2012 Atlantic Hurricane Season "Birsdeye" Discussion #2

By: NCHurricane2009 , 12:56 AM GMT on April 14, 2012

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...APRIL 13 2012...
This is the second "birdseye" discussion of the 2012 Atlantic Hurricane Season. I conceived the idea of analyzing the entire Atlantic basin from two maps that provide a "birdseye" view of the region. I found this method useful for understanding the day-to-day evolution of the Atlantic basin throughout Hurricane Season 2011.

Atlantic Hurricane Season does not start until June 1. However, this discussion is issued due to persistent computer model support for subtropical cyclone formation in the central Atlantic during the next days.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-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.

...THERMODYNAMIC 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).

...WEST-TO-EAST DISCUSSION...
Today's discussion will propagate from the west to east while referring to the above two birdseye charts.

System over the western United States has become more complex in last 24 hours. Aloft, the system is anchored by an upper trough axis currently positioned along the eastern borders of California and Nevada that has become stationary and more amplified. The vigorous 993 mb surface cyclone it supported yesterday has broadened into multiple pressure centers below 1000 mb, including the 999 mb center marked in the atmospheric features chart in northern Nebraska. The vigor and expanse of this surface cyclone has created mass cool air advection on its west side that is repsonsible for the amplifcation of the west US upper trough. Of note, low-level southerly flow east of the vigorous surface cyclone is driving warmth and moisture from the south, which collides with cooler westerly flow aloft favoring instability and directional wind shear for continued severe weather across portions of the central US.

Central US upper ridge axis has amplified over the last 24 hours, and expect it to remain amplified over the next days as warm air advection ahead of aforementioned vigorous west US surface cyclone continues. Central US upper ridge continues to support surface ridge with its eastern convergence. The surface ridge centers have shifted east into a 1024 mb center over SE North Carolina, and 1026 to 1027 mb center over southern Ontario and western New York.

As predicted during yesterday's discussion, upper trough/low near the eastern US has become further amplified in part by equal amplification of upstream central US ridge (driven by warm air advection ahead of west US cyclone), and by amplification of upstream Quebec upper shortwave ridge (driven by warm air advection ahead of 1008 mb cyclone that has moved across Hudson Bay into northern Quebec). The upper low along the upper trough axis has moved toward Newfoundland, and so has the 1004 mb cyclone it supported yesterday which has deepened only slighlty to 1003 mb as it moves into a less divergent environment beneath the upper low.

Expect the above-discussed amplification pattern of the upper trough/low near the eastern US to continue, which could allow subtropical cyclone formation along the cold front extending from the 1003 mb cyclone. The amplification process will be essential in this development as it will reduce the westerly vertical shear currently over the surface cold front. Thermodynamics are a disadvantage for development as this system will stay north of the 26 deg C SST isotherm. However the resulting amplification of the upper trough/low could lead to a large-scale upper low over the surface front whose cold temperature could allow for instability despite the mild water temps.

A non-frontal surface trough located just north of the Dominican Republic yesterday is present just north of Puerto Rico today. The surface trough is also supported by divergence ahead of the amplifying upper trough/low near the eastern US. However, the cold front extending from the 1003 mb cyclone should absorb it, shifting the potential for subtropical cyclone development to be somewhere along the cold front. Also getting help from the eastern divergence of the amplfiying upper trough/low near the east US is a new 1008 mb center along the cold front and east of Bermuda, over water temps of 20 to 21 deg C. It is not clear if this 1008 mb center could emerge as the subtropical feature of interest, but an increase in instability from present is certainly needed with those water temps. Amplification of the upper trough/low could lead to a large-scale upper low aloft whose cold temperature could allow for that instability and also a reduction of the immense westerly vertical shear in the area.

It appears Northern South Amercian monsoon upper ridge has been a bit choked by the amplifying upper trough/low near the east US (due north). Animating the 200 mb wind barbs over the last day suggests the amplificaiton of the upper trough/low has compressed the monsoon upper ridge a bit southward. If this is the case, the reduction of outflow from this upper ridge has calmed the thunderstorm intensity, and also this has resulted in the surface monsoonal 1012 mb low over north Colombia (present 24 hrs ago) to dissipate.

Central Atlantic still domianted by deep-layered ridging. The upper ridge axis in the area has greatly amplified due to continued warm air advection ahead of above-mentioned 1003 mb Newfoundland cyclone, and also warm air advection ahead of newly-formed 1008 mb cyclone east of Bermuda also mentioned above. Weak shortwave upper trough in the north-central Atlantic that was embedded in the upper ridge has contineud east to a position just south of the Azores today. Strong upper convergence east of the upper ridge axis and west of shortwave upper trough axis supports a continued 1032 mb surface ridge SW of the Azores. The convergence east of the upper ridge axis and surface divergence of the 1032 mb ridge is resulting in a large swath of sinking, dry air (seen in the above thermodynamics chart) and hence stable weather across the eastern Atlantic.

Mentioned breifly in yesterday's discussion was a 1006 to 1008 mb disorganized cyclone near south Greenland and beneath the north side of the above-mentioned central Atlantic upper ridge. Visible satellite suggests the eastern 1008 mb center is skirting ESE toward west Europe under the WNW flow from the central Atlantic upper ridge. Western 1006 mb center has weakened to 1012 mb and stationary near south Greenland. This has been a decaying system with no supportive divergence to be found at the north crest of the upper ridge axis.

In the far east Atlantic, yesterday's upper trough near the Cape Verde and Canary Islands has becoem further amplified and its postion has retrograded slighlty westward. The amplfication/retrogression of this upper trough is in part due ot the equal amplifcation of the upstream central Atlantic upper ridge discussed above. 200 mb wind barbs suggest a vort max developing midway between the Cape Verdes and Azores, suggesting that a cut-off upper low is expected to form at that location along the amplifying upper trough. Even though such an upper low will support divergence on its east side, abudandtly dry air generated by the neighboring central Atlantic ridge (also discussed above) will prevent active weather with this system.

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1. nigel20
2:35 AM GMT on April 14, 2012
Thanks again NCHurricane2009
Member Since: November 6, 2010 Posts: 12 Comments: 8472

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