2012 Atlantic Hurricane Season Birdseye Discussion #98

By: NCHurricane2009 , 11:30 AM GMT on September 06, 2012

...SEPTEMBER 6 2012...4:00 AM EDT...
This discussion was intended for release at 4 AM EDT as titled here...but actually became released three-and-a-half hours later (7:30 AM EDT) due to technical difficulties I experienced. Also...the past couple of released discussions had the incorrect date listed in their title...but have now been corrected.

Tropical Storm Leslie has intensified into a hurricane during the last 24 hours while still lurking north of the Lesser Antilles and south of Bermuda...forecast to gradually accelerate northwestward than northward across the west Atlantic and pass very close or over Bermuda over the next days. She could intensify into a strong hurricane...and hurricane watches may go up for Bermuda around Friday morning (based on the possible arrival time of hurricane force conditions and the standard practice for issuing watches 48 hours in advance of those conditions). Tropical storm conditions could reach Bermuda as early as Saturday evening. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...expected to eventually spread into Atlantic Canada. Direct impacts (high winds and rain) to Atlantic Canada can also be expected by late Monday and Tuesday. See Leslie special feature section for further details.

In the last 24 hours...Tropical Storm Michael has briskly intensified into a hurricane at a location well east-northeast of Leslie. It is expected to stay over open waters and potentialy gets absorbed by the east side of Leslie beyond 5 days. See Michael special feature section for details on this new hurricane.

A tropical disturbance southeast of Hurricane Michael (in the eastern tropical Atlantic) is currently disorganized but still in favorable upper winds...and therefore I still have a special feature section for this system. See third special feature section below for details on this tropical wave.

A new tropical disturbance has emerged in the northern Gulf of Mexico as it sags southward from the Mississippi-Alabama area. The disturbance has been upgraded to Invest 90-L and continues to be mentioned in this morning's NHC tropical weather outlook. I am not yet convinced that tropical cyclone formation will occur with this system...and therefore still do not have a special feature section on 90-L. This system is currently mentioned in paragraph P3 in the mid-latitudes discussion below.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1402Z-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).

Note that my updated forecast in Figure 1 below was done on the 5 PM EDT NHC forecast track graphic from last afternoon. The 11 PM EDT NHC track forecast did not change by much since that time.

Leslie remains quasi-stationary over the western tropical Atlantic...but is soon expected to accelerate generally to the north as the next frontal system in the mid-latitude westerlies (paragraph P1) knocks out the blocking Atlantic Canada low-level ridge (paragraph P2) to the north. My updated forecast in Figure 1 is a continuation of my previous...as I see no reason at all to change the forecast points based on the output of last afternoon's 18Z GFS model. This means right now I am a bit left of the NHC track...which has shifted a bit to the right in the last 24 hours. I continue to agree that a wiggle to the left will occur when she first accelerates northward...caused by a shortwave upper trough ejecting eastward well in advance of the main bulk of this frontal system (the western convergence of that shortwave supporting a trasient low-level ridge to the north of Leslie that causes her to wiggle left). This shortwave system was first mentioned in my previous Leslie discussion...but now I realize this shortwave is associated with the remnant of Isaac and mentioned in paragraph P3! It is the presentation of the transient low-level ridge's expanse (in 18Z GFS) that causes me to maintain my previous forecast track (even though as said above this is now left of NHC and even left of 18Z GFS itself).

Longer term track bends to the right as the southwesterly flow ahead of the paragraph P1 frontal system finally hits Leslie. I am still to the left of the NHC during that time...as my starting track is left of NHC. This means Leslie should pass near but offshore of Nova Scotia. Just beyond 120 hrs (not shown in Figure 1)...Leslie should aid in amplifying the paragraph P1 upper trough while advecting the cool air associated with the upper trough southward. As the upper trough amplifies...the steering flow ahead of it should become more southerly (as opposed to southwesterly)...causing Leslie to hook northward into all of Newfoundland and then the east coast of mainland Canada.

