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By: NCHurricane2009 , 5:09 AM GMT on May 19, 2012
...MAY 18 2012...
Yet again...early activity pops up in the Atlantic Basin before hurricane season starts June 1...but this time with two areas of interest. See both special feature sections below for further details. There are no imminent signs of tropical development in either area at this time. If this changes...special updates or additional full birdseye discussions will be written. Otherwise...this will be the final discussion until the season starts June 1.
This is the twelvth birdseye discussion of the 2012 Atlantic Hurricane Season. See previous discussion #3 concerning the idea behind this new type of discussion forum.
Any feedback on how to improve these discussions can be left in the comments section below. In particular...any suggestions on how to improve the standard two "birsdeye view" charts below will be much appreciated. Are you able to understand the markings in the charts? Are you able to relate what is said in the discussions to these charts?
One improvement being attempted tonight is the implementation of paragraph (P) numbers in general discussions. Paragraphs will cross-reference each other to make the discussion easier during complex weather patterns.
...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 DISTURBANCE OFFSHORE OF CAROLINAS
Mentioned in previous discussion #11 (five days ago) was a central US upper trough supporting a 1016 mb frontal low over Mississippi. The supporting upper trough collapsed...causing the front to decay into a surface trough across the Gulf of Mexico...Yucatan Peninsula...and over the SE US by May 16. On this day...a quickly amplifying upper trough from Canada was digging toward the SE US portion of the surface trough...which re-invigorated this portion into a cold front by May 17.
Amplification of a central US upper ridge (paragraph P2 of mid-latitudes discussion below) caused the south end of the upper trough from Canada to cut-off into an upper low over Georgia on May 17...which has moved eastward into South Carolina on May 18.
It is the development of this cut-off upper low that has triggered this special surface feature...which is located along the re-invigorated cold front. On May 17...divergence along the NE boundary of the upper low triggered a broad surface low along this cold front. As this broad surface low spun up...it triggered some heavy shower/T-storm activity in the eastern Carolinas...which has since abated to being only on the immediate coast of North Carolina as this broad surface low has tightened.
The present structure of this broad surface frontal low is shown in Figure 1. It features twin LLCs (low-level circulations)...marked as 1012 mb in the above atmospheric features birdseye chart. The satellite appearance of the northeastern LLc is less interesting while looking like a classical frontal boundary low. It is the southwestern LLC off of the South Carolina coast that is gaining attention.
The NW half of this southwestern LLC has been producing persistent convective clouds (perhaps helped by the cold upper air temps of the cut-off upper low which helps to de-stabilize things). Very recently...the cut-off upper low has weakened to an upper trough due to adjacent warming caused by these convective clouds. Newly-developed upper-level anticyclonic fanning (as shown in Figure 1) suggests this warming is allowing for a more tropical structure with the southwestern LLC. Meanwhile...the SE half of the southwestern LLC is suppressed by a dry air shield (paragraph P2 of mid-latitudes discussion below).
Bottom line is that favorable conditions include the recent development of upper-level anticyclonic outflow (or fanning) and warm Gulf stream waters. Unfavorable conditions include the ingestion of the dry air shield and the elongated surface structure of this system with twin LLCs instead of a singular LLC. Its hard to say at this time if the favorable or unfavorable conditions will start/stop further tropical development. At the very least...a special update or another full birdseye discussion will be issued when this system shows a definitive trend toward developing or not developing.
Figure 1: Structure of subtropical disturbance off of Carolina coast as of the late evening of May 18
Where will this system go? Right now...nowhere with the system hugged by low-level ridging to its west...north...and east as shown by the surface analysis in the atmospheric features chart above. The GFS model suggests that what is now a west US 996 mb cyclonic system (paragraph P1 in mid-latitudes discussion below) will erode the ridging to the NW and accelerate the system parallel to shore towards the NNE...the acceleration beginning late on May 21. Between now and May 21...the system should be stationary. The effects this system brings to the coasts of mid-Atlantic US...NE US...and eventually Atlantic Canada depends on how close to shore the system develops a consolidated LLC. Extreme case scenario in my mind would be weak tropical storm gusts...some rain...and light surf/rip currents on the west side...that is if the system acquires tropical characteristics and strengthens a bit in the first place.
...SPECIAL FEATURE...wEAK WESTERN CARIBBEAN TROPICAL DISTURBANCE...
Mentioned in previous discussion #11 (five days ago) was a central US upper trough supporting a 1016 mb frontal low over Mississippi. The supporting upper trough collapsed...causing the front to decay into a surface trough across the Gulf of Mexico...Yucatan Peninsula...and over the SE US by May 16.
The Yucatan portion of this surface trough has been recently enhanced by the outflow of Caribbean Sea upper ridging (mentioned in paragraphs P4 and P6 below). This enhancement is to the degree that surface pressures have dropped to 1009 mb as of 1800Z TAFB...and a LLC (low-level circulation) has appeared as shown in Figure 2. There is also a curved band of T-storms east of this LLC...but this band is well removed from the center due to westerly vertical shear present on the north side of the Caribbean upper ridging.
