AOPA Pilot Magazine - June 2021 - 78

PIVOTAL WEATHER WEBSITE (above)

can show many forecast models,
and the one here is the GFS model
with " composite reflectivity "
selected from the menu at the
lower left. With the model
initialized at 12Z on April 13,
clicking on the 246-hour forecast
time gives you a view of what the
precipitation situation might be
10 days in the future, on April 23
at 18Z. No way is this a bulletproof forecast, but it's a starting
point for comparisons with future
updates as the target day draws
near. Clicking on other parameters
listed on the menu gives you other
variables, like temperature, dew
point, surface pressure.

78 | AOPA PILOT June 2021

18 hours out. The graphical forecast for aviation
(GFA) tool is even more helpful in identifying-or avoiding-instrument meteorological
conditions and other adverse weather, and it
predicts 18 hours into the future as well. For
longer-range guidance, convective outlooks
from the Storm Prediction Center (spc.noaa.
gov) go out as far as eight days, and the Weather
Prediction Center's projected surface analyses (ncep.noaa.gov) give seven days' worth of
charts. Meanwhile, those who use ForeFlight
can take a look three days in the future by clicking on the " MOS " link under the Weather tab on
an airport page. MOS (Model Output Statistics)
usually appears in a coded, text format but
ForeFlight provides plain-language TAF-style
summaries. The nice thing about ForeFlight's
MOS is that they're provided for airports that
may not be served by TAFs.
But what if you're planning for a trip
three,five, or more days away? Are there ways
of gaining a sense of what's in store? Are conditions trending toward severe-clear, or is a
moisture-laden frontal system taking aim on
your destination? TAFs can't tell you, nor can

the progs, aviation surface forecasts, or the GFA
tool. And the WPC's forecast surface analysis
charts begin to drop precipitation symbols after
two days out, leaving the next five days' chart
with fronts, highs, and lows only.
Here's where numerical weather prediction-computer-generated forecast models-can
provide an extra dimension of long-range planning. You won't find them on the AWC website,
but they thrive on the internet. These models
crunch all manner of atmospheric data-wind
flow patterns, thermodynamics, climatological
factors, remote sensing from weather balloons
and satellites, and much, much more-and can
come up with graphic representations of surface
and upper-air patterns, temperatures, moisture
contents, instability indices, and even cloud
layers. In the case of one forecast model-the
Global Forecast System, or GFS-information
like this is presented at three-hour intervals up
to 14 days in advance.
There are dozens of forecast models out
there, but most of them aren't as long-legged
as the GFS. The NAM (North American
Mesoscale) goes out three days, for example,

http://ncep.noaa.gov

AOPA Pilot Magazine - June 2021

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