Tag Archives: Florida

Ana Transitions to a Tropical Storm

As Subtropical Storm Ana sat over the relatively warm Sea Surface Temperatures of the Gulf Stream, it drew energy from the upper ocean.  More thunderstorms developed near the center of circulation and condensation released latent energy which warmed the middle and upper levels.  As a result, the circulation became more circularly symmetrical and eventually some upper level outflow developed at the top of the system. As a result of the structural changes, Ana made a transition from a subtropical storm to a tropical storm.

At 11:00 a.m. EDT on Saturday the center of Tropical Storm Ana was located at latitude 32.7°N and longitude 77.9°W which put it about 90 miles southeast of Myrtle Beach, South Carolina and about 105 miles south of Wilmington, North Carolina.  Ana was moving toward the northwest at 3 m.p.h.  The maximum sustained wind speed was 60 m.p.h. and there were gusts to 70 m.p.h.  The minimum surface pressure was 1001 mb.

A Tropical Storm Warning remains in effect from South Santee River, South Carolina to Cape Lookout, North Carolina.  A Tropical Storm Watch is in effect from Edisto Beach to South Santee River, South Carolina.

As the high pressure system that has been blocking the forward progress of Ana moves eastward, the tropical storm will move toward the coast.  It is likely to make landfall near the border of South Carolina and North Carolina on Sunday morning.  An upper level trough approaching from the west will turn Ana toward the northeast and sweep it out to sea after it makes landfall.

Ana is still over the relatively warm water of the Gulf Stream and it is likely to maintain its intensity on Saturday.  When Ana moves north of the Gulf Stream, it will cross over cooler water.  In addition, the approaching upper level trough will begin to generate some vertical wind shear over Ana.  The effects of cooler water and more wind shear mean that Ana will likely be weakening as it approaches the coast.  It will bring some wind, locally heavy rain and beach erosion, but the impacts should be minor in most locations.

Tropical Storm Warning Issued for Ana

The National Hurricane Center has issued a Tropical Storm Warning for the portion of the coast from South Santee River, South Carolina to Surf City, North Carolina.  A Tropical Storm Watch remains in effect from Edisto Beach to South Santee River, South Carolina and from Surf City to Cape Lookout, North Carolina.

At 11:00 a.m. EDT on Friday the center of Subtropical Storm Ana was located at latitude 31.5°N north and longitude 77.3°W which put it about 250 miles east of Savannah, Georgia, about 190 miles east-southeast of Charleston, South Carolina and about 180 miles south-southeast of Wilmington, North Carolina.  Ana was nearly stationary.  The maximum sustained wind speed was 45 m.p.h. and there were gusts to 60 m.p.h.  The minimum surface pressure was 1001 mb.

Thunderstorms are forming on the southern and eastern sides of the circulation and Ana is beginning to look more like a tropical cyclone on satellite images.  It could slowly make a transition to a tropical storm during the next several days.

A high pressure system north of Ana is expected to move slowly eastward and steer Ana toward the coast during the next two or three days.  After that time an upper level trough approaching from the west is expected to steer Ana to the northeast and out to sea.

A combination of marginal Sea Surface Temperatures, drier air and some vertical wind shear is expected to limit the potential for intensification.  However, some modest intensification is possible and the surface pressure has decreased slightly in recent hours.

 

Subtropical Storm Ana Forms Off Southeast U.S. Coast

Thunderstorms developed closer to a broad area of low pressure off the southeast U.S. coast and the National Hurricane Center (NHC) classified it as Subtropical Storm Ana.  At 11:00 p.m. EDT on Thursday the center of Subtropical Storm Ana was located at latitude 31.5°N and longitude 77.6°E which put it about 250 miles east of Savannah, Georgia, about 180 miles southeast of Charleston, South Carolina and about 200 miles south of Wilmington, North Carolina.  Ana was moving toward the north at 2 m.p.h.  The maximum sustained wind speed was 45 m.p.h. and there were gusts to 60 m.p.h.  The minimum surface pressure was 1004 mb.  A Tropical Storm Watch has been issued for the portion of the coast from Edisto Beach, South Carolina to Cape Lookout, North Carolina.

More thunderstorms are being generated near the center of circulation, but the strongest winds are about 90 miles from the center.  So, the structure does not match a classical tropical cyclone where the strongest winds are closer to the center and NHC classified it as a subtropical storm.  The center of circulation is over Sea Surface Temperatures (SSTs) near 26°C which are warm enough to support the development of a tropical cyclone.  So, Ana could make a transition to a tropical storm during the next 24 to 48 hours.  The upper level winds are lighter and vertical wind shear has decreased.  However, because of the proximity to the coast some drier air is being pulled around the western side of the circulation.  Only modest intensification is expected due to marginal SSTs, some vertical wind shear and drier air.

