METEOROLOGICAL SERVICE JAMAICA CLIMATE BRANCH

Transcription

1 METEOROLOGICAL SERVICE JAMAICA CLIMATE BRANCH PRELIMINARY MONTHLY RAINFALL SUMMARY FOR JANUARY 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 January Island Monthly Rainfall January 2016 saw most parishes receiving below normal rainfall with the exceptions being Trelawny and St. James. The month saw the island rainfall averaging 66mm which corresponds to 64% of the 30-year ( ) mean. This was 28mm less than that of January In the early part of the month of January the weather was dominated by Troughs, however in the latter part of the month High Pressure Ridges were the dominant features. The cumulative rainfall for the island to date (January 2016) was 66mm, or 64% of the 30-year ( ) mean or 36% below what is normal for this time of the year. Parish Mean Rainfall for January 2016 (Rainfall in mm) JAN JAN JAN % OF 30 YR NORMAL YR NORMAL ( ) NOV DEC JAN 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 January 2016 (Rainfall in mm) 2

3 Climate Branch January Assessment of Parish Figures Trelawny was the only parish recording a rainfall total that was above the 30-year ( ) mean. St. James equaled its 30-year ( ) mean, while all other parishes recorded rainfall totals that were below their 30-year ( ) means. The parish rainfall figures indicate the following: Trelawny recorded the highest with 147% or 135mm. St. James was next recording 100% or 67mm. The other parishes which recorded rainfall below their 30-year mean figures were; St. Ann with 94% or 100mm, Westmoreland with 86% or 60mm, St. Catherine with 64% or 34mm, St. Elizabeth with 61% or 61mm, St. Mary also with 61% or 110mm, Manchester with 57% or 35mm, Hanover with 50% or 47mm, Portland with 46% or 160mm, St. Thomas with 41% or 39m, KSA with 32% or 22mm and Clarendon with 21% or 10mm. (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 few years. 3

4 Climate Branch January 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 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 October 2015 to January 2016 Parishes Oct/Nov Nov/Dec Dec/Jan 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 October 2015 to January 2016 Table 3 is calculated as follows: Values = {(Month 1 + Month 2) / (Normal month 1 + Normal month 2)} x 100 4

5 Climate Branch January Drought Assessment St. Mary recorded a second bi-monthly period with normal drought conditions. KSA joined St. Mary this bi-monthly period in also recording normal drought conditions. All other parishes were above drought conditions. FORECAST Precipitation Outlook for the period February to April 2016 As we complete the first month in the dry season for Jamaica, the rainfall outlook for February to April 2016 indicate that areas of eastern and western parishes are likely to be drier than normal with warmer days. Constant drought monitoring will therefore be important especially for eastern and western parishes to ensure that our critical and sensitive sectors such as agriculture which depends heavily on rainfall can plan accordingly to handle the possible shortfall and damage to crops. VERIFICATION OF MODELS For the same period last year the models predicted below normal to near normal rainfall for most sections of the island. A comparison with actual conditions indicates that most parishes recorded below (less than what is expected) or near normal (expected) rainfall for the three-month period February to April. 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 January Table 4. Climate Predictability Tool (CPT) Station Outlook- FMA 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. Cath) 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. Cath) Jamaica

7 Climate Branch January Summary Most parishes received below-normal rainfall for the month of January 2016, with Jamaica receiving 63% of its normal rainfall. St. Mary and KSA were the parishes recording drought conditions, while other parishes were above drought conditions. Based on the precipitation outlook for the period February to April 2016, some eastern and western parishes are at the greatest chance of receiving below normal rainfall. With KSA and St. Mary having already returned to drought conditions, the situation could worsen in these parishes. Meanwhile, sections of northern St. James and Trelawny along with southern St. Elizabeth could be at risk of returning to drought conditions again before the end of the dry season. 7