2012 Atlantic Hurricane Season Birdseye Discussion #82

By: NCHurricane2009 , 11:56 PM GMT on August 18, 2012

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...AUGUST 18 2012...8:00 PM EDT...
While still moving eastward toward southern Azores region...Tropical Storm Gordon has rapidly intensified into a category 2 hurricane in the last 36 hours as stated in special update #81B. As a result...the southern Azores are much more likely to see a fully-tropical Gordon...and see more intense winds than previously thought. Gordon is still expected to reach the southern Azores by late Sunday (late tomorrow) through part of Monday. See the 1st special feature section below for further details.

Elsewhere...remnant of tropical depression seven strengthened to Tropical Storm Helene yesterday afternoon in the Bay of Campeche...just offshore of Mexico...as stated in special update #81A. It has since made landfall and weakened to Tropical Depression Helene...and additional details on Helene are in the 2nd special feature section below. See 3rd special feature section for info on strong tropical wave Invest 94-L in the eastern tropical Atlantic. I am also monitoring pop-up weather in the south-central Caribbean Sea (see paragraph P8 for details).

...ATMOSPHERIC FEATURES BIRDSEYE CHART...

This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, 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...HURRICANE GORDON...
Gordon is confounding at this hour. While my previous forecast did say he would briefly become a hurricane today...he rapidly intensified into a category 2 of 105 mph winds as of 5 PM EDT. As predicted during the Gordon special feature section in discussion #81...the two upper troughs in paragraph P2 are merging...with a more zonal upper westerly jet connecting the south sides of both upper troughs and overspreading Gordon's general region. So why is Gordon doing so well when there is a 200 mb upper westerly jet right above it that should be shearing it? Perhaps its entire vertical structure (surface spin and upper outflow) has become tucked below the 200 mb upper winds. The last time we saw this was during Hurricane Chris in June (see Chris special feature section in discussion #33). But we know at one point Gordon was tall enough to be sensitive to 200 mb shearing...as we cited westerly shear in the Gordon special feature section of discussions #81 and #80.

It is also possible that the cool temps of paragraph P2 upper troughing are de-stabilizing the atmosphere despite surface water temps that are lukewarm...which is also something similar we saw during Chris. Furthermore...it is possible that the eastward acceleration in Gordon's track makes it more resilient to the 200 mb upper westerly jet's shear.

My latest forecast (versus the NHC's) is shown in Figure 1 below.

Track-wise for Gordon...beyond 24 hrs...we are still expecting the two upper troughs in paragraph P2 to be fully merged...with upper convergence behind the merged upper trough causing the 1024 mb ridge S of Greenland (paragraph P2) and Atlantic surface ridge (paragraph P4) to strengthen west of Gordon (this development is also discussed in paragraph P2). In discussions #80 and #81...I had a southward bias relative to NHC as I gave more credence to this low-level ridge development W of Gordon. Today...the NHC track has become aligned with the track forecast I showed in discussions #80 and #81...so I see no reason today to disagree with the NHC track.

Intensity-wise...weakening has got to be imminent as Gordon will still be crossing the 26 deg C isotherm into cooler waters by 5 AM Sun. Since I am confounded by the recent intensification to category 2...I am just going to go along with the NHC's weakning rate shown in their advisory at 5 PM EDT. He is still en route to merge with the frontal zone draped near the Azores (hence eventually becoming a non-tropical frontal low)...courtesy of the Atlantic high seas upper trough/surface cyclone offshore of Europe (paragraph P2). How strong Gordon is during its non-tropical phase depends on how much upper divergence he sees from the east side of the high seas upper trough.


Figure 1: Forecast for Hurricane Gordon this afternoon.

Impact swath in Figure 1 is drawn based on the tropical storm wind radius shown at the NHC 5 PM EDT advisory...then extrapolating that along the forecast track. I shrink it later on based on the forecast weakening.

...SPECIAL FEATURE...TROPICAL DEPRESSION HELENE...
Yesterday afternoon...warm waters in the Bay of Campeche...coupled with supportive upper outflow from W Atlantic upper ridge (paragraphs P1 and P2)...allowed the remnants of depression Seven to finally regenerate into Tropical Storm Helene (special update #81A). She has since made landfall in eastern Mexico today...where she has weakened to a tropcial depression. As of 5 PM EDT today...a small t-storm burst just west of her center caused the NHC to maintain advisories on this system...and scattered t-storms in her circulation continue to bring a flood/mudslide threat along the mountain slopes of east-central Mexico. As a weaker/shallow type of tropical cyclone...she has been tracking WNW to NW in last 36 hrs...steered by low-level flow around 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. As paragraph P1 mentions...this low-level ridge has not been steadily-defined...which is why Helene has been tracking slowly and erratically.

Her continually degrading satellite appearance tells me she will dissipate as a tropical cyclone anytime now...and I think her remnant low will quickly become ill-defined within the low pressure field of the Mexican/SW US summer monsoon. That is why I am not doing a forecast or prolonged outlook for Helene (like I did for Gordon).

...SPECIAL FEATURE...TROPICAL WAVE INVEST 94-L...
Strong tropical wave SE of the Cape Verde Islands 36 hrs ago is now SW of the Cape Verde Islands...still showing impressive t-storm activity and now showing cyclonic turning in those clouds. Within the paragraph P5 upper ridge...t-storm latent heat release has been attempting to generate a warm core upper anticyclone aloft....which is reducing the easterly shear on the south side of that upper ridge. As a result...tropical cyclone formation is more likely...and I am predicting a 100% chance of a tropical depression or tropical storm out of this system sometime Sunday (i.e. in the next 24 hrs). Due to the high levels of tropical activity elsewhere in the Atlantic basin...I have not had time to make my own assessment on a longer-term outlook for this system. However...latest computer model spread on www.wunderground.com/tropical are gung-ho about pushing this system west to WNW into the Lesser Antilles and northern Caribbean region in the next days...so interests in these regions should begin monitoring this system carefully.

