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2012 Atlantic Hurricane Season Birdseye Discussion #106

By: NCHurricane2009 , 6:41 AM GMT on September 14, 2012

...SEPTEMBER 14 2012...2:42 AM EDT...
Tropical Storm Nadine has stopped strengthening in the last 24 hours...therefore avoiding hurricane strength. The tropical cyclone could threaten the Azores beyond 5 days out...so interests in the Azores should monitor the progress of this system. See Nadine special feature section below for further details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0126Z-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.


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).

Track-wise...once again as in the previous two discussions...the short-term NHC recorded storm track has a slight leftward angle with respect to the NHC track forecast. The NHC short-term track forecast has continued to require ever-so-slight leftward shifts in the last 24 hrs...and therefore I am predicting another such slight leftward shift as shown by my forecast track in Figure 1 below. This means I am currently slightly left of NHC for the short-term track forecast.

With that said...in order for my and the NHC's short-term track forecast to verify...Nadine would have to turn straight north in the next 24 hours. Nadine is probably strong/tall enough in structure to "feel" steering influence from the paragraph P4 upper vorticity to the SW and paragraph P2 W Atlantic cut-off upper vortex to the NW. Along ex-Leslie's long front...a broad western Atlantic surface low is forming with the support of paragraph P2 W Atlantic cut-off upper vortex peripherial divergence. For the next 24 hrs...00Z GFS still develops a new low-level ridge west of Nadine which I surmise is from upper convergence as Nadine's upper outflow clashes with paragraph P7 Caribbean-to-C Atlc upper ridge. This low-level ridge prevents Nadine from progressing further westward...so Nadine has would have no choice but to go straight north while attracted toward the western Atlantic low to the NW. All of the facts in this paragraph support Nadine tracking straight north by 24 hours from now.

The steering picture still gets more complicated for the timeframe that is now between 24 and 48 hrs...with Nadine between the western Atlantic low to her west...paragraph P2 W Atlantic low-level ridge arriving to her north...the paragraph P6 low-level ridge to her southeast...and the ex-Isaac deep-layered vortex (paragraph P5) to her northeast. With all this conflicting steering...I honestly prefer to keep Nadine stationary between 24 and 48 hrs. But because the models and NHC forecast still want to move Nadine faster to the NE than what I am thinking for that timeframe...I still show some NE progress (but still slower than the NHC forecast).

After 48 hrs...00Z GFS shows a shortwave fragment of paragraph P2 upper trough re-enforcing the ex-Isaac deep-layered vortex...which helps drag Nadine eastward towards it. But what really inclines me to agree with an eastward motion by that time is that the low-level ridge to her north getting knocked out by the paragraph P1 upper trough. Coupled with paragraph P6 low-level ridge to her south and re-enforced ex-Isaac deep-layered vortex to her east...it makes sense to show eastward motion after 48 hrs. I support an increasingly slowing eastward track between 48 and 120 hrs (which makes me even further behind the NHC track forecast)...due to Nadine catching up to what is now the paragraph P2 W Atlantic low-level ridge which will be in the NE Atlantic by this timeframe. In fact...I would prefer to stall out the track towards 120 hrs with the ex-Isaac vortex diminishing...leaving Nadine trapped in a narrowing gap between the paragraph P6 low-level ridge to the south and NE Atlantic low-level ridge. Interestingly...this morning's 00Z GFS prefers to reverse the track WNW away from the Azores after 120 hrs (not shown in Figure 1). I suppose 00Z GFS is giving more credence to the NE Atlantic low-level ridge which should be stronger than the paragraph P6 low-level ridge by that time.

Due to uncertainty beyond 120 hrs and Nadine being closer to the Azores by that time...interests in the Azores should still watch Nadine even though there are emerging scenarios as posed above that suggest the Azores would have no impact.

Figure 1: My forecast for Tropical Storm Nadine created this morning.

Intensity-wise...Nadine has stopped strengthening in the last 24 hrs...maintaining a flat intensity of 70 mph max sustained winds (just below hurricane force). This is contrary to earlier intensity forecasts which made Nadine a hurricane by now. In hindsight...it appears all prior forecasts ignored the western upper outflow blockage that Nadine is currently experiencing from paragraph P4 upper vorticity. At this time...I no longer expect Nadine to ever become a hurricane...but the NHC suggests Nadine will have a fast enough eastward track such that it is buried below a more favorable upper outflow environment beneath an upper ridge wave just ahead of the paragraph P1 upper trough. In fact...the NHC suggests Nadine becoming a hurricane later on by day 5. Because of my Nadine track forecast continuing to show a slower eastward progression than NHC's...I still prefer being below the NHC intensity guidance with the assumption that Nadine will be closer to the westerly shear of the paragraph P1 upper trough. I slightly weaken Nadine to 60 mph max winds by 11 PM Saturday as the westerly shear initially hits Nadine...then I keep Nadine modeled as a steady-state tropical storm balanced by unfavorable paragraph P1 upper trough westerly shear over her west half and favorable upper ridge wave outflow over her east half. This is contrary to my earlier handling of Nadine...which showed continuous long-term weakening...but now I do not want to stray too far away from the newly-elevated NHC intensity guidance for day 5.

