NORTH AMERICA July - December 2002
Issued: June 2002
The IRI has prepared this experimental Climate Outlook for North America for
July - December 2002.
Of relevance in the preparation of this outlook is the
prediction of warmer than average conditions in the
equatorial Pacific for the next 6 to 9 months.
Currently the sea surface temperatures (SSTs) across
much of the eastern and central equatorial are warmer than
their long-term average
particularly in the central and western part of the basin,
where warm SST anomalies have been present for nearly
a year, but now also closer to the coast of Peru and Ecuador.
Weakly warm equatorial Pacific SST conditions
(approximately 0.5 C) are in effect for the first seasons
of the forecast,
The central equatorial Pacific is
currently observed to be warmer (SSTs over
1 degree C above normal) than these predictions, and this discrepancy
has been considered in developing the climate forecasts. The predictions
attain the level of a weak El Nino (SSTs between 0.5
and 1 degree C above normal) by the end of the forecast period,
Note that this prediction for a weak
El Nino is that given by a particular ENSO prediction model. It is
described here because it served as the boundary forcing for the
following climate forecast, which is primarily based on dynamical
global climate models. This should not be confused with the
IRI's ENSO Statement
September-November 2002, which while indicating an enhanced potential
for an El Nino to develop in 2002 also highlights the uncertainty
still present in the prediction at this time.
In the other tropical oceans, warmer than average SSTs continue to
dominate much of the Indian Ocean, and are not expected to decrease as
rapidly as suggested by the SST predictions. However, the actual
long-term evolution of the Indian Ocean SSTs is difficult to foresee
at this time. The area of above-average temperature in the tropical
south Atlantic Ocean is expected to persist through at least the
first half of the forecast period.
This Outlook was prepared using the following procedures and
A) Coupled ocean-atmosphere model predictions of tropical Pacific SST
covering the forecast period. Particularly heavy weighting has been
given to predictions from the coupled model operated by the NOAA
National Centers for Environmental Prediction, Climate Modeling Branch.
This model suggests a continuation of near-average conditions during the
first forecast season. The forecast for near-neutral conditions is
consistent with some, but not all, numerical and statistical forecasts
of central and eastern Pacific SSTs.
B) Forecasts of the tropical Indian ocean using a statistical model
developed by the IRI.
C) Global atmospheric general circulation model (GCM) predictions of the
atmospheric response to the present and predicted sea-surface
D) Other sources of information include
Seasonal to Interannual Prediction Project (GSFC-NASA)
and also seasonal prediction research at COLA.
The procedures, models, and data used to derive this Climate Outlook may
be somewhat different from those used by the national meteorological
services in the region. Thus, this product may differ from the official
forecasts issued in those areas. The Climate Outlook for
July - December 2002
is dependent on the accuracy of the SST predictions. For the
tropical Pacific, these predictions can be expected to provide useful
information, but there is some uncertainty concerning the evolution of
SSTs. Spread in global SST predictions is a source of uncertainty in the
Outlook provided here. Note that even if perfectly accurate SST
forecasts were possible, there would still be uncertainty in the climate
forecast due to chaotic internal variability of the atmosphere. These
uncertainties are reflected in the probabilities given in the forecast.
It is stressed that the current status of seasonal-to-interannual
climate forecasting allows prediction of spatial and temporal averages,
and does not fully account for all factors that influence regional and
national climate variability. This Outlook is relevant only to seasonal
time scales and relatively large areas; local variations should be
expected, and variations within the 3-month season should also be
expected. For further information concerning this and other guidance
products, users are strongly advised to contact their National
This Outlook covers four seasons: July-September 2002,
August-Octover 2002, September-November 2002 and October-December 2002.
