Montag, 1. Dezember 2014

Temporada de huracanes concluye tranquila

La temporada de huracanes de este año en la cuenca atlántica, que terminó ayer domingo, ha sido “relativamente tranquila, tal como se predijo”, y supone nueve años consecutivos sin que un ciclón azote Florida, algo que sí resulta “extraordinario”, resaltó un meteorólogo.

“Ha sido una temporada relativamente tranquila”, pero lo que sí es “sorprendente” es que en los últimos nueve años ningún ciclón haya impactado las costas de Florida. Eso es todo un récord”, dijo Dennis Feltgen, meteorólogo y portavoz del Centro Nacional de Huracanes (CNH), dependiente de Administración Nacional de Océanos y Atmósfera de Estados Unidos (NOAA).

En junio pasado, al comienzo de la temporada, los cálculos de la NOAA preveían una actividad menor de lo normal, con la formación de entre ocho y trece tormentas tropicales, de las cuales entre tres y seis iban a llegar a huracanes, y uno o dos de ellos iban a ser de categoría mayor (3, 4 o 5 en la escala Saffir-Simpson).

El vaticinio fue exacto: hubo ocho tormentas tropicales, de las que seis se transformaron en huracanes

Montag, 6. Oktober 2014

Quietest Atlantic Hurricane Season Since 1986

The traditional busiest month of the Atlantic hurricane season, September, is now over, and we are on the home stretch. Just three weeks remain of the peak danger portion of the season. September 2014 ended up with just two named storms forming--Dolly and Edouard. Since the active hurricane period we are in began in 1995, only one season has seen fewer named storms form in September--1997, with Category 3 Hurricane Erika being the only September storm. Between 1995 - 2014, an average of 4.3 named storms formed in September. With only five named storms so far in 2014, this is the quietest Atlantic hurricane season since 1986, when we also had just five named storms by the beginning of October. In terms of Accumulated Cyclone Energy (ACE), activity in the Atlantic up until October 1 has been only about 43% of the 1981 - 2010 average.

 Tracks of Atlantic named storms in 2014. Note how all of this year's hurricanes (tracks in red) have occurred well north of the tropics, north of 24°N latitude--a testament to how hostile for development conditions have been in the tropics, due to dry, sinking air. Image credit: National Hurricane Center.

 Forecast for the remainder of hurricane season

Looking at climatology, since 1995, we have seen an average of 3.6 named storms form in the Atlantic after October 1. Two of those years--2006 and 2002--saw no storms form after October 1. The most post-October 1 storms was eleven, which occurred in 2005--no surprise there! The latest 2-week forecast from the GFS and European models show a continuation of the basic atmospheric pattern we've seen over the tropical Atlantic this season, with plenty of dry, sinking air. These conditions should lead to lower than average activity into mid-October, which is when historically, Atlantic hurricane activity begins to drop sharply. I expect we'll see at least one more named storm in the Atlantic this year, with two a more likely number. It's unlikely we'll get three or more post-October 1 named storms.

During October, the focus of Atlantic tropical cyclone genesis shifts to the Western Caribbean, Gulf of Mexico, and the waters between the Bahamas and Bermuda. The Lesser Antilles typically see very few tropical cyclones after October 1, and I expect their hurricane season is over. Sea Surface Temperatures over the Caribbean are currently 0.2°C above average, and 0.4°C above average in the Gulf of Mexico.

 Quiet in the Atlantic

A tropical wave predicted to come off the coast of Africa on Saturday is forecast by the UKMET and GFS models to develop by Monday in the vicinity of the Cape Verde Islands. An upper-level trough of low pressure over the Eastern Atlantic will bring high wind shear to this region early next week, though, making developing difficult. Another major invasion of dry air from the Sahara is currently in progress over the Tropical Atlantic, which will make it difficult for any tropical storms to make the crossing from Africa to the Lesser Antilles intact.

Freitag, 1. August 2014

Tropical Storm Bertha

Bertha is a sheared tropical cyclone with the center located on the
western edge of the deep convection as indicated by satellite. The
low-level circulation continues to be vigorous, but given the
current westerly shear the outflow is very limited. There are no
reasons to change the initial intensity which is kept at 40 knots.
Another Air Force reconnaissance plane will investigate Bertha in
the next several hours.
The current shear environment is not particularly favorable for
strengthening, but there are some indications by the global models
that the shear could decrease some as Bertha moves across the
northeastern Caribbean Sea. This should allow some slight
intensification during the next 24 to 36 hours. Once Bertha's
circulation moves away from Hispaniola beyond 48 hours, there is an
opportunity for additional strengthening if the shear becomes
lighter as suggested by global and statistical models. At this time,
the NHC forecast keeps Bertha with 45-knot winds over the
western Atlantic until it becomes more certain that the shear could
Bertha continues racing toward the west-northwest or 290 degrees
at 17 knots. The cyclone is being steered by the flow around the
Atlantic subtropical ridge which is forecast to persist. Once in
the western Atlantic near the eastern Bahamas, Bertha will be
steered by the southerly flow between the subtropical high
and a mid-level trough over the eastern United States. This pattern
will force Bertha to turn northward with a decrease in forward speed
and to eventually recurve northeastward over the Atlantic. 
The confidence in the track forecast, primarily in the next 2 to 3
days, is high since most the guidance is tightly clustered. The
confidence decreases after 3 days when the guidance becomes
divergent. The track envelope, however, brings Bertha northward
and then northeastward over the Atlantic, and the NHC forecast
follows closely the multi-model consensus.
Forecast positions and Max winds
Init  01/0900z 13.0n  57.0w   40 kt  45 mph
 12h  01/1800z 14.0n  59.6w   40 kt  45 mph
 24h  02/0600z 15.5n  63.0w   40 kt  45 mph
 36h  02/1800z 17.2n  66.0w   45 kt  50 mph
 48h  03/0600z 19.5n  68.8w   45 kt  50 mph
 72h  04/0600z 24.5n  73.5w   45 kt  50 mph
 96h  05/0600z 29.0n  74.0w   45 kt  50 mph
120h  06/0600z 35.0n  67.5w   45 kt  50 mph

Dienstag, 1. Juli 2014

Tropical Depression One Forms off the Coast of East Florida

The Atlantic's first tropical depression of 2014 is here, as Tropical Depression One finally formed at 11 pm EDT Monday evening from disturbance 91L. TD 1 was drifting southwest at 2 mph towards the east coast of Central Florida early Tuesday morning. Long-range radar out of Melbourne, Florida on Tuesday morning showed that bands of heavy rain from TD 1 were affecting the Northwest Bahamas, and sustained winds of 33 mph gusting to 36 mph were observed at Settlement Point in the Northwest Bahama Islands at 2 am EDT.

Satellite loops showed heavy thunderstorms were limited to the south side of TD 1's center of circulation, and were slowly increasing in intensity and areal coverage. The counter-clockwise circulation of an upper level high pressure over Florida was bringing northerly winds over TD 1 at high altitude, and these winds were creating moderate wind shear of 10 - 15 knots. Water vapor satellite loops showed very dry air to the north of TD 1, and the northerly winds were driving this dry air in the heart of the storm, interfering with development, and keeping any heavy thunderstorms from developing on the north side of the circulation. The Air Force Hurricane Hunters will investigate TD 1 on Tuesday morning, and the NOAA jet is scheduled to fly Tuesday afternoon.