Elongated area of low pressure likely to bring higher-than-usual levels of rainfall to normally arid region

Unusual levels of rainfall are due to hit the southern Arabian peninsula this week, as a tropical wave – an elongated area of low pressure – progresses across the region, triggering a marked increase in convective activity across western Yemen and south-western Saudi Arabia this week. There is a risk of thunderstorms until Wednesday, with forecast rainfall totals reaching about 50mm for Al Hudaydah region in Yemen and the Jazan province of Saudi Arabia.

Although 50mm is not a particularly large rainfall total in most places, in this region it is enormous: Al Hudaydah typically records only 65mm annually, while Jazan averages about 150mm.

Orographic enhancement – when air rising over mountains or other upland terrain can invigorate or produce areas of heavy rain – along the Red Sea coastal escarpments further amplifies the risk of high rainfall totals here and this, combined with the arid surface conditions, brings a significant risk of flash flooding; runoff from upland rainfall is efficiently channelled into valleys, often toward more densely populated coastal settlements.

Hail and strong outflow winds are likely from these thunderstorms, with the outflow winds potentially producing short-lived but impactful intense dust storms owing to the dry desert terrain, leading to sudden reductions in visibility and localised transport disruption.

While the impacts are likely to be reserved for the Red Sea coast, there is a suggestion that moisture transferred from the Arabian Sea in association with this tropical wave could extend further inland, with the potential for rare rainfall within the Rub’ al-Khali (Empty Quarter) desert.

Meanwhile, Hurricane Erin tracked across the Caribbean over the weekend and is now progressing on a northward path following the US east coast. Erin underwent a rapid and significant intensification phase on Saturday, strengthening from category 1 to category 5, with US Air Force hurricane hunters recording sustained winds of 160mph.

Not only is Erin the first of the five named storms this hurricane season to reach hurricane status but it jumped straight to the strongest category of storms on the Saffir-Simpson scale.

Thankfully the category 5 status was short-lived, and Erin passed north of Puerto Rico and the British Virgin Islands on Sunday before adopting the gradual northward trajectory on Sunday night. While the Bahamas and the Turks and Caicos Islands have escaped the impacts of the hurricane, many islands over the coming days are expected to face large ocean swells, strong winds and heavy rainfall.

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Erin’s extratropical remnants may influence the UK during the late August bank holiday weekend, particularly on Monday, as the system accelerates north-eastwards and is steered by the jet stream. By this stage, Erin will have completed its extratropical transition – known as an ex-hurricane in name – and impacts are expected to be limited to periods of wet and windy weather across north-west Europe.

Source: www.theguardian.com