Maps are given showing tercile probabilities of
precipitation and temperature. The maps for precipitation indicate the
probabilities that the seasonal precipitation will fall into the wettest
third of the years (top number), the middle third of the years (middle
number), or the driest third of the years (bottom number). The color
shading indicates the probability of the most dominant tercile -- that
is, the tercile having the highest forecast probability. The color bar
alongside the map defines these dominant tercile probability levels. The
upper side of the color bar shows the colors used for increasingly
strong probabilities when the dominant tercile is the above-normal
tercile, while the lower side shows likewise for the below-normal
tercile. The gray color indicates an enhanced probability for the
near-normal tercile (nearly always limited to 40%). As before, numbers
and their associated histograms show the probabilities of the three
terciles. In areas with lots of spatial detail, there may not be
sufficient room on the map, to allow histograms for each region. In
those cases, some idea of the probabilities may be gained from the color
alone. A qualitative outlook of climatology ("C") indicates that there
is no basis for favoring any particular category.
Areas that are marked by "D" represent regions for which less than 3cm of
precipitation typically occurs over the season.
Otherwise, for example, in the case of
July-September 2002 (Map A), there
is a 25% probability that the precipitation will be in the wettest third
of the years, a 35% chance it will be in the near-normal third of the
years, and a 40% chance that the precipitation will be in the driest
third of the years in most of southern Mexico and Central America.
Maps of temperature show expected probabilities that the seasonal
temperatures will fall into the warmest third of the years, the middle
third of the years, or the coldest third of the years
The numbers for each region on the temperature maps
indicate the probabilities of temperatures to fall in each of the three
categories, above-, near-, and below-normal.
additional precipitation map
is provided for the first season indicating probabilities for extreme
precipitation anomalies. Extremes are defined as anomalies that fall
within the top and bottom 15th percentile of the observed records. A
priori, there is a 15% probability of being within the extremely wet
category, and a 15% probability of being within the extremely dry
category, leaving a 70% probability that the precipitation will not be
extreme. The maps indicate areas of increased risk of extreme
precipitation totals. Three levels of increased risk are defined:
slightly enhanced risk, enhanced risk, and greatly enhanced risk. For
slightly enhanced risk, there is a 25-40% probability that precipitation
will be within the indicated extreme, i.e. wet or dry. This represents
an approximate doubling of the climatological risk. For enhanced risk,
there is a 40-50% probability that precipitation will be within the
indicated extreme. This represents an approximate tripling of the
climatological risk. For greatly enhanced risk, the probability that
precipitation will be within the indicated extreme exceeds 50%, i.e. the
indicated extreme is the most likely outcome. A similar map is provided
in the first season indicating probabilities of
Boundaries between sub-regions should be considered as transition zones,
and their location considered to be only qualitatively correct.
July-September 2002 through October-December 2002:
The following discussion briefly describes the probability anomaly forecasts:
Enhanced probabilities for below normal precipitation are
forecast for parts of southern California (U.S.) and northern Baja
California (Mexico) for all four forecast period, for Central America
and southern Mexico for the first forecast period, and for much of
the Caribbean Island vicinity for the first two forecast periods.
Slightly enhanced probabilities for above normal precipitation are
forecast for the southern Rocky Mountain and southern Great Basin
regions of the United States during the second forecast period,
for parts of the Aleutian Islands of Alaska for the third period
and for southern Florida (U.S.) and northern Cuba for the fourth
Enhanced probabilities for above normal temperature are forecast
for significant parts of North America for the first two forecast periods,
including all of Central America and Mexico, most of the U.S., much of
the Caribbean region, and much of southern and south-central Canada.
During the last two forecast periods a tendency toward warmth continues
but with more limited strength and coverage. Slightly enhanced
probabilities for below normal temperature are forecast for the
first three forecast periods, in small pockets mainly along coastal
portions of Alaska and Canada. During the first two forecast periods,
a slightly enhanced probability for below normal temperature is forecast
for the eastern Caribbean, from Puerto Rico eastward.
OBSERVED CLIMATOLOGY DATA for Jul-Aug-Sep,
Aug-Sep-Oct, Sep-Oct-Nov and Oct-Nov-Dec
TERCILE THRESHOLDS (33%-ile & 67%-ile):
EXTREME THRESHOLDS (15%-ile & 85 %-ile):