METEOROLOGICAL SERVICE JAMAICA CLIMATE BRANCH

Transcription

1 METEOROLOGICAL SERVICE JAMAICA CLIMATE BRANCH PRELIMINARY MONTHLY RAINFALL SUMMARY FOR MARCH 2016 Introduction This rainfall summary is prepared by the Climate Branch of the Meteorological Service, Jamaica. The Meteorological Service maintains a network of over two hundred (200) rainfall stations located across the island. Rainfall is usually read at 7am by a cadre of paid but mainly voluntary dedicated observers and reported for the previous 24 hours. General Jamaica s bimodal rainfall pattern consists of two peak periods with higher values of rainfall and corresponding periods of lower rainfall amounts. The primary peak occurs in October and the secondary in May. The lowest amounts are at a minimum during the period February to March and the month of July. This is based on long-term reports and deviations from this pattern do occur year to year. A comparison of the old 30 year mean ( ) with the mean by the Meteorological Service has shown that the island s rainfall patterns and values have not changed significantly for the current thirty-year ( ) period. The main changes noted are that of wetter dry periods and drier wet periods. This has however not affected the overall rainfall pattern for the island. Figure 1: Precipitation Pattern for for the island of Jamaica.

2 Climate Branch March Island Monthly Rainfall March 2016 had all thirteen (13) parishes recording below normal rainfall. Overall the island s average rainfall was 41mm which corresponds to 47% of the 30-year ( ) mean. These values were well below those of March 2015 which had 108mm or 123% respectively. The cumulative rainfall for the island to date (March 2016) was 258mm, or 91% of the 30-year ( ) mean or 9% below what is normal for this time of the year. Parish Mean Rainfall for March 2016 (Rainfall in mm) MAR MAR MAR % OF 30 YR NORMAL YR NORMAL ( ) JAN FEB MAR Parishes KEY Hanover HAN Westmoreland WES Manchester MAN St. Elizabeth STE Clarendon CLA St. Catherine STC Trelawny TRE St. James STJ St. Ann STA St. Mary STM Portland POR St. Thomas STT Kgn. & St. And. KSA Jamaica JAM Table 1: Parish Mean Rainfall for March 2016 (Rainfall in mm) 2

3 Climate Branch March Assessment of Parish Figures All parishes recorded rainfall totals that were below their 30-year means. The parish rainfall figures indicate the following: Clarendon with the highest of 39mm or 69%, followed by Hanover with 64mm or 62%, Portland with 126mm or 60%, St. Elizabeth with 51m or 52%, KSA with 34mm or 51%, St. Ann with 36mm or 51%, Manchester with 40mm or 44%, St. Mary with 42mm or 39%, St. James with 23mm or 38%, Westmoreland with 33mm or 38%, St. Catherine with 20mm or 32%, St. Thomas with 17mm or 24% and Trelawny with 12mm or 19%. (Kindly note that Kingston and St. Andrew (KSA) are combined and reported as one. Drought Conditions Definition Drought is defined as a long period of weather without rain (Heinemann English Dictionary). The more precise definitions for specific areas of concern that are most commonly used are: Agricultural drought a period when soil moisture is inadequate to meet the demands for crops to initiate and sustain plant growth. Hydrological drought period of below average or normal stream-flow and/or depleted reservoir storage. Meteorological drought a period of well-below average or normal precipitation (rainfall) that spans from a few months to a few years. Methodology and Index Locally, the onset and the duration of a meteorological drought is determined by comparing the average rainfall over a period of two consecutive months with the 30-year historical averages (normal) for a similar bi-monthly period for each parish and the 3

4 Climate Branch March island. The percentage value that is generated is used to quantify the thresholds of the drought index that is presented in Table 2. This index is similar to that used by the Australian Meteorological Service except for the duration of eight consecutive weeks instead of bi-monthly periods that is used locally. Percentage of Normal for 2 Drought Condition or Status Consecutive Months 20% or less Extreme Drought 21% to 40% Severe Drought 41% to 60% Normal Drought Above 60% No Drought Table 2: Meteorological Drought Index Drought Indices (%) for December 2015 to March 2016 Parishes Dec/Jan Jan/Feb Feb/Mar Hanover Westmoreland Manchester St. Elizabeth Clarendon St. Catherine Trelawny St. James St. Ann St. Mary Portland St. Thomas Kgn. & St. And Jamaica Table 3: Drought indices (%) for December 2015 to March 2016 Table 3 is calculated as follows: Values = {(Month 1 + Month 2) / (Normal month 1 + Normal month 2)} x 100 Drought Assessment Manchester and St. Thomas were the parishes to record (normal) drought conditions this bi-monthly period. Clarendon which recorded drought conditions the previous period was 4

5 Climate Branch March above drought conditions this time around. All other parishes remained above drought conditions. FORECAST Precipitation Outlook for the period April to June 2016 Through the period April to June the models continue to indicate an increase in the forecast rainfall amounts across most areas. As we approach the first rainfall season (April-June), the expected increase in rainfall activities is likely to ease most of the current dry patches affecting sections of most southern parishes. The forecast is for the rains to start very late April into May, and therefore the timely arrival of these rains should ease the water stress currently affecting key agricultural areas. However due to the variability in the start date of the rains, we would advise that monitoring activities be continued to reduce any possible impacts that could result should there be any delays in the start of the rains. VERIFICATION OF MODELS For the same period last year, the models predicted near normal to above normal rainfall for most sections of the island. A comparison with actual conditions indicates that most parishes recorded below normal (less than what is expected) or near normal (expected) rainfall for the three-month period April to June. Although forecast confidence can be low at this time of the year, we remain very optimistic that there is a strong realization of the forecast going into the first rainfall season, (April-June). Key A: Above normal rainfall means greater than 66 percentile of the rank data N: Near normal rainfall means between 33 and 66 percentile of the rank data B: Below normal rainfall means below 33 percentile of the rank data 5

6 Climate Branch March Table 4. Climate Predictability Tool (CPT) Station Outlook- AMJ Stations Below (B) % Normal (N) % Above (A) % Manley (Kingston) Sangster (St. James) Savanna La Mar (Westmoreland) Beckford Kraal (Clarendon) Serge Island (St. Thomas) Cave Valley (St. Ann) Tulloch Estate (St. Catherine) Y.S. Estate (St. Elizabeth) Hampstead (St. Mary) Orange Valley (Trelawny) Langley (Kingston) Mount Peto (Hanover) Shirley Castle (Portland) Suttons (Manchester) Potsdam (St. Elizabeth) Frome (Westmoreland) Worthy Park (St. Catherine) Jamaica 6

7 Climate Branch March Summary All parishes reported below-normal rainfall for the month of March 2016, with Jamaica receiving only 47% of its normal rainfall. Two parishes experienced drought conditions, while the other eleven parishes remained above drought. The current projections still indicate an increase (near to above normal) in rainfall from April to June, and it is expected that the rains should ease the dry conditions affecting sections of the island. 7