2012 Atlantic Hurricane Season Birdseye Discussion #113
...SEPTEMBER 21 2012...5:15 AM EDT...
Tropical Storm Nadine beginning to move southward away from the Azores. Tropical storm warnings remain in effect across the islands...but could be dropped later today if Nadine moves sufficiently away. The future of Nadine remains uncertain...but there is a possibility that the storm could recurve southward then southwestward back into the Atlantic tropics in the next 48 to 72 hours. Currently it appears there will be upper troughing over the cyclone thru that time...so its re-emergence into the tropical belt is more likely as a subtropical storm rather than a tropical storm. See Nadine special feature section below for details.
Tropical wave moving across the Caribbean Sea...formerly Invest 92-L...is producing disturbed weather over Cuba and or the Bahamas as expected previously. South Florida is also covered by some of this activity. If the disturbed weather does not show signs of tropical development...then I will be finally dropping this system as a special feature on this blog. For now...see the ex-92L special feature section below for details on this wave.
Cut-off deep-layered vortex has west of Nadine and east of the Bermuda...Invest 94-L....still has the potential to become a subtropical depression or storm. See Invest 94-L special feature section below for details.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0131Z-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...TROPICAL STORM NADINE...
Nadine has actually slightly strengthened in the last 24 hours...which I diagnose is due to supportive divergence directly over Nadine (supplied by east side of paragraph P3 upper trough)...and supportive divergence just NE of Nadine (supplied by east side of upper trough over Nadine which has recently amplfied further into an upper vortex). With Nadine in the last day not showing upper anticyclonic outflow in the 200 mb wind barbs in the above atmo birdseye chart...nor showing cirrus outflow clouds on satellite...this indicates to me a shallower warm core system that is supported by divergence from cold core upper troughs (i.e. a subtropical storm). However...I have never heard of a case where the NHC downgrades a tropical cyclone to a subtropical cyclone operationally. Some examples of systems transitioning from tropical to subtropical (according to NHC post-storm analysis) are Lee 2011...Gordon 1994...and Allison 2001. The recent NHC discussions state that they may soon downgrade Nadine into an extratropical (non-tropical) low as the cold front of the paragraph P3 system to the north penetrates the circulation. However...I currently observe Nadine maintaining independence from this cold front cloud band while firing a well-curved t-storm band in her south half...so I prefer to maintain Nadine as a tropical or subtropical cyclone thru the forecast period.
Due to the W Atlantic deep-layered ridge to the west (described in paragraph P2)...Nadine is beginning to turn southward according to the most recent segment of the NHC recorded storm track in Figure 1...and according to infrared satellite loops. With the surface component of the deep-layered ridge forecasted by 00Z GFS to be greater than 1030 mb for the next days...I am not shy to show a track that curves south then southwest (based on the low-level flow around the ridge shown in 00Z GFS). However...I show my south and SW track at a slower pace than the previous in case I am wrong...because the NHC official forecast has not budged toward that solution. I also am not forecasting beyond 48 hours in case the scenario I posed in Figure 1 is incorrect. My forecast track is still further south than the 00Z GFS itself...as the GFS prefers to slow the southward track of Nadine by 48 hours than curve her slowly ENE by 72 hours. I suppose the GFS gives more credence to stagnating swirling upper flow of the upper troughing above Nadine...and or gives more credence to the paragraph P3 upper trough passing by to the north (which will become very impressive to the NE of Nadine as it amplifies to the east of the deep-layered ridge). The GFS solution of an ENE track by 72 hours however does not make sense to me...as it simultaneously shows the trough de-amplifying during that time (I would expect the ENE turn would be caused by the trough amplifying further or at least maintaining amplitude).
Figure 1: My forecast for Tropical Storm Nadine created this morning.
Previously my thoughts on Nadine continuing SW into the tropics (i.e. following my forecast track in Figure 1) is that it would eventually get blasted by northerly shear while sliding beneath the west side of the upper troughing. But now the GFS this morning shows a portion of the upper troughing splitting off while retrograding about the deep-layered ridge. Now if Nadine follows my forecast track...this retrograding upper troughing would keep the vertical shear over Nadine low. Therefore thru the forecast period...I show Nadine maintaining strength. I do not declare it non-tropical by 24 hours like the NHC does...as I model Nadine diving southward into increasing instability as she re-enters warm waters above 26 deg C (instability further enhanced by cold core upper troughing above the cyclone).
The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I maintain through the forecast period based on the intensity forecasts from me and the NHC. The swath is kept symmetrical through the forecast as she remains under low shear directly under the upper troughing.
...SPECIAL FEATURE...TROPICAL WAVE EX-92L...
