2012 Atlantic Hurricane Season Birdseye Discussion #38

By: NCHurricane2009 , 11:50 AM GMT on June 28, 2012

...JUNE 28 2012...12:18 PM EDT...
Debby becomes extratropical (non-tropical) much sooner than predicted. See first special feature seciton for further details.

Caribbean tropical wave in paragraph P10 (in the tropical belt discussion) is at less risk for developing.

Tropical wave southwest of the Cape Verde Islands has been upgraded to a special feature on this blog. See second special feature section for further details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1931Z-released HPC analysis.

In light blue is upper air analysis, with 200 mb wind barbs calculated by GOES satellite imagery showing the upper-level wind direction. Based on the 200 mb wind barbs, blue-dashed lines are locations of upper troughs, blue-zig-zag lines are locations of upper ridges. Blue Ls are locations of upper lows, blue Hs are locations of upper ridges.

In red is surface analysis, with solid lines indicating locations of surface fronts, dashed lines indicating locations of surface troughs, and zig-zag lines indicating surface ridge axes. Ls indicate surface lows, Hs indicate surface highs.


This chart is generated using GOES water vapor satellite imagery. Brown indicates dry air. White, blue, and purple indicates moist air. An increase in moisture indicates slower air parcel lapse rates with elevation and hence an increase toward instability.

Sea-surface temperatures are overlaid with light blue isotherms. The 26 deg C isotherm is highlighted in red. Waters at and south of the 26 deg C isotherm indicate low-level warmth and hence faster environmental lapse rates with elevation (more instability). Waters north of the 26 deg C isotherm indicate slower environmental lapse rates with elevation (less instability).

The National Hurricane Center has downgraded Debby to a remnant frontal boundary (non-tropical) low in the last 24 hours. The front attached to the 997 mb cyclone in paragraph P2 has been sagging southward...and Debby as a result has merged with it. As a non-tropical entity...the remnant of Debby is supported by divergence ahead of the upper trough/low mentioned in paragraph P2.

Expect the remnants of Debby to continue northeastward while steered by the southwesterly flow ahead of the system in paragraph P2...and southwesterly flow will be re-enforced by the frontal system in paragraph P1 in the coming days. Her northeastward track is a tad faster than previously forecast by me and the NHC...and is following the NHC's forecast track much closer than my forecast track (which had a southward bias). Debby's remnants will deliver weather to Bermuda (rain with gusty winds possible)...but because she is entagled with the frontal system in paragraph P2...it will be hard to say which is directly Debby's impact and which is the front's. The strongest weather on Debby's northeast half however looks to pass just northwest of Bermuda if current satellite trends continue. Should air mass contrasts diminish across the frontal segment Debby is within...she may be re-upgraded to tropical status at any time now.

Tropical wave SW of Cape Verde Islands continues to fight off dry air mentioned in paragraph P9...because it has received enhanced poleward upper outflow thanks to upper vorticity embedded in the east Atlantic upper ridge (this upper vorticity also mentioned in paragraph P9). Paragraph P9 also explains how there is plenty of deep-layered easterly flow ahead of this tropical wave...promoting a low shear environment also supportive for development in the coming days. The tropical wave still shows cyclonic banding features...has been upgraded with a 1013 mb low...and the National Hurricane Center continues to gives this a 10% chance of developing in the next 48 hours.

P1...Warm air advection ahead of surface frontal system in western North America continues to support an upper ridge...which is now moving into the central US. This upper ridge is associated with intense heat that has started over the central US and moving toward the eastern US. The western North America surface frontal system has its strong low pressure in SW Canada. Meanwhile...upper convergence on the east side of the west US upper ridge conitnues supporting dry air.

P2...Frontal system and longwave upper trough over the eastern US has consolidated into an upper low/trough associated with cool air advection of a 998 mb surface cyclone that has made landfall in the Maine/Nova Scotia area..and is currently 997 mb. This 997 mb cyclone should now begin weakening thanks to a much less divergent environment directly below the upper trough/low axis. The cut-off Gulf of Mexico upper vortex that stemmed from this weather system has weakened into a south Texas upper trough while swinging westward about the anticyclonic center of the upper ridge mentioned in paragraph P1. Of final note...upper convergence on the back side of this upper trough supports a 1018 mb ridge over the eastern US that moved southward from the Great Lakes...but this surface ridge will be diminishing once low surface pressures from the frontal system in paragraph P1 arrives.

P3...Upper ridge over the western Atlantic and south Greenland remains supported by warm air advection ahead of the complex frontal system outlined in paragraph P2 above. The eastern convergence of this upper ridge supports surface 1026 mb ridging in the vicinity of Greenland that is connected to the 1030 mb center mentioned in paragraph P5.

P4...Large upper trough in the north-central Atlantic is moving NE toward Europe...and the surface frontal cyclone it once supported is decaying with a lack of divergence beneath the upper trough axis. Position of the surface cyclone in the above birdseye charts is based on satellite imagery since it is outside the scope of the TAFB and HPC analyses. Cut-off upper trough east of Bermuda in the previous discussion continues to be pushed eastward by growing upper ridge mentioned in paragraph P7. It has now arrived into the open central Atlantic.

P5...Open Atlantic surface ridge has 1024 mb center SW of the Azores which has recently grown to 1030 mb and is now NW of the Azores. The 1030 mb center is supported by upper convergence on the back side of the north-central Atlantic upper trough (paragraph P4). South end of north-central Atlantic frontal cyclone's cold front (paragraph P4) has split off into a surface trough retrograding westward around the south side of the 1030 mb center.

P6...1013 mb NE Atlantic surface low (whose formation was described in paragraph P6 of previous discussion) has moved into Spain ahead of the north-central Atlantic weather system in paragraph P4. It has been downgraded to a surface trough.

P7...Upper ridge over the southern Gulf of Mexico continues to be expansive....and its inflated state is attributed to previous latent heat release from Debby's t-storm clouds. This upper ridge has gone further eastward expansion thanks to warm air advection ahead of complex frontal system outlined in paragraph P2.

P8...Cut-off upper vorticity in the eastern Caribbean has dissipted.

P9...Expansive east Atlantic upper ridge continues. In conjunction with the surface ridge in paragraph P5 and upper ridge in paragraph P7...deep-layered easterly flow exists across much of the Atlantic tropics that is advecting African desert dry air (brown shading in the above thermo birdseye chart) westward. Remaining embedded in the upper ridge is a westward-tracking upper low that has recently weakened to an east-west upper trough.

P10...Tropical wave in the eastern Caribbean Sea in the previous discussion is now in the central Caribbean. It remains suppressed by dry air mentioned in paragraph P9 above. Because the upper vorticity in paragraph P8 has dissipated...the upper divergence between it and the upper ridge in paragraph P7 is gone...and so are the associated t-storms over Panama. With the Panama activity dissipated...this tropical wave's only potential for development is if it gets closer to the favorable low shear/enhanced outflow center of the paragraph P7 upper ridge.

P11...Tropical wave SW of the Cape Verde Islands has been moved to above 2nd special feature section.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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1. weatherh98
11:45 PM GMT on June 28, 2012
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