Cut-Off Over the Atlantic


12z May 17th ECMWF model output 300mb heights and winds at 15z May 17th. Closed low detached from mid-latitude jet stream over central north Atlantic.
12z May 17th ECMWF model output 300mb heights and winds at 15z May 17th. Closed low detached (cut-off) from mid-latitude jet stream over central north Atlantic. Another cut-off low exists over southern Europe where historic flooding is ongoing. -Wunderground’s WunderMap

A weak extra-tropical cyclone tracked southeastwards from Newfoundland then eastwards across the north-central Atlantic, strengthening slightly. It pushed a weak ‘back door’ cold front across Bermuda from northeast to southwest on Wednesday with some isolated light showers. The cyclone then stalled over the north-central Atlantic as it became detached from the west-east flow aloft associated with the mid-latitude jet stream. As it stalled, the surface cyclone became occluded, and the associated upper trough closed off and became nearly co-located with the surface cyclone. We are now left with a modest surface cyclone, analyzed by the Bermuda Weather Service with a minimum central pressure at 997mb as of 12z, and a very deep upper level cyclone. 12Z GEFS showed that 500mb heights associated with this upper low were more than 5 standard deviations below normal.

12z May 17th ECMWF at 15z May 17th centered on the deep layer cyclone in question. Top 300mb heights and winds (kts); 2nd from Top 500mb heights and vorticity; 2nd from Bottom 850mb heights, winds, and temperatures in C; Bottom Sea Level Pressure and 6hr Precipitation Accumulation.
12z May 17th ECMWF at 15z May 17th centered on the well-stacked, deep layer cyclone in question. Top 300mb heights and winds (kts); 2nd from Top 500mb heights and vorticity; 2nd from Bottom 850mb heights, winds, and temperatures in °C; Bottom Sea Level Pressure and 6hr Precipitation Accumulation. (click picture for larger image) – Wunderground’s WunderMap

This deep layer cyclone (as in its circulation extends almost uniformly to a notable height) has now begun to move west-southwestward, and as it retrogrades it is beginning to lose its frontal characteristics. The cyclone has been cut off from the mid-latitude jet stream and its associated strong horizontal temperature gradients for over a day. Further, being over the sub-tropical Atlantic, cool air brought equatorward and warm air brought poleward are both being modified by the ocean due to the cyclone’s slow motion. As these air masses homogenize, the fronts associated with the cyclone become weaker and eventually dissipate. Some organized convection has developed near the center of the cyclone, and if it loses frontal characteristics and can sustain deep organized convection near its center it may have a small window of opportunity to transition to a sub-tropical cyclone Sunday into Monday. Sea surface temperatures along its track are marginal for sub-tropical development but anomalously cold air aloft may provide enough instability to sustain deep convection. It is generating gale force winds in a broad area extending from its center, so if it makes this transition, it could get a name (Arthur) from the National Hurricane Center. Regardless of development, it should remain several hundred miles away from Bermuda.

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An ASCAT pass this afternoon caught the center of this cyclone. These satellite derived winds indicate an area of gales near the center.
An ASCAT pass this afternoon caught the center of this cyclone. These satellite derived winds indicate an area of gales (the brown color) to the west of the center.

Meanwhile, in Bermuda, a narrow ridge of high pressure has remained in control of our weather. Fair, very slightly cooler and much less humid than normal weather has been in place since entering the cool air mass behind that back door cold front that passed on Wednesday. Easterly winds today backed northerly as the narrow north-south oriented ridge over the island narrowed further also retrograded to our near west. Significant swells generated by the cyclone to our distant east should begin to reach Bermuda  on Sunday. However, expect to remain in fair weather associated with the ridge for much of the rest of the weekend, save for an increase in cloudiness as a front approached from the west.

That slow moving cold front (responsible for heavy rain and flooding in Eastern United States) to our west is inching eastwards – the squeeze between the low pressure moving west-southwestward over the central North Atlantic and this approaching front from the west is resulting in the narrowing of our ridge. Eventually, this approaching front will make its way into our area bringing a chance for isolated showers and thunderstorms starting late Sunday night. But the front is weakening on its approach and is expected to fall apart in our area on Monday leaving a lingering threat for isolated showers.  But, it should introduce stronger south-north deep layer flow to our east and allow the cyclone there to move with a little more purpose. We should monitor the remains of this front post-Monday for potential development of an extra-tropical cyclone nearby later in the week.

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