When severe weather is imminent across the country, forecasters at the SPC must consider all of the possible areas. However, here at the SFE where we are considering multiple experimental NWP ensembles, we have to select our domain of interest. When the severe weather is spread across an area larger than a single domain, we must choose which region we want to focus on. To do this, the facilitators consult not only our experimental numerical weather prediction models, but also the upper-air data collected by radiosondes. Typically, the domains have quite a bit of overlap, but on days like last Thursday (17 May), a decision must be made between two very different areas.
After evaluating the previous day’s forecasts, the first forecasting activity for the current day that participants do is the hand-analysis of upper air maps at six different levels: 250 mb, 500 mb, 700 mb, 850 mb, 925 mb, and the surface. This activity ensures that participants get their hands onto the observed data, and develop a thorough understanding where a convectively favorable environments will occur. A map discussion follows (pictures of which can be found on the NSSL Flickr account) where participants share what they’ve learned through their contours. On Thursday, the conversation was far-reaching, as we had to decide between two domains that had very little overlap:
For this case, the upper-air information helped us decide which domain to choose, since the surface information didn’t favor one area over another. Ridging was a concern for the southern area, where rising heights could have suppressed convection. Adequate moisture was present at 925 mb and 850 mb for severe convection in the northern areas, with the total precipitable water value at Rapid City exceeding the 75th percent of climatology at 1200 UTC.
Soundings from Amarillo, TX, in the southern domain, did not have quite the same amount of moisture. Finally, difluence over Colorado and ongoing height falls at the 250 mb level led us to our final decision of the northern domain. The distribution of the preliminary reports shows that both the northern and the southern domains produced severe weather, but that the coverage of reports in the northern domain was slightly higher, and was more varied between wind and hail reports, presenting a challenging forecast scenario. While there was no “wrong answer” as to domain choice for us last Thursday, we had to make a selection, and the upper air maps were our guide.