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2012 Atlantic Hurricane Season Birdseye Discussion #84

By: NCHurricane2009 , 4:05 AM GMT on August 22, 2012

...AUGUST 21 2012...10:00 PM EDT...
Note this discussion was intended for release at 10 PM EDT...but has been released a full two hours later. This delayed release is due to technical difficulties with my computer.

Note this discussion has been created approximately 12 hours later than the following atmospheric features chart...thermodynamics chart...and TD 9 (Tropical Storm Isaac) forecast graphic. I will release a special update early tomorrow morning with an updated forecast graphic for Isaac. This special update may also include updates on any other system in the Atlantic discussed here...in case they have changed signficantly since the release of these outdated graphics.

Strong tropical wave Invest 94-L has strengthened into tropical depression 9...then into Tropical Storm Isaac. The delay in its development shifts the major hurricane threat from the northeast Caribbean to the north-central Caribbean. See Isaac special feature section for details.

Area of Interest Invest 95-L...This disturbance remains stalled in the western Gulf of Mexico as expected. See 2nd special feature section for further details.

Area of Interest Invest 96-L...Tropical wave behind Invest 94-L (now Isaac) has also become organized...and is likely to become yet another trpoical storm. See 3rd special feature section for further details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0600Z, and the 0600Z-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 94-L has finally strengthened into Tropical Depression Nine, then into Tropical Storm Isaac...as it nears the Lesser Antilles...Puerto Rico...and the Virgin Islands. Conditions will detereorate in this areas over the next several hours...where preparations should be underway. Preparations in the Lesser Antilles will need to near completion sooner as Isaac will strike them first. As of 8 PM EDT (a couple of hours before the release time of this discussion)...Isaac was centered at 15.5N-54.9W...or 453 miles...east of Guadeloupe in the Lesser Antilles...with maximum sustained winds at the center of 40 mph. The forecast graphic (labeled as tropical depression nine) in Figure 1...and associated forecast discussion below...was created a little over 12 hours ago as of this writing. Because of this...I will be doing an updated forecast graphic in a special update in the next several hours (early this upcoming morning).

Track-wise for Isaac...in Figure 1...my first and short-term forecast points basically followed the old forecast I created in special update #82A...because of near-perfection in how this system's center has been following that track over the last days. This leads to a hair of a southward bias in the short-term points in Figure 1.

Medium-term...my track forecast re-joins the NHC's...and represents a southward shift from #82A (which had the system pass closer to Puerto Rico and pass right over Hispaniola). I suppose this makes sense...as #82A stated the northward bend toward these areas would be caused by Isaac being strong/vertically taller and being steered more poleward by paragraph P6 upper vortex...but now the delay in Isaac's formation makes him shallower/less strong than I thought he would be by now. But also...GFS (at the time of Figure 1's creation) was showing a weaker paragraph P6 upper vortex pushed bullishly away by Isaac's growing upper outflow...which also makes it less likely that this upper vortex would have a poleward influence on Isaac's track.

Longer-term (days 4 and 5)...there is an extremely tight model consensus on www.wunderground.com/tropical that makes one shy to disagree with...especially as the NHC official track also agrees with that consensus. That is why in Figure 1 I still follow NHC's track by days 4 and 5. Even by days 4 and 5...the Atlantic low-level ridge (paragraph P4) still has no weaknesses far south enough to influence Isaac. So logic says the NW turn shown should then be caused by an upper-level weakness that a very strong/very tall Isaac can feel. By days 4 and 5...models show an upper-level ridge weakness caused by the large-scale paragraph P1 upper trough moving into the W Atlantic...featuring a narrow extension across the Carolinas to E Gulf of Mexico.

The track forecast for days 4 and 5 is threatening to Hispaniola (Haiti and the Dominican Republic)...Jamaica...and Cuba. The next watches or warnings are likely to be raised in these areas...so these areas should get ready to prepare when advisories are raised.

