Posts Tagged ‘eye of the storm’

Katia storms across the North Atlantic Basin – Tracking toward Scotland with ETA 12 UTC Monday

Katia has been called upon much earlier than expected, with the average season tracking 12 tropical storms in total. Currently a category 2 hurricane on the Saffir-Simpson scale, and with wind strengths of 96 to 110mph, she is expected to swing toward northern Britain by Monday 12th September as an extra-tropical storm. The deep and compact low pressure system, with a central pressure of around 960mb, is likely to give rise to severe gale to storm force 10 winds as it charges rapidly westward. According to the NOAA National Hurricane Center in the USA there is currently a 30 to 40% risk of storm force winds wreaking damage in the Western Isles and the North of Scotland.

Weather Logistics Ltd anticipates that the remnants of Katia will reach the UK from Monday morning onward. It will be combined with wave heights of up to 12 metres and storm surges on the north and west coasts, particularly on the south-east quadrant of the system. Throughout Scotland and Northern Ireland severe gales may also cause widespread structural damage to buildings, with estimated risks of 20 to 30% that sustained winds will reach storm-force and with gale force winds or stronger very likely.

Hurricane Katia storms toward the British Isles during mid-September

An intense tropical storm in the Atlantic basin is called a hurricane, often possessing an “eye” of clear-sky that can be viewed from space-borne instruments. The tight isobars that pack around these storms are characterised by sustained wind speeds above 76 mph. Hurricanes are a prominent feature of the north-west Atlantic basin and Gulf of Mexico, where tropical disturbances gather latent heat energy from clusters of thunderstorms that infrequently make land-fall in the USA during the early summer to late autumn season. Initially forming as localised depressions, these cloud masses move northward into the sub-tropics where they can spin-up into deep low pressure systems. They are often steered in a north-westerly direction by the sub-tropical jet stream where they are fed by moisture over warm sea surface temperatures, developing in their viciousness and in their girth, with compact radial pressure gradients. In addition, spiralled bands of thundery rain develop around these tropical storm disturbances, twirling outward from the central low pressure. The resulting weather conditions consist of tornadoes and convective instability, causing gusty winds combined with thunderstorms and heavy blustery showers.

Our final seasonal weather prediction for autumn 2011 is currently on Amazon Kindle and by subscription (using the links on the right of this page). Although we expect stormy and wet weather for the British Isles during September, we anticipate that the general weather pattern will settle down dramatically during the latter part of the season with high pressure dominating from late September onward. Early signs from the global forecasting system (GFS) indicate that sunny but cool conditions will begin to set in by mid-September, with the jet stream slipping southward blocking the passage of Atlantic storm systems. From the mid-autumn period onward Weather Logistics Ltd predicts that hurricanes, and the remnants they leave as extra-tropical storms, will dissipate over the USA as they begin to track further southward.

For further details, please contact the Project Manager @ Weather Logistics Ltd


m. 07949187732

seasonal weather forecasts for the united kingdom and ireland

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Typhoon Yasi makes clockwise landfall

One aspect of the cool waters of the La Niña is a tendency toward the inter-tropical convergence zone (ITCZ) to migrate southward around Indonesia and Australia, leading to an overall rise in humidity and a tendency toward wetter conditions than the climate average. The ITCZ is a region of strong lower-level convergence that results from general ascent of warm moist tropical air. Winds draw north-easterly and south-easterly trade winds into a discontinuous band of high tropical clouds characterised by intensive thunderstorms that surround the Equatorial regions. During La Niña periods, the ITCZ is positioned much farther south in the East Pacific Ocean than in the West, where high pressure conditions are currently dominating the weather of Chile. The ITCZ migrates northward and southward in a seasonal cycle, bringing more thunderstorms, heat and humidity to higher latitudes in the south during the southern-hemisphere summer that peaks from December to February. When thunderstorms group together to form larger systems over warm waters (typically above 28C), they transport sufficiently large amounts of heat and moisture into the upper atmosphere to form deep regions of low pressure known as tropical storms. Under the right meteorological conditions these may develop into tropical cyclones as they track toward higher latitudes. Localised regions of warm and moist air result in further instabilities in the atmosphere, through latent heat exchanges between the ocean surface and atmosphere, that drive the vertical motions within a heat engine of a tropical cyclone.

Below is a Geo-Stationary satellite image of Typhoon Yasi, just off the coast of Australia at 1500 UTC today (2nd February, 2011). The colours indicate the location of the ocean (blue) and the land (light green) as viewed in a 12 micro-metre window channel. At this frequency the satellite instrument views the surface and cloud-tops. The grey-scale indicates the temperature of the cloud, where pale grey colours indicate cooler surfaces than black or dark grey.

Hurricane Yasi makes landfall on Australia coast
Hurricane Yasi (MTSAT 1500, 02.02.11). Src: NEODAAS

Typhoon Yasi is associated with a circular region of high clouds with cold tops, with a central “eye”. The visibility of the eye is only possible in super-Typhoons where maximum winds are strong and extremely destructive to most infrastructure. Storms that possess a well defined eye are usually categorised as 4 or 5 (on a hurricane scale). The lowest surface pressure of a Typhoon is experienced at the eye wall, on the outer circumference of the eye. Paradoxically subsiding (sinking) air at the eye causes a local region of relatively higher pressure at the surface, a lull in surface winds and clear-skies. The winds of a Typhoon are typically strongest around the edge of the eye-wall. The bands of high clouds that circulate the central low pressure in a clockwise direction (visible on the image above) are associated with intensive rainfall, which can lead to extreme flooding.

When the Typhoon makes westward landfall, driven by upper level easterly motion, the south-west quadrant of the system (close to the eye wall) experiences much of the intensity in the winds. This is because the clockwise motion of the airflow and the easterly track of the storm combine in the same orientation. Similarly the north-west quadrant usually experiences the lowest intensity in wind strength as the vectors oppose.


02 2011