2012 Atlantic Hurricane Season Birdseye Discussion #112

By: NCHurricane2009 , 9:09 AM GMT on September 20, 2012

...SEPTEMBER 20 2012...5:15 AM EDT...
Tropical Storm Nadine producing gusty winds for the western and central Azores. Tropical storm warnings remain in effect across the islands. The future of Nadine is uncertain...but there are increasing indications that the storm could recurve southward then southwestward back into the Atlantic tropics in the next 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...could produce a disturbance over Cuba and or the Bahamas in the next 72 hours...and therefore I still am not eliminating it as a special feature on this blog. See the ex-92L special feature section below for details.

Anticipated cut-off deep-layered vortex has formed west of Nadine and east of the Bermuda and has been upgraded to Invest 94-L due to its potential in becoming a subtropical depression or storm. See Invest 94-L special feature section below for details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0123Z-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).

As long anticipated...the paragraph P3 mid-latitude system and its upper trough has approached Nadine from the west while decomposing into two features. The first is a cut-off deep-layered vortex west of Nadine (special feature Invest 94-L). The second is an upper trough passing by to the north. What I did not anticipate is Nadine maintaining strength 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 what I previously described in the last two Nadine special feature sections as a "pesky" upper trough W of Nadine...which is now over Nadine). With Nadine no longer showing upper anticyclonic outflow in the 200 mb wind barbs in the above atmo birdseye chart...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...we shall see. Some examples of systems transitioning from tropical to subtropical (according to NHC post-storm analysis) are Lee 2011...Gordon 1994...and Allison 2001.

Due to developing deep-layered ridge to the NW (described in paragraph P2)...Nadine is beginning to accelerate southeastward according to infrared satellite loops. The track on infrared satellite seems both faster and slightly left of the 11 PM EDT NHC forecast...so my track forecast in Figure 1 has a slight left bias and is faster with respect to NHC from the get-go. With the surface component of the deep-layered ridge forecasted by 00Z GFS to be greater than 1030 mb for the next 72 hours...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). My forecast track is even faster than the 00Z GFS itself...as the GFS prefers to slow the southward track of Nadine by 48 and 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). I do not forecast beyond 72 hours in case my aggressive south and SW track is wrong.

Figure 1: My forecast for Tropical Storm Nadine created this morning.

It is worth noting that if Nadine continued on my forecasted brisk pace shown in Figure 1...it would enter hostile northerly shear on the west side of the upper troughing by 120 hrs. But for the next 72 hours...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 my new intensity forecast. The swath is kept symmetrical through the forecast as she remains under low shear directly under the upper troughing. This swath covers the western and central Azores...but all of the Azores should continue to experience outer rain squalls from Nadine's north side in the next 24 hours...these squalls containing wind gusts that could reach tropical storm force (my impact swath represent tropical storm sustained winds as opposed to gusts).

The tropical wave in the eastern Caribbean in the previous discussion is now moving west across the central Caribbean. The tropical wave is sliding out of the paragraph P5 upper ridge cell....and into the less favorable environment beneath the W Caribbean upper trough (also mentioned in paragraph P5). However...there are t-storm clouds (albeit scattered) persisting beneath the favorable upper ridge cell over Hispaniola and Jamaica. Moreover for the next 72 hours...there is potential for the favorable upper ridge cell to become amplified over Cuba and the Bahamas by warm air advection ahead vigorous paragraph P2 weather system. The surface frontal zone of the paragraph P2 system may also contribute. Therefore...expect the t-storm activity to spread into Cuba and the Bahamas...where a tropical disturbance could emerge beneath the favorable upper ridge cell.

Paragraph P3 in the mid-latitudes discussion best describes the formation of this deep-layered subtropical low currently east of Bermuda. Sea-surface temps are in 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 symmetric ring of t-storms around the surface spin). I currently expect a 100% chance that this will become a subtropical depression or subtropical storm by early tomorrow morning (if not sooner).

