2012 Atlantic Hurricane Season Birdseye Discussion #109

By: NCHurricane2009 , 7:20 AM GMT on September 17, 2012

...SEPTEMBER 17 2012...3:30 AM EDT...
Hurricane Nadine weakenes to a tropical storm as I forecasted 24 hours ago. The cyclone will be in the vicinity of the Azores by 5 days out...so interests in the Azores should monitor the progress of this system. The worst case scenario at this time appears to be a high wind and heavy rain non-tropical cyclone for the islands. See the Nadine special feature section below for additional details.

Tropical wave Invest 92-L approaching the Lesser Antilles and Caribbean Sea appears quiet fragile at this time. Due to potential for more favorable upper winds in the next days...I maintain this as a special feature on this blog at this time. See the second special feature section below for details.

Disturbed weather Invest 93-L in the western Gulf of Mexico...the formation of which was described in special update #108A...remains unimpressive from a tropical perspective and therefore is still not considered a special feature on this blog. See paragraph P2 for details on this system.

As the Nadine special feature section highlights...a cut-off deep-layered vortex is forecast to form west of the storm and east of Bermuda by 72 hours. This location features water temps in the 28 deg C range...and coupled with what should be a vigorous cold upper vortex...the atmosphere could become unstable enough for subtropical cyclone development beneath the cold core upper vortex by that time.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0126Z-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).

Nadine's eastward motion continues to be slightly faster than the previous forecasts even though those forecasts upped the pace of the eastward track. Therefore it would seem my updated forecast in Figure 1 should be another rightward adjustment for all forecast points. However...my previous forecasts had a southward bias for the timeframe that is the next 24 hours as I saw no reason to bend the track to the north until later on...but Nadine is turning more northward already. Since the NHC has adjusted their track a bit southward...I suppose it is time for me to adjust my track northward and meet the NHC in the middle...therefore agreeing with the NHC forecast track for the first 48 hours. In the end...my northward adjustment does not require me to have a rightward adjustment despite the initial faster than expected eastward motion. Also...Nadine's eastward track is finally slowing down.

The developing slowdown in Nadine's eastward track is the result of resistance from the Atlantic high seas low-level ridge (end of paragraph P2) entering NE Atlantic. The developing northward deflection in the track is the result of the paragraph P5 low-level ridge already bridging with the NE Atlantic low-level ridge...the bridge forming at a location east of Nadine. I begin disagreeing with the NHC track forecast in between 48 and 60 hours....when I think Nadine will wiggle left toward a low-level ridge weakness associated with what is now the 1001 mb low in paragraph P2. As the weakness passes by to the north between 60 and 72 hrs...I believe Nadine will then wiggle back rightward while trying to link with that weakness.

The aforementioned 1001 mb system and its upper trough approach Nadine while decomposing into into two upper vortices...thanks to strong and deep-layered ridge developing in warm air advection ahead of the paragraph P1 frontal system. The first is a cut-off deep-layered vortex west of Nadine (which could become a subtropical cyclone as mentioned in the intro of this full discussion). The second is an upper vortex to the north which now appears to have the potential to drag Nadine eastward between 72 and 96 hrs. The strength of the deep-layered ridge in the 00Z GFS causes it to shoot this second upper vortex southward such that Nadine would whirl counter-clockwise about the upper vortex in between 96 and 120 hrs.

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

Intensity-wise...Nadine in the last 24 hrs has weakened to a 70 mph max wind tropical storm as I previously forecasted...so I maintain the weakening rate I showed in my previous forecast. The developing slowdown in track for the next 24 hours should allow the less shearing (more favorable) north Atlantic upper ridge wave...propped up by warm air advection ahead of the paragraph P2 1001 mb low...to catch up to and overspread Nadine...so I flatten the weakening rate during this time. This warm air advection should cause the new and unfavorable shortwave upper trough W of Nadine to diminish as remarked in paragraph P4. I accelerate the weakening rate (faster than the NHC shows) by 48 to 96 hrs as Nadine's forecast track crosses the 26 deg C isotherm into cooler waters...and experiences unfavorable SW vertical shear delivered by the two upper vortices mentioned in the above track forecast discussion. I suppose the NHC shows a slower weakening rate as they expect Nadine to more gradually transition into a non-tropical low supported by divergence from this shearing SW jet...but alternatively I see Nadine in unfavorable upper converence on the SW and south sides of the northern upper vortex. In fact...I don't see Nadine receiving supportive upper divergence until it enters the east side of the northern upper vortex in between 96 and 120 hrs...which is finally when I flatten the weakening rate. I agree with the NHC on a non-tropical Nadine by 120 hrs.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius...which I lean rightward with respect to the storm track by 48 to 96 hrs to account for a forecasted increase in SW vertical shear for that timeframe. The swath is also shrunken to represent a weakening Nadine in the latter part of the forecast. My swath only covers the western and central Azores at this time...but could easily cover all of the Azores if the radius of the counter-clockswise loop track at 96 to 120 hrs becomes larger...or if Nadine's wind field actually grows or maintains size if the more vigorous above-discussed NHC intensity forecast verifies.

