I created this graph as a tool to help forecast precipitation for Fayetteville, AR. It displays some common items found elsewhere like temperature and dew points every three hours as forecasted by the GFS model. Both temperature plots are 980MB temps, not surface temperature forecasts. The data is parsed from BUFKIT files generously provided by Penn State University. This data is a point forecast for my favorite airport, Drake Field, or KFYV. I tweet this thing out occasionally when it seems worth it.
Starting from the bottom up, the next seven days are listed with times adjusted to local time. Above that is the GFS precipitation type, R=rain, S=snow, P=sleet and hopefully never, Z=freezing rain.
The yellow line is a crude wind shear measure, simply wind speed at 3KM - wind speed at surface.
The green dots display accumulated precipitation in units that vary depending upon seasons and events. This is data from the most recent model.
Red stars indicate CAPE as provided by the GFS. It is displayed as CAPE/50 for better resolution on a 0 to 100 Y axis. A red star at 40 is equal to 40*50, or 2000 j/kg.
Finally, the black triangles boldly claim to forecast percent chance of precipitation. The GFS model either says it will rain or not. It doesn't list a percentage. So I came up with my own formula, and frankly, it works pretty well. Many times there will be trace outputs of rain from the GFS. Including these trivial amounts weighted the chance of rain unrealistically high. So I arbitrarily chose a small threshold as "actual rain".
The next step was to save the last 12 GFS runs, and compare the hourly results. The thinking is trying to gauge consistency. Initially I compared 16 runs, but the results became "noisy".
Finally, I sum the comparisons, weighted to give preference to the most recent run, this way:
Most Recent Run =50
Run Before That =25
Run Before That =12.5
Run Before That =12.5/2
Run Before That =Half of the last Run
and so on.
What you'll see, is when the graph shows a POP at 50% or above, the most recent run shows rain.
If the last 2 show rain, its 75%.
If every run shows it, its nearly 100%.
If every run except the last one shows it, it is below 50%.
Although this method is far from flawless, at least now you have an idea of what you are looking at.
Hope you enjoy it!
Steve
