By: J Malherbe, R Kuschke

Transcription

1 By: J Malherbe, R Kuschke Contents Summary... 2 Overview of conditions over South Africa... 3 Rainfall... 3 Maximum wind speed... 4 Latest status of indicators of global climate... 5 SSTs... 5 El Niño-Southern Oscillation (SSTs and SOI)... 6 ENSO probabilistic forecast... 7 Southern Annular Mode (SAM)... 7 Overview of significant weather conditions during the coming week globally (Sourced from the output of Global Coupled Climate Models, published online)... 8 Tropical Cyclones... 8 Conditions over the USA, Europe and Australia (28 July 3 August)... 9 USA... 9 Europe Australia Overview of expected conditions over South Africa during the next few days Significant weather events (28 July 3 August) Conditions in main agricultural production regions (28 July 3 August) Daily rainfall and temperatures across South Africa Possible extreme conditions - relevant to agriculture

2 Summary Unstable conditions continued over much of the country during the monitoring period until Saturday the 25 th after which settled cool conditions started dominating. After being semi-stationary over the southwestern parts of the country for a few days, a deep cut-off low started moving eastwards during the second half of the week, resulting in widespread thundershowers with small hail in places over the central to eastern and southern parts. Heavy falls were recorded in places along the southern and eastern coastal belts due to a strong ridging high-pressure system to the south. The ridge also resulted in strong southeasterlies over the southwestern parts of the winter rainfall region by Tuesday and Wednesday. As a cold front at the surface accompanied the low pressure system in the upper air, snow occurred over many of the southern high-lying areas and Drakensberg in the east. Widespread rain along the eastern coastal belt brought some relief to drought-stricken areas in KZN, but totals further inland were relatively low lessening the potential positive impact on water resources. Conditions this coming week will be much more stable over the interior with the upper air fairly flat and with relatively low temperatures. The westerlies will be relatively far north, a reflection of conditions over much of the Southern Hemisphere, as the Southern Annular Mode will become negative. Precipitation (rain or snow) is expected especially over the southwestern and western parts of the winter rainfall region with a strong cold front moving in by tomorrow. Even though cold air will invade the entire country by Thursday/Friday, anticyclonic conditions over the subcontinent will keep it mostly dry over the interior, except for the southern and southwestern parts where frontal activity and some upper air support may result in snow over high-lying areas. During winter, similar years in the decadal rainfall cycle tended to be somewhat more unstable with precipitation over the interior during periods equivalent to 5-10 May, 1-13 June and around 30 June 2015 and around 19 July. Going forward, the periods with a stronger tendency for more unstable conditions and possible precipitation over the interior include the period from early to middle August (see graph below for May to August 2015, days of the months indicated on the x-axis, noting that the observed (bar graph) is valid until 28 July). Similar years were associated in the past with normal to below-normal precipitation over the western winter rainfall region, mainly a result of a sharper seasonal rainfall curve (normal rain in mid-winter and less during autumn and spring). The 2015 El Niño continues to strengthen according to both atmospheric and oceanic indices. Circulation patterns towards summer will be monitored closely as the negative impact of ENSO on summer rainfall may be of concern. 2

3 Overview of conditions over South Africa Rainfall Widespread precipitation occurred over the central, southern and eastern parts. Heaviest falls occurred along the southern and eastern coastal areas, where several stations recorded daily totals in excess of 50mm (grey circles) or even in excess of 100 mm (black circles). 3

4 Maximum wind speed Parts of the winter rainfall region as well as the central pats of the country experienced maximum wind speed in the moderate gale category. 6 Strong breeze km/h Large branches in motion. Whistling heard in overhead wires. Umbrella use becomes difficult. Empty plastic bins tip over. 7 High wind, 8 moderate gale km/h Whole trees in motion. Effort needed to walk against the wind. Gale km/h Some twigs broken from trees. Cars veer on road. Progress on foot is seriously impeded. 9 Strong gale km/h Some branches break off trees, and some small trees blow over. Construction/temporary signs and barricades blow over. 10 Storm km/h Trees are broken off or uprooted, structural damage likely. 4

5 Latest status of indicators of global climate SST (Sea Surface Temperature) anomalies in the Central Eastern Equatorial Pacific are above normal and increasing, indicating ENSO Warm conditions, SSTs in the Southern Hemisphere still indicate the effect of a positive SAM, usually associated with normal to above normal rainfall towards late summer while The SAM has reached high positive values, but is decreasing and expected to become negative SSTs SST map: NOAA Climate Prediction Centre - SSTs across the equatorial Pacific are above average. The 2015 El Niño continues to develop. Weakened (or reversed) trade winds have resulted in further warming over much of the tropical Pacific Ocean. All key ENSO ocean monitoring areas have been more than 1 C above average for 10 successive weeks two weeks longer than the record in The eastern tropical Pacific is now at or exceeding +2 C. In the atmosphere, the past week has seen the Southern Oscillation Index (SOI) drop to around 20, the lowest values of the event so far. - Australian Bureau of Meteorology- Anomalously low SSTs visible towards the Southern High Latitudes and relatively high SSTs over the Mid Latitudes reflect the positive tendency in the SAM, usually associated with above-normal rainfall over the summer rainfall region. A positive SAM may also be associated with below-normal rainfall over the winter rainfall region during most of autumn and parts of winter. 5

