March 2018 Introduction This rainfall summary is prepared by the Climate Branch of the Meteorological Service, Jamaica. The Meteorological Service maintains a network of approximately one hundred and seventy (170) rainfall stations located across the island. Rainfall is usually read at 7:00 a.m. and reported for the previous 24 hours. These readings are done by a cadre of paid but mainly voluntary dedicated observers. current thirty-year (1971-2000) 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 as seen in Figure 1 below. General Jamaica s bimodal rainfall pattern consists of two peak periods with higher values of rainfall and corresponding periods of lower rainfall. 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 but deviations from this pattern do occur year to year. Figure 1: Precipitation Pattern from 1971-2000 for Jamaica. A comparison of the old 30-year mean (1951-1980) with the 1971-2000 mean by the Meteorological Service has shown that the island s rainfall patterns and values have not changed significantly for the Prepared by the Climate Branch Meteorological Service, Jamaica 65 ¾ Half Way Tree Road Kingston 10 Telephone: 929-3700/3706 Email: datarequest@metservice.gov.jm
HIGHLIGHTS FOR MARCH Eight of thirteen parishes received below-normal rainfall. Some southern parishes have experienced significant decreases in rainfall percentages compared to the previous month. Some northern areas are still experiencing wet conditions. Near-normal to above-normal rainfall is forecast for the island, during the early wet season. Parish Mean Rainfall and Comparison with 30-YR Averages Parishes KEY 2018 2017 MAR MAR MAR % OF 30 YR NORMAL 30 YR NORMAL (1971-2000) 2018 2018 2018 JAN FEB MAR Hanover HAN 63 104 104 224 29 61 Westmoreland WES 76 110 88 162 60 86 Manchester MAN 89 179 91 171 23 98 St. Elizabeth STE 103 167 99 165 69 104 Clarendon CLA 29 109 56 209 98 52 St. Catherine STC 28 97 61 222 179 47 Trelawny TRE 71 104 61 278 79 117 St. James STJ 57 114 60 482 107 94 St. Ann STA 79 196 71 428 165 111 St. Mary STM 113 150 108 208 117 105 Portland POR 212 580 209 291 112 101 St. Thomas STT 28 123 68 256 115 41 Kgn. & St. And. KSA 28 108 67 215 84 43 Jamaica JAM 75 165 88 264 96 86 Table 1: Parish Mean Rainfall and Comparison with 30-YR Averages 2
Rainfall Assessment For March 2018, eight (8) of thirteen (13) parishes 1 recorded below-normal rainfall while, the other five (5) parishes recording above-normal rainfall. It was noticeable that all parishes received less rainfall in March 2018 when compared to March 2017, even for those parishes which recorded 100% or more of their 30-year mean monthly rainfall. Eight (8) of thirteen (13) parishes have recorded 2 consecutive months with deceasing rainfall percentages, however, three of these parishes still recorded 100% or more of their monthly means. The rainfall percentages in March were down significantly for 3 of the 8 parishes when compared to those of February. Overall, the island s average rainfall for March was 75 mm, which is 90 mm less than that received a year ago, and which corresponds to 86% of the 30-year (1971-2000) monthly mean value.the accumulated rainfall for Jamaica for the first quarter of 2018 was 154% of what is expected, thanks to the above-normal rainfall received in January. On the parish level, St. Thomas, Kingston & St. Andrew and St. Catherine, recorded rainfall that were less than 50% of their 30-year March monthly means. Across the island, the parish percentages of the 30-year monthly means were from 41% for St. Thomas to 117% for Trelawny. Fig.2. Distribution of Jamaica s Rainfall for March 2018 1 Note that Kingston and St. Andrew (KSA) are combined and reported as one parish. 3
Fig.3. Thirty-year (1971-2000) Mean Island Rainfall for March Drought Conditions Meteorological Drought Methodology and Index The Standardized Precipitation Index (SPI), developed by T.B. McKee, N.J. Doesken, and J. Kleist in 1993, is a tool used to monitor drought conditions based on precipitation. The SPI can be used to monitor conditions on a variety of time scales ranging from a 1-month to 12-months. This temporal flexibility allows the SPI to be useful in both short-term meteorological, agricultural and long-term hydrological applications by providing early warning of drought and for making assessments on the severity of a drought. The Meteorological Service, Jamaica (MSJ) calculates an observed SPI Drought Index (see Table 2) using a 2-month time interval. 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. 4