The impact swath in Figure 1 is initialized based on the 11 PM EDT NHC tropical storm wind radius. The impact swath remains generally the same size thru the forecast period due to the flat intensity forecast. Note the rightward bias in the initial impact swath (with respect to forecast track)...due to the upper outflow blockage caused by paragraph P4 upper vorticity limiting the activity in the west half of Nadine. The impact swath is drawn with the assumption that Nadine remains lopsided like this thru the forecast period...especially with potential westerly vertical shear from the incoming paragraph P1 upper trough later on.

P1...Upper trough and surface frontal system in mid-latitude westerlies has entering the upper-left corner of above atmo birdseye chart. The surface front is advancing into the eastern US...which curls into a frontal cyclone that has ejected northeastward from Hudson Bay and toward Greenland in last 24 hrs. Warm air advection ahead of the front supports upper ridge wave over the NE US and Atlantic Canada. Upper convergence of the upper trough supports a 1030 mb low-level ridge building over the central US. Finally...a fragment of this upper trough has dropped off a cut-off upper vortex over the SW US.

P2...Upper trough has advanced from W Atlantic and into the north Atlantic in last 24 hrs...and its eastern divergence supports the still-strong remnant low of Leslie which has zipped NE toward northern Europe and out of the scope of the above atmo birdseye chart. Even though Ex-Leslie has exited the picture...it should be noted she will continue to bring elevated winds and surf to the Atlantic high seas shipping lanes as she heads toward northern Europe. The associated surface frontal zone tailing from ex-Leslie is stretched across the central Atlantic and the Bahamas. Elsewhere...the western upper convergence of the upper trough used to support large area of dry air and a 1029 mb surface ridge over the W Atlantic...but now is supported by SE convergence of upper ridge mentioned in paragraph P1. The upper trough has left behind a new cut-off upper vortex over the W Atlantic in last 24 hrs. Another old fragment of this upper trough remains cut-off...now located over the SE US/Gulf of Mexico area.

P3...To the southeast of ex-Leslie...the remnant of Michael has continued NE along the cold front extending from ex-Leslie...also out of the scope of the above birdseye charts like ex-Leslie. Ex-Michael appeared intermittently in HPC analyses to the east of S Greenland...but now has also left the scope of the HPC analyses. I am currently assuming that ex-Michael has lost its identity within the larger low pressure field of ex-Leslie.

P4...Cut-off upper vorticity remains in the open Atlantic...still established as SW-NE upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P7). An embedded upper vortex is currently just N of Puerto Rico.

P5...While currently positioned just south of the Azores...remnant deep-layered low of Isaac is turning slowly northwestward toward break in paragraph P6 and P7 lower and upper ridges...the break generated by paragraph P2 mid-latitude system.

P6...Atlantic surface ridge has been eroded out of the western Atlantic as ex-Leslie's Atlantic cold front continues advances in. Its remaining eastern Atlantic portion is supported by the SE convergence of lengthy paragraph P7 Caribbean-to-C Atlc upper ridge cell. Easterly flow on the south side of this surface ridge is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics.

P7...Upper ridging across the tropical Atlantic persists. The Caribbean to Central Atlantic upper ridge cell has been stretched into the NE Atlantic toward Europe thanks to low-level warm air advection ahead of the ex-Leslie (paragraph P2) system. Like a mid-latitude upper ridge wave...southeastern convergence of this Caribbean-to-C Atlc upper ridge supports pockets of dry air in the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa and above Tropical Storm Nadine.

P8...The tropical wave SW of the Cape Verde Islands in the previous discussion is now midway between the Lesser Antilles and Cape Verde Islands. Its t-storm cluster has slightly increased in the last 24 hrs...but so far is embedded in straight upper easterly flow on the south side of paragraph P7 upper ridging. Until the t-storm latent heat release increases to support an embedded warm core upper outflow structure in the upper ridge...not considering this an area for tropical development. It is also headed toward paragraph P7 dry air and less favorable paragraph P4 upper vorticity.

P9...Satellite imagery suggests a tropical wave has emerged from the west coast of Africa and into the waters SE of the Cape Verde Islands. The t-storm activity is not as impressive as it was earlier when it was along the African coast.

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. wxchaser97
11:07 AM GMT on September 14, 2012
Thanks NC09, great job again.
Member Since: December 31, 1969 Posts: Comments:

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