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| NWS Disclaimer |
3. Intermediate Tools
As the day progresses, periodic reassessment of the initial outlook will be necessary as the environment changes. During this intermediate forecast interval, the forecaster will be making the transition from initially assessing the environment (Outlook) to the warning decision (Nowcasting).
Updating the outlook indices (MDPI and WINDEX) during the day are a crucial part of the intermediate forecasting interval. Even if your outlook includes forecast values for these indices, it is important to regularly update these indices to evaluate, and possibly update, your forecast. Both of these indices rely on sounding data, so updating these values requires some assumptions. Under synoptically quiescent conditions, midlevel variables will not change much during the day. Hence, using the midlevel parameters from the morning sounding, combined with updated surface observations, can provide a fairly accurate estimate of these indices. Under synoptically active conditions, using model forecast values for midlevel parameters will result in a better estimate.
Another useful way to update these indices is with GOES-Sounder products (Ellrod, 1998). These products, available at the NOAA/NESDIS/ORA GOES Microburst Products Page, help the forecaster understand how the middle and upper levels of the atmosphere change in between sounding launches. The three products available at this site [the Wind Index (WINDEX), the Microburst Day Potential Index (MDPI), and the Dry Microburst Potential Index (DMPI)] are updated approximately every 30 minutes. There is also an archive of the previous 24 hours worth of images. The former two indices will be the most useful intermediate tools for wet microburst forecasting. Users of this data should note that the MDPI GOES-Sounder product has no local tuning in its calculations. Hence, a value of 30, not 1, would be the key threshold in eastern Florida. Examples of these two products can be seen in Fig. 3 and Fig. 4. A drawback to the GOES data is a need for relatively clear air to collect sounder data. Regions with widespread convection or large areas of clouds will result in very few measurements of MDPI and WINDEX. The best system for updating the outlook indices will contain some redundancy, so relying on both locally calculated values and using GOES-sounder products is recommended.
A major factor on how (or if) your initial outlook is updated will be the initiation and evolution of convection, and how it effects the local envrionment. Previous convection will tend to increase midlevel moisture which make microbursts less intense or less likely. Convection may also produce regions of strong, deep outflow. If the outflow is sufficiently deep, it can slow a downburst or even prevent it from reaching the ground altogether. It should be noted that strong convective outflows can also be a very good indicator of future downburst development. If an approaching cluster of storms has produced repeated, significant downbursts outside of your warning area, the storms (or subsequent convection) will likely continue to produce downbursts when it enters your warning area if a similar environmental exists there.
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