EZTip No. 13 - HREF probabilistic ceiling forecasts
The High Resolution Ensemble Forecast (HREF) model found in the EZImagery is extremely useful when you are proposing a VFR or IFR flight. It provides a number of forecasts for ceilings including the probability that the ceiling will be below 3,000 feet such as shown below.
The HREF describes an “ensemble of opportunity,” meaning that several independently-designed, deterministic convection-allowing models (CAMs) are collected and post-processed as an ensemble. When thinking about an ensemble, imagine an ensemble of instruments used in an orchestra. An analogy to a global numerical weather prediction ensemble would be processing models like NCEP's GFS and ECMWF’s global model together as an ensemble. That's an ensemble of two members. Many ensembles have many models. At the Storm Prediction Center (SPC), they currently process HREFv2.1, which contains 10 ensemble members. The members are diverse with respect to dynamical core, physics parameterizations, and initial/boundary conditions so that you get a more diverse result. The figure and table below illustrate HREFv2.1’s membership design.
To access the HREF model runs for ceiling in the EZImagery, select the HREF Model collection and then you will see the options for ceiling that include CIG below 3K ft, CIG below 2K ft and CIG below 1K ft as shown below. You can also view these for various regions.
Each of these options provides a forecast that is based on probability from the ensemble mean. The legend is on the left of the diagram with red/orange colors implying the high probability that the ceiling will be below the specific threshold (e.g., 3,000 feet AGL). Colors such as blue, green and white indicate that there's a low probability of ceilings being below the threshold. It's important to understand that it does not provide absolute ceiling information (e.g., 300 feet). Instead, it provides the confidence that the ceilings will be below the threshold for the forecast.
In the forecast above, notice that there is plenty of red depicted over southern Ontario, Canada as well as Indiana, Ohio and Pennsylvania. This means there's a near certainty at the valid time of the forecast (10Z) for the ceiling to be below 3,000 feet in these areas. So this means that the ceiling will likely be MVFR, IFR or possibly LIFR in these regions making VFR flight difficult.
On the other hand, the forecast above is the probability of ceilings below 1,000 feet implying IFR or low IFR conditions. So pilots flying to Georgia arriving at 13Z should expect to do an instrument approach given there's a high confidence that the ceiling will be below 1,000 feet.
Interested in learning more about the Skew-T? You can order your copy of The Skew-T log (p) and Me book today in soft cover or eBook format.
Most pilots are weatherwise, but some are otherwise™
Dr. Scott Dennstaedt
Weather Systems Engineer
Founder, EZWxBrief™
CFI & former NWS meteorologist
