2012 Atlantic Hurricane Season Birdseye Discussion #114

By: NCHurricane2009 , 8:38 PM GMT on September 22, 2012

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...SEPTEMBER 22 2012...4:50 PM EDT...
This is the first day since August 15 with no active tropical cyclones in the Atlantic basin.

In the last 36 hrs...Nadine has transitioned from a tropical storm to a subtropical storm...then into a remnant low. See Nadine special feature section below. If Nadine stops showing potential to recurve southward then southwestward back into the Atlantic tropics...then I will cancel it as a special feature section on this blog.

Tropical wave moving across the Caribbean Sea...formerly Invest 92-L...leaves behind a disturbance offshore of the SE US while the wave itself enters hostile upper winds while moving into southeastern Mexico. The disturbance and the wave do not show signs of tropical development...and therefore have been cancelled as a special feature on this blog. See paragraph P7 for latest statement on this system.

Cut-off deep-layered vortex east of Bermuda...Invest 94-L....no longer expected to become a subtropical depression or storm...and therefore is no longer a special feature on this blog. See paragraph P3 for latest statement on this system.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0723Z-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).

...SPECIAL FEATURE...REMNANT LOW OF NADINE...
Nadine's central pressure has been slowly rising in the last 36 hrs...and correspondingly its winds have gradually been weakening. Previously Nadine had been maintaining strength from supportive divergence directly over Nadine (supplied by east side of paragraph P4 upper trough)...and supportive divergence just NE of Nadine (supplied by east side of upper trough over Nadine). The current weakening appears to be Nadine losing touch with these upper divergence sources while becoming tucked entirely below the less-divergent upper trough axis above it. Nadine continues to have the characteristics of a subtropical low with no upper anticyclonic outflow...indicating a shallower warm core system below cold core upper troughs. This is why the NHC downgraded Nadine to a subtropical storm last afternoon before cancelling advisories altogether last night. Cancellation in advisories came from lack of intense enough storm bands in Nadine's circulation to consider it a subtropical cyclone.

Nadine is currently located west of the Canary Islands and south of the Azores. The central Atlantic strong and deep-layered ridge to the west (described in paragraph P1) has been pushing Nadine southward until recently...when the new paragraph P4 surface cyclone offshore of Portugal and Spain has begun dragging Nadine eastward. It remains to be seen if Nadine continues eastward...or if it resumes a southward track once the surface cyclone lifts out to the NE. Currently the center of Nadine is over the 26 deg C sea-surface temp isotherm. If the track resumed on a southward course...the instability will increase with more low-level heat content coupled with cold temps of upper troughing above...potentially allowing for t-storms to regenerate...hence allowing Nadine to regenerate into a subtropical storm.

...MID-LATITUDES DISCUSSION...
P1...Weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes persists across central Canada and the central US. The surface cyclone has weakened from 995 mb to a diffuse 1004 mb center over Hudson Bay in the last 36 hrs while stalled beneath the less-divergent upper trough axis. The central US front extending from the cyclone has moved into the eastern US...while the cyclone drives a second front from Canada and into the central US. Western convergence of the upper trough supports dry air across the southern US/northern Gulf of Mexico and a building surface ridge over the western US. Southerly flow ahead of this weather system supports a W Atlantic upper ridge formerly supported by southerly flow ahead of the paragraph P2 system. This upper ridge has shifted east into the central Atlantic...and remains stacked with a strong central Atlantic surface ridge (mentioned in paragraph P4) to produce a deep-layered ridge.

P2...Upper trough moving across Greenland has entered the NE Atlantic and merged with paragraph P4 upper trough in the last 36 hours. Upper trough leaves behind surface cold front stretched across the north Atlantic and into the NW Atlantic...while the remainder of the front leaves behind a pair of surface troughs mentioned in paragraph P7. Days ago...western convergence of this upper trough used to support a low-level ridge across the eastern US...but since the the upper trough is now part of the paragraph P4 upper trough...this surface ridge has been weakening and is becoming replaced by building western US low-level ridge mentioned in paragraph P1.

P3...Deep-layered subtropical low east of Bermuda...Invest 94-L...is beginning to move NW about the deep-layered ridge mentioned in paragraph P1. It will soon pass NE of Bermuda....then curve and accelerate northward toward Atlantic Canada in advance of the paragraph P1 mid-latitude system. At the surface...the subtropical low should be merging with one of the cold fronts of the paragraph P1 system...so it will likely be non-tropical by the time it reaches Atlantic Canada. The upper vortex will eventually become absorbed by the paragraph P1 upper trough.

P4...Upper trough approaching Europe has stalled while becoming energized in amplitude thanks to amplifying central Atlantic upper ridge to the west (paragraph P1) and thanks to upper trough from the NW (paragraph P2) merging with it. Eastern divergence of the amplifying upper trough has quickly spun up a surface frontal cyclone offshore of Portugal and Spain. NW flow on the back side of this upper trough converges with W flow on the north side of the upper ridge...supporting a greater-than-1032 mb strong surface ridge that has moved from the W Atlantic to central Atlantic in the last 36 hrs.

P5...What is left of the eastern Atlantic surface ridge has merged with the central Atlantic surface ridge mentioned in paragraph P4. Associated dry air in the eastern tropical Atlantic is now being wafted westward by the south side of the paragraph P4 central Atlantic surface ridge...and by south side of the paragraph P1 central Atlantic upper ridge.

...TROPICAL BELT DISCUSSION...
P6...SE Mexico upper ridge has been eroded by amplifying paragraph P1 upper trough to the north. Yesterday's retrograding upper vorticity crossing the Lesser Antilles has spread across the SE half of the Caribbean Sea. Upper ridge cell over tropical wave ex-92L (paragraph P7) remains amplified across Cuba...the Bahamas...and W Atlantic in advance of paragraph P1 upper trough. Easterly flow across north side of SE Caribbean upper vorticity is converging with northerly flow from this upper ridge cell to produce sinking dry air in the E half of the Caribbean. Tropical upper ridge cell in the central tropical Atlantic remains. In the vicinity of the Cape Verde Islands...upper troughing over Nadine appears to be cutting-off to the east of this central tropical Atlantic upper ridge cell.

P7...Tropical wave ex-92L in the western Caribbean in the previous discussion is now moving into SE Mexico. Flanking the tropical wave is a surface trough in the Bay of Campeche and another over the W Bahamas...originating from the decaying surface front in paragraph P2. Outflow of the paragraph P6 upper ridge cell overhead and surface forcing of W Bahamas surface trough is creating a disturbance offshore of the SE US. The tropical wave itself and Bay of Campeche surface trough under hostile westerly shear from south side of paragraph P1 upper trough.

P8...The tropical wave nearing the Lesser Antilles in the previous discussion is dissipating while moving into the NE Caribbean.

P9...Tropical wave west of the Cape Verde Islands in the previous discussion is now midway between the Cape Verdes and Lesser Antilles. It has overcome the paragraph P5 dry air in the last 36 hours thanks to enhanced poleward upper outflow streaming into the deep-layered low in paragraph P3.

P10...Satellite imagery suggests a potent tropical wave with a low pressure spin has stalled out along the W coast of Africa. The stalled motion is due to absence of low-level steering easterlies thanks to ridge weakness from paragraph P4 weather system and remnant low of Nadine.

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