2012 Atlantic Hurricane Season Birdseye Discussion #16

By: NCHurricane2009 , 6:17 AM GMT on May 25, 2012

...MAY 25 2012...
A subtropical cyclone is possible this upcoming Memorial Day weekend. Residents in the Carolinas...SE Georgia...and NE Florida should keep tabs on this developing situation through the weekend. See special features section for further details.

This is the sixteenth 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 this early morning 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.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1930Z-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.


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).

As special update #15A concluded...this disturbance has been days in the making. A more brief statement on how this disturbance emerged is found in paragraph P6 of tropical belt discussion below. The surface low of the disturbance has moved NE out of the W Caribbean...across the Florida Straits...and now into the Atlantic waters between SE Florida and the west Bahamas. As it did so...there has been some increased definition to the spin of the surface low in the low-level clouds. The surface low of this disturbance was 1008 mb as of 1800Z TAFB. The NE motion of the surface low means that the rains have ended in the W Caribbean...and now are concentrated over portions of Cuba and the Bahamas.

Special update #15A concluded two challenges for 94-L...dry air in the Gulf of Mexico that would wrap into the circulation (paragraph P3 below mentions source of this dry air)...and westerly vertical shear from the frontal system also covered in paragraph P3 below. Westerly vertical shear has been maintained by trapped upper vorticity (over E Gulf of Mexico/SE US) left behind by the frontal system covered in paragraph P3. Because the National Hurricane Center and computer models are more "gung-ho" about development...I am upgrading my forecast for 94-L...summarized in Figure 1 below.

To say the least...atmospheric pattern is complicated...and so is the forecast. The central US frontal system in paragraph P2 is putting out plenty of warm air advection ahead of itself...which is amplifying a Texas-Arkansas upper ridge...and in turn this will cause equal amplfication of the trapped upper vorticity SW of 94-L. As the upper vorticity SW of 94-L amplifies...this will tend to (a) bend the track more directly northward as shown in Figure 1 and (b) lead to subtropical cyclone formation by late Friday/early Saturday (0000Z May 26). Subtropical cyclone formation will be possible by this window as the amplified upper vorticity allows for relaxation in westerly vertical shear...but more importantly there will be plenty of upper divergence to work with between the NE side of the amplified upper vorticity and adjacent upper ridging. I do not expect full-on tropical cyclone formation at this time because 94-L has deeply-swallowed the dry air from the Gulf of Mexico...which will cause a struggle for central convective T-storms and associated vertical warm core formation.

Note how I briefly slowed-down the northward speed of 94-L by 0000Z May 26...the slow-down I believe will be created by a blocking pattern consisting of (a) surface ridging in the Gulf of Mexico mentioned in paragraph P3...(b) upper anticyclone mentioned in paragraph P2...and (c) 1028 mb ridge center mentioned at the end of paragraph P3.

By 0000Z May 26...I expect the frontal system in paragraph P2 to be marching across the Ohio valley...to the NW of 94-L. From this position...I expect the frontal system to bend the track of 94-L eastward after 0000Z May 26. I expect upper westerlies from the frontal upper trough to shear whatever subtropical cyclone emerges out of 94-L....the center of the sheared cyclone passing just south of Cape Lookout, NC by 0000Z May 27 (late Saturday/early Sunday).

Afterwards...I predict 94-L (the subtropical cyclone) to make a clockwise loop turn and begin accelerating westward toward the Carolinas by late Sunday/early Monday (0000Z May 28). This westward acceleration is caused by the NE side of the trapped upper vorticity and south side of surface ridging that builds behind the frontal system of paragraph P2. I state the system as "recovering" by this point as it escapes the westerly shear imposed by frontal system of paragraph P2.

Numerical models such as the GFS (Figure 2 below) have a different take on how 94-L's track concludes. This is because they give less credence to paragraph P2's frontal system...and I think the models are under-estimating how much this frontal system will steer 94-L. The end result is that my forecast 0000Z May 27 and 0000Z May 28 positions are to the northeast of the numerical models. The GFS for example brings the system into S Georgia/N Florida by 0000Z May 28...while I am accelerating the system westward toward the Carolinas by 0000Z May 28.

Residents in the Carolinas...SE Georgia...and NE Florida should keep tabs on Invest 94-L through the Memorial Day weekend.

