TTMS: “Concerns For Impactful Dry Conditions For Most Of Tobago”

The Trinidad and Tobago Meteorological Service (TTMS) is forecasting borderline, short-term dry conditions across Tobago through February 2022, through the first half of the 2022 Dry Season.

According to the TTMS, accumulated rainfall totals during September and October 2021 mitigated the impacts of the significant rainfall deficit of November 2021. However, November’s lackluster rainfall led to borderline dry conditions across much of Tobago.

The dryness indicator for September to November shows values between 0.1 and -0.4 in Trinidad and -0.3 and -0.5 in Tobago. Concerns are raised for dryness impacts in northeast Tobago.

Observed dryness levels based on the rainfall differences from average, which have been standardized and expressed as the number of standard deviations less than average over September 2021 to November 2021 compared to the historical average for the same 3-month period. (TTMS)
Observed dryness levels based on the rainfall differences from average, which have been standardized and expressed as the number of standard deviations less than average over September 2021 to November 2021 compared to the historical average for the same 3-month period. (TTMS)
Predicted dryness levels for December 2021 to February 2022 based on the standardized rainfall difference between what is forecasted and the historical average rainfall, for the period of the forecast, while also considering actual observed rainfall for the last three months. (TTMS)
Predicted dryness levels for December 2021 to February 2022 based on the standardized rainfall difference between what is forecasted and the historical average rainfall, for the forecast period, while also considering actual observed rainfall for the last three months. (TTMS)

The Dryness Outlook for the three months from December to February 2022 indicates little to no dryness impacts in Trinidad and borderline dryness conditions in Tobago, as a mixture of near-normal rainfall in December and January and dryness in February act against each other.

There are concerns for impactful dry conditions for most of Tobago. Dryness indicator values for the period are likely to be between -0.21 and -0.43 on the Standardized Precipitation Index (SPI) scale for Trinidad, which are outside the levels for dryness concerns. In Tobago, the SPI values are likely to range from -0.5 to -0.7, which raises concerns for impactful dryness during the period.

According to the TTMS, the outlook is based solely on rainfall and should be used only for guidance. In the TTMS’ Rainfall and Temperature Outlook issued in January 2022, the Office called for “enhanced signals for suppression of rainfall that tilts the odds towards drier than usual conditions with below-normal rainfall totals favored at this time for both islands. January is likely to get near-normal rainfall,” and “the 3 months, February to April, are likely to be the driest months of the season, with February likely to be the driest of these.”

What is the Dryness Indicator?

The Dryness Indicator is a tool for keeping track of the level of dry conditions, rainfall deficit, and length of the dry period across Trinidad and Tobago. It is based on the TTMS’s Dry Spell and Meteorological Drought Policy which uses the Standardized Precipitation Index (SPI) to indicate dryness levels. The SPI is a simple measure of dryness based solely on the total rainfall for a given period, for example, over the last 60 days, compared with the long-term average rainfall for that period.

The colors on maps indicate observed levels of dryness based on the difference between observed and/or predicted rainfall and the average rainfall, which is then standardized and expressed as the number of standard deviations less than average. The colors moving from yellows to reds show increased levels of dryness. The grey areas indicate areas where the dryness is either very close to the average, average, or on the wet side. In general, dryness concerns and negative impacts are expected when the dryness indicator’s value lies near -1.0. Concerns for dryness impacts increase as the size of the negative value increases.

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