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By: NCHurricane2009 , 1:55 AM GMT on May 20, 2012
...MAY 19 2012...
Special feature off of the Carolina coast becomes Tropical Storm Alberto. Meanwhile...I am no longer considering the western Caribbean disturbance a special feature due to unfavorable upper winds.
This is the thirteenth 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 1200Z, and the 1336Z-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...TROPICAL STORM ALBERTO...
The rapid genesis of this tropical storm off of the Carolina coast is covered by discussions #12 and #12A written in the last 24 hours. My track and intensity forecast (versus the National Hurricane Center's) this evening is shown in Figure 1 below.
Track-wise...my best guess leans to the right (or more offshore) when compared the hurricane center's projected track...as I expect the 1006 mb cyclone to the northeast (in the above charts and in paragraph P3 below) to keep the system tugged on a more southward track than the hurricane center shows in the short-term. The eventual acceleration towards the NE is caused by the 999 mb frontal cyclone covered in paragraph P1. This frontal cyclone is expected to erode the low-level ridging to the NW as it nears Alberto...the southwesterly flow from the frontal cyclone causing that NE acceleration.
Intensity-wise...let me first say that I bet no one in the meteorological community was expecting a 60 mph wind tropical storm to develop so fast when this was such a disorganized disturbacne 24 hours ago! What continues to favor Alberto's strengthening is warm water Gulf Stream waters and its warm core anticyclonic outflow it has established...extending northward into E North Carolina and Virginia as remarked later in paragraph P3. I am reluctant however to suggest any further strengthening as Alberto's upper anticyclone (its source of vital upper outflow) is being squeezed tightly between an upper low vortex over Georgia and another E of Virginia (these upper vortices covered in paragraphs P3 and P7). Another negative factor preventing strengthening is how dangerously close Alberto is to a Gulf of Mexico/N Florida dry air mass covered at the end of paragraph P2. My forecast intensity points are covered in Figure 1 below...with a maintainence of 60 mph max winds at the center...and then a weakening to 50 mph max winds as Alberto exits the warm swath of Gulf stream waters.
Impact-wise...surf and rip currents for the next few days as Alberto parallels the east US shore. Based on my best-guess forecast track and current wind radii from the National Hurricane Center...I drew out how the swath of Alberto heavy weather (high winds/heavy rain) should evolve. First it is generally symmetric about the center...and then notice how I later lean this swath towards the east of center as it accelerates to the NE. This is because the stronger winds are always to the right of center during an acceleration...and because I expect Alberto to get some upper-level southwesterly shear induced by the frontal cyclone that will steer it northeastward...the shear also acting to bias weather east of the center. All-in-all...this small diameter swath of strong Alberto weather remains offshore using my current best-guess track. Radar imagery and a recent northwestern flare up in convective clouds on satellite imagery suggests some rain will spread onshore in NE SC and SE NC...with this rain potentially spreading into the NC Outer Banks when Alberto accelerates towards the NE later in the week.
Figure 1: My best guess forecast for Tropical Storm Alberto as of May 19, 2012 evening. NOTE: This chart has an error...all 11 PM positions should actually be 5 AM. "11 PM Mon" should be "5 AM Tue"...and "11 PM Tue" should read "5 AM Wed"
P1...In the above birdseye charts...an upper trough has moved into view from the western US. Divergent southwesterly flow ahead of this trough supporting a disorganized frontal cyclone with fairly low pressures...right now the lowest pressure being 999 mb in north Texas/western Oklahoma. Southerly low-level flow ahead of this 999 mb cyclone is in competition with northerly low-level flow behind a 993 mb cyclone mentioned below in paragraph P3...resulting in an east-west frontal boundary dividing air mass contrasts between the two cyclones. This east-west front runs just north of the US/Canada border featuring a 1004 mb depression on the Ontario/Manitoba border and 1019 mb depression in S Quebec.
