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Rare February Disturbance - Invest 90L

Posted by Andy Johnson under Daily updates

The National Hurricane Center identified a tropical disturbance Sunday, February 5, 2012 near the western tip of Cuba. Wind shear was moderate at 25 kts but increased to 30 to 40 knots by today, Monday, February 6. As a result, the system was sheared apart over the Florida Straights. Temperatures are still warm enough even in winter over the northwest Caribbean Sea to support tropical development but wind shear is usually so great that tropical development does not happen. Record rainfall occurred in Key West with 4.35″ of rain on February 5.

Even though this is rare, it is not unprecedented. A tropical storm moved out of the Caribbean Sea on February 2, 1952 and make landfall in extreme southwest Florida with 60 MPH winds. It has been called the “Groundhog Day Storm”.  It was not given a name at the time, however. In 1952, the phonetic alphabet was used to name hurricanes - Alpha, Baker, Charlie, Dog, etc. Even so, since it occurred out of season, it was not given a name.

This winter has been unusually mild in Florida. Tropical air has covered South Florida and the Keys recently. Part of the reason has been that the Arctic Oscillation has been in a positive phase, which keeps cold air trapped at more northern latitudes. Alaska and Europe have had a very cold winter. This is in great contrast to 2011, when the Arctic Oscillation was in its negative phase and Florida had a record cold December.

Quiet Atlantic; Kenneth Weakens

Posted by Andy Johnson under Daily updates

Invest 99L in the far eastern Atlantic transformed into an extratropical low and continued to move northeastward towards Europe. It appears that the system became occluded and even though there was some warm air in the core two days ago, the occlusion process, lifted the warm bubble above the air leaving a cold core systems. Tropical systems are warm core and extratropical systems are cold core.

The circulation of the non-tropical low can still be clearly seen in the enhanced IR satellite image below.

The rest of the tropical Atlantic remains very quiet as it normally the case this year. This is confirmed in the basin-wide satellite image below.

Just as quickly as Kenneth strengthened to a Category 4 hurricane on Tuesday, he weakened just as quickly to a tropical storm. The structure of the storm as shown in the IR satellite image below is much less defined than it was yesterday.

Kenneth should continue to weaken without any threat to land as shown below.

Invest 99L Weakens; Pacific Kenneth is Strongest Late Season Hurricane

Posted by Andy Johnson under Daily updates

The National Hurricane Center has stopped running models on Invest 99L in the far eastern Atlantic as the system has moved into progressively cooler waters. The structure has become more extratropical than subtropical today since evidently a cold front has developed to the east of the low pressure center. The extratropical remains of Invest 99L will continue to move towards the northeast.

The enhanced IR satellite image below shows all of the convection is displaced well to the north and northeast of the center of circulation. There is also an air mass change across the band feature to the east of the low. Thus, the band feature is becoming a cold front.

The visible image of 99L below just shows a swirl of clouds near the center. The organization clearly does not look tropical.

Hurricane Kenneth in the Eastern North Pacific has exploded into a Category 4 hurricane. This feat makes Kenneth a very rare hurricane, especially considering that the hurricane season in the Eastern North Pacific ends on November 15, which is fifteen days earlier than the Atlantic season ends. The records for strongest late season hurricane have been shattered! The latest hurricane on record in the Eastern North Pacific was Hurricane Winnie in 1983 during an El Niño year, but she was only a Category 1 hurricane. In addition, there has never been a hurricane as strong as Kenneth this late in the Atlantic hurricane basin. The strongest major hurricane in November in the Atlantic was Hurricane Kate on November 21, 1985. Hurricane Kate slammed into the Florida panhandle causing extensive damage and power outages in Tallahassee. Kate was a rare Thanksgiving holiday hurricane. So, Kenneth is the strongest late season hurricane in both the Eastern North Pacific and the Atlantic.

The enhanced IR satellite image below shows a classical hurricane structure with a well defined eye. Notice that there are not any feeder bands that are normally present with most hurricanes. Kenneth definitely has a circular or annular appearance. In fact, Kenneth is a rare annular hurricane in which winds in all quadrants are nearly perfectly symmetrical. Annular hurricanes are much stronger than average hurricanes and maintain their strength longer than the majority of hurricanes. Only 4% of hurricanes can be considered annular.