...MID-LATITUDES DISCUSSION...
P1...Upper trough over central US/Canada is now over eastern US/Canada. Strong surface frontal cyclone supported by divergence from this upper truogh...located over southern Hudson Bay...has intensified further to 995 mb. Local cool air advection has caused an upper low to form along the upper trough axis...and the surface cyclone is now nearly-stationary beneath this upper low. Warm air advection ahead of this surface cyclone now supports W Atlantic upper ridge that used to be supported by warm air advection ahead of paragraph P2 system (see paragraph P2 for current state of this W Atlantic upper ridge). Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...but so far the surface ridging has not been steady...with one surface ridge tracking from the west US to the Great Lakes (is currently 1018 mb over SE Michigan in the above atmo chart)...and another surface ridge developing over the NW US.

P2...Upper trough regime over eastern Canada remains is now all consolidated in the Atlantic high seas. Frontal cyclone just offshore of Europe in previous discussion is still performing a meandering loop beneath cyclonic flow of amplified Atlantic high seas upper trough. Western upper convergence of Atlantic high seas upper trough supports a 1024 mb ridge just south of Greenland. Shortwave upper trough over Atlantic Canada is merging with Atlantic high seas upper trough...therefore losing its identity. Surface cyclone supported by this shortwave has moved from E Maine to SE of Newfoundland in last 36 hrs...and in next 24 hrs should dissipate as upper covnergence on back side of high seas upper trough causes the 1024 mb ridge south of Greenland to merge with Atlantic surface ridge in paragraph P4 (hence squashing out the existence of this surface cyclone). W Atlantic upper ridge south of this upper trough regime has gained a SW-NE tilt in last 48 hrs...now stretching from E Mexico (where it supports the upper outflow of TD Helene) to the central Atlantic waters E of Bermuda. Because this W Atlantic upper ridge is now supported by warm air advection ahead of 995 mb cyclone in paragraph P1...this W Atlantic upper ridge will be moved to that system's paragraph in next discussion.

P3...Elongated upper trough E of Bermuda persists...which still extends into the central Caribbean. An upper vortex crowns the N end of this elongated upper trough...embedded in the midst of W Atlantic upper ridge mentioned in paragraphs P1 and P2.

P4...Atlantic surface ridge with 1018 mb to 1022 mb centers is supported by a few upper convergent sources while stretching from the northern Gulf of Mexico to Canary Islands....including convergence SE of the of the W Atlantic upper ridge (paragraphs P1 and P2)...convergence behind the upper trough in paragraph P1...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. Hurricane Gordon continues to be near the NW edge of this upper ridge. T-storm latent heat release from Gordon continues to locally inflate this corner of the upper ridge...which has resulted in an embedded shortwave upper trough W of Gordon. This shortwave upper trough W of Gordon was also noted in paragraph P5 of previous discussion #81...and I am still not sure if this shortwave upper trough 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...A new upper vortex has spun up in the central tropical Atlantic...thanks to relatively lower pressures in between the E Atlantic upper ridge in paragraph P5 and W Atlantic upper ridge mentioned in paragraph P1 and P2.

P7...Remnant of depression seven in the Bay of Campeche became Tropical Cyclone Helene just offshore of E Mexico last afternoon (special update #81A)...and therefore has been granted its own special feature section above. See above Helene special feature section for further details on this system.

P8...South fragment of pre-Gordon tropical wave in eastern Caribbean area was removed from NHC TAFB analyses yesterday (see paragraph P7 of discussion #81). Since then...there has been a flare up of t-storms with light cyclonic rotation in the south-central Caribbean...which I think maybe associated with this tropical wave fragment. This area of disturbed weather is located east of the upper vorticity in paragraph P3...and west of a new east Caribbean upper ridge (devloping between the paragraph P3 and paragraph P6 upper vorticities). Split flow upper divergence between the paragraph P3 upper vorticity and new E Caribbean upper ridge is also supporting the t-storms. So far...no computer models develop this system...but I will be watching this system carefully due to the favorable upper divergence. If signs of development occur before this moves into Central America (and before my next full discussion)...then I will write a special update.

P9...Tropical wave expected to cross the Lesser Antilles in the previous discussion has done so...and is now in the eastern Caribbean. Although it has escaped the dry air (paragraph P4)...westerly shear on north side of E Caribbean upper ridge (paragraph P8) is preventing development with this system.

P10...Tropical wave headed into the waters midway between the Cape Verdes and Lesser Antilles has arrived there. It has been producing t-storms extending hundreds of miles west of Invest 94-L. There is enough cyclonic turning is in these clouds such that the NHC TAFB analyzed a 1013 mb low on the south end of this tropical wave as of 1200Z TAFB (and as shown in above atmo chart). The development of its t-storms is due to supportive outflow beneath the E Atlantic upper ridge (paragraph P5)...coupled with enhanced poleward outflow streaming into paragraph P6 upper vortex.

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1. KoritheMan
1:24 AM GMT on August 19, 2012
Still don't know why these don't garner more attention from your readers. I think they're better than mine.
Member Since: March 7, 2007 Posts: 575 Comments: 20573

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