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By: NCHurricane2009 , 3:52 AM GMT on April 18, 2012
...APRIL 17 2012...
998 mb low east of Bermuda weakens to 1002 mb...but is upgraded to Invest 91-L due to continued potential for subtropical cyclone formation.
This is the sixth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1929Z-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).
...NORTH AMERICA DISCUSSION...
A frontal system and associated upper trough, new to this discussion, is crossing the NW US after emerging from SW Canada. In the above charts, this frontal system is marked in the upper-left corner and has a 1012 mb surface low over NW North Dakota.
Vigorous 994 mb Great Lakes cyclone has moved into SE Canada, and now is weaker at 1007 mb while positioned just north of Maine in the above charts. As it continues racing eastward, the northern fragment of the supporting upper trough has fractured from the southern portion. Expect the northern portion of the upper trough to continue distancing itself from the southern portion while becoming associated with cold air advection just behind the 1007 mb cyclone racing eastward. Presently, upper convergence behind both upper trough portions is supporting a lengthy surface ridge across central North America with a 1026 mb center over eastern Hudson Bay, 1030 mb center over the Great Lakes, 1023 mb center over Colorado, and 1024 mb center in north-central Mexico.
Meanwhile, the southern portion of the upper trough remains strewn across the central US, and its eastern divergence is supporting a lengthy cold front extending from the aforementioned 1007 mb cyclone. The 1018 mb low along this cold front that was over E Texas 24 hours ago is now over E Louisiana. A new 1019 mb frontal low has developed over North Carolina.
Upper ridge over eastern US is no longer a North American feature as it gets pushed into the west Atlantic, but the upper ridge continues to amplify even further due to warm air advection ahead of aforementioned 1007 mb, 1019 mb, and 1018 mb frontal lows. The effect of this upper ridge on the Atlantic environment is covered later in today's Open Atlantic Waters Discussion below.
...DISCUSSION OF OPEN ATLANTIC WATERS...
Yesterday's North America discussion mentioned a 992 mb cyclone emerging from Labrador, which is now over SE Greenland in the above charts at a weaker pressure of 1001 mb. The shortwave upper trough supporting it has been assimilated into the upper trough fragment supporting the 1007 mb cyclone near Maine mentioned in today's North America discussion. As a result, it now shares upper divergence support with the 1007 mb cyclone.
997 mb low heading from Greenland to British Isles mentioned yesterday has made landfall across the British Isles. Its center is now outside the scope of the TAFB and HPC analyses used in these discussions, so its central pressure value is not disucssed here today. However, TAFB analysis still includes this cyclone while showing its tailing cold front in the far NE Atlantic just N of the Azores.
Deep-layered ridge over the Azores is undergoing a complex evolution. Cool air advection/upper trough associated with the aformentioned British Isles cyclone has eroded the upper ridge center, and now only a surface 1031 mb center remains as marked in the above charts. Upper convergence behind the British Isle cyclone's upper trough presently supports the 1031 mb center. The eastern US upper ridge has been pushed into the W Atlantic as mentioned in today's North America discussion while still amplifying, resulting in a new upper-level high pressure E of Newfoundland and just NW of the 1031 mb center. Expect the new upper-level high to align over the 1031 mb center, resulting in the reformation of a deep-layered ridge near the Azores. Leftover central Atlantic upper ridge axis to the SW of the deep-layered ridge also persists, still supported by warm air advection ahead of developing subtropical cyclone E of Bermuda. The sprawling curvature of the fronts attached to the developing subtropical cyclone has shifted the warm air advection eastward, and as a result the upper ridge axis has also moved eastward into the E Atlantic.
E Atlantic cut-off upper low midway between Azores and Cape Verdes has accelerated eastward toward Morroco. This is because it has become pushed by the central Atlantic upper ridge axis which has been driven into the E Atlantic as described above.
The eastern US upper ridge has been pushed into the W Atlantic as mentioned in today's North America discussion while still amplifying. The west Atlantic 1028 mb surface ridge supported by the eastern covnergence of the upper ridge remains stationary and has weakened to 1024 mb. The weakening of the surface ridge is the result of it getting eroded by surrounding surface low pressure features, including the developing subtropical cyclone to its east and Louisiana-North Carolina-Maine frontal system to its west mentioned in today's North America discussion.