With warm waters and a moist air mass seen in the thermodynamics chart above...the only inhibiting factor against tropical cyclone formation at this moment is exposure to the shear on the north side of the Caribbean upper ridging. The GFS model suggests this shear pattern will continue for the next days...with the upper ridge remaining suppressed to the south due to the remaining upper vorticity associated with the Carolina coast special feature....followed by upper vorticity (an upper trough) swinging in from the northwest associated with what is now a 996 mb cyclonic west US system mentioned in paragraph P1 below. Therefore...I do not expect this system to develop into a tropical cyclone.
Figure 2: Structure of tropical disturbance in the western Caribbean Sea during the afternoon of May 18.
P1...Moving into the western US is an upper trough (not marked in the above charts)...with divergent southwesterly flow ahead of this trough supporting a surface 996 mb cyclone over E Wyoming. Southerly low-level flow ahead of the 996 mb cyclone is in competition with northerly low-level flow behind a 984 mb cyclone mentioned below in paragraph P3...resulting in an east-west frontal boundary dividing air mass contrasts between the two cyclones extending eastward from the 996 mb cyclone. This east-west front runs just north of the US/Canada border featuring a 1002 mb depression in south-central Manitoba and 1014 mb depression near Nova Scotia.
P2...Upper ridge dominates the central US...supported by warm air advection ahead of 996 mb cyclone in Paragraph P1. Embedded in the upper ridge is a shortwave upper trough just west of Missouri in the above charts. Divergence east of this embedded shortwave supports a weak surface trough over Kentucky/the Ohio Valley. Convergence on the east side of the upper ridge supports surface ridge centers of 1024 mb near New York and 1025 mb just offshore of Maine. Upper convergence/surface divergence from the ridge system covered in this paragraph has created a dry air shield seen propagating eastward across the southern tier of the US over the last days. This dry air shield is strongest over the northern Gulf of Mexico and Florida in the above thermodynamics chart.
P3...Upper trough/surface frontal system dominates the eastern edge of North America and western edge of the Atlantic tonight. 996 mb cyclone in paragraph P1 had caused warm air advection across the central US...which has pumped up the ridge system in paragraph P2...and in turn this ridge system has caused the frontal system covered in this paragraph to split into two main areas. The first is an upper low vortex over Labrador whose divergence has supported a rapidly intensifying 984 mb cyclone just offshore. The second is a cut-off upper low vortex over E South Carolina supporting a pair of 1012 mb frontal lows that are beginning to gain tropical characteristics as covered in the first special features section above.
P4...Dome of upper ridging has spread eastward into the Caribbean Sea over the last few days...an eastward extension of an upheaval in tropical activity in the eastern Pacific. This caribbean upper ridging has been spread NE for several-hundred miles by warm air advection ahead of the frontal system in paragraph P3. Convergence on the east side of this upper ridge supports a strong surface 1034 mb ridge center NW of the Azores...and this surface ridge coveres much of the open Atlantic with low-level anticyclonic flow. The divergent anticyclonic flow at low-levels and eastern covergence of the upper ridge supports a tongue of dry, sinking air extending southward into the Lesser Antilles.
P5...Large upper trough dominates the open Atlantic from the Lesser Antilles to the Azores. This upper trough formed in the last five days from a consolidation of three features mentioned in previous discussion #11...first the parent upper low vortex of last week's subtropical disturbance 92-L...the second was an upper trough over the Lesser Antilles...and the third was the upper trough of a frontal system moving toward 92-L. The remnants of 92-L believe it or not are still out there...with the surface low finally being downgraded to a surface trough just recently at 1800Z TAFB. The remnant surface trough of 92-L was located just north of the Canary Islands and is supported by divergence on the NE side of the upper trough.
...TROPICAL BELT DISCUSSION...
P6...As mentioned earlier in paragraph P4...dome of upper ridging has spread eastward into the Caribbean Sea over the last few days...and this upper ridge has been extended NE into the Atlantic for several hundred miles. There is an embedded weak upper trough south of Jamaica located in relatively lower pressures between this NE extension and upper ridge centers over Central America. Low surface pressures in the western Caribbean are being maintained by the outflow of this Caribbean upper ridging...see second special features section above for further details...
P7...It is possible a weak tropical wave is over the Lesser Antilles tonight. This tropical wave was dropped from TAFB at 0600Z May 18 with a statement of its landfall in South America. However, a small batch of clouds is present just NE of the Lesser Antilles...suggesting that the tropical wave is producing surface convergence in this area. It is also possible that the tropical wave is indeed dissipated and this batch of clouds is due to directional upper divergence with the northerly upper flow branching between the upper trough mentioned in Paragraph P5 and Caribbean upper ridging in Paragraph P6. If the tropical wave is not resurrected soon in TAFB analyses...I will drop it from my analyses as well.
P8...To the southeast of the upper trough in Paragraph P5...upper ridging has flared up in the eastern tropical Atlantic...perhaps due to relatively higher upper air pressures as the upper trough in paragraph P5 is gradually weakening. This eastern tropical Atlantic upper ridge is promoting favorable low shear with easterlies on its south side aligned with eastelies generated by the 1034 mb center mentioned in paragraph P4. Animation of water vapor imagery suggests strong dry air pulses from Saharan Africa being wafted westward by this devloping deep-layered easterly flow. So desipte the favorable low shear in this deep-layered easterly flow...the unfavorable dryness will prevent a tropical wave to the SE of the Cape Verde Islands from developing.
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