2012 Atlantic Hurricane Season Birdseye Discussion #104

By: NCHurricane2009 , 6:52 AM GMT on September 12, 2012

...SEPTEMBER 12 2012...2:55 AM EDT...
See paragraphs P2 and P3 in the mid-latitudes discussion for details on the remnants of Leslie and Michael entering the north Atlantic high seas.

Eastern Atlantic tropical wave Invest 91-L has finally organized into a tropical cyclone. It has strengthened into tropical depression fourteen then Tropical Storm Nadine in the last 24 hours. Nadine is expected to strengthen into a hurricane while also recurving northward then eventually eastward...hence keeping the storm over open waters. See Nadine special feature section below for further details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1924Z-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).

Strong tropical wave Invest 91-L with t-storms and low pressure spin...now midway between the Cape Verde Islands and Lesser Antilles...has strengthened into tropical depression fourteen...then into Tropical Storm Nadine...in the last 24 hours. My forecast on this new tropical storm versus the NHC's is shown in Figure 1.

Track-wise...Nadine was in its formative stages earlier...featuring a tighter swirl center wobbling inside of a larger outer circulation. The wobbling motion of the swirl center created an unsteady initial motion as shown by the NHC recorded storm track in Figure 1. When smoothing out the wobble (by picking the first and last points on the NHC recorded storm track)...Nadine should continue WNW to 50W-20N by 24 hrs...and the 00Z GFS also supports this idea. Therefore my initial WNW track in Figure 1 has a hair of a southward-bias from the get-go...and therefore much of my forecast in Figure 1 has some form of a leftward bias (with respect to NHC) thru the forecast period.

The initial WNW track is supported by paragraph P6 low-level ridge. Between 24 and 48 hrs...the paragraph P2 upper trough develops a cut-off upper low whose eastern peripherial divergence supports the gradual formation of a broad western Atlantic surface low along ex-Leslie's cold front. Based on the 00Z GFS low-level wind field of this surface low...it is reasonable to show a NW track between 24 and 48 hrs. Interestingly between 48 and 72 hrs...00Z GFS 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 between 48 and 72 hrs...so Nadine has no choice but to go straight north while attracted toward the western Atlantic low to the NW.

The steering picture gets even more complicated between 72 and 96 hrs...with Nadine between the western Atlantic low to her west...paragraph P2 east US 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 72 and 96 hrs...but out of respect for the NHC track and models I move Nadine slowly NE.

Between 96 and 120 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 by 120 hrs.

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

Figure 2: Morning (0415Z) Infrared Satellite Image of Tropical Storm Nadine

Intensity-wise...Nadine's t-storms are quiet healthy and symmetric about the center judging by infrared satellite image in Figure 2...with the storm under low shear and enhanced upper outflow as she has developed an embedded warm core upper anticyclone within the upper ridge cell located toward Africa (paragraph P7). This low shear condition should last thru the next 48 hrs...making it difficult for paragraph P6 and P7 dry air to penetrate. I like and currently agree with the NHC intensification forecast shown at 11 PM EDT...which first shows a gradually intensifying system whose broad core has not yet consolidated...then a more rapidly intensifying system who reaches hurricane strength (75+ mph max winds)...and then a system who stops intensifying on Friday as it encounters upper outflow blockage from the west courtesy of incoming paragraph P2 cut-off upper vortex (which currently does not exist but should by 48 to 72 hrs)...and paragraph P1 upper trough right behind that. Because of my westward bias in track from 72 thru 120 hrs (in part because of my suggested slow track between 72 and 96 hrs)...Nadine would be closer to westerly shear generated by incoming paragraph P1 upper trough...so I weaken Nadine faster than the NHC shows with the assumption that Nadine follows my forecast track.

The impact swath in Figure 1 is initialized based on trying to identify circular core of t-storms seperate from the outer spiral bands. This circular core I identified is highlighted in Figure 2. I think this core represents the area in which the tropical storm and hurricane force wind field will gradually develop in...but I do mention in impact statement (b) the rain squalls outside of the core due to the large and sprawling nature of Nadine. The impact swath is gradually grown in size until peak intensity...and then shrunken in accordance with weakening late in the foreast period. Note the rightward bias in the impact swath (with repsect to my forecast track) beginning Friday...associated with westerly vertical shear that should be starting by that time.

P1...Next upper trough and surface frontal system in mid-latitude westerlies is entering the upper-left corner of above atmo birdseye chart...from western Canada and the NW US. Warm air advection ahead of the front supports upper ridge wave over central US.

