2012 Atlantic Hurricane Season Birdseye Discussion #130
...OCTOBER 11 2012...12:35 AM EDT...
An outage persists with GOES-E satellite imagery in the last several days. GOES-W has been extended to cover much of the view in the two birdseye charts below. However...the east edge of the GOES-W scan has a bias for showing cold cloud tops that are not actually present. Therefore...I have patched the east side of the atmospheric birdseye chart with Meteosat-9 grafts. The east side of the thermodynamics birdseye chart is left unrepaired...so be mindful that the moisture content on the east side of this chart has a positive bias due to the false illusion of cold cloud tops.
As stated in special update #129A...tropical disturbance Invest 97-L just north of the eastern Bahamas has re-organized to the degree it could become a tropical cyclone in the short-term. See special feature section below for additional details.
Attention continues turning to tropical wave Invest 98-L en route to the Lesser Antilles. Computer models suggest tropical cyclone formation potential as the wave moves into the eastern Caribbean area by 48 to 96 hours...but the tropical wave must first survive a current onslaught of westerly vertical shear. In addition to the eastern Caribbean...early computer model runs suggest that Bermuda should also monitor the progress of this system. See paragraph P5 for additional details on this feature.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
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 analysis, 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 indicate surface lows, Hs indicate 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 DISTURBANCE INVEST 97-L...
After beginning to accelerate NE with the surface front of the 969 mb gale in paragraph P1 24 hrs ago...tropical low 97-L (now 1013 mb) is drifting SE at a location just north of the eastern Bahamas while becoming trapped between the central US ridge in paragraph P1 and central Atlantic ridge in paragraph P2. With the two ridges shown in models to be of equal strength and opposing influence...coupled with the fact that the gap between the two ridges will be re-enforced by the 985 mb and 1010 mb depressions in paragraph P1...expect 97-L to be generally stationary at its current location for the next 48 hours. Invest 97-L has become well-organized while taking advantage of upper outflow and reduced shear under the paragraph P4 W Atlantic upper ridge...so the best window of opportunity for tropical cyclone formation is in the next 24 hrs while this condition persists. After 24 hrs...the aforementioned 1010 mb frontal depression will re-amplify the paragraph P1 upper troughing such that the favorable W Atlantic upper ridge gets booted out and unfavorable westerly vertical shear pushes in.
After 48 hrs...the models agree that the central US ridge in paragraph P1 will eject eastward and pass to the north of 97-L...and tropical wave Invest 98-L (paragraph P5) should be pushing in from the east by that time. Between 48 and 120 hrs...CMC and NOGAPS show 97-L tracking WSW across the Bahamas...Cuba...and into the W Caribbean while steered by the ridge and 98-L. A check with this evening's 18Z GFS shows 97-L remaining under hostile westerly vertical shear if such a WSW track occurred...so if the CMC and NOGAPS verify...we will likely be tracking a remnant surface trough by then rather than a tropical cyclone.
P1...Upper troughing over North America is being re-enforced by cool air advection from strong surface frontal cyclone that has intensified from 994 to 985 mb while nearly stationary over eastern Hudson Bay in the last 24 hrs. A developing 1010 mb frontal depression from Montana will soon dive eastward across the northern US and also re-enforce this upper trough. Western convergence of this upper troughing is supporting a central US surface ridge (currently 1026 mb). Intense 977 mb frontal cyclone just east of Greenland in the previous discussion has intensified further to 969 mb with the eastern divergence of its shortwave upper trough (meanwhile the western convergence of this same upper trough supports 1024 mb ridge moving offshore from SE Canada). A long frontal zone extends from the 969 mb center...reaching as far SW as the W Atlantic where a new 1009 mb depression has formed offshore of the NE US with the support of eastern divergence from the above-mentioned North America upper troughing.
P2...Surface and upper-level trough in the NE Atlantic continues. The surface trough is becoming absorbed into the 969 mb gale east of Greenland mentioned in paragraph P1. The upper trough has amplified into an upper vortex south of the Azores thanks to equal amplification of W Atlantic upper ridge into the north Atlantic as discussed in paragraph P4. Western convergence of this upper vortex is supporting 1026 mb central Atlantic surface ridge.
...TROPICAL BELT DISCUSSION...
P3...SW-NE tilted upper trough in the central Atlantic and eastern Caribbean persists...currently linked with upper vortex south of the Azores in paragraph P2. In relatively higher pressures west of the upper trough...a western Caribbean upper ridge persists whose upper outflow supports scattered t-storm clouds across Central America and adjacent Caribbean waters. Yet another upper ridge persists in relatively higher pressures east of this upper trough...located in the eastern tropical Atlantic. This eastern tropical Atlantic upper ridge remains de-amplified by the aforementioned central tropical Atlantic upper trough...and by upper trough W of the Cape Verde Islands. Even though I have no satellite-derived 200 mb (upper wind) barbs in this area from the GOES-E satellite outage...I am using the adjacent wind barbs and the curvature of the cloud band SE of the Cape Verde Islands to deduce the aforementioned upper trough W of the Cape Verde Islands. I surmise the cloud band SE of the Cape Verde Islands is supported by the eastern divergence of this upper trough.
P4...W Atlantic upper ridge has amplified into the north Atlantic due to warm air advection ahead of 969 mb and 985 mb gale centers mentioned in paragraph P1. SW end of this upper ridge supports upper outflow of tropical disturbance Invest 97-L north of the eastern Bahamas (see above special feature section for additional details on 97-L).
P5...Tropical wave Invest 98-L with 1008 mb low pressure spin is currently approaching the Lesser Antilles..and has not gotten any better organized in the last 96 hours. It continues to be enhanced by upper outflow of paragraph P3 eastern tropical Atlantic upper ridge. As described in that paragraph...the upper ridge has de-amplified such that zonal shearing upper westerlies have increased across the tropical wave. The CMC...Euro (ECMWF)...GFS...and NOGAPS models insist on tropical cyclone formation from this system as it enters the eastern Caribbean region in the 48 to 96 hr timeframe. A study of this evening's 18Z GFS shows that the SW-NE upper trough in paragraph P3 should be diminishing during that timeframe...allowing the upper ridge over the tropical wave to expand and support such development. First waiting to see how well the tropical wave survives the current and unfavorable westerly vertical shear. If the tropical wave turns out to be in good shape as we approach the 48 hr timeframe...then I will be upgrading it to a special feature on this blog. Models support 98-L tracking NW across the eastern Caribbean region while rounding the SW edge of paragraph P2 central Atlantic surface ridge...the NW track also supported by expanding ridge weakness from 985 mb and 1010 mb depressions mentioned in paragraph P1. The central US surface ridge in paragraph P1 is then progged to eject eastward to the north of 98-L...supporting a more west track toward the Bahamas...but the models show the next mid-latitude system quickly kicking in behind that ridge so that 98-L misses the Bahamas and turns northward toward Bermuda.