The plot below shows that Kenneth has probably peaked in intensity and will slowly weaken over the next 5 days as he moves farther out to sea.

Time for Tammy? Rare Late Season Kenneth Forms

Posted by Andy Johnson under Daily updates

Invest 99L is very similar in structure to the precursor system of the last named storm, Sean. Invest 99L is in an area of marginally cool waters for subtropical development and began as a purely extratropical system. Over the last few days convection has been increasing solely on the east side of the low level circulation. Moderate southwesterly shear is pushing and keeping all the of the convection on the east side of the circulation. Today, the appearance has become more comma shape, which indicates a trend from extratropical to subtropical. Just like Sean, Invest 99L may develop into a subtropical storm and eventually could become tropical. Tropical storm force winds are already occurring on the northeast side of the low level circulation.

The enhanced IR satellite image below shows that Invest 99L has a fairly large cloud shield and is located in the middle of the Atlantic Ocean. Since the steering currents are from the southwest, Invest 99L is no threat to the U.S.  If Invest 99L becomes a subtropical storm it would be named Tammy.

The image below shows the comma shaped band of convection more clearly.

The visible image below shows the low level clouds more clearly than the IR satellite image. Upon close inspection, the low level cumulus cloud field indicates a circulation center on the the southwest edge of the heaviest convection.

The hurricane season in the Eastern North Pacific runs from May 15 through November 15. Ironically, now that hurricane season is over there, a new hurricane has developed, Kenneth. Hurricane climatology in the Eastern North Pacific is generally considered available back to 1949 but sketchy before that. Since 1949, there have only been three tropical storms that have formed after November 18th. Of those three storms, only one became a hurricane - Winnie. Hurricane Winnie formed on December 5, 1983 during a major El Niño year. In fact, 1983 was one of the strongest El Niño’s of recent history. However, we are currently in a La Niña, which means cooler Pacific waters and fewer Pacific hurricanes. So, it is very ironic to see Hurricane Kenneth develop so late, especially in a La Niña year.

The enhanced IR image below shows the well developed circulation of rare late season Hurricane Kenneth.

2011 Season Winding Down

Posted by Andy Johnson under Daily updates

Tropical Storm Sean came close enough to Bermuda to bring some late season tropical storm conditions on Friday, November 11, 2011. Winds at Kindley field in Bermuda were sustained at 37 MPH with a maximum gust of 56 MPH. Amazingly, only .10″ of rain fell in Bermuda as dry air was entrained into the circulation of late season Sean. 

Now that Sean is long gone, the tropics remain fairly quiet. By mid-November there are limited areas that tropical cyclones can develop due to the shrinking areas of warm water. Activity is generally confined to the Caribbean. In many years, no storms develop in November. The Eastern North Pacific hurricane season ends November 15th and the Atlantic hurricane season ends November 30th.

The enhanced satellite image below shows a disturbed area in the southwestern Caribbean Sea. Wind shear is light in the southern Caribbean but from Jamaica northward wind shear is moderate to high.

A closer view below shows that while there are large areas of thunderstorm clusters, there is no circular organization to the Caribbean disturbance at this point in time.

Sean Could Bring Gales to Bermuda; No Threat to U.S.

Posted by Andy Johnson under Daily updates

Despite all odds, Sean has held together today and has actually intensified. Even though the models have not been indicating that Sean would strengthen to hurricane force, it now appears possible that Sean could attain hurricane strength within the next 36 hours. A coiled band of clouds has wrapped around an eye-like feature today. The wind field has also expanded today. Gale force winds extend outward up to 250 miles to the northeast and 120 miles to the southwest. Yesterday, there were hardly any gale force winds in the southwestern quadrant.

Sean is no threat to the United States, but could bring gale force winds to Bermuda Thursday. As a result, Bermuda is under a tropical storm warning as it has been several times this year.

The visible satellite image below does not show a large cloud area. In fact, the gales extend farther than the central core of clouds. However, the presentation is much more circular and organized than it was yesterday.

The enhanced IR satellite image below shows convection wrapping around the center almost all the way in a coiled fashion.

The satellite image below shows the approaching cold front to the west which will cause Sean to accelerate to the northeast.