6 El Niño-Southern Oscillation (SSTs and SOI) Australian Bureau of Meteorologyhttp:// SST anomalies over the central-toeastern Pacific continue to increase (+148) as average for the last week, indicating El Niño conditions. The anomalies have increased and are the highest since the 2009/10 warm event. Australian Bureau of Meteorologyhttp:// The 30-day moving average of the SOI (-14.7) is an indication of atmospheric response to warm ssts over the Equatorial Pacific. (The Walker circulation refers to the location of upward and downward large scale air flow along the equator. When the SOI becomes negative, it is an indication that one of the ascending limbs of this circulation pattern, usually located over Africa, may be shifted off the continent, usually leading to lower rainfall over southern Africa). 6

7 ENSO probabilistic forecast The consensus probabilistic forecast by the Climate Prediction Center (CPC) and International Research Institute for Climate and Society (IRI) indicates a 100% chance for the occurrence of warm ENSO conditions during Austral autumn and winter. During late June through mid-july 2015 the SST was at a moderate El Niño level. All atmospheric variables support the El Niño pattern, including weakened trade winds and excess rainfall in the east-central tropical Pacific. The consensus of ENSO prediction models indicate continuation of moderate, and likely becoming strong, El Niño conditions during the July-September 2015 season in progress. Further strengthening is likely, with the event lasting into early IRI - CPC ENSO outlook - Southern Annular Mode (SAM) The Annular Mode Website - The SAM (Southern Annular Mode / Antarctic Oscillation) is an indication of the position and strength of pressure anomalies over the Southern Hemisphere. When this index is positive (negative), it is usually an indication of highpressure (low-pressure) anomalies over the mid-latitudes. As positive anomalies over the mid-latitudes may result in strong subtropical high pressure regions over the oceanic regions surrounding South Africa, the SAM is positively (negatively) related to South African summer (winter) rainfall. Significant rainfall events in the past have been associated with strong decreases or increases in the SAM. The SAM is currently positive, but expected to decrease drastically over the next few days while a relatively zonal circulation pattern and a northward displacement of westerlies will favor the western parts of the winter rainfall region for precipitation. 7

8 Overview of significant weather conditions during the coming week globally (Sourced from the output of Global Coupled Climate Models, published online) Tropical Cyclones Currently, 2 active tropical systems are being tracked: o Located over the Eastern Tropical Pacific, two depressions are expected to track westwards, not affecting the Americas, but possibly Hawaii. Weather Underground - Tropical Cyclone Centre La Reunion - (CIMMS) - Tropical Cyclone Group - In any event of a tropical system posing a threat to southern Africa, the South African Weather Service ( will issue the relevant warnings. 8

9 Conditions over the USA, Europe and Australia (28 July 3 August) USA Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) - For the next 2 days: There is a Slight Risk of severe thunderstorms Tuesday afternoon and evening across parts of the Midwest, from southeastern NE and extreme northeastern KS across northern MO and much of IA and into extreme portions of southeastern MN and southwestern WI. A Marginal Risk of severe thunderstorms extends from southwestern KS to northern MN and Lake Superior, as well as across northern New England

10 This U.S. Drought Monitor week saw improvements in the Southwest as overall conditions continued to improve across parts of the region. Improvements primarily were focused upon west-central New Mexico and northern Arizona where most long- and short-term indicators have pointed toward improvements, although reservoir storage levels in various drainage basins remain below normal. During the weekend, residual moisture associated with Hurricane Dolores fueled showers and thunderstorms across southwestern California and western Arizona leading to locally heavy rainfall accumulations and flash flooding. Despite well-above-average precipitation in southern California during the past 90 days, recent rainfall has had little impact on the overall drought situation in the state. In the Pacific Northwest, above average temperatures and precipitation deficits continue to mount across the region with growing concern about potential crop losses in central and eastern Washington. Moving eastward, short-term precipitation deficits led to slight deterioration of conditions in the northern Plains while locally heavy rainfall was observed across drought-free areas of Illinois, Indiana, Ohio, Kentucky, and Missouri. In the Southeast, conditions continued to deteriorate across portions of Alabama, Georgia, and South Carolina where excessive heat and lack of rainfall dried soils and reduced streamflows. According to the National Centers for Environmental Information (NCEI) climatological rankings, the contiguous U.S. average temperature for June was the second hottest in the observational record ( ). On a state level, California, Idaho, Oregon, Utah, and Washington all experienced their hottest average-temperature Junes on record since United States Drought Monitorhttp://droughtmonitor.unl.edu This information is relevant for the week ending 21 July

