Figure 1. 13-hour WRF-NMM forecast of simulated reflectively at 1 KM AGL and valid at 1300 UTC 8 May 2009 (left), Verifying observed base reflectively, and severe thunderstorm warning polygon, valid at 1300 UTC 8 May 2009 (right). Both images are centered over middle Tennessee/south-central Kentucky.

The 13-hour WRF-NMM forecast of the Missouri Bow Echo (see earlier post with this title) is remarkable in both its accuracy and structure, particularly given the severity of the event. As model forecasts go, it appears to be a perfect piece of numerical guidance. But shifting the grid about 450 miles to the east, the exact same 13-hour WRF-NMM completely missed the MCS (albeit less severe) moving through eastern Tennessee. So while there is little doubt that the model provided an essentially perfect prediction of the intense bow echo over southern Missouri, in a purely deterministic sense, the same model provided little-to-no short-term convective guidance with respect to convective mode and QPF over much of Tennessee.

The information provided by these high-resolution NWP models is revolutionary, and will likely lead to a quantum increase in high-impact services provided by the NWS. But let’s be careful not to oversell the capabilities of a single, deterministic model forecast. In order to fully realize the potential of future NWS forecasting and warning services, an ensemble of convective-resolving models will be required to address the uncertainty that accompanies all weather forecasts. The HWT has evaluated convective-resolving models over a large portion of the CONUS for the past several years, and it is encouraging to see the improvements these models have made in high-impact convective guidance and in their ability to predict intense, realistic convective structures such as the bow echo over southern Missouri. But a single high-resolution NWP forecast, regardless of its ability to reproduce intense convective structures, is unlikely to meet the future uncertainty requirements of the entire NWS at all times and locations. That said, the development and evaluation of these convection resolving models is and will continue to be an essential part of future high-impact, life saving, decision support services provided by the NWS, likely realized through a blend of deterministic guidance, well constructed ensemble systems, and related ensemble interrogation tools.

Dr. David Bright, Storm Prediction Center, Science Support Branch, Norman, OK

Above (left) is an example of a forecast from numerical models being evaluated in this year’s SPC/NSSL Spring Experiment. The actual radar image on the right shows the intense bow echo that raked across southwest Missouri early last Friday morning. These images are centered on Joplin, Missouri (JLN). The 13-hour forecast of simulated reflectivity from the WRF-NMM (Weather Research and Forecast Model – Non-hydrostatic Mesoscale Model) is exceptionally accurate and depicts the form and intensity of the bow very well. This is a 4km resolution numerical model that was initialized at 00Z 08-May-2009 and valid at 13Z that Friday morning.

The verifying 1km base reflectivity (radar mosaic) image is on the right with the model fields for winds overlaid on the radar. The barbs in each of the images are the model’s instantaneous 10m winds in knots (the grid is skipped to lessen the clutter of the wind barbs). The isotachs (lines of constant wind speed at 10m) are plotted from the WRF “history variables” (no grid skip). The wind history variables are the maximum 10m wind speeds in the model over the past hour ending at 13Z.

Instantaneous 10m winds in the model at 13z, near the rotating bow head, are at least 50 knots. The maximum model 10m winds over the past hour range from 60-70 knots near and north of the weak echo channel and around the comma-head of the model bow. The intensifying low was coexistent with deep convection later in its life-cycle and produced significant wind damage in parts of Missouri and Illinois. Tornadoes also developed near and ahead of this feature, killing one person in Missouri in the morning, and two people in Kentucky later in the afternoon.

This was only one of several exceptional forecasts of this severe event from the models being evaluated in this year’s SPC/NSSL Spring Experiment. To see more output on this case and more, check out the Spring Experiment website.