TTMS: Dry Conditions Across T&T To Recover

The Trinidad and Tobago Meteorological Service (TTMS) is forecasting dry conditions subside across Trinidad and Tobago through May 2021, through the end of the 2021 Dry Season.

According to the TTMS, from December 2020 through February 2021, rainfall was close to average across most of Trinidad, with large areas experiencing more than average rainfall in February. This led to recovery from slightly dry conditions at the end of January to borderline dry conditions at the end of February, which also widened in spatial coverage.

The Dryness Indicator for December 2020 through February 2021 shows a range between -0.22 and -0.75 on the Standardized Precipitation Index scale for Trinidad. In contrast, most of Tobago got less than average rainfall from December to February. This resulted in short-term rainfall deficiencies emerging across the island and a worsening of dryness levels. The Dryness Indicator shows that at the end of February, Tobago was in a dry spell in the southwest and borderline dry conditions in the northeast.

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 December 2020 to February 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 December 2020 to February 2021 compared to the historical average for the same 3-month period. (TTMS)
Predicted dryness levels for March to May 2021 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)

The TTMS’ Dryness Outlook from March through May 2021 indicates that current borderline dry conditions in Trinidad and the emerging dry spell in Tobago are likely to recover to levels of no significant dryness concerns by the end of May, with negative Dryness Indicator levels forecasted to decrease to levels between -0.1 and -0.4.

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 March 2021, the Office called for “slightly wetter than usual over most areas of Trinidad, with above-normal rainfall most likely, in large areas.” for the second half of the 2021 Dry Season.

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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