Figure 1: My current best-guess forecast for the evolution of disturbance Invest 94-L during Memorial Day weekend.

Figure 2: GFS model output for Invest 94-L during its 1800Z May 24, 2012 run.

P1...Frontal system covered previously in discussion #15 (paragraph P2) has undergone complex evolution. The cold front it was driving southward from Canada in discussion #15 has since expanded southward into the central US (see paragraph P2 below). The remainder of this frontal system had zoomed east...absorbing the upper vorticity near Alberto along the way (see paragraph P3 below for details).

P2...Frontal system currently in the central US originated from southern Canada as paragraph P1 above mentions. As this front pushed south and ushered in colder air...this created a new upper trough currently present in the upper-left corner of the above birdseye charts. Meanwhile...warm air advection ahead of this frontal system has created upper ridging...with an upper ridge axis from Texas to Arkansas...and an upper anticyclone presently over the NE US. The lowest pressure at this surface with this frontal system was 995 mb over SW Iowa as of 1930Z HPC. The north end of this frontal system was anchored by a single cyclone over S Canada 24 hrs ago...which has since split into 997 mb and 996 mb centers as shown in the above birdseye charts.

P3...Frontal system currently over the high seas S of Greenland (and whose tail end reaches into the NE US) had a complicated evolution over the last days. Its upper trough had absorbed the upper vorticity near what was once TS Alberto. It subsequently disconnected from this upper vorticity while the upper vorticity became trapped by building upper ridging covered in paragraph P2 above. This trapped upper vorticity is currently over the eastern US and E Gulf of Mexico and could aid in subtropical cyclone formation beginning 24 hrs from now (see special features section above for details). Meanwhile...northwesterly flow on this upper vorticity's west flank still heavily converges with westerly flow on the north side of Caribbean upper ridging...which firstly has begun to create surface ridging in the Gulf of Mexico...and secondly has done a great job in re-enforcing sinking dry air over the Gulf of Mexico over the last several days. Finally...divergence on the east side of the upper trough S of Greenland supports a broad surface frontal cyclone with lowest pressure of 1000 mb...and convergence on the west side of this upper trough supports a 1028 mb ridge S of Newfoundland.

P4...Upper ridge axis in north-central Atlantic supported by warm air advection ahead of frontal system discussed in paragraph P3. Eastern convergence of this upper ridge supports a surface 1027 mb subtropical ridge center.

P5...In paragraph P4 of discussion #15...a large upper vortex was marching ESE while located S of Greenland...and another upper vortex was NE of the Lesser Antilles in paragraph P8 of discussion #15. Both of these upper vortices have since merged into a singular massive upper trough in the E Atlantic...from N of the Azores...to the tropical waters midway between the Lesser Antilles and Cape Verde Islands. Over the last 48 hours...divergence on the NE side of this E Atlantic upper trough has been supporting a new surface trough NE of the Azores...and a depression over Spain (1011 mb in the above charts).

P6...Upper vorticity remains trapped over the SE US and now into the E Gulf of Mexico (see paragraph P3 above for details). Southwesterly upper flow has been directionally divergent between this upper vorticity and Caribbean upper ridging over the last several days...which ultimately has led to the development of disturbance Invest 94-L and its massive area of clouds (see special feature section above for details). The convective cloudiness of 94-L has maintained a good chunk of the Caribbean upper ridging via latent heat release. Convergent northerly flow on the south side of this upper ridging is leading to sinking dry air across the Caribbean's SE half and into the tropical waters east of the Lesser Antilles.

P7...A pair of tropical waves are in the tropical belt of the Atlantic...one approaching the Lesser Antilles...the other following behind hundreds of miles to the east. Both waves are suppressed by dry air mentioned at the end of paragraph P6 above...although the southern Lesser Antilles is seeing some convective cloudiness from the western of the two tropical waves.

P8...Upper ridge over the E tropical Atlantic continues as it has for the last days. In water vapor animation over the last days...noticed some moistening of the air beneath this upper ridge...perhaps as the upper ridge's outflow is allowing for some rising of air below.

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. nigel20
1:19 AM GMT on May 26, 2012
Thanks again...you guys maybe dealing with a moderate to strong tropical storm by Sunday
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