Intense Tropical Cyclone Bejisa

[Vorticity0123]

Just a quick post: a new tropical depression has formed NE of Madagascar.The system is forecast to move slowly southward to southeastward (in the long term) and impact La Reunion in about 5 days while intensifying to hurricane strength.

 

The JTWC forecasts the cyclone to reach 65 knots, before weakening as it recurves into the midlatitudes.

 

There is some uncertanity in when the cyclone will speed up to the southeast, as given from comparing JTWC to La Reunion's forecast).

 

I'll provide a more extensive post later today.

 

Sources:

http://www.meteo.fr/temps/domtom/La_Reunion/meteoreunion2/

http://www.usno.navy.mil/JTWC/

[Somerset Squall]

Tropical Depression 04 has become Moderate Tropical Storm Bejisa, with winds of 40kts according to MeteoFrance. Convection is persisting over the LLCC, with fragmented banding taking shape stretching from the south to the west quadrant of the storm. Bejisa is moving slowly as competing ridges result in very limited net steering currents. Bejisa should eventually head south as ridging to the east becomes more dominant, where it looks like it will threaten Mauritius and La Reunion.

Edited December 29, 2013 by Somerset Squall

[Vorticity0123]

Indeed, Bejisa has been organizing quite rapidly during the day. Last visible imagery shows that the cyclone is well-organized, as pointed out by Sommerset Squall. A nicely defined CDO (central dense overcast) completely covers the LLCC.

 

 

The organization of the cyclone also shows that the intensity could be higher than the 40 kt as analyzed by JTWC. Last CIMSS ADT satellite intensity estimate confirms this, with a final estimate of 53 kt. AMSU and SATCON satellite estimates are somewhat lower (40 and 39 kt respectively), but those were of 9 hours ago.

 

The intensity trend of ADT satellite estimates show a steadily increasing intensity associated with Bejisa. 

 

CIMSS ADT Satellite intensity trend

 

Wind analyses from earth.nullschool.net show that Bejisa has two inflow channels, a strong one from the north (very moist air from the equator) and one weak channel from the east (also plenty of moisture in that location).

 

What can also be seen there is that Bejisa is located in an area of upper level divergence. (diverging winds at high altitudes), which is favorable for continued development. Moreover, the upper-level winds are very weak (possibly an anticyclone?), which means shear levels are very low. Such low shear values usually are accompanied by rapid intensification phases, as seen with Bejisa. Both are confirmed by CIMSS, as seen below:

 

Wind shear map from CIMSS. Note the very low shear values over Bejisa.

 

Upper level divergence map of CIMSS. There are currently very high values of upper level divergence over Bejisa, favorable for intensification.

 

Sea surface temperatures are currently about 28-30 *C, which is also very favorable for continued development. This can be seen in the SSTS map from NOAA:

SSTS map from NOAA.

 

Given the current organizational state of the cyclone, and the very favorable conditions as stated above, I think we will see a continued steady phase of intensification in the short term, possibly even rapid.

 

Regarding track, the latest run GFS shows a track over La Reunion in about 5 days, consistent with the official forecasts. The intensity, however, might be a little too conservative in the short term. (see reasoning above).

 

GFS forecast track of Bejisa.

 

However, the UKMET model shows the cyclone moving to the east of La Reunion in the long term, increasing the uncertanity in the long term. 

 

UKMET forecast of Bejisa.

 

Concluding, La Reunion could be impacted by a significant tropical cyclone in about 5 days. There are some uncertanities regarding track and intensity. A slight deflection in the track forecast could be the difference between a direct hit by the TC to possibly no impact at all. It will be interesting to see how the track of Bejisa unfolds.

 

Sources:

http://tropic.ssec.wisc.edu/#

http://www.ssd.noaa.gov/PS/TROP/floaters/06S/06S_floater.html

http://earth.nullschool.net/#current/wind/isobaric/250hPa/orthographic=31.21,-11.62,289

http://polar.ncep.noaa.gov/sst/ophi/

http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/13122912/33.html

Edited December 29, 2013 by Vorticity0123

[Somerset Squall]

Bejisa has rapidly strengthened overnight, and now has winds of 60kts. The central dense overcast has persisted, and there are hints of a small eye developing. Banding features have become more prominant, now in all quadrants of the storm. As shear remains low and outflow good, Bejisa should continue to strengthen over the coming few days before weakening occurs as the cyclone runs into higher shear.

[Somerset Squall]

I can't post an image because I'm on my phone but Bejisa is clearly bombing. It has developed a pinhole eye in latest satellite imagery. The next JTWC update will be interesting!

