The IRI has prepared this experimental Climate Outlook for North America
for July - December 2000. Of
relevance in the preparation of this Outlook has been the weakening remains of the 1998-2000
La Niņa
conditions (lower than average sea-surface temperatures (SSTs) in the
central equatorial Pacific Ocean).
Thus, near neutral but slightly below-average equatorial Pacific SST conditions are in effect for the first season of the forecast,
July-August-September 2000, and near-normal
conditions are expected for the second forecast season,
October-November-December 2000.
The SSTs of the tropical Indian Ocean were assumed to remain near-normal.
Tropical Atlantic SSTs were not expected to change significantly.
METHODS -
This Outlook was prepared using the following procedures and information:
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 final demise of La
Niņa conditions and a return to near-average conditions during the first forecast season.
The forecast for 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 and Atlantic oceans using statistical models developed by the IRI and CPTEC,
respectively.
C) Global atmospheric general circulation model (GCM) predictions of the atmospheric response to the present and predicted
sea-surface temperature patterns.
D) Other sources of information include
CPC ;
CMC
.
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 2000 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 Meteorological Services.
OUTLOOK -
This Outlook covers two seasons: July - September 2000 and October - December 2000. 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). 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 experiencing
their dry season (less than 15% of annual precipitation) and for which less than 5cm of precipitation typically occurs
over the season. Otherwise, for example, in the case of region I in July - September 2000
(Map A), there is a 25% probability that the precipitation will be in the wettest third of the years, a 40% chance it will be in the near-normal third
of the years, and a 35% chance that the precipitation will be in the driest third of the years.
An
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.
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 (Map A). 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.
Boundaries between sub-regions should be considered as transition zones, and their location considered to be only
qualitatively correct.
July - September 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region I) slightly enhanced probabilities of below-normal precipitation over the Hawaiian Islands;
Region II) slightly enhanced probabilities of above-normal precipitation in part of the desert
Southwest of the United States;
Region III) slightly enhanced probabilities of below-normal precipitation in the western portion
of the Midwest of the United States;
Region IV) slightly enhanced probabilities of above-normal precipitation over much of the
southeastern and mid-Atlantic sectors of the United States;
Region V) slightly enhanced probabilities of below-normal precipitation over southern Florida,
Cuba, Jamaica, Haiti, Dominican Republic, Puerto Rico and the Virgin Islands;
Region VI) slightly enhanced probabilities of normal precipitation over southern Mexico (except
for Yucatan), El Salvador, and the western (Pacific) halves of Guatemala, Honduras and Nicaragua.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region I) enhanced probabilities of normal temperatures over the Alaskan panhandle and adjacent
western Canada;
Region II) slightly enhanced probabilities of normal to above-normal temperatures over a large
area occupying much of western Canada;
Region III) slightly enhanced probabilities of normal temperatures over the northern Rocky
Mountains of the United States;
Region IV) slightly enhanced probabilities of above-normal temperatures in much of the desert
Southwest, southern California and southern Great Basin of the United States;
Region V) greatly enhanced probabilities of normal temperatures in the region of southern Nevada,
eastern Arizona and southeatern California of the United States; this region is a stronger inner
coure surrounded by region IV.
October - December 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation anomalies are indicated:
Region I) slightly enhanced probabilities of above-normal precipitation over the southern Great
Plains and southern Rocky Mountains of the United States, and northern Mexico.
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies are indicated:
Region I) slightly enhanced probabilities of below-normal temperatures in western Canada, the
Alaskan panhandle, and the extreme northwestern corner of the United States;
Region II) slightly enhanced probabilities of above-normal temperatures over part of the desert
Southwest of the United States;
Region III) slightly enhanced probabilities of above-normal temperatures over the Bahamas and the
state of Florida of the United States;
Region IV) enhanced probabilities of above-normal temperatures over southern Mexico, Beleze,
Guatemala, Honduras, El Salvador, Nicaragua, Costa Rica and Panama.
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