2012 Atlantic Hurricane Season Birdseye Discussion #117

By: NCHurricane2009 , 9:13 AM GMT on September 26, 2012

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...SEPTEMBER 26 2012...5:15 AM EDT...
An outage persists with GOES-E satellite imagery in the last 48 hours. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the temporary GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.

Tropical Storm Nadine expected to track erratically in the Atlantic subtropics for the next five days. See Nadine special feature section below for details.

A currently disorganized disturbance in the vicinity of the Bahamas has been upgraded to Invest 95-L. The disturbance is not mentioned in the NHC Tropical Weather Outlook due to its current lack of impressiveness. A check with computer models (GFS...CMC...ECMWF...NOGAPS) does not suggest tropical cyclone formation in this area for the next days. Therefore I am not considering this area a special feature on this blog. See paragraph P6 for details on the weather in this area.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 1922Z-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...
In the last 24 hrs...Nadine has followed my previous & more conservative intensity forecast which predicted no strengthening during that time. I speculated that an eastern arm of the paragraph P5 E Atlc upper vortex would be parked south of Nadine and provide continued easterly shear. Its hard to say if this eastern arm currently exists...but it is clear that Nadine has not been under easterly shear like I thought it would be as their has been a symmetric ring of t-storm clouds about the center thru much of the last 24 hrs. Perhaps instead its the lack of instability over cool waters below 26 deg C that has prevented Nadine from strengthening. Eventual re-strengthening still appears imminent as models still show favorable upper anticyclonic outflow becoming symmetrical about the storm...becoming enhanced to the SW by retrograding paragraph P5 E Atlc upper vortex...and becoming enhanced to the NE by departing paragraph P4 upper trough. I then show weakening at the end of the forecast...as the paragraph P5 E Atlc upper vortex merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting southerly vertical shear across Nadine with its east side. My intensity forecast in Figure 1 below is a copy-paste of my previous as my previous has been accurate in the last 24 hrs. My intensity forecast is similar to the NHC's 11 PM EDT forecast.


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

Nadine is currently located west of the Canary Islands and south of the Azores. She has almost perfectly followed the NHC's short-term previous track forecast which predicted she would first turn southward while ramming into the brickwall that is the paragraph P2 deep-layer ridge to the west. The previous short-term track forecast also agreed the south track would be slow as it has been...thanks to resistance from the paragraph P9 low-level ridge. Because the NHC southward short-term track forecast has not changed in the last 24 hrs...and because that short-term track forecast has done well...I continue to agree with it.

In the longer-term solution (just after 24 hrs)...the paragraph P2 ridge passes to the north...which should force Nadine westward by 48 hours. Previously for the timeframe that is 48 hours (11 PM Thu)...I had a westward bias relative to NHC...as the 00Z GFS shows Nadine's low-level low pressure field having eroded the paragraph P9 ridge to the south such that I see only an impressive paragraph P2 ridge to the north that should in my opinion push Nadine more west than the NHC showed for this time. The NHC in the last day has adjusted leftward more in alignment with my thoughts for this time...but I am still a tad more left. As Nadine re-curves northward late in the forecast...00Z GFS re-develops the paragraph P9 ridge to the south such that Nadine gets trapped again between the paragraph P2 and P9 ridges...which is why I re-agree with a slower track by 72 to 120 hrs.

My northward hook in the forecast track in the longer-term is to the right of the NHC's latest track forecast...and the hook is related to how the paragraph P1 upper trough evolves late in the forecast. By the latter part of the forecast...the upper trough has split into two impulses...with the eastern divergence of the eastern impulse driving a frontal cyclone from Atlantic Canada...and the western convergence of the eastern impulse driving yet another low-level ridge. The frontal cyclone's low-level ridge weakness is not particularly wide...so I show Nadine drifting northward toward the weakness (between the paragraph P2 ridge now to the east and other low-level ridge to the NW). I currently expect this weakness to leave behind Nadine...so we will have to wait for another frontal cyclone in the very long range that will ultimately accelerate Nadine northward to her demise. Some of this morning's models (GFS and CMC) show Nadine moving northward faster than what I showed in Figure 1 to the degree it threatens the western Azores...but this solution does not make sense to me as the paragraph P2 low-level ridge to the east and other low-level ridge to the NW are shown to be of equal strength and opposing influence. It would take a paragraph P2 low-level ridge to be a lot stronger than the other ridge for something like the CMC and GFS to verify.

Impact swath in Figure 1 is based on extrapolating the 11 PM EDT tropical storm wind radius along my forecast track. Impact statement (b) highlights that surf will linger on the shores of the Azores..due to the large wind radius of Nadine capable of stirring a large amount of water. However..the Azores surf this week will be less intense compared to last week...as Nadine should be further away from the islands this time around.

