Pre-Season Sub-Tropical Disturbances!


So far, three pre-season sub-tropical disturbances have formed, each showing more promise than the last. In today’s post I will recap these disturbances. A sub-tropical cyclone has characteristics of both extra-tropical and tropical cyclones in that it has an associated upper level cold pocket or upper level low like an extra-tropical cyclone, but doesn’t have fronts and therefore doesn’t derive its energy from differences in temperature from north to south at the surface like a tropical cyclone.

When an upper level low is aligned with a surface low over warm water, instability due to the difference in temperature between the surface and at the altitude of the upper level low (which is essentially a pocket of cold air), allows convection to develop. Condensation in this convection is a warming process, releasing latent heat into the upper level low – weakening it. If the surface low can sustain the convection, it can warm the upper level low enough that it completely dissipates and it replaced by an upper level high.

Upper level high pressure is typically associated with upper level divergence – this is the process that helps fully tropical cyclones to sustain their convection; when there is wind shear this process is disrupted, the cyclone cannot sustain itself and dissipates. Sometimes, an extra-tropical cyclone can go straight to being tropical if the upper level low dissipates before the surface low loses its frontal characteristics.

April 15th-20th 2012:

MODIS Satellite imagery of invest 91L (April 18th 2012)

A stationary front was invigorated by an upper level cut off low pressure and a surface reflection developed. It began as an extra-tropical storm – that is a storm with baroclinic origins and therefore cold and warm fronts. It was however, trapped in an area with little steering and so only slowly drifted westward. As it did so, the airmasses around the storm modified due to their presence over the central Atlantic for so long and the low pressure began to lose its frontal characteristics. This is a key stage in a transition to a sub-tropical storm. Winds as observed by ASCAT satellite showed that when the low was as close to sub-tropical as it got, winds of 40-45mph were present. However, the low failed to develop deep convection over its center because the upper level low over it weakened and the water temperatures were still too cold for full sub-tropical transition.

May 11th-14th 2012:

GOES Satellite image from NAVY NRL Tropical Cyclone page (Invest 92L May 12th 2012 1615 UTC)

A complex area of low pressure dipped southwest from the jet stream and cut away from it to the south of the Azores islands. The complex low quickly lost its frontal characteristics as the airmasses surrounding the storm were fairly similar to begin with. However, the fronts, instead of dying completely, transitioned to troughs of low pressure. This kept the low center from becoming truly sub-tropical. However as the fronts transitioned into troughs, significant convection developed over the center of the surface low and quickly wrapped around it and at times formed an eye-like feature.

ASCAT satellite caught the system multiple times, and showed the storm in decline after the eye-like feature disappeared. At its peak, winds of 50-60mph were observed near its center through the satellite. However, because it could not sustain the convection – as the upper level low weakened, the instability decreased below what was necessary to sustain the convection because the water was still too cold at around 20C (68F). It quickly degenerated to a low cloud swirl and was picked up by an approaching front after its upper level low degenerated.

May 19th 2012:

A cut-off area of upper level low pressure over Texas drifted eastward and  began to interact with a stationary front over the east coast of the United States. The combined system continued eastward and developed a surface trough  near the coast. Overnight on the 18th, the trough moved offshore and, with the instability created between the warm waters of the gulf stream and the upper level low, moderate-deep convection developed over the trough and a surface circulation began to develop.

Long Range Base National Weather Service NEXRAD Radar out of Wilmington, NC (May 19th 2012)

NEXRAD Radar from sites in North and South Carolina show low level banding structures developing and at times an eye-like feature. However, satellite imagery does not show this eye-like feature. This feature is common in sub-tropical transitions as there is typically a lot of dry air nearby. ASCAT Satellite passed over the storm observing 40-50mph winds near the center of a closed surface circulation.

This disturbance has the highest potential to become a sub-tropical or tropical storm so far because it is over the warm waters of the gulf stream, currently at 26C (80F) – this means convection won’t suffer as much as in the previous disturbances as its upper level low weakens, and there is a higher probability that convection will sustain itself over the center of the surface low. It is, however, pressed for time for additional development and strengthening as the forecast steering currents have the low coming ashore in North Carolina in 36hours.

Current mesoscale models are not handling the system very well, with some showing complete  tropical transition and intensification to hurricane strength, and other showing complete degeneration as the low detaches from its precursor trough of low pressure.

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