2012 Atlantic Hurricane Season Birdseye Discussion #31

By: NCHurricane2009 , 11:24 AM GMT on June 19, 2012

...JUNE 19 2012...7:30 AM EDT...
Subtropical low that was over Bermuda in previous discussion #30 was upgraded to Invest 95-L during discussion #30A. See 1st special feature section for current details on this system.

Threat of emerging disturbance pulling NE out of the eastern Caribbean...as mentioned in special update #30A...has diminished.

Tropical disturbance has emerged in the western Caribbean Sea while heading toward the Bay of Campeche and southern Gulf of Mexico...a long-awaited event as computer models have been forecasting this scenario for several days. See 2nd special feature section for current details on this system.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1928Z-released HPC analysis.

In light blue is upper air anlaysis, 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 indicating surface lows, Hs indicating 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...SUBTROPICAL LOW INVEST 95-L...
After suprirising us with impacts over Bermuda during discussion #30...then getting better orgainzed through discussion #30A and throughout much of yesterday...the subtropical low suddenly lost t-storm activity last night when it became exposed to southwesterly shear on the SE side of its supporting upper vortex. The subtropical low is back to a favorable (and less shearing) position with respect to the upper vortex this morning...and is taking advantage of split flow upper divergence on the east periphery of said upper vortex. Some small pockets of t-storm clouds have even re-fired up on the north side of the subtropical low. The north and west sides of this system are more favored for firing t-storms...as this side is located closer to the warm Gulf stream waters.

Now what? As the computer model consensus has been showing...the NE track of the subtropical surface low is curving on a more eastward slant along 40N latitude while rounding the north side of the subtropical ridge in paragraph P4. This is taking it parallel to the Gulf stream...so I think this has a small chance to become a subtropical or tropical cyclone (or at least fire some last rounds of t-storm clouds) until it leaves the Gulf stream. Computer models suggest the eastward curving track of 95-L will soon be blocked as the 1025 mb ridge to the north of the system (paragraph P2) passes to the NE of the system. This will allow the third Canadian frontal cyclone and its supporting upper trough (paragraph P1) to catch up with 95-L as said frontal cyclone ejects eastward. Models seem to want to cyclonically loop the track of 95-L to the ESE of Newfoundland while it transitions to an intensifying non-tropical low supported by divergence of the frontal cyclone's upper trough. It is after this cyclonic loop that 95-L continues east. So even if this feature doesn't become a named storm or numbered depression...it looks to be around as a remnant non-tropical low for the next days.

...SPECIAL FEATURE...WEST CARIBBEAN DISTURBANCE...
See paragraph P6 in tropical belt discussion for how this system came together. This is something the models have been predicting for the last several days to develop into a tropical cyclone. And as we get closer to the expected timeframe of development...the way the models evolve the atmosphere is becoming increasingly complex...so the situation is not as clear cut as previously thought. So interests in the Gulf of Mexico should keep paying attention until it is clearer how this will evolve.

Figure 1 below is my current best guess on how this system evolves over the next days...after extensively studying computer model outputs of surface maps and upper wind maps. I initially take the system NW into the Yucatan...on a track further north than what the TAFB is suggesting (because the best-curved clouds are north of the TAFB central position of the surface low as if the surface low could regenerate northward). The NW track then bends north into the southern Gulf of Mexico as the steering surface ridge in paragraph P2 gets knocked out by fourth frontal cyclone mentioned in paragraph P1.

It is during the Yucatan timeframe that the models show the best upper wind conditions...with the system's warm core upper ridge having pushed out the Gulf of Mexico upper trough (paragraph P3)...and the warm core upper ridge merging with the east US upper ridge (paragraph P2) such that favorable upper outflow is tremendous. That is why I expect a tropical depression when it pulls north into the southern Gulf of Mexico.

Just as this system enters the Gulf...models show the upper winds become unfavorable...with westerly upper winds from the fourth frontal cyclone's upper trough shearing this system. The track bends west as a small low-level ridge passes by behind the fourth frontal cyclone...with a brief bend to the NW once again as a fifth frontal cyclone swings into the central US. In general...the fourth and fifth frontal cyclones merge in the computer models...and then a strong surface ridge builds behind them to steer this tropical cyclone westward toward the end of the Figure 1 forecast. My best guess (emphasis on guess) is a Texas/Mexico border landfall. Towards the end of the forecast...it seems the computer models dump in a west Gulf of Mexico upper level low (ULL)...a cut-off feature from the upper troughing that supports the fourth and fifth frontal cyclones. I expect this tropical cyclone to interact with the north and east sides of this ULL towards the end of the forecast. This interaction may lead to a more subtropical looking system at the end of the forecast (if it plays out this way).


