2012 Atlantic Hurricane Season Birdseye Discussion #81

By: NCHurricane2009 , 11:14 AM GMT on August 17, 2012

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...AUGUST 17 2012...7:15 AM EDT...
Tropical Storm Gordon trucking eastward toward the southern Azores region...likely to affect the southern islands with some tangible weather (gusty winds and rain) as a non-tropical system late Sunday through part of Monday. See special feature section for details.

Elsewhere...remnant of tropical depression seven has arrived to the Bay of Campeche while becoming a bit better organized...but I am not yet re-upgrading it to a special feature on this blog until I see a more imminent trend of tropical cyclone reformation (see paragraph P6 for details). A strong tropical wave has just emerged from Africa...and is already upgraded to disturbance Invest 94-L. I believe this tropical wave will eventually become a tropical cyclone...and therefore have added it as a special feature on my blog this morning. See second special feature section for details.

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0130Z-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 STORM GORDON...
Tropical Storm Gordon initially surprised us in the last 24 hrs by strengthening more than expected...but now has weakened a bit from earlier-than-expected westerly shear induced by SW side of Atlantic high seas upper trough (paragraph P2)...and induced by shortwave upper trough just west of Gordon (paragraph P5). My latest forecast (versus the NHC's) is shown in Figure 1 below.

Track-wise for Gordon...beyond 48 hrs...models still show east US shortwave upper trough (now entering Atlantic Canada) joining Atlantic high seas upper trough (both upper troughs mentioned in paragraph P2)...with upper convergence behind the merged upper trough causing the 1026 mb ridge S of Greenland (paragraph P2) and Atlantic surface ridge (paragraph P4) to strengthen west of Gordon. By 96 thru 120 hrs (5 AM Tue thru 5 AM Wed)...GFS still shows an impressive low-level ridge stomping in from the west and into the Azores. Therefore...I still show my southward bias in the later part of the track forecast...as I still am giving more credence to the forecast low-level ridge development just west of Gordon. The NHC has also done some southward adjustment of the short-term track forecast as of 5 AM EDT...as they note Gordon will be exposed to some deep-layered NW flow on the back side of the Atlantic high seas upper trough/surface cyclone offshore of Europe (paragraph P2). This also increases my confidence in showing a southward bias in the later part of the track forecast.

Intensity-wise...warm enough waters and paragraph P5 upper ridge outflow initially strengthened Gordon more than expected such that he got ahead of my previous intensity forecast. This early morning...we are finding that earlier-than-anticipated westerly shear (1st paragraph of this special feature section) is weakening Gordon...so now he has just fallen behind my previous intensity forecast. My intensity forecat in Figure 1 below is similar to my previous...but a little less vigorous (given that he has just fallen behind). My intensity forecast below suggests strengthening to a minimal hurricane (75 mph max winds) by 24 hrs...as I think the shortwave high-latitude uppper ridge between the two upper troughs in paragraph P2 could transiently reduce the westerly shear. After 24 hrs...the GFS model 200 mb (upper) wind forecast shows the two upper troughs merged...with a more zonal upper westerly jet connecting the south sides of both upper troughs and overspreading Gordon's general region. So after 24 hrs...westerly shear will pick up in earnest...so I forecast Gordon to weaken after 24 hrs. By 5 AM Sun (48 hrs)...Gordon will be crossing the 26 deg C isotherm into cooler waters...and with a frontal zone already draped across the Azores this morning...we can expect Gordon to quickly become a non-tropical frontal low around or just after that time. The frontal zone is courtesy of Atlantic high seas upper trough/surface cyclone offshore of Europe (paragraph P2)...and how strong Gordon becomes as a non-tropical low depends on how much upper divergence he sees from the east side of the high seas upper trough. An alternate scenario was shown by 00Z GFS this morning...which shows Gordon as a dissipating surface trough located N of the Canary Islands as the forecast strong low-level ridge stomps in from the west.


Figure 1: Forecast for Tropical Storm Gordon this morning.

Impact swath in Figure 1 is drawn based on my forecast track. Its initial shape is based on the NE-biased storm canopy seen on infrared imagery...then I extrapolate that along my forecast track. Taking my impact swath literally means only the southeasternmost Azores islands get clipped by tangible weather from Gordon...but the central Azores may get some effect if it follows closer to the NHC's forecast track. Mention of strongest winds being biased south of center is due to Gordon's forecast brisk eastward track...and we know that cyclonic circulations tracking eastward in the northern hemisphere have their winds enhanced on the south side and weakened on the north side.