Sean Becomes Tropical; Rare Mediterranean Hybrid “Rolf” Near Riviera

Posted by Andy Johnson under Daily updates

Thunderstorm activity increased near the center of Sean which aided in the transition from a subtropical storm to a tropical storm. Tropical Storm Sean has been nearly stationary and will wait until a upper level trough over the Central U.S. picks it up and moves it northeastward on Thursday. Until then, he will meander towards the west or northwest. Bermuda was getting gusts to 43 MPH yesterday and today sustained winds remain about 28 MPH. Bermuda is under a tropical storm watch even though it is a little late in the season for them to experience tropical storm conditions. Sean’s wind field is highly asymmetrical. Gale force winds extend outward 300 miles to the northeast but 0 miles to the southwest. So, it will make a big difference which side of Bermuda Sean’s center passes as to the effects on Bermuda. If Sean passes to the west or near Bermuda, winds will be higher in Bermuda. If Sean passes to the east, the effects will be minimal. Large waves will continue in the northeast facing beaches of the Bahamas and the East Coast from Florida to North Carolina for the next few days.

Sean is not expected to become a hurricane. If that occurs, it will continue the odd ratio of hurricanes to tropical storms this year. We have had 18 named tropical storms but only 6 have become hurricanes, which is only a 30% ratio.

The visible satellite image of Sean below shows a low level circulation present in the cumulus cloud field but but there is still a lot of dry air on the south side of the system. It does not look very impressive.

The enhanced IR view below shows the the vast majority of convection in only in the northeastern quadrant, which is the same quadrant that has the largest wind field.

Waves up to 18′ are being produced by Sean near and northeast of the center. The wave shadow effect of the Bahamas is shielding the Miami Beach area from big waves but from Palm Beach northward to North Carolina, large swells are rolling ashore.

Sean is located in a col in the steering currents. The next system that will cause forward acceleration can be seen back over Texas in the enhanced IR satellite image below. Shear is relatively low now but will increase dramatically on Thursday as the current Texas front moves of the Florida East Coast.

Some extratropical European low pressure areas are named. There has never been a named Mediterrean tropical system, although there have been several other hybrid storms that could have been at least subtropical. The NHC has no responsibility for forecasts  in the Mediterranean Sea. A system that developed yesterday off of the coast of Spain started developing tropical-like characteristics. It has already been named Rolf when it was purely extratropica. Water temperatures are still cooler than what is considered the minimum for pure tropical cyclone development. However, the wind flow shown below (courtesy of MeteoFrance) clearly shows a circulation with an “eye-like” feature in the wind field to the south of Marseille, France. Rolf seems to have moved closer to the French Riviera today where Nice has been reporting gusts to 39 MPH. Buoys offshore of the Riviera have seen sustained gale force winds.

The image below is a high resolution visible MODIS image of Rolf.

Sub-Tropical Storm Sean Forms

Posted by Dave Osterberg under Daily updates

The The swirling area of low pressure east of the Bahamas has gathered enough convection and wind speed to classify as a sub tropical storm.  Since Sean isn’t purely tropical it has been given the name sub-tropical. In a true tropical system the strongest winds and convection are at the center of the storm, whereas in subtropical systems sometimes the strongest winds could be displaced from the center by hundreds of miles.  To be honest 45 mile per hour winds are the same whether it be tropical or sub-tropical. Over the course of the next couple of days there is so room for Sean to strengthen and become purely tropical. Where Sean has formed is not conducive for it to strike the lower 48. In November constant cold fronts move off the East Coast and that would tend to push Sean away from the SE U.S. All the computer models are pointing to this scenario of Sean drifting West, then being picked up by a cold front and eventually being pulled out to sea.

Could SE Atlantic System Become Sean? Rare Mediterranean Invest

Posted by Andy Johnson under Daily updates

Over the weekend, an extratropical (non-tropical) system moved in an unusual path off towards the east-southeast off the southeast U.S. Coast. Most of the time, November East Coast extratropical lows move north or northeastward towards New England or the Canadian Maritimes, like the rare pre-Halloween snow storm. In this case, however, the extraptropical low moved into warmer waters at a more southern latitude and has gradually started to acquire convection and rain bands closer to its center. As a result, the NHC has identified the systems as Invest 98L. There is a broad area of gale force gusts occurring over much of the southwestern Atlantic. Bermuda has been reporting sustained winds of 31 MPH this afternoon with gusts to 44 MPH. Strong upper level westerly winds are creating a sheared environment over Invest 98L. Tropical systems have warm cores and extratropical systems have cold centers. A subtropical storm is a hybrid in which there is not always much temperature difference across the systems and can act as a bridge from an extratopical storm to a tropical one. Most of the models, keep the system meandering offshore for a few days then racing off the the northeast towards the Canadian Maritimes by Friday.

