Explicit forecasting of bow echoes/derechoes with the WRF-ARW model

During the past six springs and summers, 36 hour realtime forecasts

were conducted daily with WRF-ARW using 3 to 4 km horizontal grid

resolution and explicit convection over the central and eastern US.

Initial and boundary conditions have been supplied by the larger-scale

operational models such as the NAM and GFS.

The goal has been to assess the potential benefits of using such

resolutions for forecasting severe convection.

Although such model resolutions and initialization procedures cannot

be expected to predict individual convective cells out to 36 h, larger convective systems,

such as bow echoes and derechoes, are readily produced in such forecasts, sometimes

quite accurately. In this paper, we will review both the successes and failures in

predicting such larger, longer-lived bow-echo type systems, with emphasis on

clarifying whether there are specific meteorological regimes (e.g., daytime versus nocturnal, strong forcing versus weak forcing, etc.)

that may be more systematically associated with the good versus bad forecasts. For instance,

one common failure mode can be associated with more isolated supercellular convection that

triggers relatively early in the day in the vicinity of frontal forcing, and builds upscale as it propagtes

away from the forcing features. Also, while 3-4 km grid resolutions seem capable of

reproducing the system-scale features associated with such events (e.g., rear-inflow jets, bookend

vortices, etc.), such resolutions do not seem to be sufficient to reproduce the intensity of the

surface winds often associated with such systems. These results will be discussed in the context

of what modeling improvements may be needed to remedy these more systematic forecast errors.