...SPECIAL FEATURE...TROPICAL WAVE INVEST 94-L...
Yet another tropical wave has just come off of Africa into the Atlantic tropical belt...currently SE of the Cape Verde Islands with impressive t-storm activity. Within the paragraph P5 upper ridge...t-storm latent heat release has been attempting to generate a warm core upper anticyclone aloft. For now...most of this latent heat release has been south of this upper ridge axis...resulting in intensified upper easterly winds shearing the t-storms west of the tropical wave axis. Once an upper anticyclone becomes more established overhead of the wave and within the paragraph P5 upper ridge...the shear will reduce and tropical cyclone formation will become likely.

...MID-LATITUDES DISCUSSION...
P1...Upper trough from W Canada has arrived to central US/Canada. Divergence east of this upper trough is supporting a vigorous 998 mb surface frontal cyclone just south of Hudson Bay. Expect the upper trough to support this frontal cyclone over the next days...and in turn cool air advection behind this frontal cyclone will cause this upper trough to amplify southward across the eastern half of North America in the next days.

P2...Upper trough regime over eastern US and eastern Canada remains spread into all of the Atlantic high seas. Frontal cyclone just offshore of Europe in previous discussion is performing a meandering loop beneath cyclonic flow of amplified Atlantic high seas upper trough. Cool air advection behind this cyclone continues enforcing this Atlantic high seas upper trough...whose western upper convergence supports a 1026 mb ridge just south of Greenland. Shortwave upper trough over the eastern US is now pushing into Atlantic Canada. Divergence east of this shortwave upper trough has intensified the NE US surface cyclone in the previous discussion...which is now 1004 mb over E Maine and spreading into the NW Atlantic. Low-level warm air advection ahead of this cyclone supports W Atlantic upper ridge.

P3...Mid-ocean upper vorticity persists. West Gulf upper vortex has been pushed into E Mexico via upper anticyclonic outflow of the remnants of Seven (paragraph P6). Elongated upper trough E of Bermuda persists...which still extends into central Caribbean upper vorticity.

P4...Atlantic surface ridge with 1016 mb to 1023 mb centers is supported by a few upper convergent sources while stretching from the northern Gulf of Mexico to waters offshore of W Europe....including convergence SE of the of west Atlantic upper ridge (paragraphs P2)...convergence behind the shortwave upper trough entering Atlantic Canada (paragraph P2)...and convergence behind Atlantic high seas upper trough (paragraph P2). In conjunction with south sides of W Atlantic upper ridge (paragraph P2) and E Atlantic upper ridge (paragraph P5)...south side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics.

...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the eastern tropical Atlantic persists. Its northwestern corner continues aiding the upper outflow of Tropical Storm Gordon. T-storm latent heat release from Gordon has locally inflated this corner of the upper ridge...which has resulted in an embedded shortwave upper trough W of Gordon and some upper vorticity SE of Gordon. I am not sure of the shortwave upper trough W of Gordon was the result of Gordon's cyclonic circulation advecting in some of the mid-upper level cooler air associated with the upper vorticity mentioned in paragraph P3.

P6...Remnant tropical wave of depression seven has entered the Bay of Campeche...and gained some hints of organization on satellite imagery. Highly divergent SW quad of W Atlantic upper ridge (paragraph P2) is helping to produce t-storms over the remnants. The NHC has increased its probability of tropical cyclone reformation from this system given the developing organization...and may send out a recon airplane to investigate later today. Currently...I am forecasting the ridge weakness associated with the paragraph P1 system will have no effect...and therefore I expect this to continue WNW into eastern Mexico in the next 36 hrs while steered by the low-level ridge in paragraph P4 (and also by the low-level ridge building beneath the convergent back side of paragraph P1 upper trough). I am also predicting this to become close to tropical depression status (but not quiet get there) before landfall. GFS model is not delivering a clear-cut solution...somewhat showing a surface low tracking WNW into E coast of Mexico...then keeping a semi-permanent surface low along the E coast of Mexico/near S Texas. Also...surface cold front from paragraph P1 system is expected to dive toward the Gulf of Mexico in the next days...which could also add to disturbed weather in this area.

P7...South fragment of pre-Gordon tropical wave in NE Caribbean area has been dropped from NHC TAFB maps in last 24 hrs...as if it dissipated.

P8...Tropical wave midway between the Cape Verdes and Lesser Antilles in the previous discussion continues rapidly westward...and will be crossing the Lesser Antilles later today. Dry air (paragraph P4) is suppressing this tropical wave.

P9...Tropical wave that passed over the Cape Verde Islands in the previous discussion is now west of the islands...already headed into the waters midway between the Cape Verdes and Lesser Antilles. Dry air (paragraph P4) is suppressing this tropical wave.

P10...Yet another tropical wave has just come off of Africa into the Atlantic tropical belt...currently SE of the Cape Verde Islands with impressive t-storm activity. Unlike the previous two tropical waves (paragraphs P8 and P9)...this wave is doing much better against dry air and looks poised to become the next Atlantic tropical cyclone. It has also been upgraded to disturbance Invest 94-L. See 2nd special feature section above for further details on this system.

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