1) MARS is a prefect prog approach. That prog happens to be a special version
of the GFS ensemble mean. This is a version that is being tested here at SPC.
The ensemble includes weighted time-lags from prior operational GFS, and GFS ensemble
runs. There are 19 GFS members and MARS uses the overall
mean for the height fields and the precipitable water field at each forecast hour.
The ensemble mean can "wash out" significant features such as sharp troughs and
ridges, especially at medium to extended ranges as the spread between the individual
ensemble members increases. Verification statistics for the SPC Medium Range
Ensemble forecasts are not yet available.
2) MARS uses the NCEP Reanalysis data with a rather course grid resolution of 2.5 degrees. This is far less
detail than what is needed to discern the mesoscale features that play a huge role
in severe weather. However, synoptically evident cases may be captured *if*
the model forecast is correct.
3) Gridded severe weather output is based on arbitrary probability values ranging from
5% to 45%, with 15% roughly corresponding to a SLGT risk in past SPC Outlooks.
We have not verified what a 15% contour means when generated on a MARS forecast map. If you look at the
historical outbreaks using MARS you'll see values of 25% inside areas that verified at
over 45%. Is this a "good" or a "bad" forecast? We hope to have statistical verification after
the 2005 severe weather season. In the meantime, use the link below for a graphical verification
for the latest MARS forecasts.
4) Only 3 components go into the MARS scheme; 500 height gradient,
850 height gradient, and precipitable water. Obviously,
many more ingredients that go into making a severe weather episode!
5) The MARS gridded severe weather output is based on the SPC rough logs from 1979 to
2004. There are huge problems with these logs including report inflation during
the last 15 years. There may be incredibly good pattern matches from the
earlier part of this period that may have very few reports associated with them.
A poor match on a later date may contain 50-100 small hail events. Remember,
"garbage in-garbage out".
6) Again, be careful! Look at the date matches on the web page and make
sure (if you can) that the pattern being forecast is similar to the analog
pattern found by MARS. The RMS numbers help a little but investigation continues on
ways to better define whether a match is "good" or "bad". Right now, only
the top 4 "matches", or analogs are used. It's entirely possible that some days will have 8 great
analogs and other days may have none. On those bad days it is possible that
a lot of unreliable severe weather data is being gridded anyway. Also, the RMS
values usually decrease in the extended period as the GFS ensemble grid changes
to a lower resolution.