2012 Atlantic Hurricane Season Birdseye Discussion #166
...THURSDAY NOVEMBER 22 2012...3:20 PM EDT...
This discussion has been released 60 hours since the previous...due to interruptions from me traveling on Thanksgiving holiday.
Still monitoring weather in the western Atlantic Ocean for potential subtropical cyclone development. As expected...first phase of the western Atlantic surface low has not developed into a subtropical cyclone due to increased westerly vertical shear below de-amplified paragraph P3 southern upper trough. This first phase has since exited stage right and is moving across the Azores this afternoon.
Second phase has begun in association with broad 1004 to 1008 mb surface low offshore of the southeastern United States as mentioned in paragraph P2 below. As remarked in that paragraph...this surface low is supported by eastern divergence of a cut-off upper trough...a feature which currently has an upper vortex at its north end that is reducing the vertical shear and whose de-stabilizing cold upper air is supporting increasingly organizing t-storms across the northwest half of the 1004 to 1008 mb surface center. In addition...the formation of this surface center just north of the 26 deg C sea-surface temperature isotherm supports low-level warmth for additional instability. Given all this information...chances for subtropical cyclone development are increasing. This feature may warrant a special update on my blog later today...or may be upgraded to a special feature in my next full blog update. This feature and cut-off upper trough will be headed eastward toward Bermuda in the next 24 hours...then hook northward toward Atlantic Canada by 72 hours while steered by the southeast half of the paragraph P1 upper trough/surface frontal system.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-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.
...THERMODYNAMICS BIRDSEYE CHART...
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).
P1...Next upper trough and surface frontal system...new to this discussion...has entered the top-left of the above charts. The upper trough is currently over the western US. Eastern divergence of the upper trough supports 999 mb surface frontal depression over north-central Minnesota. Western convergence of the upper trough supports 1023 mb SW US surface ridge.
P2...Upper trough and surface frontal system crossing the central US in paragraph P1 of the previous discussion is now over eastern Canada. Eastern divergence of the upper trough supports marine 994 mb surface frontal depression just SW of Greenland. South fragment of this upper trough has cut-off offshore of the SE US...where eastern upper divergence of this cut-off supports a 1004 to 1008 mb surface low that is an extension of the west lobe of the Azores surface low mentioned at the end of paragraph P3 below. Western convergence of the upper trough and its cut-off south fragment supports lengthy 1020 to 1028 mb surface ridging extending from east Mexico...across the eastern US...and into Atlantic Canada.
P3...Northern upper trough crossing the Atlantic high seas in the previous discussion has entered the NE Atlantic (a south fragment of this upper trough has just exited the picture into NW Africa where its eastern divergence supports NW Africa surface trough seen in top-right of above atmo chart). Southern upper trough in the last 60 hours has moved from the W Atlantic/W Caribbean and into the open Atlantic waters near the Azores. Lengthy surface frontal/troughal activity across the Atlantic persists while supported by eastern divergence of the northern and southern upper troughs. This frontal/troughal activity continues to curl into surface cyclone that has moved into the waters east of Greenland in the last 60 hours...and this cyclone remains associated with the northern upper trough. Western convergence of the northern upper trough supports strong surface ridge that has long since moved offshore of the NE US in the past 60 hrs. This surface ridge was 1027 mb just south of the Azores as of 0000Z Nov 21...and is 1025 mb just north of the Canary Islands as of 1200Z today. The 1013 mb low offshore of the SE US in the previous discussion continues to be supported by eastern divergence of the southern upper trough...and because this upper trough has moved toward the Azores as mentioned above...it is now the less-than-1004 mb low approaching the Azores at this hour.
...TROPICAL BELT DISCUSSION...
P4...Upper ridge over the central tropical Atlantic persists. Upper divergence west of this upper ridge supports scattered t-storm activity across the central Caribbean Sea. Meanwhile...upper convergence east of this upper ridge axis formerly supported a north Atlantic surface ridge 60 hours ago that has been long-absorbed by the 1025 mb Canary Islands ridge mentioned in paragraph P3.
P5...Cut-off upper vortex midway between the Cape Verde Islands and Lesser Antilles has weakened into an upper trough that has been pushed east toward the Cape Verde Islands by growth of the paragraph P4 upper ridge. Upper divergence east of this cut-off upper trough supports some cloudiness. What is left of surface troughing formerly supported by this upper divergence has moved west while steered by 1025 mb Canary Islands ridge mentioned in paragraph P3. North fragment of this surface troughing is currently NE of the Lesser Antilles becoming assimilated into lengthy surface frontal/troughal activity mentioned in paragraph P3. South fragment of this surface troughing is well SE of the Lesser Antilles.