast of icing severity in the FIP is also like that in CIP. There are five categories: none, trace, light, moderate, and heavy. On the display this field can be shown for all icing probabilities or masked (gray areas) to show only icing severity at icing probabilities greater than 25 or 50%. Supercooled large drops (SLD) can be formed in FIP by one of three methods: collision-coalescence, classical freezing rain, or convection. FIP identifies these SLD scenarios and calculates the potential for its occurrence by using thermodynamic profiles and microphysical fields. It is represented by red hatching overlaid on top of the icing severity plot. The FIP generates a two-, three-, six-, and nine- hour forecast every hour. For example, forecasts issued at 1300Z would be valid for 1500Z, 1600Z, 1900Z, and 2200Z. Twelve-hour forecasts are issued every three hours beginning at 0000Z. |
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