Beyond day 5...the track forecast is suggestive of a Florida threat...so interests in Florida should monitor the progress of Isaac carefully. Upstream of the paragraph P1 large-scale upper trough...the models show a shortwave upper ridge followed by the next major mid-latitude upper trough immediately after. If Isaac is as far NW as shown by day 5...the next mid-latitude upper trough's low-level ridge weakness is far south enough to steer even the weakest/shallowest of tropical cyclones poleward...so a poleward turn is imminent beyond day 5 regardless of Isaac's strength or vertical depth. If Isaac remains tall/vertically deep...I interpret the brief shortwave upper ridge would nudge him more west beyond day 5. Therefore...solutions after Florida range from a threat in the east Gulf of Mexico (if the shortwave upper ridge nudges the track west)...or alternatively a track that goes up the US east coast.

Figure 1: My forecast for Tropical Depression Nine (TS Isaac) released a little over twelve hours ago.

Intensity-wise...Isaac has taken longer to become a tropical cyclone than prevoiusly thought...due to the dry air in paragraph P4. As old discussion #82A said...I believe outside of dry air...Isaac has almost unlimited strengthening potential with warm waters and a good upper atmosphere. Discussion #82A also suggested upper outflow enhacement from....

(1) Paragraph P6 upper vortex to its west.

(2) A second upper vortex that should form to its east. Indeed paragraph P5 below confirms this second upper vortex is forming.

Even better for Isaac...outflow enhacement to the north is going to come from the large-scale paragraph P1 upper trough as it moves into the Atlantic by days 4 and 5. And coupled with an impressive t-storm cluster at the time of Figure 1's forecast graphic...that is why I suggested a more bullish strengthening rate to 120 mph max winds (category 3 major hurricane)...more bullish than the NHC shows. This means I pose a possible major hurricane scenario in the north-central Caribbean area (Jamaica...Cuba...or Haiti). The good news is after Figure 1 was created...another round of dry air ingestion for Isaac weakened its t-storms and has caused him to already fall behind my intensity forecast schedule.

Impact swath in Figure 1 begins with the shape/size of the impressive t-storm cluster that was over/west of the center at the time Figure 1 was created...then me imagining the tropical storm wind field growing from there and into a generically-sized major hurricane. With today's skill...it is almost impossible to forecast the true size fluctuations that occur in a tropical cyclone...so the swath size could easily change in the next 5 days. The swath location could also undergo shifts in the next days if there is a good shift in forecast track.

Outflow from W Atlantic upper ridge (paragraph P1) continues to enhance t-storms in this area...which still remains semi-organized about a surface trough just offshore of Mexico and over W edge of Gulf of Mexico waters. Based on above thermo chart...upper convergence on the back side of the paragraph P1 upper trough has caused some sinking dry air to the north that has infiltrated into this system...numbing tropical cyclone potential for this system for now.

Satellite animations suggests Invest 95-L remains stationary...trapped in a low-level ridge weakness associated with this front...with the Atlantic low-level ridge (paragraph P4) to the east and west US/Canada low-level ridge to the north (paragraph P1) providing conflicting steering. Based on previous discussion #83...I expect 95-L to remain stationary thru a little after 24 hrs...then begin drifting northward to south Texas beyond that time as the blocking low-level ridge to the north gets pushed eastward by the next frontal system in the mid-latitudes. With upper winds staying favorable thru this time...a weak tropical cyclone is possible...so interests along the NE Mexico and south Texas coast should continue monitoring this system carefully. Even if no tropical cyclone development occurs...the risk of persistent rains could cause flooding problems.

Tropical wave located east of Invest 94-L (now TS Isaac) is now southwest of the Cape Verde Islands...continiuing to feature cyclonic turning and organized t-storm activity to the degree it now looks poised to become yet another Atlantic tropical cyclone.

