Anatomy of a Well Forecast Bow Echo


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.

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