Jan 13, 2016

Out of Season Subtropical Storm in the Atlantic

The Atlantic Ocean has its first named (sub)tropical cyclone of the year. Alex has formed over the far eastern subtropical waters of the Atlantic Ocean. Last week on the 7th of January the National Hurricane Center (NHC) issued a special Tropical Weather Outlook (TWO), highlighting that an intense extratropical storm could acquire subtropical or tropical features within a week or so as it moved over the warmer waters of the subtropical eastern Atlantic, Ocean.
NHC TWO from Jan 7 2016

Quick Discourse on extratropical, tropical and subtropical lows

EXTRATROPICAL (XT) LOWS develop when potential energy is released due to warm and cold air masses interacting with each other. (Meteorologists call this baroclinicity). XT lows have fronts within their circulation (cold fronts/warm fronts). Winds can be light or very strong. In the more intense XT lows winds can be at or above hurricane force. In addition, the pattern of wind and precipitation with respect to the center of the low can be asymmetrical, that is not necessarily concentrated near the center of the storm. Lastly the core of the storm, especially aloft, exhibits cold temperatures. XT lows can form over land or over the ocean. They also tend to form in mid or high latitudes.

TROPICAL CYCLONES (TC) form over warm ocean waters with a water temperature of 25C-26C or higher. They develop over the subtropical and tropical oceans of the world mostly during the summer and fall months.Tropical cyclones are warm core systems. The energy that allows them to intensify is the latent heat that is released when moisture in the air condenses.  TCs have NO frontal features. Temperatures warm steadily as one gets to the center of the TC. Wind and precipitation tend to be symmetrical with respect to the storm; the most intense rain and wind are near the center. (If the TC becomes a hurricane then center or EYE is often dominated by light or even calm winds and minimal cloud cover and rain).

SUBTROPICAL CYCLONES (STC) are more or less hybrid systems. They often form in one of two ways. One is when an upper air storm with cold air aloft moves over warm SSTs. Cold air over warmth is unstable and thus thunderstorms and cloud formation increases.These type of ST lows tend to be rather large in circulation with a broad area of light winds and cloudy skies in the center. The strongest winds tend to be well removed from the center along with the more intense precipitation.

The second way a ST low can form is when an XT low moves over warm SSTs  (in this case as cool as 19C-24C).  and thunderstorms increase near the center of the low. The system then "warms" and begins to lose its frontal features. These types of ST lows tend to be relatively small in size.usually no more than 300 miles in diameter and some have had diameters as small as 100 miles across (midget cyclones).

If an STC moves over SSTs of 25-26C or greater it can develop a warm core and in turn  become completely warm core or tropical.

About Subtropical Storm Alex

Our newest subtropical storm (STS) Alex has evolved and developed from the latter scenario. On January 6 and 7 2016 and intense XT storm developed between the Bahamas and Bermuda. For the next 5 days it tracked east-northeast across the central subtropical Atlantic Ocean.
Track of Low that became Subtropical Storm Alex

 In time it began to move over marginal warm SSTs and started to lose its XT features and fronts. The SST map below from Environment Canada shows the water temperatures along the track of the low and the current location of  STS Alex. SSTs are 22-24C along and beneath the storm.

Sea Surface Temperatures across the North Atlantic and in the vicinity of STS Alex
STS Alex is forecast to gradually turn NE then N-NNW over the next few days. It will be moving over colder water temperatures so transitioning to warm core or tropical is not expected.

Official NHC track for STS Alex
 However, a surge of very cold air over Eastern North America will move out across the Atlantic and interact with Alex. Once again baroclinic elements will come together and Alex will once again intensify as rejuvenated extratropical storm impacting the shipping lanes of the North Atlantic.

Below is an infrared satellite of Alex over the Eastern Atlantic. Sat pic from Environment Canada
Satellite picture of STS Alex
From the NHC's forecast storm discussion from 5 PM AST 13 Jan: Alex is the first tropical or subtropical storm to form in January since an unnamed system did so in 1978, and is only the fourth known to form in thismonth in the historical record that begins in 1851.

Fascinating! Tomorrow I'll talk more about Alex and on Hurricane Pali in the Central Pacific, far far to the southwest of Hawaii, As a matter of fact Pali is closer to the equator than it is to Hawaii.