Parish Drought Assessment Observed SPI for December2017 to March 2018 Parishes Dec/Jan Jan/Feb Feb/Mar Hanover 1.52 0.74-0.89 Westmoreland 1.01 0.69-0.29 Manchester 1.84 0.43-0.73 St. Elizabeth 1.99 0.83 0.09 Clarendon 1.00 1.60-0.04 St. Catherine 1.12 1.90 0.31 Trelawny 2.17 1.89 0.06 St. James 2.64 2.47-0.05 St. Ann 2.46 2.95 0.71 St. Mary 0.70 1.91 0.54 Portland 2.54 2.75 0.38 St. Thomas 1.84 1.84 0.11 Kingston & St. Andrew 0.09 0.22-0.45 Table 2: Parish SPI for December 2017 to March 2018 SPI Value Category SPI Value Category 0.00 to -0.50 Near Normal 0.00 to 0.50 Near Normal -0.51 to -0.79 Abnormally Dry 0.51 to 0.79 Abnormally Wet -0.80 to -1.29 Moderately Dry 0.80 to 1.29 Moderately Wet -1.30 to -1.59 Severely Dry 1.30 to 1.59 Severely Wet -1.60 to -1.99 Extremely Dry 1.60 to 1.99 Extremely Wet -2.00 or less Exceptionally Dry 2.00 or more Exceptionally Wet Table 3: Severity Classes of the SPI Drought Index Discussion Based on the SPI figures for the February-March period, all 13 parishes have shown decreases in their SPI values, with rankings ranging from moderately dry to abnormally wet conditions. Nine (9) of thirteen (13) parishes had rankings in the near-normal category. Over the last two bimonthly periods St. James and St. Ann have experienced significant decreases in their SPI values, with St. Ann s rankings moving from exceptionally wet to moderately wet, while, that for St. James moved from exceptionally wet to near-normal (dry). Most areas of Hanover, 5
Westmoreland and Manchester were showing drier conditions when compared to other parishes, with Hanover s moderately dry ranking being the lowest of all the parishes. These decreases in SPI values were the result of the lower rainfall amounts received during the last 2 months in these parishes and were reflected in areas showing less wetness, when compared to other parishes (see Fig 4 below). Reductions in wetness were noticeable across sections of most northern parishes from St. James to Portland, as well as for sections of Clarendon, St. Catherine and St. Thomas, when comparing the last 2 drought index maps. Fig.4. Drought Analysis for February-March 2018 Precipitation Outlook: April to June 2018 During the next three months (April-June), the forecast models are indicating that Jamaica should receive near-normal to above-normal rainfall, during the early rainfall season. On the parish level, Manchester, Westmoreland and Hanover which were experiencing dry conditions could receive above-normal rainfall during the period. Above-normal temperatures are still expected across the island. Table 4 below shows the precipitation outlook for selected stations across Jamaica as analysed by the Climate Predictability Tool. For the April to June 2018 period, ten (10) of seventeen (17) stations are indicating higher probabilities for above-normal rainfall, three (3) stations are 6
indicating higher probabilities for normal rainfall and the remaining four (4) stations showing higher probabilities for below-normal rainfall. Stations Parishes Below (B) % Normal (N) % Above (A)% Beckford Kraal Clarendon 40 30 30 Mount Peto Hanover 20 30 50 Manley Airport Kingston 40 30 30 Lawrence Tavern Kingston 33 34 33 Suttons Manchester 30 30 40 Shirley Castle Portland 40 30 30 Cave Valley St. Ann 20 35 45 Tulloch Estate St. Catherine 40 30 30 Worthy Park St. Catherine 25 35 40 Y.S. Estate St. Elizabeth 30 30 40 Potsdam St. Elizabeth 33 34 33 Sangster St. James 25 35 40 Serge Island St. Thomas 30 30 40 Hampstead St. Mary 33 34 33 Orange Valley Trelawny 20 35 45 Savanna-La-Mar Westmoreland 25 35 40 Frome Westmoreland 15 35 50 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 Table 4: Precipitation Outlook for Selected Stations for April to June 2018 Forecast Verification For the same period last year, April-June 2017, the models under-performed, with accuracy in the range of 30-45 percentage points. This was due to very weak signals from the oceans during 7
this transition period. The initial forecast indicated that rainfall was likely to be below-normal for the period; however, most stations recorded above-normal rainfall amounts. Summary Eight of thirteen parishes recorded rainfall that were below their respective 30-year (1971-2000) monthly means, another five parishes recorded above-normal rainfall. Overall, Jamaica recorded below-normal (86%) rainfall in March. On the parish level, St. Catherine, Kingston & St. Andrew and St. Thomas have recorded consecutively lower percentages of monthly rainfall over the last 3-months, as well as recording less than 50% of their 30-year monthly mean rainfall for March. With Hanover and Westmoreland receiving less than expected rainfall in March, this has resulted in conditions becoming drier in sections of these parishes. Although Manchester received 98% of its 30-year mean rainfall for March, dry conditions were still being experienced across most of the parish. These drier conditions are also reflected in the decreases in the SPI values for these three parishes when comparing the last three bi-monthly periods. The projection over the next three (3) months is for near-normal to above-normal rainfall across the island. On the parish level, Manchester, Westmoreland and Hanover which were experiencing dry conditions could receive above-normal rainfall during the period. 8