Wednesday, August 16, 2017

Looking at C&V in the East on Day 2

The C&V desk turned their attention east for ceiling and visibility forecasting yesterday. The initial national blend looked like this:

Immediately obvious was the broad area of IFR and even LIFR conditions. Based on the observed ceilings at the time, this was clearly over done. The RAP seemed to have a better handle, eliminating a lot of those low ceilings. Those working on the national scale opted to go with the RAP, assuming that it would be a lot easier for the local WFOs to more accurately add areas of C&V back in at their level, rather than send them a grid with a mess of widespread IFR and LIFR (accuracy debatable) that would require a large amount of editing and adjusting.
20170815 1400 UTC National Blend (left) and RAP (right) ceiling and visibility forecast, 6-hour (20 UTC) forecast
With a decent model basis to begin with, participants moved on to editing the grids. As with previous days, the discussion of the national vs. local perspective arose. Those working on the national scale noted how challenging it was to load shed and edit the grids where there are aircraft concerns. On this particular day, there was a lot going on in the Northeast, which drew the attention there and away from other areas like Atlanta. So, a lot of time was spent on the grids sent to WFOs in the Northeast, making the workload at their local level fairly light; whereas, the lack of attention south resulted in more of a messy grid that required a lot more editing and smoothing.

There was also more discussion on more versus less detail in the cloud grids. Below is the National Blend Sky Cover grid.
National Blend Sky Cover grid from 1800 UTC on August 15th
Pretty picture? Yes. Realistic? Certainly. For the most part it looks very much like a clouds would. Is this useful? It depends on who you ask and what you're forecasting. While the detail is realistic, the point was brought up again that it is perhaps too much detail for the purposes of the TAFs. Users want to see the overall trend in the TAF and not the minor ups and downs in the gridded details.

In this case, one of the mock local offices was Philly, and participants at that local level, smoothed out the Sky Cover grid in that area for better TAF generation.

National Blend Sky Cover forecast zoomed in over Philly, the mock WFO forecasting at the local level
The difference between the initial grid and the grid edited at the local scale is very clear. Again, much of the local grid area over Philly was smoothed with the goal of better TAF generation. From this the question arose, where is the balance? Would users prefer the higher detail and resolution in the overall grids? Or should they be smoothed for better TAF performance? Is there a happy medium? These are all difficult questions that have not been answered, or may not even have a good answer yet. However, feedback from the Summer Experiment and continued debated will be crucial as the Digital Aviation Services (DAS) effort evolves and continues.

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