...MID-LATITUDES DISCUSSION...
P1...Weather system remains anchored by upper trough over the eastern United States. The Hudson Bay surface cyclone once supported by this upper trough has whirled cyclonically and weakened from 980 to 990 mb in the last 24 hrs while losing touch with the eastern divergence of the upper trough. It has split from its cold front...which projects into the W Atlantic...where a NW Atlantic frontal depression has intensified (with supportive eastern divergence of the upper trough) from 1013 to 1000 mb in the last 24 hrs while shooting NNE across Atlantic Canada. Western convergence of the upper trough supports a 1023 to 1026 mb surface ridge that has slid into the eastern US and NW Atlantic. SW fragment of upper trough remains cut-off over the W Gulf of Mexico. Upper ridge wave over the SW US remains built behind this cut-off upper trough...whose W divergence supports a broad 1005 to 1009 mb frontal depression in the area (and in turn local cool air advection behind the broad surface depression has created a cut-off upper vortex over Utah). A second frontal cyclone (now 999 mb) and its upper trough has dived southward across Hudson Bay around the south side of the 990 mb cyclone...with this second cyclone becoming absorbed by the 990 mb cyclone.

P2...Deep-layered ridge across the central Atlantic...featuring an impressive 1028 mb center...persists.

P3...Upper vortex remains stationary to the south of amplified paragraph P5 upper ridge cell from Cuba/Bahamas...located SE of Bermuda.

P4...Amplified upper trough remains just offshore of Europe. Study of extended Meteosat-9 infrared imagery suggests that repeated cool air advection from surface frontal cyclones/depressions diving into the upper trough re-enforces the upper trough...similar to what is happening with eastern US upper trough in paragraph P1.

...TROPICAL BELT DISCUSSION...
P5...Large-scale upper vorticity persists across the SE half of the Caribbean Sea. Upper ridge cell remains built across SE Mexico as paragraph P1 upper trough to the north loses a bit of ground...and remains amplified across Cuba...the Bahamas...and W Atlantic due to low-level warm air advection ahead of the paragraph P1 weather system. 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 across the central Caribbean (alternatively Westerly flow across the south side of the SE Caribbean upper vorticity diverges with the same upper ridge cell to produce enhanced t-storms over Central America). In the eastern tropical Atlantic...upper troughing just SW of Nadine has consolidated into an upper vortex retrograding about the paragraph P2 ridge. In relatively higher pressures southeast of this upper vortex...upper ridge is building from the W coast of Africa as evidenced by widely scattered eastern tropical Atlantic t-storm clouds whose outflow is enhanced by this upper ridge.

P6...Surface troughing still extends from the Bay of Campeche to the waters offshore of the SE US. A good chunk of this surface troughing and widespread t-storms have been overran by south end of paragraph P1 cold front...and persists in favorable split flow upper divergence between the SE Mexico paragraph P5 upper ridge cell and incoming paragraph P1 upper trough. However...this whole area continues to be bombarded by unfavorable SW vertical shear from paragraph P1 upper trough as to prevent tropical development.

P7...Tropical wave crossing the Lesser Antilles in the previous discussion is now in the eastern Caribbean Sea. Zonal shearing upper westerlies across the tropical wave appear to be re-amplifying into a more favorable upper ridge wave due to the tremendous latent heat release in the t-storm clouds.

P8...Surface troughing persists ENE of the Lesser Antilles...which now appears to be supported by upper divergence on the SE edges of the paragraph P3 upper vortex...and E Atlc upper vortex mentioned in paragraph P5.

P9...Relatively new eastern Atlantic surface ridge of 1018 mb persists NW of the Cape Verde Islands. I currently speculate that this surface ridge is now supported by upper convergence between northerlies from W Africa upper ridge (paragraph P5) and westerlies from paragraph P5 E Atlc upper vortex and paragraph P3 upper vortex.

P10...During the last days...satellite imagery suggested a suspect tropical wave with a low pressure spin stalled 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 Tropical Storm Nadine. Based on one cluster of widely scattered t-storms in eastern Atlantic described in paragraph P5...I continue to speculate it is just offshore of W Africa and east of the Cape Verde Islands.

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2. WunderAlertBot (Admin)
9:54 AM GMT on September 27, 2012
NCHurricane2009 has created a new entry.
1. KoritheMan
5:23 AM GMT on September 27, 2012
Kinda wish you would have mentioned the possible Gulf of Mexico the GFS has been portraying, even if won't amount to much.
Member Since: March 7, 2007 Posts: 556 Comments: 19960

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