As mentioned in the old forecast discussed in special update #82A...there is an upper vortex that is expected to form east of 94-L (Isaac)...associated with relatively lower pressures east of Isaac's upper outflow. Latest 200 mb wind barbs in above atmo chart reveal this upper vortex is already forming (and this is concurred by the notes in paragraph P5 below). It looks as though this upper vortex will be just in the right spot to enhance the NW outflow of this tropical wave rather than shear it. Although I have not yet had time to make my own detailed assessment on this system...I do support a gradual NW recurvature into the open Atlantic waters if this system becomes a strengthening tropical cyclone. As it becomes stronger/vertically taller...it is more likely to be steered by the east side of aforementioned upper vortex...and the above Isaac special feature section suggests a large-scale upper ridge weakness which will cause Isaac to initially turn northwest (and the same can be said with this system). Even from a low-level steering perspective...as Isaac bends northward...the east side of Isaac's low-level circulation will also be coaxing this system into a NW track to the east of him.

P1...Upper trough over the eastern US/Canada persists. Strong surface frontal cyclone supported by divergence from this upper trough...now located over eastern Canada...has weakened to a very diffuse 1006 mb under the less divergent upper vortex that is a product of its local cool air advection. Warm air advection ahead of this surface cyclone still supports W Atlantic upper ridge. W Atlantic upper ridge still has a SW-NE tilt...now stretching from E Mexico (where it supports the upper outflow of 95-L) to the central Atlantic waters E of Bermuda. Meanwhile...upper convergence on the back side of this upper trough has been supporting surface ridging...with multiple 1012 to 1019 mb centers across North America.

P2...Upper trough regime over Atlantic high seas persists. Frontal cyclone just offshore of Europe has exited the picture. Western upper convergence of Atlantic high seas upper trough supports surface ridging south of Greenland that has joined Atlantic surface ridge in paragraph P4. Shortwave upper trough from Atlantic Canada is still merging with Atlantic high seas upper trough (as it passes over the Azores). Over the last days...surface cyclone supported by this shortwave has moved from SE of Newfoundland to the waters W of the Azores while chasing Gordon. Meanwhile...Gordon himself has quickly decayed into a non-tropical remnant low with zero t-storm activity due to its passage over cooler waters north of the Canary Islands and east of the Azores. So far...it shows no signs of surviving under any divergence produced by the Atlantic high seas upper trough. Therefore...Gordon continues to be a gradually dissipating surface feature that will be steered increasingly southward by the above-mentioned-merger between the ridge S of Greenland and paragraph P4 ridge.

P3...Elongated upper trough in Central America in last 24 hrs has moved into the Yucatan as an upper vortex...all as it retrogrades about W Atlantic upper ridge mentioned in paragraph P1.

P4...Atlantic surface ridge with 1018 mb to 1022 mb centers is supported by a few upper convergent sources while stretching from the central Gulf of Mexico to the waters offshore of W Europe....including convergence SE of the of the W Atlantic upper ridge (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.

P5...Upper ridge across the eastern tropical Atlantic persists. T-storm latent heat release from tropical wave Invest 94-L (now Isaac) is causing this upper ridge to concentrate into an anticyclonic center over the storm. The remainder of the upper ridge is toward west Africa...a portion of which is inflated by Invest 96-L's latent heat release. An upper vortex is forming between Isaac's outflow and Invest 96-L's outflow...splitting this upper ridge in half.

P6...Upper vortex in central tropical Atlantic is retrograding westward...thanks to steering about the growing W Atlantic upper ridge (paragraph P1) to its north. The upper vortex is currently entering the Caribbean Sea.

P7...Tropical wave that was in the central Caribbean during the previous discussion is now in the western Caribbean heading for the Yucatan/Bay of Campeche/SE Mexico area.

P8...Tropical wave midway between the Cape Verdes and Lesser Antilles is now crossing the Lesser Antilles into the eastern Caribbean Sea. As Invest 94-L strengthened into Tropical Storm Isaac to its east...this wave is losing the battle of low-level inflow against Isaac. Due to strengthening Isaac's proximity to the east...this tropical wave may become indistinct in the west side of Isaac's outer low pressure field.

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. KoritheMan
4:55 AM GMT on August 22, 2012
Pretty exhaustive analysis there.
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