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Model Output Statistics (MOS)
- A poor man's TAF 
Say you are flying to Effingham Count Airport
(1H2) in southern Illinois within the next eight hours. However, there
isn't a terminal aerodrome forecast (TAF) issued for that airport. What
can you reasonably do to understand the weather you might face during
your arrival at 2200 UTC? Use the nearest TAF? How about the area
forecast (FA)?
Effingham County Airport sits in a hole (see upper-right)
with terminal forecasts located on all sides for the most part.
Decatur,
Illinois (KDEC) is
the closest
airport that has a scheduled TAF issued. It is about 50
nautical miles to the north of Effingham County. Fifty miles
isn't all that far, so can a pilot expect the weather at Effingham
County to be the same as Decatur? It is certainly easy for a
pilot to leap to that conclusion.
Don't count on the nearest TAF to be accurate in
all cases, especially one that is 50 nm away. A TAF is a
point forecast that is valid only five (5) statute miles from the
center of the airport's runway complex. This leaves the pilot
with the unfortunate job of extracting site-specific weather data from
the FA. That is not an easy task considering that the FA
doesn't tell you much about surface winds unless they are strong and
gusty. The winds at an
airport can play a critical role turning a straight-in ILS approach to
a very challenging circle to land approach with higher minimums even
when the winds don't meet the criteria to appear in the FA.
Don't despair. Model Output Statistics
(MOS) may provide some much needed help. MOS (pronounced
"moss") isn’t new and has been around for several decades. In
recent years it has surfaced and now is being used regularly by some
pilots. Model output statistics, as the name implies, is
derived from the output of weather prediction
models. Numerical weather prediction models are run
on a scheduled basis on the computers at the National Centers for
Environmental Prediction (NCEP). Models such as the North
American Mesoscale (NAM) and Global Forecast System (GFS) are executed
every six hours.
These models do not automatically
produce a point forecast for a specific town or airport,
however. The job of MOS is to take the “raw” model forecast
and attempt to improve on it using a statistical method to produce an
objective and more useful site-specific forecast.
Therefore, MOS is simply a computer
program that is executed after
the underlying models have completed. The NAM and GFS both
have their own version of MOS called the MET and MAV,
respectively. While MOS may not fulfill the
regulatory role, it does provide the pilot with much needed
site-specific detail that isn’t found from using the FA alone and
provides an extended forecast well beyond the TAF’s 24 or 30 hour
period. It is not just for pilots either; it is
also used by the professional forecasters at the local weather forecast
offices (WFOs) to help them construct and amend TAFs.
These MOS forecast predicts the sensible weather
elements. These predicted elements fall into one of three
categories: probabilistic, categorical and continuous. For
example, continuous data includes, but is not limited to, temperature,
dewpoint temperature and wind (direction and speed). For some
predicted elements categories are used. Instead of providing
a cloud height forecast of 500 feet AGL, a category of “3” is assigned
that represents a range of cloud heights from 500 feet AGL to 900 feet
AGL as shown in the table on the right. Precipitation and
thunderstorm occurrence, on the other hand, are always presented as a
probability.
The output of MOS is found formally in
a tabular bulletin format called FOUSnn where nn is the specific
bulletin number. If you are like many pilots who don’t like
looking at a tabular bulletin, the MOS data can also be depicted
graphically as it is depicted on this website here.
If we go
back to our original example, Decatur's TAF was calling for TEMPO 2921/3001 5SM -RA OVC015
and the wind to be 32013KT.
The FA valid at this time was forecasting BKN030 with SCT -SHRA.
Cloud bases in the FA are shown in MSL unless otherwise
noted. Given a field elevation of 600 feet, that places the
ceilings at roughly 2,500
feet for an arrival at 2200 UTC.
You can already see that the nearest
TAF and FA are close, but don't necessarily agree. The
Decatur TAF suggests the need for an instrument approach and an
alternate whereas a pilot using the FA would not need an alternate and
should expect a visual approach. Moreover, there's no mention
of surface winds in the FA.
The good news is that a MOS site exists
for Effingham County that can help provide a clue as to the weather
upon arrival. According to the GFS MOS, at 2200 UTC the sky
is expected to be overcast (OV)
with a 59 percent chance of rain. The categorical ceiling
forecast shows a 4
which equates to a ceiling between 1,000 feet and 1,900 feet according
to the table above. Visibility is being forecast at a
category of 7
which means better than 6 statute miles. Finally, the winds
are expected to shift around from the southwest to the northwest with
winds of 260 degrees at
8 knots at 2200 UTC.
While MOS isn't always perfect and
never should be used as a replacement over the official TAF guidance,
it can help provide a site-specific forecast typically better than the
FA. In case you were wondering, the 2225 UTC surface
observation (below) for Effingham County airport turned out to be very
close to the MOS forecast above valid at 2200 UTC.
K1H2 292225Z 25008KT 10SM OVC016
A2981 RMK AO2
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