Weather Watch:  May 2004

HOW ACCURATE ARE WEATHER FORECASTS?

A well-planned paddle trip includes considering the weather forecast.
The increasing availability of weather information through the media
and internet forces us to be intelligent users of that information.

There are two reasons why weather forecasts are imperfect.  First,
computer models that predict weather need accurate representation of
the current state of the atmosphere (<initial conditions>).  These
models then advance these initial conditions forward in time.  If the
initial conditions are imperfect, because of the lack of weather data
(like over the oceans or in the mountains), then model forecasts may
diverge quickly from reality, producing greater forecast errors over
time.  Thus, forecasts for tomorrow are likely to be better than
forecasts for two or three days from now.  Second, imperfect computer
models, due to gaps in our knowledge of complex atmospheric processes,
also lead to imperfect forecasts.

Smaller weather features are likely to be less predictable than larger
features.  Although the morning forecast may call for afternoon
thunderstorms over an area the size of several states, knowing
whether your paddle trip will be hit by an individual thunderstorm,
which may only affect a few counties, is practically impossible until
thunderstorms develop.  Meteorologists communicate this uncertainty
by expressing the probability of thunderstorm occurrence for any given
location.

Is it possible to obtain a forecast for a paddle trip I'm planning
next year?  A distinction should be made between weather forecasts and
seasonal climate forecasts.  While current prediction of specific
weather events cannot be made much beyond five days at best, seasonal
patterns may have some predictability, although considerably lower
than that for individual weather events.  An example is the 1997-1998
El Nino, which was forecast many months in advance.  An El Nino
forecast meant California would receive above-normal winter
precipitation, leading to good spring whitewater conditions in the
rivers leading from the Sierra Nevada Mountains.  Although El Nino
could be forecast months in advance, individual storms producing
those mountain snowfalls could only be forecast days in advance.

Thus, use knowledge of the climate of a region to help guide your trip
planning months or weeks in advance.  Knowing the typical temperatures
and the record highs and lows, for example, will help you pack gear
and prepare for the possibilities.  As the departure date for your
trip nears, check the forecasts online, from media outlets, or on
NOAA Weather Radio to fine-tune your planning.

The former Director of the National Weather Service Jack Kelly said,
"Numerical weather modeling is the most unsung scientific achievement
of the 20th Century."  Indeed, today's two-day forecasts are as good
as one-day forecasts were 20 years ago.  Although great strides in
using computers to aid forecasting have been made, the scientific
limitations on forecasts mean paddlers and other users of weather
forecasts will always face uncertainty.  That is why it is important
to get the most up-to-date forecasts before starting your trip, and
monitor the weather during your paddle.

<-David Schultz, NOAA/National Severe Storms Laboratory, Norman, Oklahoma>