Figure 1: My current best guess forecast for west Caribbean disturbance (emphasis on guess) this morning.

...MID-LATITUDES DISCUSSION...
P1...Second Canadian frontal cyclone in recent days and its supporting upper trough have swung northward across Hudson Bay and exited the picture. Third Canadian frontal cyclone and its supporting upper trough has swung into the picture from the upper-left...featuring a 999 mb surface center in the above birdseye charts. Unlike the previous two...this third cyclone is not expected to swing north across Hudson Bay. Instead...a fourth and strong Canadian frontal cyclone will enter the arena from the west...and warm air advection ahead of this fourth cyclone will re-amplify the east US upper ridge in paragraph P2 to the SW of the third cyclone. As the east US upper ridge re-amplifies...the ridge will push the supporting shortwave upper trough of this third cyclone more eastward such that the surface cyclone itself goes more east.

P2...Upper ridge over the eastern US is becoming squeezed out between upper troughs associated with the Canadian cyclones in paragraph P1...and the W Atlantic upper trough in paragraph P3. However...this upper ridge is expected to re-amplify thanks to warm air advection ahead of a fourth and strong Canadian cyclone expected to soon enter the picture. The upper ridge's eastern convergence continues to support a strong surface ridge. This strong surface ridge currently has a 1025 mb center offshore of Newfoundland and 1020 mb center over the SE US.

P3...Upper trough over the W Atlantic and Gulf of Mexico persists. Peripherial upper diveregence on the east side of its upper vortex continues to support subtropical low (special feature Invest 95-L) which has been tracking NNE across the NW Atlantic. The Gulf portion of this upper trough has recently split off into another upper low...the split occuring as upper ridging from the latent heat release of the western Caribbean disturbance (2nd special feature above) expands.


P4...Surface 1027 mb ridge center S of the Azores has shifted west. From what I can gather from the latest 200 mb upper winds...it is now supported as upper southerlies from E Atlantic upper ridge (paragraph P8) converges with upper westerlies from NE Atlantic upper trough (paragraph P5).

P5...Longwave upper trough persists in the NE Atlantic...with divergence ahead of this upper feature supporting a surface front stretching from W Europe to the Azores. Yeserday...this front was extended from the Azores into the NW Atlantic...perhaps to mark the divide between cooler air from the 1025 mb ridge in paragraph P2 and warmer air from the 1027 mb ridge in paragraph P4. The NW Atlantic portion of the front merged with subtropical low Invest 95-L...which is probably why the NHC gave it new fronts when it did not have any during the previous discussion #30 and #30A. As of this writing...the NE Atlantic longwave has two shortwave upper troughs...one supporting a 1018 mb frontal depression W of the Azores...the second supporting a 1000 mb frontal depression over Spain.

...TROPICAL BELT DISCUSSION...
P6...Upper ridge over the Caribbean persists. 1009 mb low (which had absorbed a tropical wave in previous discussion #30...paragraph P11) has moved NW into the west Caribbean and underneath this favorable upper ridge. See 2nd special features section above for further details on this developing situation.

P7...NE Caribbean upper trough has merged with the central Atlantic inverted upper trough (as expected during discussion #30 paragraph P10). The merger has an inverted signature E of the Lesser Antilles.

P8...In the east Atlantic...the upper ridge continues to expand rapidly...an effect due to relatively higher pressures south of the NE Atlantic upper trough mentioned in paragraph P5 above...and perhaps as the jet stream and its upper troughs have quickly relaxed their influence in this area.

P9...Tropical wave midway between the Cape Verdes and Lesser Antilles in the previous discussion has been steered rapidly westward toward the Antilles by deep-layered eastery flow S of the upper ridge in paragraph P8 and S of surface 1027 mb ridge in paragraph P4. Tropical wave remains suppressed by upper troughing mentioned in paragraph P7. The aformentioned easterly deep-layered flow may have caused the recent dry air outbreak stemming from Africa's desert seen by the brown shading in the above thermo chart. This tropical wave continues to also be suppressed in this dry air.

P10...Vigorous tropical wave S of the Cape Verde Islands in previous disuccsion has been steered very rapidly west by deep-layered easterly flow...S of the upper ridge in paragraph P8 and S of surface 1027 mb ridge in paragraph P4. It is now midway between the Antilles and Cape Verde Islands. As it remains in favorable low shear in this deep-layered easterly flow...and as the upper ridge helping to promote this low shear in paragraph P8 expands westward...it would seem conditions are favorable for development. However...the thermodynamics are not quiet that favorable...with the tropical wave mixed with dry air originating from Africa's desert.


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. AtHomeInTX
12:40 PM GMT on June 19, 2012
Nice job!
Member Since: December 31, 1969 Posts: Comments:

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