11 Europe Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) 11

12 Australia Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) 12

13 Overview of expected conditions over South Africa during the next few days Significant weather events (28 July 3 August) A strong cold front will move into the western parts of the country by tomorrow. A westerly flow will result in most of the precipitation being confined to the western winter rainfall region, with low temperatures resulting in snow over parts of the winter rainfall region and the high-lying areas of the southern interior as the system moves eastwards during the week. Due to the surface flow and relatively flat upper air, conditions over the interior will remain dry, but a sharp drop in temperatures is expected by Thursday and Friday as cold air invade the interior from the southwest. Some rain will also occur along the southern and eastern coastal areas (moving west-east as the week progresses) and adjacent interior, associated with the front and an onshore flow due to some ridging of the Atlantic Ocean Anticyclone over and around the country during the week. Conditions will remain cool to cold over most of the country as a high pressure system becomes established over the eastern parts and towards the east by the weekend, but precipitation should be minimal. A northerly flow will however develop over the central to western parts during the weekend, resulting in an increase in temperatures over those parts. With cold air spreading across the country during the week, temperatures are expected to be below normal on average over most parts. Maps for precipitation and temperatures for the next few days are on the next page. Conditions in main agricultural production regions (28 July 3 August) Maize production region: Sunny and cool conditions will dominate. After some warming today and tomorrow, temperatures will drop sharply on Thursday with the cold front moving through. Swartland, Cape Wine Lands and Ruens: Widespread rain should occur over the entire region from tomorrow until Thursday. Westerly to northwesterly winds will dominate, resulting in highest falls over the western and northwestern parts of the region (including the Swartland) while lower falls are expected over the Ruens and Garden Route. Totals in the west may range between 20 and 50 mm. Very low temperatures are expected tomorrow and Thursday, enhancing the probability for snow over high-lying areas. Warmer conditions may set in during the weekend as a northerly flow may start dominating. 13

14 Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) 14

15 Daily rainfall and temperatures across South Africa Daily rainfall (mm) for July 2015 Precipitation in the form of rain or snow will occur over the southwestern to southern parts during the next few days from tomorrow in association with the cold front moving in from the southwest. 15

16 Daily minimum temperature ( C) Minimum temperatures will decrease over many areas as the cold front moves across during the week. 16

17 Daily maximum temperatures ( C) Maximum temperatures will also fall during the next few days with the frontal activity over the country, with lowest temperatures over the central to southern and southwestern areas, especially the high-lying regions. 17

18 Possible extreme conditions - relevant to agriculture The South African Weather Service issues warnings for any severe weather that may develop, based on much more information (and in near-real time) than the output of one single weather model (GFS atmospheric model - Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) considered here in the beginning of a week-long (starting 28 July) period. It is therefore advised to keep track of warnings that may be issued by the SAWS ( as the week progresses. According to current model projections (GFS atmospheric model) of weather conditions during the coming week, the following may be deduced: Snow is possible over the high-lying parts of the winter rainfall region by Wednesday and Thursday spreading to the southern high-lying areas and southern Drakensberg by Thursday and Friday Temperatures over the interior will drop fairly sharply by Wednesday and Thursday over the entire country with frost possible over the central to southern parts, especially the high-lying areas Temperatures will be low, most of the time, over the southern parts especially the high-lying areas A northwesterly flow over the eastern and northeastern parts by Thursday and again later (during the weekend) may enhance the potential for the development and spread of wild fires 18

19 Sources of information Rainfall, temperature and wind maps over South Africa for the past week: Agricultural Research Council - Institute for Soil, Climate and Water (ISCW) Climate Data Bank. Data recorded by the automatic weather station network of the ARC-ISCW. Vegetation condition maps: Coarse Resolution Imagery Database (CRID), ARC-ISCW. Information related to: ENSO, IOD and SOI: Australian Bureau of Meteorology - Climate Prediction Center - International Research Institute for Climate and Society- Information related to the SAM: The Annular Mode Website - SST map: NOAA Climate Prediction Center - Daily conditions over South Africa: CSIR NRE (National Resources and the Environment) CSIR NRE produces forecasts on an experimental basis, doesn t guarantee the accuracy of the daily forecasts and cannot be held accountable for the results of decisions taken based on the forecasts Tropical cyclone/hurricane/typhoon information: Weather Underground - Cooperative Institute for Meteorological Satellite Studies (CIMMS) - Tropical Cyclone Group - Tropical Cyclone Centre La Reunion - Information on drought conditions over the USA: NOAA National Weather Service - United States Drought Monitor - Precipitation and temperature outlooks for the coming week: Center for Ocean-Land-Atmosphere Studies (COLA) and Institute of Global Environment and Society (IGES) COLA and IGES make no guarantees about and bear no responsibility or liability concerning the accuracy or timeliness of the images being published on these web pages. All images are generated by COLA and do not represent the actual forecasts issued by the National Weather Service. These products are not a substitute for official forecasts and are not guaranteed to be complete or timely. The underlying data are the direct product of the various operational forecast models run by the National Centers for Environmental Prediction, National Weather Service, NOAA and are supplied without interpretation or correction. 19