The enhanced IR satellite image below shows the system in the Atlantic far off the east coast of Florida. It does not resemble a tropical system in that it has an arc shape rather than a circular or comma shape.

The visible satellite image below shows the low level structure better than the IR images. There is some convection near the center but there is also a lot of dry air to the west and south of the system.

The IR satellite image below shows the large area covered by low level (gray) cumulus clouds organized into circular bands.

Waves up to 19′ are being generated by Invest 98L to the northeast of the Bahamas. So, the main impact to Florida from this system will be breezy conditions and large breaking waves along the east coast mainly from Palm Beach northward (due to the wave shadow effect of the Bahamas).

An unusual sub-tropical “looking” system has developed in the Mediterranean Sea. It is centered off the coast of northern Spain and south of the French Riviera. The closest land area to center is the island of Minorca, one of the Balearic Islands which belongs to Spain.

The IR satellite image below shows the system in the Northwest Mediterranean. The Naval Research Laboratory’s (NRL) Monterrey Marine Meteorology Division has identified this system as Invest 99L.

 The Satellite Services Division (SSD) seems to have classified this system as a tropical cyclone 01M (See the quote below). However, the NHC is not mentioning this system on their web site. Another blog posted an email from Dennis Feltgen with NHC stating “Despite the “L”, Invest 99 is not under the jurisdiction of the National Hurricane Center, as the location of the Invest is not in the range of our ATCF software. The request for an Invest came from the U.S. Navy, which went to the Satellite Analysis Branch for a classification. An Invest simply means that more information on a particular weather system is being requested, and in no way implies there is any threat of tropical cyclone development…which does not occur in the Mediterranean Sea”. At any rate, it is surely an unusual system and could cause high winds and seas near the French Riviera.

TXMM21 KNES 071819
TCSMED

A. 01M (NONAME)

B. 07/1800Z

C. 41.1N

D. 5.3E

E. THREE/MET-9

F. T2.5/2.5/D1.5/24HRS

G. IR/EIR/SWIR

H. REMARKS…DT=2.5 BASED ON .5 BANDING ON LOG10
SPIRAL. PT=2.5. MET=2.0. FT IS BASED ON DT. DEEP CONVECTION HAS PERSISTED
LONG ENOUGH AROUND THE LOW LEVEL CENTER FOR A TROPICAL CLASSIFICATION.

I. ADDL POSITIONS

NIL

La Niña Set to Strengthen

Posted by Andy Johnson under Daily updates

The La Niña pattern (cooling of the equatorial Pacific Ocean) helped to cause a very active 2011 Atlantic hurricane season, at least, in terms of the number of storms that developed. The graph below shows La Niña vs. time. The zero line is a neutral point and the negative numbers are cooler than normal Pacific water temperatures, which are indicators of a La Niña. If the line goes into positive territory (which it didn’t), that would be the beginning of an El Niño. However, the forecast models are indicating a stronger La Niña developing and lasting into July of next year, which is the first part of hurricane season. ENSO (El Niño - Southern Oscillation) is just one of many factors that affect the hurricane season.

Also, in the wintertime a La Niña pattern is usually associated with a drier than normal winter in Florida. However, as we saw last winter, other effects such as the Arctic Oscillation (AO) can overpower the effects of the La Niña. So, Florida generally had a wetter than normal winter. The image below also shows that we were actually in neutral conditions (not a La Niña) during the first part of the 2011 hurricane season.

As we  move into the final month of the Atlantic basin hurricane season, as might be expected, the tropics are very quiet. The disturbance that has been lingering in the western Caribbean has continued to weaken today. The enhanced IR satellite image below shows that the Intertropical Convergence Zone is somewhat active in the Central and East Atlantic.

Even though the areal coverage of convection has increased since yesterday in the eastern Atlantic, shear is moderate to high. So, no development is expected in this part of the Atlantic in the near future.