Sub-Tropical Woes


A weak trough of low pressure expanded northwards across the island late on the 11th of April. This was out ahead of a weak slow-moving cold front that would cross the island the next day.  The front consisted of a line of clouds, a sharp wind shift, and a sudden drop in temperature and humidity.

My PWS records a sharp drop in temperature just after 3pm April 12th, the dew point also drops off after this time.

This same front continued to the east, met up with the pre-frontal trough and became a stationary front about 600 miles to our east. A massive blocking ridge of high pressure over the far Eastern Atlantic remained in place allowing the front to remain stationary. During this time a wave of low pressure developed along this front as an upper level low became cut off from the jet stream in the area. The surface low deepened and became a strong mid-latitude cyclone, however, with the blocking high to the east, it moved very little.

NOAA ASCAT (April 17th 2012, 21:14 UTC) descending pass shows a region of gale force winds on the North side of the low as it was declared an invest by the National Hurricane Center for a slight chance of sub-tropical transition.

The low began a slow drift westward as high pressure to the east shifted to the northeast. A weak upper level shortwave trough was absorbed by the upper level low over top our system, injecting mid and upper level moisture. The airmass to its west became modified by the warmer waters over the western Atlantic and the surface low pressure’s fronts began to weaken into troughs while the upper level low remained aligned with it.  Despite sea surface temperatures around 70F, the system may have briefly had an asymmetric, shallow warm core at this point early on the 18th. However, after this time, a new tongue of mid-upper level dry air made it into the system hurting deep convection; this halted any further transition to a sub-tropical cyclone.

NASA's MODIS satellite image of the low at its closest approach to Bermuda on April 18th 2012. A circle of shallow water around Bermuda appears as a lighter shade of blue-green on the left hand side of this image.

The surface low began to weaken as the upper level low was knocked out of alignment by an increase in vertical wind shear associated with the approach of the next front late on the 19th . The surface low began to drift back towards the east and eventually northeast as it degenerated into a low cloud swirl, void of any deep convection. It eventually dissipated over the northern Atlantic by the 24th.

It was unlikely to get a full sub-tropical transition out of this because of the relatively cold water and unfavorable environment this time of year. But this feature was interesting to track and forecast as there was an unusually high amount of model agreement in terms of the track and intensity of this low before it even formed. Its impacts on Bermuda were limited to a stiff northerly breeze, and an increase in cloud cover at times. The AG Show was not affected :).

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