The IRI has prepared this Climate Outlook for North America for January
- June 2000. Of relevance in the preparation of this Outlook has been
the current
La
Niņa conditions (lower than average sea-surface temperatures (SSTs)
in the central equatorial Pacific Ocean) that have been amplifying
through the end of 1999. Thus La Niņa conditions are in effect for the
first season of the forecast,
January-February-March 2000, and are expected to decline slowly
through the second season of the forecast period,
April-May-June 2000. The SSTs of the western tropical Indian Ocean
were assumed to remain near-normal, and the above-average SSTs of the
eastern tropical Indian Ocean are expected to return to normal during
the forecast period. The forecast of tropical North Atlantic SSTs
suggests a cooling trend through June 2000.
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 the continuation of La Niņa conditions. The
persistence of colder that average conditions in the central and eastern
equatorial Pacific is consistent with most other numerical and
statistical forecasts.
B) Forecasts of the tropical Indian and Atlantic oceans using
statistical models developed by the IRI and CPTEC respectively.
C) Global atmospheric circulation model (GCM) predictions of the
atmospheric response to the present and predicted sea-surface
temperature patterns.
D) Other sources of information include (
CMC ;
COLA ;
CPC ;
).
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 January - June
2000 is dependent on the quality of the SST predictions. For the
tropical Pacific, these predictions can be expected to provide useful
information, but there is some uncertainty concerning the detailed
evolution of SSTs. Spread in global SST predictions is a source of
uncertainty in the Outlook provided here.
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. 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: January - March 2000 and April - June
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 Cuba in January - March
2000 (Map B), there is a 15%
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 50% 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.
January - March 2000:
PRECIPITATION
The following regions of enhanced probabilities of precipitation
anomalies are indicated:
P.A) slightly enhanced probabilities of above-normal precipitation over
the Pacific Northwest, covering northern Oregon, Washington, and
southern parts of British Columbia and Alberta;
P.B) enhanced probabilities of below-normal precipitation in a zone over
the southwest United States, extending slightly into northwest Mexico
and Baja;
P.C) enhanced probabilities of above-normal precipitation over an area
extending over the central/north eastern United States, east of about
90E from Tennessee, to southeastern Canada and including the New England
States;
P.D) enhanced probabilities of below-normal precipitation of the central
Atlantic states of the US, from Georgia through southern Maryland;
P.E) greatly enhanced probabilities of below-normal precipitation along
the Gulf Coast of the United States starting at Texas, and including all
of Florida, Cuba, and the Bahamas. There is a slightly enhanced risk of
the seasonal precipitation being in the extreme below-normal category
over Florida;
P.F) enhanced probabilities of below-normal precipitation over the
southern tip of Texas down through central eastern Mexico;
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies
are indicated:
T.A) greatly enhanced probabilities of below-normal temperatures over a
zone extending from the northwest corner of the United States and the
entire west coast of Canada eastward to Hudson Bay;
T.B) slightly enhanced probabilities of above-normal temperatures over
an area extending from the south-western region of the United States,
down through the western half of Mexico and extending esatward though
the central United States;
T.C) enhanced probabilities of above-normal temperatures over the
eastern United States, covering all Atlantic states and inland;
T.D) greatly enhanced probabilities of above-normal temperatures over
the eastern half of Mexico, and the gulf coast of the United States,
including all of Floria;
T.E) enhanced probabilities of below-normal temperatures over Cuba,
Jamaica, and the Bahamas;
T.F) slightly enhanced probabilities of near-normal to below-normal
temperatures over southern Guatemala, Honduras, El Salvador, and
Nicaragua;
April - June 2000:
PRECIPITATION
No coherent regions of anomalous precipitation probabilities are
indicated:
TEMPERATURE
The following regions of enhanced probabilities of temperature anomalies
are indicated:
T.i) slightly enhanced probabilities of above-normal temperatures over
the east coast of the United States;
T.ii) slightly enhanced probabilities of below-normal temperatures over
southern Alaska;
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