2012 Atlantic Hurricane Season Birdseye Discussion #96

By: NCHurricane2009 , 7:33 AM GMT on September 04, 2012

...SEPTEMBER 4 2012...3:32 AM EDT...
See paragraph P3 of mid-latitudes discussion for updated assessment of Isaac remnant on this blog.

Tropical Storm Leslie lurking north of the Lesser Antilles and south of Bermuda...forecast to move slowly over western Atlantic waters for next few days. As she does so...she could creep toward Bermuda and eventually Atlantic Canada...so interests in these areas should monitor the progress of Leslie over the next days. In addition...Leslie could bring surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...and eventually Atlantic Canada. See Leslie special feature section for further details.

Subtropical disturbance Invest 99-L becomes tropical depression thirteen at a location well east-northeast of Tropical Storm Leslie. See TD thirteen special feature section for details.

A tropical wave southeast of tropical depression thirteen is becoming better organized in favorable upper winds...and therefore has a special feature section. See third special feature section below for details on this tropical wave.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1933Z-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).

As expected...Leslie has become stalled with the storm trapped between the Atlantic Canada low-level ridge to her north (paragraph P2)...Gulf of Mexico low-level ridge to her west (paragraph P1)...and Atlantic low-level ridge to her east (paragraph P4). Prior to becoming stalled...the surface center underwent a large westward wobble in the last 24 hrs that has caused her to be more south and west of where all forecasts predicted her to be at this point. Leslie is a tropical cyclone under northerly shear with a tremendous t-storm mass biased to the southeast of the surface center. The t-storm mass is where the greatest amount of latent-heat-release-driven upper outflow is occurring...so there maybe a mid-level center more embedded in the t-storm mass supported by the pressure falls of the upper outflow. Such a mid-level center would be southeast of the current surface center position...so maybe the mid-level center has dragged the surface center westward...perhaps explaining the unexpected westward wobble. Notice my new forecast track in Figure 1 from the get-go has a rightward bias to NHC's...as I think the surface center could easily regenerate southeastward or eastward into the pressure falls generated by the upper outflow of the t-storm mass...particularly as Leslie is currently quasi-stationary with no other net forces to push her one way or the other at this time.

Longer-term...models show the next frontal system in the mid-latitude westerlies (paragraph P1) knocking out the blocking Atlantic Canada low-level ridge to the north (paragraph P2)....so my forecast in Figure 1 shows a northward acceleration ahead of this frontal system based on how the morning's 00Z GFS model animates the low-level flow. I show no eastward bend thru 11 PM Sat (120 hrs) as their should be enough paragraph P4 surface ridging to her east. My earlier forecasts showed a wiggle to the left as Leslie became attracted toward the initial break in the blocking low-level ridging from the incoming frontal system...but now I think this will not occur since Leslie is further south than thought earlier...so its possible for her to not feel this initial break. Without a leftward wiggle...this means my new forecast in the longer-term is to the right of my previous...passing Leslie (and most of her impacts) east of Bermuda. However...Bermuda should not take their eye off of Leslie because of my opinion. My track forecast is also slower to the north than the 00Z GFS and the NHC's (unlike my previous)...to account for the fact that Leslie could regenerate toward her sheared-off t-storm mass early in the forecast period...and because I see no incentive to move Leslie at all thru 11 PM Wed (thru 48 hrs) based on the way 00Z GFS shows the blocking low-level ridge pattern persisting.

Beyond 120 hrs (not shown in Figure 1)...Leslie should eventual curve eastward depending on how much southwesterly flow she experiences ahead of the incoming frontal system. Because of her more southward-than-expected initial position...she is more likely to link with southwesterly flow on the SE side of the system (rather than southerly flow E of the system)...so at this time I would lean toward a more rightward track toward Newfoundland or offshore of Atlantic Canada...as opposed to a more Nova Scotia solution.

