Geoscience Reference
In-Depth Information
long-range forecasts is the realization that El Niño-
Southern Oscillation has documented statistical effects
in many parts of the globe. For any particular El Niño
or La Niña it is generally not realistic to forecast
increased/decreased precipitation at most points in the
globe but many regions show a statistical tendency
towards more or less precipitation or higher/lower
temperatures depending on the phase of ENSO.
Long-range forecasts make use of these statistical
relationships. ENSO has a fairly regular periodicity
allowing for some skill in predicting changes in phase
just from climatology. Several dynamical models also
try to predict the future phase of ENSO, though these
have not been dramatically more sucessful than a
knowledge of the climatology. The phase of ENSO is
the single most important factor going into long range
forecasts today.
The United States NCEP is again typical of the
methodology used globally. It currently issues thirty-
day and three-month seasonal forecasts up to one year
into the future. The primary information used in these
outlooks is the phase of ENSO, recent and extended
climate history, the pattern of soil moisture which can
affect temperature and precipitation far into the future,
and an ensemble of twenty GCM model runs driven
with predicted SSTs from an AOGCM simulation
over the period. This information is used to produce a
variety of indices which predict the probability of three
equally likely categories of temperature (near normal,
above/below normal) and precipitation (near average,
Figure 8.7
Forecast of North
American weather for December
1985 made one month ahead (A)
Predicted 700-mb contours (gp dam).
Solid arrows indicate main tracks
of cyclones, open arrows of anti-
cyclones, at sea-level. The forecasting
of such tracks has recently been
discontinued. (B) and (C) Forecast
average temperature (B) and average
precipitation (C) probabilities. There
are three classes of temperature,
above normal, normal and below
normal, and similarly heavy, near
normal and light for precipitation. Each
of these classes is defined to occur
30 per cent of the time in the long
run; near-normal temperature or
moderate precipitation occur 30 per
cent of the time in the long run; near-
normal temperature or moderate
precipitation occur 40 per cent of
the time. The 30 per cent heavy lines
indicate indifference (for any departure
from average), but near-normal values
are most likely in unshaded areas.
Source
: From
Monthly and Seasonal
Weather Outlook
, 39(23) (28 November
1985), Climate Analysis Center, NOAA,
Washington, DC.
