(Invited Talk) Severe-weather forecast guidance from the first generation of large domain convection-allowing models: Challenges and opportunities

Guidance from numerical weather prediction models is typically presented as a series of snapshots in time (an exception is accumulated precipitation). Traditionally, this has been adequate for most applications because common features of interest evolve slowly compared to the time between outputs. Furthermore, the path followed by these features can be implied with reasonable precision by assuming temporal continuity between output times. However, output in this format can leave many questions unanswered when the phenomena of interest develop, move, and vary in intensity on times scales shorter than the output period. Under these circumstances, it can be useful (while remaining computationally efficient) to track features, or phenomena of interest every time step during the model integration, then plot the average or extreme values of the data at normal output intervals.

In collaborations between scientists at the NOAA/OAR/National Severe Storms Laboratory (NSSL) and forecasters at the NOAA/NWS/Storm Prediction Center (SPC), this concept was applied in mesoscale models several years ago. For example, upward mass flux in parameterized convective updrafts was checked at each grid point, at every time step, and the maximum value at each point was saved at normal output intervals. The 2D field of grid-point maxima provided unique information about the magnitude of parameterized convective overturning, which in turn provided useful guidance for the intensity of convective activity in the real atmosphere.

Recently, this concept was applied to the real-time, convection-allowing 4-km grid length, large domain Advanced Research WRF forecasts at NSSL. In particular, it was used to extract sub-output-time information about convective storms in the model, initially focusing on five different output fields. The first two are maximum updraft and downdraft velocities in the lowest 400 hPa of the model atmosphere. These two fields have obvious implications regarding the intensity of convective overturning. The third field is maximum simulated reflectivity. For simplicity, this is computed at the lowest model level, but without much additional computational effort, other levels (or a composite reflectivity field) could be used. As with updraft and downdraft velocities, this field is related to the intensity of convection and may provide some guidance for forecasting large hail, even though hail is not explicitly represented in the current microphysical scheme (WSM6). The fourth field is maximum 10-m wind speed, which may be helpful in predicting the magnitude of convectively generated surface wind gusts. The last field is maximum updraft helicity, a diagnostic field that was developed before the 2005 Hazardous Weather Testbed Spring Experiment to detect mid-level mesocyclones in simulated convection.

While these fields are useful for diagnosing maximum values, they also provide valuable clues about storm tracks, wind swaths, and the longevity of individual convective features. For example, on several days this spring the 4 km WRF-ARW model produced long-lived (multi-hour), strongly rotating mesocyclones that were revealed in hourly output as downstream-marching segments of high maximum UH. With relatively simple post-processing tools, one can concatenate such segments to reveal features such as model generated supercell storm tracks or surface wind swaths.

These unique output fields have significant potential value for severe weather forecasters at the SPC and elsewhere. Applications in operational forecasting will be discussed at the conference.