P2...Upper ridge dominates the central North America...supported by warm air advection ahead of 999 mb cyclone in Paragraph P1. The shortwave upper trough that was embedded in the ridge has merged with the upper vorticity over SC that triggered Tropical Storm Alberto. Convergence on the east side of the upper ridge supports surface ridge centers of 1024 and 1023 mb over the NE US...and a 1028 mb center over Hudson Bay. Northwesterly flow on the east side of this upper ridge converges heavily with westerly flow on the north side of Caribbean upper ridging mentioned below in Paragraphs P4 and P6. This heavy upper-level convergence is leading to a strengthening in the dry air shield over the Gulf of Mexico and northern Florida.
P3...Upper trough/surface frontal system in the western Atlantic has undergone complex evolution in last 24 hours. An upper low vortex over Labrador has moved offshore in a direction toward S Greenland...and what was a 984 mb cyclone (24 hrs ago) supported by this upper low vortex has weakened to 993 mb while beneath a less divergent environment under the upper low vortex. The cut-off upper low vortex over South Carolina 24 hrs ago supported two 1012 mb frontal lows...the northeastern of which has deepened to 1006 mb in the above charts...and the southwestern of which is now Tropical Storm Alberto (see special features section above for further details). The newly-developed vertical warm core of Alberto is supporting an upper ridge extending northward thru E North Carolina and Virigina...and this upper ridge has forked the South Carolina upper low into one upper low over Georgia and another upper low east of Virginia. The upper low east of Virginia is supporting the 1006 mb surface low with divergence on its NE periphery.
P4...Caribbean upper ridging persists...and continues to extend NE into the open Atlantic 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 1035 mb ridge center W 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 waters E of the Lesser Antilles.
P5...A Large upper trough was positioned from the Lesser Antilles to the Azores 24 hrs ago has shifted east and is undergoing a split due to upper ridging mentioned in paragraph P4. The southern split is a new upper low vortex covered below in paragraph P8. The northern split as an upper trough from the E Azores into Europe. Divergence on the east side of this northern split supports a cold front in E Spain (where the remnant trough of last week's 92-L was finally absorbed/assimilated) and another surface trough just west of Portugal.
...TROPICAL BELT DISCUSSION...
P6...As mentioned earlier in paragraph P4...upper ridging continues across the Caribbean..and this upper ridge has been extended NE into the Atlantic for several hundred miles. The embedded weak upper trough S of Jamaica is now a weak upper low. Low surface pressures in the western Caribbean (with a 1009 mb center) are being maintained by the outflow of this Caribbean upper ridging. The disturbance associated with this 1009 mb center is no longer a special feature...currently under westerly vertical shear on the north side of the Caribbean upper ridge. Shear is expected to continue with the upper ridge remaining suppressed to the south by (a) the upper vorticity that has triggered Tropical Storm Alberto...and (b) the western US upper trough mentioned in paragraph P1 that will pinch in from the northwest. Gulf of Mexico dry air mentioned in paragraph P2 is also beginning to affect the 1009 mb disturbance.
P7...The upper vorticity that triggered TS Alberto has a center over Georgia and another center E of Virginia as mentioned towards the end of paragraph P3. Southwesterly upper flow is directionally divergent between this upper voriticty and Caribbean upper ridging mentioned in paragraph P4 and P6...leading to heavy storm activity NE of the Bahamas and the formation of a weak surface trough.
P8...A new upper low vortex is NE of the Lesser Antilles...a southern split of the upper trough covered in Paragraph P5. It shows up impressively in water vapor imagery as a spinning mass of dry air...this dry air source covered at the end of paragraph P4.
P9...Upper ridging continues in the E tropical Atlantic. This upper ridge promotes favorable low shear with easterlies on its south side aligned with easterlies generated by the 1035 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....albeit the tropical wave SE of the Cape Verde Islands has moistened the area a little bit. Desipte the favorable low shear in this deep-layered easterly flow...the unfavorable dryness will prevent this tropical wave from developing. This tropical wave will also later encouter hositle southweserly shear thanks to new upper low in paragraph P8.
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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