Figure 1: My forecast for Tropical Storm Leslie this morning

Despite being under northerly shear from flow on the back side of the paragraph P2 upper trough...Leslie has gradually strengthened from 60 to 65 mph max winds in last 24 hrs. I think she is experiencing less northerly shear than expected because of her more southward-than-expected position which keeps her distant from the paragraph P2 upper trough. Because she is already strengthening under the northerly shear...and because the northerly shear is going to relax after 11 PM Tue (after 48 hrs) according to the 00Z GFS model 200 mb upper winds...the NHC strengthens Leslie thru the forecast period...and I also agree with that philosophy. Despite the shear relaxing after 11 PM Tue...I still show a slow strengthening rate thru 11 PM Wed as I think a quasi-stationary Leslie would have upwelled cooler waters beneath the sea surface with her winds. I accelerate the strengthening rate as she then moves northward into "untouched" 29 to 30 deg C waters (more 29 deg C toward Bermuda). With the 00Z GFS suggesting grand anticyclonic outflow...coupled outflow enhancement into the south Florida upper vortex to her west (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...I see a recipe for a major hurricane (115 mph+ max winds) by day 5. The 11 PM EDT NHC forecast shows 100 mph max winds by day 5. I do not strengthen Leslie past 115 mph winds out of conservativeness...as for some reason it seems no Atlantic tropical cyclone this season has wanted to take the title of a major hurricane even when conditions appear ripe to do so.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I slightly inflate in size based on the anticipated strengthening.

Subtropical disturbance Invest 99-L has been upgraded to tropical depression thirteen in last 24 hrs. Despite forming below a cold core upper vortex...it is considered fully tropical...so I imagine this is a system whose entire vertical warm core circulation (surface spin and upper outflow) is tucked below the 200 mb upper wind layer. Most recent analogues of this are tropical cyclone Chris in June this year...and Gordon in the latter part of his life in August this year. My forecast versus the NHC's is shown in Figure 2 below.

Figure 2: My forecast for Tropical Depression Thirteen this morning

Track-wise...I am not sure why TD 13 began on a NW heading...but the recent satellite animation and very recent segment of NHC recorded storm track suggest a more westward heading. An initial west track makes sense to me...with steering from the deep-layered ridge north of the Azores mentioned in paragraph P5. Albeit this westward track is slowed by the 1021 mb center seen west of TD 13 in the above atmo chart (1021 mb center part of paragraph P4 surface ridge). My observation of an initial westward track makes me to the south of the NHC forecast from the get-go. I agree with the NHC on a northward hook beginning in 24 hrs...as the system gets pulled up by cold front of NW Atlantic frontal cyclone mentioned in paragraph P2. But the ridge weakness of the front is quiet narrow as to leave behind this system by day 5 (11 PM Sat)...so I show TD 13 becoming stalled while it gets wedged between paragraph P4 low-level ridge to the east and what is now the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. After day 5...the 00Z GFS seems to dissipate TD 13 as it melds together the two low-level ridges.

To me...the intensity forecast is uncertain. This is a fragile and small tropical cyclone at the moment with very little t-storm activity...with paragraph P4 dry air lurking to the south. I think the t-storms are limited by the dry air...coupled with the temps of the cold core upper vortex not being cold enough. Therefore this upper vortex is then acting as a cap whose upper convergence is countering the upper outflow of TD 13...so I choose to keep TD 13 steady-state thru next 24 hrs while the NHC makes it a minimal tropical storm. By 48 hrs..the NHC argues that a blast of NW shear from the paragraph P2 upper trough will destroy this fragile tropical cyclone into a remnant low.

On the other hand...48 hrs is when I prefer to make TD 13 into a tropical storm...but why? I argue that this is a shallower tropical cyclone than normal tucked below the 200 mb upper wind layer...so this system could end up being less sensitive to vertical shear than the NHC is thinking (the analogues of Chris and Gordon actually strengthened under what should have been a shearing environment). Moreover...48 hrs is when 00Z GFS shoves off the capping upper vortex currently over TD 13...and replaces it with a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge...so I surmise TD 13 strengthening while taking advantage of the divergent upper westerly jet as a shallow tropical cyclone (rather than getting ripped apart by the jet as a tall tropical cyclone would). After 48 hrs...the paragraph P2 upper trough drops off a new cut-off upper vortex over TD 13. Like the current upper vortex...I think this new one won't be cold enough to de-stabilize things to TD 13's advantage. So like the current upper vortex...I view this one as a cap that prevents TD 13 from strengthening further. This new upper vortex is pushed south of TD 13 (thanks to Leslie's outflow) by 120 hrs...but with TD 13 being highly fragile right now...I don't want to strengthen TD 13 by that time as I could be wrong (i.e. TD 13 dissipates like the NHC says).

