Saturday, May 20, 2017

Mind the Gap

Continuing on the last post's theme of choosing the proper forecasting domain when we have multiple areas of convection to contend with, today's discussion will focus on the boldest of forecasting moves:

The gap.

The full period forecasts issued by each desk are a group effort, with input from participants guiding the placement of the lines. Prior to issuing the lines, participants consider observations, coarse-scale operational models such as the GFS and the NAM, and fine-scale operational and experimental models, such as the HRRR, FVGFS, and the members of the CLUE ensemble. Convection-allowing models, with grid spacing of ~3 km, provide very realistic-looking radar signatures that can give confidence in specific areas of threat beyond those of the GFS. For a quick example, see the GFS forecast for 18 May 2017 at 0000 UTC:

The echos from the HRRR suggest that these storms would be supercells, given the strong tracks of  hourly updraft helicity (as indicated by the black contours) and the individual reflectivity echoes. Images such as these can give forecasters more confidence in the location(s) of convection, particularly when compared to the larger-scale QPF precipitation products that current coarse-resolution models can provide. 

So what does this have to do with gapping the forecasts? And what does gapping the forecasts even mean?

Tuesday, May 16, 2017

Picking Areas during an Active Week

This week is gearing up to be the most active week thus far in the SFE, with every day having the chance of severe weather somewhere in the center of the country. Yesterday, we had three separate areas of potential severe weather to consider:

The first area was concentrated across northern Iowa and far southwestern Wisconsin, the second area stretched from central Nebraska south through western Oklahoma, and the third area was in western South Dakota. Since SFE forecasts cover a subset of the contiguous United States, choosing which areas to forecast for is an important part of the forecast process. In this case, the worst severe convection was anticipated within the eastern two areas, and the forecast domain was chosen to encompass as much of those areas as possible.