[Somerset Squall]

Bejisa's intensity continues to rocket up, with winds now at 90kts. MeteoFrance have declared Bejisa an "Intense Tropical Cyclone" which makes it the third cyclone out of three so far in the Southwest Indian Ocean to achieve this status. Bejisa could continue to rapidly strengthen over the next day or so whilst waters remain warm, shear reletively low and outflow excellent.

Edited December 30, 2013 by Somerset Squall

[Somerset Squall]

Winds now at 105kts, cat 3 on the SS scale. What a beaut!

 

[Somerset Squall]

Bejisa is undergoing an eyewall replacement cycle which has caused weakening to 95kts. Once the eyewall replacement is complete, Bejisa should restrengthen. On the current forecast track, La Reunion look to be in the firing line from Bejisa, with a lesser impact expected on Mauritius.

[Somerset Squall]

Looking at satellite imagery this morning, Bejisa has completed the Eye Wall Replacement Cycle. Bejisa has a new eye which is a little larger and, at least currently, not as sharply defined as the old eye. Winds are down to 90kts. Now the EWRC is complete, restrengthening should occur, and this is forecast by both JTWC and MeteoFrance.

 

 

 

Latest track from MeteoFrance shows an impact on the island of La Reunion, with Mauritius not expected to be impacted quite so much. Bejisa should be weakening by this point due to higher shear values to the south, but flooding rains and strong winds are still expected.

 

Edited January 1, 2014 by Somerset Squall

[Somerset Squall]

Bejisa has again strengthened to 105kts, cat 3 on the SS scale, according to JTWC. Bejisa is probably near peak as weakening should occur tomorrow as the cyclone runs into gradually increasing shear, however, Bejisa will still be a signifcant system as it slams in to La Reunion.

[Somerset Squall]

Bejisa has reached a new peak, 110kts according to JTWC. This image shows Bejisa very near La Reunion:

 

 

 

 

[Coast]

Placed on red alert since Thursday morning, the Reunion Island holds its breath while the cyclone Bejisa could cross the island.

 

On the shores of the island of La Réunion, wave roars. Huge waves cause sound bearings rollers on strikes that department overseas amidst the Indian Ocean. Trees bend under the blows of Hurricane Bejisa. The sky is dark, the rain is visible in bursts, the eye of the storm is near. The islanders have a question in mind: this low pressure area - where the sun shines and where the winds are contrary to the rest of the huge meteor - she will pass over their heads? Every year it's the same question during the hurricane season, which runs from November to February. Each year, at least one cyclone is announced catastrophic, but the island has not been cataclysmic for over ten years, during the passage of Dina.

 

This time, the atmosphere is special, admittedly. For two days already, Météo-France has adopted an alarmist tone and advised residents to protect their homes. Since this morning, it is even forbidden to leave the house, the prefecture has declared a red alert , the final stage of the system to prevent damage. The airport is closed and the road Littoral, which connects the north west of the island. From all corners of Reunion information back and are broadcast by local radio stations: there flooded homes, washed away here, all the rain and wind strike that blow up to 150 km / h.

 

"I'm not afraid"

 

Yet this is only the beginning and the island is not affected equally by the tropical cyclone Bejisa. At the moment, it is on the heights of the south, in the region of Saint-Pierre, the winds are the most violent and the most intense rain. In the North, the phenomenon is a little less destructive. "I'm not afraid, I have a certain age and I saw a lot of cyclones that have never destroyed my house," says Arlette, a resident of the capital, Saint-Denis. This grandmother of 80 years still caulked terrace and protected the plants winds meteor. "I made reservations water, candles and matches," is she reassures.

 

These precautions may not be necessary. Hurricane is getting closer and contains within it higher than 250 km / h winds. In the afternoon of Thursday, when it will be closest to the island, the whistling of the wind will turn into screams. "I knew the monsoon Vietnam and in rain I'm not impressed, "says Kim, a young tourist traveling to Reunion since a fortnight. Yet it is not quiet. "The wind is scary and I fear the passage of a sheet of metal that could hurt me on the terrace." The young woman reassures consultant Internet news. It has only a few hours of anguish to hold. If the eye of the storm passes over La Réunion, it will be later in the early evening.

 

 

 

http://www.lepoint.fr/societe/cyclone-bejisa-la-reunion-retient-son-souffle-02-01-2014-1776103_23.php

 

 

[Vorticity0123]

Nice to see, Sommerset Squall, how you kept updating the thread. I (and I think more do) really appreciate the continued updating!

Also the articles from Coast are always very informative, it is interesting to see what is actually happening on the areas being affected by tropical cyclones.

 

General

 

Bejisa is definitely on a weakening trend, with weakening convection to the west of the circulation and a less-defined eye. The intensity has come down to 90 knots. Moreover, the eye and eyewall are somewhat elongated WNW-ESE, as can be seen in Dvorak imagery.