Tropical wave SW of the Cape Verde Islands has finally produced t-storms and become better organized under favorable outflow beneath the upper ridge portion toward Africa (mentioned in paragraph P5). As such...due to the recent string of tropical waves that have developed into tropical cyclones...I have now considered this a special feature on this blog. Have yet to do my own analysis of where this system would head toward if it became a tropical cyclone...due to the activity with Tropical Depression Thirteen and Tropical storm Leslie.

P1...Next upper trough in mid-latitude westerlies is entering the upper-left corner of above charts...with its surface frontal zone across south-central Canada and north-central US. Warm air advection ahead of this front now supports all of the southern US upper ridge once supported by the ex-Isaac's (paragraph P3) warm air advection. Upper convergence on the SE half of this upper ridge supports Gulf of Mexico surface ridge with 1019 to 1020 mb centers.

P2...Main portion of upper trough over NW Atlantic and the Atlantic highs seas has ejected northeastward out of the picture to the east of Greenland. Correspondingly...its main 993 mb frontal cyclone from the previous discussion has also exited the picture. The upper trough leaves behind a cut-off upper vortex in the NW Atlantic whose eastern divergence has intensified a new 1015 mb frontal cyclone NE of Bermuda into less-than-1008 mb in last 24 hrs. To the north of that frontal cyclone...a shortwave upper trough and associated surface frontal cyclone moving across north Canada is entering the top-center of the above atmo chart. Upper convergence behind the north Canada upper trough and NW Atlantic upper vortex supports a 1023 mb low-level ridge over Atlantic Canada.

P3...Remnant surface low of Isaac is centered over southern Indiana as of this evening and early morning. Due to blocking effect of Atlantic Canada low-level ridge (paragraph P2)...its slow eastward speed has caused it to fall behind the nearby Shortwave upper trough...but perhaps eastern divergence of the upper shortwave will continue to cause surface pressure falls to continue attracting the remnant low eastward. Some scattered severe weather across the SE US again popped up today. I suppose the severe threat emerged from instability of daytime heating coupled with directional vertical wind shear in the east half of Isaac (low-level southerlies in Isaac's east half coupled with upper westerlies ahead of upper shortwave). Therefore...watch out for possible severe threat in Isaac's east half thru the next few days. Low-level warm air advection ahead of Isaac's circulation no longer supports the southern US low-level ridge...but instead is now supported by warm air advection ahead of the frontal system NW of Isaac (paragraph P1).

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to NW Atlantic surface cyclone mentioned in paragraph P2. Easterly flow on the south side of this surface ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft pockets of Africa desert dry air westward across the Atlantic tropics. This surface ridge is supported by convergence ahead of the central Atlantic-to-Caribbean upper anticyclonic cell mentioned in paragraph P5.

P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...now supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex diving SW into the Bahamas in previous discussion is now moving westward toward south Florida while steered about southern US upper ridge. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the NE Atlantic by low-level warm air advection ahead of the paragraph P2 mid-latitude system. The NE lobe has aligned with 1028 mb surface center of paragraph P4 ridge to make a deep-layered ridge center north of the Azores. Large and elongated upper vortex above tropical depression thirteen persists in relatively lower pressures east of this central Atlantic upper anitcyclone...while the remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of that upper vortex.

P6...Tropical wave SW of the Cape Verde Islands in the previous discussion has been moved to above 3rd special feature section as it becomes better organized. See 3rd special feature section for details on this tropical wave.

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10:14 AM GMT on September 04, 2012
Good Morning, I thought I got up early every day. :)

Great blog, I'll have to make your blog a daily read.

Member Since: September 3, 2008 Posts: 0 Comments: 28171

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