Figure 1: My forecast for Hurricane Leslie created 11 PM EDT September 5

Leslie has strengthened into a category 1 hurricane of 75 mph max winds in the last 24 hours. Her t-storm latent heat release appears to have punched out the unfavorable upper vortex over her west half mentioned in the previous discussion. Her quasi-stationary motion so far has not upwelled cooler waters...with healthy 30 deg C waters still in her area when looking at the above thermo birdseye chart. Since we expect her to accelerate northward away from any cooler waters she may soon upwell....gradual strengthening still appears likely. With the 00Z GFS developing grand anticyclonic outflow over Leslie...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 3...a little more bullish than the 11 PM EDT NHC forecast. Note that I adopted my strengthening rate from my previous forecast since it has done generally well recently. By day 3 and beyond...GFS still shows a strong southwesterly upper jet developing in advance of the paragrah P1 frontal system's upper trough at a location just north Bermuda. That is why I weaken Leslie as she approaches Bermuda's latitude...due to anticipated shear from that jet. Even though she should be under southwesterly shear and cross the 26 deg C isotherm (into cooler waters) by day 5...I only keep the weakening rate gradual as she interacts with upper divergence from the paragrah P1 upper trough. It is conceivable that just after day 5...she could easliy become a strong non-tropical low of category 1 hurricane force...supported by divergence of the upper trough.

Impact swath in Figure 1 is initialized on the 5 PM EDT NHC tropical storm wind radius from last afternoon...which I ever-so-slightly inflate in size based on the anticipated strengthening. Even though Leslie weakens later in the forecast...I keep the swath the same size as I anticipate her interacting with divergence of the paragraph P1 upper trough. This interaction will cause pressure falls over a wide area that should still keep the wind field size dilated. I lean the impact swath towards the right of the storm track by day 5 to represent possible southwesterly shear Leslie is expected to encounter by that time. Since she should be moving rapidly northeast by that time...winds will also tend to be stronger in the east half and diminished in the west half...another reason to lean the impact swath to the right. There are several hazards Leslie will bring in the next 5 days. Read all impacts statements in Figure 1 for headlines on all these hazards.

Tropical Storm Michael has strengthened briskly into a hurricane in last 24 hrs. As I anticipated in my previous discussion...Michael is under a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge...and his intensification is due to 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). This is simlar to the recent analogues of Chris in June this year...and Gordon in the latter part of his life in August this year. Chris and Gordon also strengthened in what should have been a shearing environment due to their shallower-than-usual structure for a full-fledged tropical cyclone.

Note that my updated forecast in Figure 2 below was done on the 5 PM EDT NHC forecast track graphic from last afternoon. The 11 PM EDT NHC track forecast did not change by much since that time. Michael strengthened from a 70 mph max wind tropical storm to a 75 mph wind hurricane between 5 and 11 PM EDT...as indicated in Figure 2.

Figure 2: My forecast for Hurricane Michael created at 11 PM EDT September 5.

Track-wise...we last left Michael 24 hrs ago when he was stalling...which told me at the time that he was getting jammed between the paragraph P4 low-level ridge to the east and the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. Therefore I previously downplayed the interaction with the paragraph P2 frontal cyclone's cold front...so I had a track forecast that was biased SW of the NHC's. Of course...this was wrong in hindsight as Michael has turned NE while dragged by the front...which has caused the NHC to adjust their forecast track NE in the last 24 hrs. His developing eye has been following the current NHC track forecast...so I am simply going to agree with the NHC track forecast for the time being. That track forecast assumes the paragraph P2 cyclone's cold front should soon leave behind Michael...which will cause him to northwest while steered by the Atlantic Canada low-level ridge (paragraph P2) coming in from the NW. The NW track is shown to be slow...as he will be trapped in some conflicting steering in the armpit between the paragraph P4 low-level ridge to the east and paragraph P2 Atlantic Canada low-level ridge.

I currently wonder if the slow NW track forecast is too fast. What if Michael becomes nearly stationary in conflicting steering between the two low-level ridges? This is idea is further supported by the quick passage of ex-Isaac to the north during the forecast period...which would briefly weaken the Atlantic Canada low-level ridge that is supposed to steer him NW in the first place. Beyond day 5 (not shown in Figure 2)...if the slow NW track forecast verifies...Micheal would eventually curve around the west side of the Atlantic Canada low-level...taking him northward into the east side of Leslie...where in this case he would meet his demise while getting absorbed my much larger Leslie.