The tropical wave in the central Caribbean in the previous discussion is now moving into the western Caribbean. The paragraph P5 upper ridge cell over the area has become amplified over Cuba and the Bahamas by warm air advection ahead of the paragraph P1 and P2 weather systems...the outflow of which supports scattered t-storm activity across south Florida...Cuba...and the Bahamas this morning. The surface frontal zone of the paragraph P2 system may also contribute. Currently watching to see if a tropical disturbance emerges due to surface forcing of incoming frontal zone and the tropical wave...coupled with the favorable upper outflow of the amplified upper ridge cell.
...SPECIAL FEATURE...SUBTROPICAL LOW INVEST 94-L...
Deep-layered subtropical low east of Bermuda persists. Sea-surface temps remain in the the 27 to 28 deg C range...the upper vortex above is keeping the surface spin in favorable low shear...and it appears the upper vortex is cold enough for instability above these waters (as evidenced by a continued ring of t-storms around the surface spin). I was expecting this to have become a subtropical depression or storm by now. I still forecast this to briefly become a subtropical cyclone before the conditions become less favorable to do so in the next 48 hours.
Currently the deep-layered subtropical low is trapped in meandering steering currents below its upper vortex...and is blocked from any W or NW progression thanks to W Atlantic deep-layered ridge (paragraph P2) hugging the system. In the next 48 hours...the deep-layered ridge should pass to the north...steering the upper vortex and subtropical cyclone W then NW such that is passes NE of Bermuda. It should then curve and accelerate northward toward Atlantic Canada in advance of the paragraph P1 and P2 mid-latitude systems. At the surface...the subtropical cyclone should be merging with the cold front of the paragraph P2 system (if not...then the cold front of the paragraph P1 system)...so it will likely be non-tropical by the time it reaches Atlantic Canada. The upper vortex will become absorbed by the paragraph P1 upper trough.
P1...Weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes has dived southeast while absorbing a large portion of paragraph P2 upper trough. The surface cyclone is currently at 995 mb over southern Hudson Bay while the surface front is across the central US. Western convergence of the upper trough supports dry air across the southern US and a building surface ridge over the western US.
P2...Major upper trough from eastern Canada to the Gulf of Mexico has split in the last 24 hours. The portion over the US and Gulf of Mexico has been absorbed by paragraph P1 upper trough. The east Canada portion has shifted east across Greenland. The associated surface cold front is stretched from Greenland...across the W Atlantic...and into the Gulf of Mexico. North end of the front is anchored by a surface low that weakened from 981 to 992 mb in the last 24 hours while shooting NE into Greenland from eastern Canada. This surface low remains whirled beneath the less-divergent axis of the upper trough...and therefore may continue to weaken. Southerly flow ahead of the 992 mb low is supporting a W Atlantic upper ridge via warm air advection. This upper ridge has stacked with the W Atlantic surface ridge in paragraph P3 to produce a deep-layered ridge. Finally...western convergence of this upper trough used to support a low-level ridge across the eastern US...but since the main portion of the upper trough is now over Greenland...this surface ridge is weakening and becoming replaced by building western US low-level ridge mentioned in paragraph P1.
P3...Upper trough continues progressing eastward across the Atlantic high seas and is nearing Europe. Western convergence of the upper trough supports a strong low-level ridge over the W Atlantic that currently features a greater-than-1030 mb center.
P4...Eastern Atlantic surface ridge...currently with a 1020 mb center...persists. It is supported as NW upper flow from upper troughing over Nadine and SW upper flow from paragraph P5 upper ridging convergence. As noted in the last days...dry air in the eastern tropical Atlantic is still noted with this surface ridge...perhaps as it south side wafts Africa desert dry air westward. The dry air could is also enhanced by the aforementioned upper convergence.
...TROPICAL BELT DISCUSSION...
P5...Central America and SE Mexico upper ridge cell is now just over SE Mexico...with W Caribbean upper trough moving into Central America while peristing to the east of that. Yesterday's retrograding upper vortex nearing the Lesser Antilles is about to cross the islands. Two upper ridge cells (one to the east and the other to the west) continue flanking this upper vortex...one of which is over tropical wave ex-92L...the other which has become vertically stacked with 1020 mb center of paragraph P4 surface ridge...in essence creating a deep-layered 1020 mb center. Inverted upper trough west of the Cape Verde Islands continues retrograding westward about what is now the 1020 mb deep-layered center. Another inverted upper trough is forming just NW of the Cape Verde Islands...perhaps a fracture of the upper troughing over Nadine that will soon retrograde SW then W about the 1020 mb deep-layered center.
P6...The tropical wave nearing the Lesser Antilles has lost its moisture and activity while becoming suppressed by the upper convergence of the paragraph P5 upper vortex nearby. The upper vortex is not cold enough to destabilize the atmosphere for t-storms to re-fire with the wave.
P7...Tropical wave over and south of the Cape Verde Islands in the previous discussion is now west of the islands. It remains suppressed by the paragraph P4 dry air.