 

Further weakening is anticipated by JTWC and La Reunion, with a turn back toward the southwest forecast. This will lead to the system move away from the island in about a day or so.

 

 

Steering winds

 

The turn to the southwest is due to a ridge being forecast to develop to the south of the system, blocking its recurve to the southeast. This can be seen in the GFS 500 hPa heights and MSLP charts. But first, I'll give an explanation of the relationship between TC intensity and steering winds, how troughs and ridges at 500 hPa can be identified, and to a lesser extent how those affect the steering of tropical cyclones.

 

First of all, tropical cyclones of varying intensity are being steered by winds at a different altitudes. For example, a 1010 mb tropical depression will be steered by winds at much lower altitude than a 950 mb hurricane. This can be seen in the plot below (Courtesy of CIMSS):

 

 

The current MSLP of Bejisa is 967 hPa (source: MeteoFrance), which means the TC is generally steered by winds blowing between 300 and 850 hPa. As a result, 500 hPa gepotential heights are an adequate measure for analyzing the steering environment of Bejisa.

 

For the next step, look at the geopotential heights and MSLP from the GFS chart below

 

GFS 12Z T0 (=analysis)

 

Focus on the geopotential heights at 500 hPa (the colors, not the lines. For the reason, see explanation above). Bejisa is the <990 mb low to the east of Madagascar. The first thing that shows up is a decrease of geopotential heights with increasing latitude. This is because the air is generally colder at higher latitudes (with a constant altitude), also at 500 hPa. A lower temperature also means a pressure of 500 hPa is reached at lower altitudes (ideal gas law --> lower temperature is lower pressure). I'm not 100% sure about the last two sentences, so correct me if I'm wrong.

 

The decrease of geopotential heights at higher latitiude is not of any importance right now. The thing to look at is the longitudinal difference in geopotential heights (from west to east), to the south of Bejisa. Check the above chart. What can be seen is at about 30S, 60E is a decrease in longitudinal heights. This means a mid-to upper level trough is located there. This is what provides the main steering for Bejisa at the moment. Despite high pressure at the surface, the system is being dragged toward the southeast (a recurve scenario) because of the trough at higher altitude. 

 

Now check the chart below, 36 hours later (once again GFS):

 

GFS 12Z T36

 

What can be seen clearly is a very distinct increase of geopotential heights to the south of Bejisa. This is indicative of an upper ridge being positioned to the south of the tropical cyclone. Though the trough to the southeast is still apparent, the ridge has become the dominant steering mechanism. As a result, the steering winds around Bejisa change radically, now taking the system to the southwest. The southeastward motion is essentially being "blocked".

 

Impact on La Reunion

 

Unfortunately, as seen in a combination of above charts, the island of La Reunion is currently experiencing the worst of the storm, as the island is located in the eastern eyewall of the cyclone where the most intense convection is occuring. This could lead to very severe winds and  intense flooding. Moreover, storm surge from the cyclone could also lead to severe damage.

 

Regarding the extreme rain on the island, the south of the island has received up to 153 mm in the last 3 hours¹. Given that the island will continue to be pounded by the storm or outer rainbands of it, totals could increase much further in the next day or so. Let's hope damage will not be too severe.

 

The storm surge forecast by La Reunion during the day is up to 7 meters¹. Quite incredible, given that in Holland, the maximum tide increase during the storm on the fifth of december was less than 4 meters. However, because La Reunion is a mountainous island, only a small part of the island will be impacted by the storm surge.

 

My heart goes out to the people being hit by Bejisa at the moment.

 

EDIT: because my original explanation about the steering of Bejisa was rather poor, I decided to give a better explanation of the steering. Note that the explanation might contain a few errors, as I only recently learned the basics of steering of tropical cyclones. Please accept the possible inconvenience. Remarks or improvements are always welcome.

 

 

Sources:

http://www.meteo.fr/temps/domtom/La_Reunion/meteoreunion2/# (¹)

http://www.usno.navy.mil/JTWC/

http://www.nhc.noaa.gov/

http://deutsch.wunderground.com/blog/JeffMasters/archive.html?year=2013&month=07

http://www.wetterzentrale.de/topkarten/fsavnet.html

http://cirrus.meteo.noa.gr/forecast/wrf/glossary.htm (for some brief explanation on the 500 mb charts)

Edited January 2, 2014 by Vorticity0123

[Somerset Squall]

Bejisa is moving to the southwest for reasons that Vorticity explained. The cyclone is gradually weakening, due to moderate shear and falling along track sea temperatures. Good poleward outflow is slowing the weakening trend however. Winds are down to 45kts according to JTWC. Bejisa remains very well structured but convection is on the wane. As sea temps continue to decline as Bejisa heads southwest, the cyclone should degenerate into a remnant low in a day or so.