Jim Andrews

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Jeddah Flood, Australia Cyclone

February 1, 2011; 12:04 PM ET

JEDDAH FLOODS LIKE THOSE IN 2009

As I understand from media reports, rainfall of 11 cm triggered tragic flooding in Jeddah, much as it did in November 2009.

Unfortunately, I did not have access to the weather data from Jeddah or nearly all of the Arabian Peninsula, so I do not have any good idea what was happening.

I believe it is enough to say that a strengthened subtropical jet stream ferried a few waves of low pressure across northeastern Africa and Arabia to Iran. Indeed, this was the approximate weather setting for these areas during the latter half of January.

But what happened in Jeddah on Jan. 26, the day of the flash flood?

Again, it is not possible for me to say with any detail or confidence. My guess is that one of the low pressure waves picked up enough Red Sea moisture to destabilize, thereby triggering the deep convective clouds that are thunderstorms.

The Red Sea is far too small to afford its shores any dependable rainfall. However, it is big enough to instigate infrequent, localized cloudbursts.

I believe that a careful study of area climate data (if indeed such data exists somewhere) would show that there are "hot spots" along and near the Red Sea shore that get flooding cloudbursts more often than the background frequency for the region.

ASPECTS OF FLASH FLOODING IN A DESERT CITY

Rainfall of 11 cm amounts to more than the climatological mean yearly rainfall for a year in Jeddah. This also happened within a matter of hours, if I understand correctly.

It does not matter how little rain normally falls in a given spot. When heavy rain does fall, it yields runoff that must flow somewhere before reaching a river, sea or somehow soaking into the ground.

Deserts often have hard surfaces that do not allow rainwater to soak into the ground, and this is certainly true of any modern city like Jeddah.

If there is no natural path, wide and deep enough to handle the flow of runoff, that runoff will escape its confines. If it happens in a city, then highways and buildings will inevitably lie in the path of flood.

Let us make this an arithmetic problem. Take a rainfall of 10 cm over an area 10 km by 10 km (smaller than greater Jeddah). Rainfall of 10 cm gives one cubic meter of water for each 10 square meters of land surface.

Thus, on the 100 urban square km, 10,000,000 cubic meters (if my arithmetic were to be trusted) of water would be released by the storm. Put another way, if all that water were to flow into 10 percent of the city area, it would cover it 1 meter deep. Make that 5 percent of the city area, and it would be over the head of most people (2 meters deep).

All of this is based on an assumption of 10 cm rainfall spread over 100 square km. The actual area may have been much bigger including the desert hills east of Jeddah. Readers may recall that the worst of the 2009 flooded was triggered by rainfall east of the city. Runoff then swept into the heart of the city along wadis, some of them built over with homes, businesses, streets and highways.

QUEENSLAND UNDER DEADLY THREAT FROM CYCLONE YASI

This one is nothing like Anthony, the moderate cyclone that landed at the end of last week, and Tasha, also a moderate cyclone that kicked off the devastating Queensland flooding late in December 2010.

T.C. Yasi is a Category 4 (Saffir-Simpson) hurricane over the northern Coral Sea at this writing (1800 UCT, Tuesday). Highest sustained winds are 120 knots, or above 220 km/h, according to the JTWC.


T.C. Yasi on Feb. 1, 2011 (Australia BOM).

With forward speed of 17 knots, or 32 km/h, towards the west-southwest, T.C. Yasi should storm ashore in north-central Queensland within less than 24 hours, or late on Wednesday, local time.

Yasi could sustain its major hurricane intensity right up until landfall, which is what happened in March 2006, when T.C. Larry hammered Innisfail.

However, Yasi is bigger than Larry, so the path of its destruction would be wider.

This path should cross the coast between Cairns and Townsville. The JTWC favor a path near to Cairns, maybe even at Innisfail yet again.

MAJOR IMPACT FAR BEYOND THE QUEENSLAND COAST

I believe that flooding rains will live on well after Yasi has lost its cyclone status and thus the lash of its high winds.

As the center of its remnant low tracks west and south and interacts with one or more southern cold fronts, its flooding rain could reach a significant area of Northern Territory, South Australia, New South Wales and Victoria. For that matter, these could even reach Tasmania.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com

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