The tropical wave approaching the Lesser Antilles and Caribbean Sea is struggling despite a vast area of supportive split flow upper divergence on the SW quad of paragraph P6 central Atlantic upper ridge cell. Perhaps the upper divergence region is too vast...creating a large area of east-west pressure falls as visible satellite imagery last afternoon suggested an east-west elongated system. With an elongated surface center (instead of a single tight center)...the surface convergence to drive t-storms is not focused enough for tropical development. Perhaps the Caribbean dry air mentioned in paragraph P6 is also not helping this system.

Computer models still insist that the Caribbean upper trough (paragraph P6) would continue to retrograde westward as to allow the SW quad of the central Atlantic upper ridge cell to split off into a favorable shear-reducing and outflow-enhancing upper anticyclone directly over this tropical wave. Moreover...model runs suggest this favorable upper anticyclone becoming enhanced by warm air advection ahead of what is now the paragraph P1 frontal system as this tropical wave crosses the Caribbean Sea. Therefore...interests in the Caribbean should monitor this tropical wave over the next days. However...development could be obstructed if the tropical wave gets too close to the less favorable retrograding east Caribbean upper trough...or later on gets too close to what is forecast by models to be a highly-amplified upper trough associated with the paragraph P1 frontal system.

P1...Next system in the mid-latitude westerlies continues entering from the upper-left corner of the above birdseye charts...with the upper trough over western Canada...and the associated surface frontal zone across the NW US...north-central US...and along the east coast of Hudson Bay.

P2...Cut-off upper trough over the SW US has re-amplified into an upper vortex...its eastern divergence formerly supporting a frontal low that has dissipated over the SE US (although there was some residual rain from this system in the last 24 hrs). The eastern divergence of this upper vortex now supports a new surface trough Invest 93-L in the western Gulf of Mexico...the origin of this surface trough mentioned in special update #108A. In advance of this upper trough (and the upper trough in paragraph P1)...it is expected in the next 60 hours that Invest 93-L will be steered NE into the northern US Gulf coast and across the SE US while it enhances rainfall. I currently think SW vertical shear ahead of the upper troughing will prevent 93-L from becoming a subtropical or tropical cyclone. Elsewhere...shortwave upper trough over eastern Canada has amplified into an upper vortex due to the cool air advection of the intensifying 1001 mb frontal low it supports with its eastern divergence. The western convergence of the upper vortex supports a 1020 mb low-level ridge across the eastern US. Warm air advection ahead of the 1001 mb low supports a north Atlantic upper ridge wave...and this 1001 mb system has absorbed all layers of western Atlantic low pressure system in paragraph P3. Shortwave upper trough is still sliding eastward toward Europe...with its western convergence supporting a low-level ridge moving eastward across the Atlantic high seas.

P3...Cut-off upper trough and associated 1007 mb frontal low over the W Atlantic has been absorbed by the 1001 mb frontal low and its upper vortex in paragraph P2. Cut-off upper trough in the Gulf of Mexico is de-amplifying thanks to warm air advection southeast and ahead of the paragraph P1 frontal system.

P4...Deep-layered vortex persists south of the Azores...but is becoming pushed eastward as the north Atlantic upper ridge wave (supported by warm air advection ahead of the 1001 mb frontal low in paragraph P2) pushes in from the west. A portion of this deep-layered vortex has split off into an upper trough west of Nadine...the western convergence of this upper trough increasing the dry air to the west of Nadine. This upper trough west of Nadine could easily de-amplify thanks to the warm air advection ahead of the 1001 mb frontal low in paragraph P2.

P5...Eastern Atlantic surface ridge...currently with a 1018 mb center... persists with support from upper convergence as the northerly flow from the paragraph P6 central Atlantic upper ridge cell clashes with southerly flow from the paragraph P6 upper ridge cell located toward Africa. This upper convergence has also increased the dry air in the eastern tropical Atlantic in the last 24 hours.

P6...Upper ridging across the tropical Atlantic persists. A western Caribbean and SE Mexico upper ridge cell is split from the central Atlantic upper ridge cell via eastern Caribbean upper trough retrograding into the central Caribbean. Dry air has collected below this retrograding upper trough. Embedded upper vortex NW of the Cape Verde Islands persists...with the remainder of the upper ridging located toward west Africa in relatively higher pressures SE of this upper vortex.

P7...The tropical wave south of the Cape Verde Islands in the previous discussion is now SW of the Cape Verde Islands. Despite being below low shear and favoarable upper outflow beneath the paragraph P6 upper ridge cell toward Africa...it appears suppressed by adjacent dry air mentioned in paragraph P5.

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