2012 Atlantic Hurricane Season Birdseye Discussion #103

By: NCHurricane2009 , 8:13 AM GMT on September 11, 2012

...SEPTEMBER 11 2012...4:15 AM EDT...
Tropical Storm Leslie bearing down on Newfoundland while just under hurricane force. Weather across much of Newfoundland should soon detereorate through the morning hours...then rapdily improve during the afternoon and evening. Tropical storm and hurricane advisories are in effect for a large part of Newfoundland...see www.nhc.noaa.gov for latest watches and warnings. As a large non-tropical (extratropical) gale...it will then affect the Atlantic high seas shipping lanes after striking Newfoundland. See Leslie special feature section for further details.

Michael continues to weaken. It is expected to stay over open waters and potentially lose its identity along the cold front of non-tropical (extratropical) Leslie in the next 48 hours. See Michael special feature section for details on this hurricane.

I expect Leslie and Michael to no longer be tropical entities by the next full discussion 24 hours from now. If so...there will no longer be special feature sections on either storm on this blog...but this does not mitigate the high winds and surf that both storms will contribute to shipping lanes as they race into the Atlantic high seas.

Eastern Atlantic tropical wave Invest 91-L has continued to become better organized...and is about to become the next Atlantic tropical cyclone. See third special feature section below for details.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0123Z-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).

Leslie continues to accelerate north-northeastward ahead of the next frontal system in the mid-latitude westerlies (paragraph P1). She has so far been perfectly on track with the previous NHC forecast...although the NHC in the last 24 hrs has adjusted the track a bit more leftwards for Leslie's non-tropical phase across the Atlantic high seas later on. This adjustment does not affect anything about the short-term track across Newfoundland...so the 11 AM Tue forecast point in Figure 1 is unchanged from the previous. I still believe Leslie will be non-tropical by 11 AM Tue (11 AM this morning)...while the NHC still forecasts transition to non-tropical for 11 PM Tue.

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

Intensity-wise...Leslie was previously forecast by NHC to become a hurricane (75 mph max winds). The NHC argued that the rapid acceleration to the northeast would increase the winds in the east half of the cyclonic circulation (FYI...this same effect decreases the winds in the west half of the cyclonic circulation). This indeed has occurred...with max winds now at 70 mph...but she did not quiet become a hurricane. I forecasted previously for Leslie to stay a tropical storm...and even though I was correct...my intensity error at this point it time is bigger than NHc's because I predicted 60 mph max winds when Leslie is now at 70 mph. The NHC predicts Leslie to stay at 70 mph max winds thru transition to non-tropical...and I agree as Leslie should continue to get supportive divergence from the east side of the paragraph P1 upper trough.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I do not shrink nor expand with the forecast intensity remaining flat. I previously leaned the impact swath to the right-of-track...but I no longer do this as Leslie has amazed me with her wind field symmetry even though she is moving rapidly NE. However...the NHC does note that within this current wind field...the winds are in fact stronger in the east half and diminished in the west half thanks to the rapid NE motion. Without my impact swath leaning to the right as I showed before...Newfoundland is now much more covered by the swath than previously...but the swath remains within the tropical storm warnings and hurricane watches that have been in effect for Newfoundland.

Updated NHC track forecast is in Figure 2 below. Michael in the last 24 hours has tracked a little to the left of the previous NHC track forecast...but is now finally turning northward around the west side of the paragraph P3 low-level ridge. The NHC forecast track is now a hair to the left and faster than the previous...I suppose the faster aspect also making sense since Michael's more westerly position puts him a bit closer to the faster SW flow in advance of Leslie. The rightward bend in forecast track later on remains as Michael should now experience SW flow on the SE half of Leslie instead of getting absorbed by the E side of Leslie as we thought days ago. With the NHC foreast track making sense...I see no reason to disagree with it...especially as infrared satellite imagery shows Michael's surface swirl tracking straight north as the NHC track forecast shows Michael should be. I also continue to agree with the NHC solution of making Michael non-tropical by 24 hrs...coincident with when Michael should merge with the cold front extending from what should be a non-tropical Leslie.