Hot topic today is aformentioned cut-off upper low vortex that formed E of Bermuda yesterday...and the subtropical cyclone formation its supporting at the surface. The surface low its supporting has weakened from 998 to 1002 mb as the surface low is now directly beneath the non-divergent upper low center, and the system as a whole is marked as a deep-layered cyclone in the above charts. With a lack of divergence at the upper low center, expect the 1002 mb surface low to continue weakening.
Yesterday's discussion mentioned how the thermodynamics had become less favorable for subtropical cyclone formation due to dry air ingestion and cold waters. The surface low (now 1002 mb) has made the W and SW turn toward warmer 20 to 21 deg C waters (predicted yesterday), but has not quiet yet made it to those waters. 24 hours ago, yesterday's discussion mentioned the surface low had just finished warding off dry air by wrapping in moist air induced by the upper low's divergence. The flow around the upper low's rim is no longer divergent, and so the moisture bands around the surface low have thinned when animating water vapor imagery over the last 24 hours. This may be allowing the rather dry air mass to its SW to re-penetrate into the circulation. As mentioned in yesterday's Gulf/Caribbean discussion, the source of this very dry air is upper convergence/surface divergence from east US upper ridge (now in the W Atlantic) and surface W Atlantic ridge.
To further complicate things, mixed messages are occuring with official meteorological agencies. This system has been upgraded to Invest 91-L by the Naval Research Laboratory (NRL), but the National Hurricane Center (NHC) TAFB analyses still show fronts attached to the surface low as if it is still non-tropical. The surface low has not quiet yet made it to the 20 to 21 deg C waters as remarked above. If persistent convective cloud tops do not develop when the surface low arrives at 20 to 21 deg C waters, this means the cut-off upper low above it is not cold enough to de-stablize the atmosphere at those water temps, and subtropical cyclone formation will no longer be possible. On the other hand if persistent convective cloud tops begin developing near the center, a special brief update will be issued on this blog declaring imminent risk of subtropical cyclone formation.
Yesterday's forecast delayed possible subtropical cyclone formation to as late as midday April 18 (tomorrow). Today's North America discussion mentioned a lenghty cold front from Louisiana-North Carolina-Maine and associated central US upper trough. Based on the frontal system's current eastward pace, the frontal system could begin absorbing all vertical layers of this potential subtropical cyclone beginning April 19 into early on the 20th. This timeframe marks the end of subtropical cyclone potential as it gets absorbed. It is possible I will be calling off subtropical cyclone potential sooner than those dates if persistent convective cloud tops do not develop when the surface low arrives at 20 to 21 deg C waters as remarked above.
...DISCUSSION OF GULF OF MEXICO AND CARIBBEAN SEA...
Yesterday's east-to-west oriented shortwave upper trough over the Bahamas and just N of all the Caribbean islands has been absorbed by large upper low vortex over possibly developing subtropical cyclone.
Upper convergence southeast of the east US upper ridge (which has moved into the W Atlantic today), and surface divergence of the west Atlantic 1024 mb ridge continue to induce dry sinking air across the N Gulf and west Atlantic waters S of Bermuda. The dry air in the N Gulf is being replaced by moist air in a west-to-east fashion due to the arrival of 1018 mb frontal E Louisiana low (mentioned in today's North America discussion).
The E Lousiana 1018 mb low is supported by divergence ahead of central US upper trough (also mentioned in today's North America discussion) Divergence well-ahead of the central US upper trough also supports a new surface trough in the Bay of Campeche as marked in the above charts and as of 1800Z TAFB.
Upper ridge persists over the Yucatan peninsula of Mexico due to warm air advection ahead of aforementioned 1018 mb frontal low.
Shortwave upper trough that formed in central Caribbean (mentioned in yesterday's Gulf/Caribbean discussion) is being tugged slowly eastward by upper-level cyclonic flow associated with possibly developing subtropical cyclone to the NE.
As central Caribbean upper shortwave trough has dug eastward, the South American monsoon upper ridge to its south has been pinched away. This upper ridge is reforming westward more toward Central America and may merge with Yucatan Peninsula upper ridge mentioned above.
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