Blog by Jo Farrow
Issued: 21st August 2020 18:20
Updated: 22nd August 2020 09:26

Hurricane season update - Fujiwhara Effect and the trouble with warnings

Weather forecasting is tricky. We are looking into the future, with the help of military-grade supercomputers but it is not an exact science and the further ahead you look the more uncertainty grows. Developments on the other side of the Atlantic are spiking interest with the possibility of two potential hurricanes interacting next week.

Warning procedures need to be clear and simple. We’ve just seen Storm Ellen affecting Ireland and then Britain with a series of weather warnings. The Name our Storms project was put in place to highlight severe weather and join up national warning messages to create louder communication, to keep people safe and assist their daily decisions.

TS LAura and TD 14 atlantic hurricane

The Atlantic hurricanes and cyclones around the world are given names to help identify storms and clarify the information around them. However, each warning system or cyclone centre has its own nuances and needs some understanding. Often familiarity helps. Using the same language, colours and criteria help the public and other forecasters to share information and understand what might happen. Alongside how to discuss and display any uncertainty within the forecast.

Warnings need to be useful and timely but also impart information about likelihood, the possible impacts and the uncertainty. Coming up in the Gulf of Mexico is a forecasting nightmare combination of these.

For example, the cone of uncertainty around a cyclone path. Over time it gets wider. The text in the National Hurricane Centre’s discussion area gives more insight into the different model outputs and possible leanings west or east. With talk of shear, dry air and the sea surface temps. There could be interactions with land and other synoptic features.

Fujiwhara Fujiwara effect

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This is the situation facing forecasters in this coming week: not just one tropical cyclone heading towards the Gulf of Mexico but two, both of which could make hurricane status. This hasn’t been recorded before, although there have been two tropical cyclones, just not both at hurricane strength.

So, we have the uncertainty of path from various models, interactions with islands, the Yucatan peninsula and then the US coast along with the atmospheric and oceanic conditions. It is a complex picture and for a bit of excitement the possibility of the rare Fujiwhara Effect. (Fujiwara – American)

When you have two tropical cyclones moving close to each other they can interact, and their motion becomes linked. So, if there were two hurricanes nearby (less than 870miles) they will influence each other. Low pressures which we see in the Atlantic near the UK, extratropical lows, can also demonstrate this effect from an even greater distance apart.

A Japanese meteorologist, Dr Fujiwhara, found that two storms will sometimes pivot around a common centre point.

“the tendency of two nearby tropical cyclones to rotate cyclonically about each other”  "a binary interaction where tropical cyclones within a certain distance (depending on the sizes of the cyclones) of each other begin to rotate about a common midpoint. " NWS

The interaction can cause two cyclones to dumbbell around each other with an unusual motion. Their proximity can disrupt the environment for development, and one would usually weaken but it could stall and that brings the risk of persistent rainfall.

Examples of Fujiwhara Effect

“Hurricane Diane 1955- reached its peak intensity at Category 3 status on August 12 and remained at Category 3 status for three days until cooler air behind Hurricane Connie, a Category 1 hurricane (North Carolina) became entrained in Diane’s circulation. As a result, Hurricane Diane weakened to Category 1 strength. Floods in New England led to Diane becoming the first hurricane with $1 billion in damage, with about 200 people losing their lives.” NWS.  The quick succession of heavy rains from Connie then Diane was disastrous with the two almost appearing like one huge hurricane event for a few days in mid-August

NASA Irwin HIlary Fujiwara

July 2017 NASA – ‘Irwin and Hilary were a little unusual because they travelled so close to each other and interacted. The Fujiwara Effect radically changed the direction and speed of tropical storm Irwin's movement as their centres will sometimes begin orbiting. The larger tropical cyclone, Hilary in this case, often dominates the interaction. The smaller tropical cyclone, Tropical Storm Irwin, orbits around the larger tropical cyclone. Irwin slowed down and moved to the northeast and Hilary moved over colder waters and lost energy before the two could combine.’ Irwin’s track was radically changed and there was significant rainfall.

Fujiwara Effect Nancy Olaf

Image-  A strengthening Cyclone Olaf and a weakening Cyclone Nancy eye each other across the South Pacific February 15, 2005 NASA

What about TD14 (would become Marco) and TS Laura?

From current forecasts, these two cyclones could be in the northern part of the Gulf of Mexico Tuesday into Wednesday. We’ll have to see if they manage to reach hurricane status but in this bizarre year and with a forecast of an active Atlantic season from NOAA, you can’t dismiss anything.

The Atlantic hurricane season had an early start in mid-May with TS Arthur but only two Cat 1 hurricanes so far.

“The five named storms ties the record for the most named storms forming during the month of July, which was previous set in July 2005. The season has been considerably more active than average so far, as typically only 1 or 2 named storms form prior to August. So far in 2020, twelve named storms have formed, including 2 hurricanes -- Hanna and Isaias.” NHC

Tropical cyclone forecasts always have a fair amount of path uncertainty with the potential for big impacts. This set up although intriguing, will be very difficult. Quite something to watch if the storms do manage to strengthen and get close in the Gulf of Mexico.

More  -Complete guide to Atlantic Hurricanes-

Tropical Storm Laura in the eastern Caribbean 21st Aug
Tropical Depression 14 off the north coast of Honduras 

More discussion on the Netweather Forum

Tags: World Weather  Severe Weather

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