Intensity-wise...as a shallower than usual tropical cyclone seemingly invincible against the effects of vertical shear...Michael has briskly strengthened under a 200 mb divergent westerly jet between the paragraph P2 upper trough and paragraph P5 C Atlc upper ridge. Normally a jet like this would shear apart a taller tropical cyclone. If I look at the 5 PM EDT intensity of 70 mph...I can say that Michael has strengthened by 20 mph in the past 24 hrs...which is what my new intensity forecast in Figure 2 is based on. Realistically...he has strengthened by 25 mph in 24 hrs (considering the 11 PM EDT intensity of 75 mph)...so my intensity forecast in Figure 2 could be conservative. I saturate him as a category 3 hurricane of 115 mph max winds out of conservativeness in case I am wrong. His more NE than expected track makes me think he will now stay north a cut-off upper vortex to be dropped off by the paragraph P2 upper trough...dodging a potential upper convergent source that would cap his intensification...another reason I show continuous strengthening. One could argue that Leslie's outflow could induce weakening thru northerly shear late in the forecast period...but again he appears shallower than usual and invincible to vertical shear at the moment...and his more NE than expected track increases the chances of him staying away from this shear source anyway. The above forecast track keeps Michael over 28 to 27 deg C waters thru the forecast...and with him currently strengthening over 28 deg C water...I have no choice but to keep him at 115 mph thru day 5. I think what will ultimately destroy Michael beyond day 5 is absorption into Leslie's east side...or acceleration northward over cooler waters below 26 deg C.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 5 PM EDT yesterday afternoon...which I only slightly inflate in size on the presumption Michael remains a compact tropical cyclone (but the forecast intensification is what forces me to inflate the size slightly). In Figure 2...I added a statement about surf in the vicinity of the storm. Previously I thought he would be a small & weakish tropical cyclone not capable of stirring much water. Now...he looks like he could become a strong small cyclone that stirs water in the vicinity.

Tropical wave SW of the Cape Verde Islands in previous discussion has been cancelled from NHC TAFB maps recently...due to the NHC's struggle in finding evidence of its existence. However...plenty of t-storms persist east of dissipated tropical wave and beneath this upper outflow of the upper ridge located toward Africa (mentioned in paragraph P5). Wihtin these t-storms...a surface 1012 mb low was removed and then re-added west of the Cape Verde Islands in the last 24 hours. It has been a while since a tropical wave emerged from Africa...so this new surface low could be the next tropical wave that has emerged from Africa...a feature left behind by the above-mentioned dissipated tropical wave...or a surface low that has spun up along the ITCZ due to pressure falls generated by the upper outflow. Because of the favorable upper outflow environment and persistence of this surface low...I still am considering this a special feature on this blog.

P1...Next upper trough in mid-latitude westerlies has entered the upper-left corner of above charts...with its surface frontal zone across south-central Canada and north-central US. Western upper convergence of the upper trough supports buidling 1020 mb surface ridge over the western US. Warm air advection ahead of this front supports the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a Gulf of Mexico surface ridge.

P2...Cut-off upper vortex in the NW Atlantic has de-amplified into an upper trough...and its eastern divergence continues to support a surface frontal cyclone located SE of Newfoundland and evaluated at less-than-1012 mb. To the north of that frontal cyclone...a shortwave upper trough and associated second surface frontal cyclone has moved across southern Greenland. Upper convergence behind this shortwave upper trough and the NW Atlantic upper trough supports a 1020 mb low-level ridge just south of Atlantic Canada....and another 1023 mb low-level ridge on the east coast of Canada.

P3...Remnant surface low of Isaac is centered over the NE US. Its local cool air advection has carved out a new supporting shortwave upper trough in advance of the paragraph P1 upper trough. Upper convergence behind this new shortwave supports 1014 mb ridge over the eastern Great Lakes. Meanwhile...the cut-off upper vortex over the western Carolinas mentioned in the previous discussion has merged with the upper vortex near south Florida mentioned in paragraph P5. Now...split flow divergence between the west side of south Florida upper vortex and southern US upper ridge (paragraph P1) supports a 1010 mb low (newly upgraded to tropical disturbance Invest 90-L) and associated t-storms shifting southward into the northern Gulf of Mexico. The system is under vertical shear as evidenced by the surface low being centered near the Florida panhandle while the t-storms are blown southwestward by northerly flow west of the south Florida upper vortex. The system will remain generally stationary while trapped between the new western US surface ridge and Gulf surface ridge (both ridges mentioned in paragraph P1)...and has 72 hours to develop before the surface front in paragraph P1 sags south and absorbs it. It is conceivable that the shear relaxes before that time if the south Florida upper vortex weakens (as cut-off upper vortices typically do over time)...but until that occurs...I am not considering this a special feature on this blog...especially as some of the t-storms have died off during these overnight hours.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to NW Atlantic surface cyclone mentioned in paragraph P2. What is left is in the eastern Atlantic centered below the NE lobe of the paragraph P5 upper ridging to create a deep-layered ridge center north of the Azores. 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.

P5...Upper ridging across the tropical Atlantic persists. Southern US upper ridge persists...supported by warm air advection ahead of cold front in paragraph P1. Embedded upper vortex in relatively lower pressures south of this upper ridge persists...located near south Florida. 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. Large and elongated upper vortex above Hurricane Michael has broken into a few upper troughs now located southeast of Leslie and Michael...while the remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of these upper troughs.

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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