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Reversed Tendency Between Forecast and 12-Month Precipitation Observation

Shading on this map identifies regions where there has been below-normal or above-normal precipitation during the previous 12 months AND where the IRI seasonal precipitation forecast for the next 3 months indicates an enhanced likelihood of a reversal of these precipitation conditions.

Please refer to our licensing agreement for permission to use any IRI forecast material.

NCEP gridded monthly precipitation data for a 30-year period are used as the observational data reference, and CAMS-OPI monthly gridded precipitation is used for the observations for the past 12 months. Observed precipitation over the past 12 months at each grid point is ranked on a scale from 0 to 1, with 0 as the driest event for the same 12-month period of the year as compared to the 30-year observational reference period, and 1 as the wettest event. A value of 0.333 and less is considered dry (below normal), and a value of 0.667 and greater is considered wet (above normal). Secondly, the forecast for the next 3-month season is examined for the same location. A forecast with a 40% or greater probability for the below-normal category is considered an "enhanced" likelihood of dry forecast, and a forecast of 40% or greater for the above-normal category is considered an "enhanced" likelihood of wet forecast.

Based upon the observed and forecast precipitation, green areas on the map indicate where precipitation for the past 12 months has been below normal AND where the IRI seasonal forecast for the next 3 months indicates an enhanced probability of above-normal precipitation. Yellow areas on the map indicate where precipitation for the past 12 months has been above normal AND where the IRI seasonal forecast for the next 3 months indicates an enhanced probability of below-normal precipitation.

The date shown on the map is the calendar month in which the forecast was issued. The 12-month period of observed precipitation ends in the calendar month before the month in which the forecast was issued. The three-month seasonal forecast period begins in the calendar month after the month in which the forecast was issued.

Dataset Documentation

Historical Precipitation

Data
CAMS monthly gridded precipitation from Jan 1969 to Dec 1998 on a 2.5° lat/lon grid
Data Source
NCEP

Observed Precipitation

Data
CAMS_OPI monthly gridded precipitation on a 2.5° lat/lon grid
Data Source
NCEP Climate Prediction Center (CAMS_OPI)

IRI Net Assessment Seasonal Precipitation Forecast

Data
Dominant precipitation tercile probability on a 2.5° lat/lon grid
Data Source
International Research Institute for Climate and Society (IRI Net Assessment Forecast)

Dataset

Access the dataset used to create this map.

Helpdesk

Contact help@iri.columbia.edu with any technical questions or problems with this Map Room.