Flight Training - February 2015 - 45

catch anyone who's paying attention by
surprise. In recent years forecasters at the
National Weather Service's Storm Prediction Center in Norman, Oklahoma, have
been quite successful at alerting parts of
the nation that will be targets of severe
thunderstorms as much as two or even
three days in advance.
DON'T EXPECT PERFECTION. Despite all of
the advances since the middle of the twentieth century, no meteorologist expects to
ever be able to make perfect forecasts.
"My profession gets a lot of flack about
not being an exact science," said Michael
Pat Murphy, a meteorologist at the NWS
Aviation Weather Center in Kansas City,
Missouri. "Well, it is an exact science. The
equations of dynamics and thermodynamics perfectly describe how the atmosphere
moves and changes over time.
"The problem we have is measuring that
atmosphere. If we could measure every
molecule and track that molecule as it
travels through the atmosphere we could
perfectly forecast the weather. But we
can't, so we have to make assumptions,"
Murphy said. "As one of my professors
said, we measure with a micrometer, and
then cut with an ax."
While the goal of measuring the temperature and tracking the movements of every
molecule in the atmosphere is impossible,
the more such data that can be fed into
models, the better forecasts will be.
Temperature, wind speed and direction,
and dew point measurements are easy
to feed into a model. The color images
you see depicting weather radar levels of
energy scattered back by precipitation and
other tiny objects in the air, or satellite
images with colors representing cloud-top
temperatures, also are data.
Since computers don't know the difference between the colors red and yellow,
for instance, this information has to be
turned into numbers for computers to use
it. Scientists refer to this process as "data
assimilation."
Louis W. Uccellini, who is now director
of the National Weather Service, said in
2008, "I like to use the analogy that you go
to a supermarket to buy food, you go to a

Figure 1

Figure 2
FIGURE 1. A forecast made without the
High-Resolution Rapid Refresh (HRRR) model
shows a general area where thunderstorms are
expected.
FIGURE 2. A forecast made with the aid of the
HRRR model gives a more detailed picture of where
thunderstorms are expected at the time of the
Figure 1 forecast.

restaurant to buy a meal. The chef has put
in effort to take food and make it a meal.
The men and women who design and
operate data assimilation systems are the
chefs that prepare the food [data] as easily
digestible meals: forecasts."
Successful data assimilation from radar
and other sources is one of the reasons
that forecasters are excited about the
High-Resolution Rapid Refresh (HRRR)
model the NWS put into operation with its

Weather and Climate Operational Supercomputing System on September 30, 2014.
It is updated with the latest data and
churns out predictions for the next 15
hours. The HRRR is designed to forecast
critical weather events such as thunderstorms, flash flooding, and localized bands
of heavy precipitation.
The model's output is for those who
need frequent updates of short-range forecasts such as pilots and airline dispatchers,
severe weather forecasters, and those
involved with renewable energy production with wind or solar farms. Knowing
when the wind should change or the sun
will disappear behind clouds helps them
manage their operations.
When it went into operation on September 30, "No one said, 'ah, that's cool,'
everything is cool," Murphy said. "It's been
experimental since 2010, and improvements were made during this time."
Pilots aren't likely to immediately see
huge changes in the forecasts they use,
Murphy said. Forecasters at the AWC
have been using the model for about four
years. Pilots "will see a gradual increase in
accuracy and timeliness. What makes the
HRRR so powerful is that every hour it's
reassessing what it's doing."
Forecasters have measured improvements in the "probability of detection,"
which shows how often what happens
had been forecast. They've also seen
improvements in the false alarm rate for
weather events, such as thunderstorms,
that do not occur.
No matter how much forecasts improve,
pilots should realize that all weather predictions are probabilities. You can't count
on them working out 100-percent correct
every time. When you make a go or no-go
decision about a flight, you are playing the
odds. Improvements such as the HRRR
model increase the odds the forecast will
work out. But you always need to be alert
for things to start to go wrong and have a
strategy for how to react, such as by landing sooner than you had planned.
Jack Williams is an instrument-rated private
pilot. His latest book is The AMS Weather Book:
The Ultimate Guide to America's Weather.

FEBRUARY 2015 FLIGHT TRAINING

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