P2...Eastern divergence of upper trough in mid-latitude westerlies once supported a strong surface frontal cyclone between Canada and Greenland 24 hrs ago...but that cyclone has whirled beneath the less-convergent upper trough axis and has now weakened from 976 to 986 mb in last 24 hrs. The associated surface frontal zone is stretched across the west Atlantic and Gulf of Mexico...where it has overspread Tropical Storm Leslie racing NNE from Newfoundland and hence transitioned her to a non-tropical frontal cyclone. In fact...ex-Leslie is now the dominant frontal cyclone of this system...getting the supportive eastern divergence of the upper trough. Hence...ex-Leslie remains a high wind...high surf threat for the Atlantic high seas shipping lanes as she races NE to the south of Greenland and towards north Europe in the next days. Western upper convergence of the upper trough supports a large area of dry air and a 1028 mb surface ridge over the eastern US. A fragment of this upper trough has cut-off over TX/LA/NW Gulf of Mexico...to the south of the central US upper ridge wave mentioned in paragraph P1.

P3...To the south-southeast of Leslie...Michael has dissipated quickly into a remnant low in the last 24 hrs while losing its t-storms in dry air created by western upper convergence of the upper trough in paragraph P5. Warm air advection ahead of Leslie created an upper ridge wave west of Michael that initiated Michael's demise 24 hrs ago as that upper ridge wave applied northerly shear to Michael. That upper ridge wave is now over ex-Michael (which has reduced the shear)...and ex-Michael may soon intersect the eastern divergence of the ex-Leslie upper trough (paragraph P2) to survive as a non-tropical entity. However...the aforementioned dry air has prevented any t-storms from refiring...so ex-Michael may very well dissipate before it gets the chance to survive as a non-tropical frontal low. Because it is much weaker than ex-Leslie...ex-Michael should not contribute to the high wind...high surf that ex-Leslie will bring to the Atlantic high seas shipping lanes.

P4...Cut-off upper vorticity remains in the open Atlantic...now established as an east-west upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P7)...with leftover upper vortex east of the Lesser Antilles.

P5...Remnant surface low of Isaac is diving southward to the east of the Azores...steered by east side of intensifying paragraph P6 surface ridge supported by eastern convergence of amplifying Caribbean-to-C Atlc upper ridge cell mentioned in paragraph P7. This Caribbean-to-C Atlc upper ridge cell is amplifying thanks to warm air advection ahead of the ex-Leslie system (paragraph P2)...and as a result the shortwave upper trough that has been supporting ex-Isaac over the last several days has equally amplified into a cut-off upper vortex superimposed over ex-Isaac (hence ex-Isaac is now a deep-layered vortex).

P6...Atlantic surface ridge has been eroded out of the northern Caribbean Islands as ex-Leslie's western Atlantic cold front advances in. However...it 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 north Atlantic by 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 a large area of dry air across the Caribbean and east of the Lesser Antilles. Remainder of the upper ridging is located toward the west coast of Africa.

P8...I added a tropical wave in the lower-right corner of birdseye atmo chart 24 hrs ago...surmising a tropical wave had emerged from Africa to the SE of the Cape Verde Islands. This was based on a westward-moving cluster of t-storms seen on satellite. The NHC has added this tropical wave to their TAFB maps...located just south of the Cape Verde Islands. Associated t-storm cluster in last 24 hrs has significantly weakened...and using the above thermo birdseye chart...this appears to be due to paragraph P6 dry air.

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4. NCHurricane2009
2:19 PM GMT on September 12, 2012
Quoting aspectre:
So why isn't Nadine attracted to the Low to its west?
Then why doesn't the surface ridge force Nadine to stay more south until it hits the PuertoRico region?

It actually is attracted to the low between 24 and 48 hrs. During that time...the 00Z GFS showed it as a large inverted surface trough whose flow across it looks like the letter "A"...and Nadine is just east of that...which makes it moves NW into the east side of the "A".

Quoting aspectre:
So why isn't Nadine attracted to the Low to its west?
Then why doesn't the surface ridge force Nadine to stay more south until it hits the PuertoRico region?

The 00Z GFS places the low-level ridge exactly west of Nadine (between 48 and 72 hrs)...which acts like a force field that stops westward progression. Had Nadine been south of that low-level ridge...then yes she would have gone straight WNW into say Puerto Rico.

The steering picture is complicated by 72 hrs and beyond...with the low to the west...the ex-Isaac low to the east...and one ridge to the north...the other ridge to the southeast. That's 4 things trying to push Nadine. What tips the balance for an eastward turn is that ex-Isaac gets re-enforced by a shortwave...and the northern low-level ridge gets knocked out.
Member Since: September 15, 2009 Posts: 896 Comments: 3957
3. originalLT
1:13 PM GMT on September 12, 2012
Had the same thoughts, post#1.
Member Since: January 31, 2009 Posts: 0 Comments: 8012
2. wxchaser97
11:24 AM GMT on September 12, 2012
Very good, thanks NC09! I like how I can compare your forecast to the NHC and your birds eye view map.
Member Since: March 16, 2012 Posts: 134 Comments: 8005
1. aspectre
8:01 AM GMT on September 12, 2012
So why isn't Nadine attracted to the Low to its west?
Then why doesn't the surface ridge force Nadine to stay more south until it hits the PuertoRico region?
Member Since: August 21, 2007 Posts: 0 Comments: 4860

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