2012 Atlantic Hurricane Season Birdseye Discussion #116

By: NCHurricane2009 , 8:04 AM GMT on September 25, 2012

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...SEPTEMBER 25 2012...4:04 AM EDT...
An outage has developed on GOES-E satellite imagery in the last 24 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.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0127Z-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 36 hrs...Nadine has weakened while ingesting the paragraph P2 dry air to her south...and while experiencing some unpredicted easterly vertical shear produced by north side of E Atlc upper trough/vortex just SW of her (paragraph P5). This weakening was not predicted by the previous NHC intensity forecast...and therefore the newer NHC intensity forecasts are dampened down from the previous. 00Z GFS this morning shows an eastern arm of the shearing upper trough persisting just south of Nadine thru the next 24 hrs...so in Figure 1 I prefer to be slightly below the 11 PM EDT NHC intensity guidance while delaying the re-strengthening of Nadine until after 24 hrs. This re-strengthening 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 trough...and becoming enhanced to the NE by departing paragraph P4 upper trough. These very favorable upper winds suggest the need to show aggressive re-strengthening. But one has to remember that Nadine will be above waters cooler than 26 deg C if she follows the forecast track in Figure 1. I agree with the NHC's weakening shown at the end of the forecast...as the paragraph P5 E Atlc upper trough merges with mid-latitude westerlies...hence turning eastward toward Nadine and imparting southerly vertical shear across Nadine with its east side.


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. SW side of 1019 mb low-level ridge that built over the Canary Islands in the previous Nadine discussion imparted more of a northward turn than I thought it would...and so Nadine will soon have to turn southward while ramming into the brickwall that is the paragraph P2 deep-layer ridge to the west. I agree with the NHC on an initial slow southward track as it incurs resistance from new paragraph P9 low-level ridge. Eventually the paragraph P2 ridge passes to the north...which should force Nadine westward by 72 hours. It is by 72 hours I begin disagreeing with the NHC forecast with me showing a westward bias...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 shows for this time. 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 96 and 120 hrs.

My northward hook in the forecast track in the longer-term is related to how the paragraph P1 upper trough evolves late in the forecast. By 120 hrs...the upper trough has split into two impulses...with the eastern divergence of the eastern impulse driving a frontal cyclone over 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.

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 eastern North America. The Hudson Bay surface cyclone strengthened from 993 to 980 mb in the last 24 hours while sliding into favorable eastern divergence of the upper trough. Cold front projecting from the cyclone extends into the W edge of the Atlantic...where a new 1013 mb frontal depression has formed in the NW Atlantic while supported by eastern divergence of the upper trough. Western convergence of the upper trough supports a 1022 to 1023 mb surface ridge that has slid into the eastern US. SW fragment of upper trough has become cut-off over Louisiana and W Gulf of Mexico. Upper ridge wave over the SW US has built behind this cut-off upper trough..whose W divergence supports a broad 1006 to 1007 mb frontal depression in the area. A second frontal cyclone (now 995 mb) and its upper trough has dived southward across Hudson Bay on the back side of the 980 mb cyclone.

P2...Deep-layered ridge across the central Atlantic...featuring an impressive greater-than-1028 mb center...persists. South side of this deep-layered ridge is wafting dry air from Africa's desert westward across the eastern Atlantic tropics. This dry air in this area may be under-represented due to the unrepaired thermo birdseye chart as explained in the intro statement of this full discussion.

P3...Deep-layered subtropical low east of Bermuda...formerly Invest 94-L...has become vertically decoupled. The surface vortex continued northward while becoming absorbed by NW Atlantic 1013 mb frontal depression in paragraph P1. The upper vortex meanwhile remains held back by amplifying paragraph P5 upper ridge cell from Cuba/Bahamas...located stationary SE of Bermuda.

P4...Amplified upper trough is moving into Europe according to Meteosat-9 infrared satellite imagery.

...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 much of the 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 Costa Rica and Panama). In the eastern tropical Atlantic...upper troughing just SW of Nadine has consolidated and is retrograding about the paragraph P2 ridge. In relatively higher pressures southeast of this upper trough...upper ridge is building from the W coast of Africa as evidenced by an increase in eastern tropical Atlantic t-storm clouds whose outflow is enhanced by this upper ridge.

P6...Tropical wave ex-92L has exited the picture from SE Mexico. Extending NE from the tropical wave...surface troughing still extends from the Bay of Campeche to the waters offshore of the SE US. This surface troughing and widespread t-storms have been overran by paragraph P1 cold front (where there is a 1015 mb frontal depression E of Florida)...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 nearing the Lesser Antilles in the previous discussion is now crossing those islands. Its t-storm activity was enhanced by favorable upper ridge cell overhead 36 hrs ago...but now is enhanced by divergence on the SE edge of paragraph P5 SE Caribbean upper vorticity. Meanwhile...the upper ridge cell overhead has de-amplified into zonal shearing upper westerlies thanks to paragraph P5 E Atlc upper trough moving in from the NE.

P8...Eastern Atlantic surface trough continues west and persists ENE of the Lesser Antilles (its formation discussed in paragraph P9 of previous discussion #115).

P9...New eastern Atlantic surface ridge of 1016 mb has formed just NW of the Cape Verde Islands. Based on 00Z GFS model's 200 mb (upper) wind initialization...it appears this surface ridge is supported by upper convergence between northerlies from W Africa upper ridge (paragraph P5) and westerlies from paragraph P4 upper trough.

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. Due to the increase in eastern Atlantic t-storms described in paragraph P5...there is no longer a distinct area of t-storms to mark this suspect tropical wave...so without it being acknowledged by NHC TAFB...I speculate it is just offshore of W Africa and east of the Cape Verde Islands.

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