A high pressure system north of Ana is likely to limit the motion of Ana for the next day or two.  Eventually, the high will shift eastward and an upper level trough approaching from the west will accelerate Ana toward the northeast.  Persistent northeasterly winds along the coast could create problems with beach erosion.  The slow movement also means some locations could receive locally heavy rainfall.

 

Possible Development of Low Pressure East of Florida on Wednesday

A number of numerical models have been predicting the development of some type of low pressure system east of Florida this week.  The surface pressure has decreased by about 4 mb during the past 24 hours at several locations in the northern Bahamas, which could be an early sign that a surface low is in the formative stages.  When the low forms, it will likely be classified as either an extratropical cyclone (a typical mid-latitude low with a cold core) or a subtropical cyclone (a type of hybrid low with some tropical characteristics).  An upper level trough over the eastern Gulf of Mexico is generating strong southwesterly winds across the region.  At the same time a surface high pressure system north of the area is producing northeasterly winds in the lower levels.  The combination of southwesterly winds in the upper levels and northeasterly winds in the lower levels means the vertical wind shear over the likely development region is very high.  Since tropical cyclones form in regions of little vertical wind shear, the low is not likely to be a tropical cyclone when it develops.

However, the surface high north of the system is likely to inhibit the northward motion of the low.  If the low moves slowly while it is over warm Sea Surface Temperatures, the thunderstorms in the circulation could release enough latent energy to warm the upper levels of the circulation.  If, at the same time, an upper level ridge over the Central U.S. moves eastward and causes the upper level wind speed to diminish, then the wind shear over the low could be reduced.  Less shear and warmer temperatures in the upper levels could cause the structure of the low to taken on a more tropical form.  It is possible that it could make a transition to a tropical cyclone if the environment is just right.

The forecast track of the low is uncertain because it is unknown precisely where the low will form initially.  It does appear that after the low forms it will move northward slowly for several days.  The expected track could bring the low near the Mid-Atlantic Coast by the end of the week.  Given the Sea Surface Temperatures, the wind shear, and the time of year, the low is likely to be be of tropical storm strength by the end of the week.  However, both the track and intensity forecasts will be highly uncertain until the low pressure system organizes.  Even if the center of the low stays off the coast, northeasterly winds at the surface could generate some beach erosion.

Tropical Depression Nine Forms Over Bay of Campeche

More thunderstorms formed near the center of a small low pressure system over the Bay of Campeche and the National Hurricane Center classified the low as Tropical Depression Nine (TD9) at 11:00 p.m. EDT on Tuesday.  The center of TD9 was located at latitude 19.4°N and longitude 92.9°W which put it about 160 miles west of Campeche, Mexico and about 850 miles west-southwest of Key West, Florida.  TD9 was moving toward the east at 6 m.p.h.  The maximum sustained wind speed was 35 m.p.h. and the minimum surface pressure was 1000 mb.  The government of Mexico has issued a Tropical Storm Warning for the portion of the coast from Celestun to Frontera.

Westerly winds in the upper levels are still creating wind shear over the top of TD9 and most of the thunderstorms are located in the eastern half of the circulation.  However, the upper level wind speed is expected to diminish and TD9 is likely to intensify into Tropical Storm Hanna on Wednesday.  The westerly winds are also likely to continue to push TD9 toward the east or east-northeast during the next 24 hours.

 

Potential Tropical Cyclone Developing Over Bay of Campeche

A reconnaissance plane found a small center of low pressure within a broader area of low pressure over the Bay of Campeche on Tuesday afternoon.  The plane also found winds to near tropical storm force.  At the 3:00 p.m. EDT the low was centered at latitude 19.3°N and longitude 93.7°W which put it about 350 miles northwest of Campeche, Mexico and about 850 miles west-southwest of Key West, Florida.  The low was moving toward the east at 6 m.p.h.  The maximum sustained wind speed was 38 m.p.h. and the minimum surface pressure of 1001 mb.

Thunderstorms are forming mainly on the southern and eastern sides of the center.  The low could be pulling in some of the drier air over the northern Gulf of Mexico, which may be limiting convection on the northwest side of the low.  Westerly winds in the upper levels are also generating wind shear, which is inhibiting the development of the low.  On the other hand, the low is over Sea Surface Temperatures (SSTs) that are warmer than 29°C and there is plenty of energy in the ocean to support the development of a tropical cyclone.  The pressure has fallen about 5-6 mb in the past 12 hours and so the low appears to be getting stronger.  If more thunderstorms form around the center of the low, it could be classified as a tropical depression or a tropical storm.

The westerly winds in the upper and middle levels are pushing the low slowly toward the east.  Guidance from the forecast models seems to be divided into two main possibilities.  One group of models keeps the low relatively weak and forecasts it to move eastward across the Yucatan peninsula and into the northwestern Caribbean Sea before turning it northeastward and moving the low across the Bahamas.  A second group of models predicts that the low will be a bit stronger and that the upper level flow will turn it northeastward sooner and move the low across southern Florida.  If that second scenario occurs, the low could bring stronger winds and heavy rainfall to parts of south Florida.