Climate Outlook - Rainfall & Temperature
NORTH AMERICA April - September 1998
The IRI Experimental Forecast Division has prepared this Climate Outlook for North America for April - September
1998. Of relevance in the preparation of this Outlook has been the strong El Niņo
episode now subsiding in the tropical Pacific, and the expectation that warm El Niņo
conditions will persist for the next few months. Also relevant to
this Outlook are the warm sea surface temperature (SST) anomalies in the Indian Ocean basin and in the tropical
Atlantic Ocean.
METHODS -
This Outlook was prepared using the following procedures and information:
A) Coupled ocean-atmosphere model predictions of tropical Pacific sea surface
temperatures (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 moderate, but
weakening, El Niņo conditions in the tropical Pacific through the forecast period.
B) Statistical predictions of the tropical Atlantic SST's.
C) Global atmospheric circulation model (GCM) predictions of the atmospheric response to the present and predicted
SST patterns.
D) Statistical analyses of the observed climate over North America.
E) Other sources of information include NCEP,
CPC-HawaiianIslands
Canada, and
Central America.
The procedures, models, and data used to derive this Climate Outlook may be somewhat different from those used by
the national meteorological services in North America. Thus, this product may differ from the official forecasts issued
in those areas. This Outlook has been prepared in the time available, using all information
that was reasonably accessible. Inclusion of other climate information and guidance requires
further arrangements. The IRI is engaged in establishing such collaborative arrangements with
the goal of improving its capability to provide the best and most
complete global climate guidance. The Climate Outlook for April-September is dependent of the quality of the SST
predictions. For the tropical Pacific, these predictions can be expected to provide useful
information, but there is considerable spread in coupled model predictions concerning the
evolution of SSTs. This spread in predictions is a primary source of uncertainty in the
Outlook provided here, which assumes that tropical Pacific SSTs will continue to
decline at a modest rate. Also, it is also known that tropical and/or mid-latitude Atlantic
SSTs may play some role in
modulating rainfall changes over parts of eastern North America. Thus, the uncertainties in
Atlantic SST values during the forecast period may lead to additional uncertainty over some
parts of eastern North America.
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: April-June 1998 and July-September 1998.
Maps are given showing expected probabilities of precipitation. The maps 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). An additional 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. 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. A qualitative outlook of climatology
("C") indicates that there is no basis for favoring any particular category. Otherwise, for example, in the case of
Northern California in April-June 1998 (Map A), there is a 50% probability that the rainfall will be in the wettest
third of the years, a 30% chance it will be in the near-normal third of the years, and a 20% chance that the rainfall will
be in the driest third of the years. The numbers for each region on temperature maps indicate the probabilities of tem-
peratures 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.
April-June 1998:
PRECIPITATION
The map is dominated by the following main regions of enhanced probabilities of rainfall anomalies:
P.A) Elevated probabilities for normal to above-normal rainfall across the southern tier and central United States, and
along and east of the Rocky Mountains. Greater confidence is given to probabilities that
suggest an increased likelihood for above-normal precipitation along the west coast. There is
also a perceived enhancement of risk for extreme
above-normal precipitation (precipitation values reaching values only seen in the wettest 15% of years in the
observed record) along the west coast of the United States, extending northward into British
Columbia.
See map.
P.B) Probabilities slightly favor near-normal to above-normal conditions over central Canada.
P.C) Enhancement of probabilities for above-normal rainfall along the eastern seaboard of the United States and up
into eastern Canada. This region along the eastern seaboard carries a slight enhancement of
risk for extreme above-normal precipitation.
See map.
P.D) Slightly enhanced probabilities for below-normal precipitation over most of Central America. An enhanced risk
of extreme below-normal precipitation (precipitation values reaching values only seen in the driest 15% of years in
the observed record) is also seen for this region.
See map.
TEMPERATURE
The main features of the pattern of probabilities for temperature field are:
T.A) Enhancement of probabilities for above-normal temperatures along the eastern seaboard of the United States and
up into eastern Canada.
T.B) Enhanced probabilities for warmer than normal temperatures over the Canada, especially over the region along
the western coast.
T.C) Enhanced probabilities for the below-normal temperatures over the south-western part of the US and in Mexico.
July-September 1998:
PRECIPITATION:
The main features of this season are as follows:
P.i) Slightly elevated probabilities for above-normal precipitation over the eastern Canada.
P.ii) Enhanced probabilist for dry conditions over the western part of Canada.
P.iii) Enhanced probabilities for above-normal rainfall over the south-western part of Canada and across the western
tier of the United States.
P.iv) Slightly elevated chances for below-normal rainfall over the south-eastern tier of the United States.
P.v) Normal conditions over Florida and Cuba.
TEMPERATURE
The main features of the pattern of probabilities for temperature field are:
T.i) Slightly enhanced probabilities for warm temperatures over the west coast of Canada.
T.ii) Elevated probabilities for the above-normal temperature across the Central America.
Cutoffs and Max/Min
extremes for AMJ and JAS.
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