Currently the developing subtropical cyclone is trapped in meandering steering currents below its upper vortex...and is blocked from any W or NW progression thanks to developing deep-layered ridge (paragraph P2) hugging the system. In the next 72 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 P2 mid-latitude system. At the surface...the subtropical cyclone should be merging with the cold front of the paragraph P2 system...so it will likely be non-tropical by the time it reaches Atlantic Canada. The upper vortex will become absorbed by the paragraph P2 upper trough.

P1...The next weather system (surface frontal cyclone supported by upper trough) in the mid-latitudes is diving southeast while attracted to the low pressure field of the immense weather system in paragraph P2 below. The surface cyclone is currently at 998 mb over south-central Canada with its front across the central US.

P2...Vigorous system in the mid-latitude westerlies anchored by a major upper trough remains across the eastern US and eastern Canada...and its south extent has reached the Gulf of Mexico. The associated surface cold front is also stretched across this region and slowly moving into the W Atlantic. Along the front...the remnant low of former disturbance Invest 93-L dissipated at 1014Z yesterday in the vicinity of NE NC/SE VA. Most impressive feature along the front is a surface low that intensified from 991 to 981 mb in the last 24 hours while shooting northward into eastern Canada from the NE US. This surface low has whirled beneath the less-divergent axis of the upper trough...and therefore may start to weaken. Southerly flow ahead of the 981 mb low is supporting a building W Atlantic upper ridge via warm air advection. This upper ridge will soon stack with W Atlantic surface ridge in paragraph P3 to produce a deep-layered ridge. Finally...western convergence of the major upper trough supports plenty of dry air across the southern US and a low-level ridge across the eastern US.

P3...Upper trough in the NW Atlantic in the previous discussion is now shooting eastward across the Atlantic high seas. Eastern divergence of the upper trough supported the formation of a new surface low east of Bermuda...and now this surface low is deep-layered as the south end of this upper trough became a cut-off upper vortex over the surface low. This deep-layered low has a chance of becoming a subtropical depression or storm. See Invest 94-L special feature section above for details. Meanwhile... western convergence of the upper trough supports dry air and a strong low-level ridge over the W Atlantic. The weakening surface vortex being steered into Spain and Portugal by this system was along the coats of Portugal at 1014 mb as of 0000Z TAFB.

P4...Eastern Atlantic surface ridge...currently with a 1019 mb center...persists. It is supported as NW upper flow from paragraph P3 upper trough (and upper trough over Nadine noted in Nadine special feature section) and SW upper flow from paragraph P5 upper ridging converge. As noted in the last 72 hrs...increased 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.

P5...Central America and SE Mexico upper ridge cell persists...with Caribbean upper trough (now in the W Caribbean) persisting to the east of that. Yesterday's retrograding upper trough midway between the Cape Verdes and Lesser Antilles (mentioned in paragraph P5 of discussion #111) is nearing the Lesser Antilles as an upper vortex. 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. Inverted upper trough near the Cape Verde Islands is now west of the Cape Verde Islands. Upper ridge cell toward Africa persists in relatively higher pressures NE of this inverted upper trough and SE of the upper trough over Nadine (noted in Nadine special feature section).

P6...The tropical wave midway between the Cape Verdes and Lesser Antilles in the previous discussion is nearing the Lesser Antilles while still producing a batch of moisture in between the paragraph P4 dry air to the east and dry air of nearby paragraph P5 retrograding upper trough/vortex to the west. This moisture is helped by split flow upper divergence at the boundary between this retrograding upper trough/vortex and one of the upper ridge cells in paragraph P5.

P7...Suspect tropical wave emerging from Africa in the previous discussion has been added into NHC TAFB maps in the last 24 hours...positioned over and south of the Cape Verde Islands as of 0000Z TAFB. It is suppressed by the paragraph P4 dry air.

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. wxchaser97
10:17 AM GMT on September 20, 2012
Thanks NC09, I like the analysis on Nadine and 94L.
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