ISSUED AT: 03:08 PM

Date: Friday 19th of May 2023

Dryness Indicator from February to April 2023 & 3-month Dryness Outlook for May to July 2023

Key Message

The 2023 dry season gradually developed over Trinidad and Tobago where below-normal rainfall occurred over Trinidad while near-normal rainfall occurred over Tobago. The driest month for both islands was in March 2023, when well below rainfall amounts occurred. However, a drastic increase in rainfall amounts in April for both islands where over 250% of the long-term rainfall averages had occurred. Piarco’s monthly totals for February-March-April range from 61.9 mm, 6.9 mm and 140.1 mm respectively, while Crown Point, Tobago's monthly totals for February-March-April range from 51.9 mm, 17.7 mm and 123.0 mm respectively. The outlook for the next three months, May-June-July 2023 shows no concern for dryness at this time.

Dryness Indicator for February-March-April 2023

The month of February had below-normal rainfall levels for Trinidad while it was near-normal for Tobago. In February, Trinidad had seven relatively wet days (1.0-10.0 mm), which gave a monthly total of 31.9 mm. While, Tobago had eleven relatively wet days (1.0-10.0 mm), and one wet day (>10.0 mm) and stayed within the near-normal range. The month of March had well below-normal rainfall in Trinidad. Trinidad had four relatively wet days (1.0-10.0 mm), which gave a monthly total of 6.9 mm while Tobago had three relatively wet days (1.0-10.0 mm) which gave a monthly total of 17.7 mm. The month of April 2023 had a drastic change in rainfall with three consecutive weeks being typically dry followed by substantial rainfall events that occurred during the late part of April giving above-normal rainfall for Trinidad and Tobago. Trinidad had three relatively wet days (1.0-10.0 mm) and one wet day (>10.0 mm) and two excessively wet days (> 50.0 mm) which gave a monthly total of 140.1 mm, while Tobago had six relatively wet days (1.0-10.0 mm) and one wet day (>10.0 mm) and one excessively wet day(> 50.0 mm) which gave a monthly total of 123.0 mm.

The 3-month dryness indicator for February-March-April 2023 shows dryness indicator values that ranged from -1.7 to +1.2, which was within the cause of concern for the impactful dryness category (see Figure 1).

The colour-coded dryness indicator map showing the observed level of dryness is based solely on rainfall and can be used for decision-making or for heightening awareness of dryness.

Figure 1. The colours on this map show 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. The period used to compute the dryness is  the  3-month period February to April 2023 compared to the historical average for the same 3-month period. The yellow to red colours on the map are used to indicate areas with borderline-dry to extremely-dry levels of dryness. The grey colour indicates areas where there are no significant dryness concerns.

Dryness Outlook for May to July 2023:

The accumulated rainfall received nationwide, during the month of April 2022, together with the predicted volume of rainfall for the transition month of May and rainy months of June to July 2023 has shown no cause for concern at this time across the islands. Frequent rainfall events are expected to continue into July. The Dryness Outlook shows dryness indicator values between +0.2 and +0.3 are likely to develop across all of Trinidad and Tobago by the end of July 2023 (see Figure 2). The outlook is based solely on rainfall and should be used only as a guidance tool.

Figure 2. The colours on this map show the predicted levels of dryness for the period May to July 2023.  It is based on the difference between standardized accumulated rainfall (observed and predicted) from May to July 2023 and the historical average rainfall for the same period. The yellow to red colours indicate borderline dry to extremely dry levels of dryness. The grey colour indicates areas where there is no significant dryness concerns.

Standardized Precipitation Index   

The Standardized Precipitation Index (SPI) is an index showing the severity and rarity of dryness or wetness of an area. Negative values of SPI indicate less than median rainfall and drier conditions; positive values indicate greater than median rainfall and wetter conditions. In general, dryness impacts are expected when the value of the 3-month SPI lies near -1.0. As the negative SPI value becomes larger in amount than -1.0, the severity of impacts increases. For Trinidad and Tobago, extreme dryness occurs in the dry season when negative SPIs are larger than -1.25.