Figure 2: My forecast for Hurricane Michael created this morning.

Intensity-wise...Michael continues to weaken...and is probably no longer a hurricane (75+ mph max winds) based on his disheveled satellite appearance. I had previously predicted that Michael would avoid northerly shear as Leslie's upper outflow would get sheared eastward to a position north or over Michael. Indeed...this upper outflow shows itself as a distinct upper anticyclone due north of Michael (see blue H north of Michael in above atmo chart). However...there is an upper ridge wave due west of Michael (blue zig-zag line in above atmo chart...which is a NE extension of C Atlc-to-Caribbean upper ridge supported by warm air advection ahead of paragraph P1 frontal system). This upper ridge wave is now applying northerly shear across Michael as evidenced by the infrared satellite showing a meager cluster of t-storms to the south and an exposed swirl center to the north. Based on this satellite appearance...and the fact that Michael should be crossing over cooler waters soon...I prefer to weaken Michael faster than I previously showed and faster than what the 11 PM EDT NHC advisory showed for the next 12 hours. Between 12 and 24 hours...I think Michael will be getting supportive eastern upper divergence from the paragraph P1 upper trough as he becomes non-tropical...so I weaken him slower...but my suggested intensity for 24 hours is still lower than what the 11 PM EDT NHC advisory showed.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I slightly shrink in size as Michael weakens. Only a slight shrink is needed because Michael is already a tropical cyclone with a compact wind field that cannot get much more compact from the current.

Strong tropical wave with t-storms persists west of the Cape Verde Islands. It features a 1008 mb low pressure center whose t-storms have continued to become better organized...and this system is now close to becoming the next Atlantic tropical cyclone. The t-storms are still ever-so-slightly biased to the west side...indicative of easterly vertical shear. I diagnose that the shear is due to the tropical wave's t-storm latent heat release being mostly on the south side of the paragraph P5 upper ridge...which enhances the easterly flow on the south side of the upper ridge. Continued latent heat release should allow a relaxation in the shear when an embedded warm core upper anticyclone forms directly over the tropical wave...and in fact we are beginning to see this. The easterly shear did let in some of the paragraph P4 dry air located to the east...which has delayed the tropical cyclone formation of this system.

Computer model runs suggest that a tropical cyclone that forms from this wave would first track westward to WNW about the south side of paragraph P4 ridge...followed by a northward recurvature into the open Atlantic beginning in 48 hrs. A strong eastern US low-level ridge (paragraph P1) should soon enter the Atlantic. However...what prevents the models steering this tropical wave west under the strong ridge is a cut-off upper vortex (and associated surface low supported by peripheral divergence of that vortex) to be delivered by non-tropical Leslie's upper trough. The models agree that this cut-off low pressure system will curve this system northward after 48 hrs. Statistically by this point in the season...a recurvature to the north for an eastern Atlantic tropical cyclone is more likely anyway...as mid-latitude systems such as this forecast cut-off low pressure system are more prominent as the jet stream sags southward beginning in mid-September and onwards.

P1...Eastern divergence of upper trough in mid-latitude westerlies continues to support a strong surface frontal cyclone (currently 976 mb) as the cyclone pushes from the east coast of Canada and into Greenland. The associated surface frontal zone is stretched across the west Atlantic and southern Gulf of Mexico. Western upper convergence of the upper trough supports a large area of dry air and a 1022 mb surface ridge over the eastern US. Relatively warmer air south of the surface front supports northern Gulf of Mexico upper ridge...which in the last 24 hours has merged with the Caribbean-to-C Atlc upper ridge cell mentioned in paragraph P5. The surface ridge once supported by this north Gulf upper ridge has now joined the Atlantic surface ridge mentioned in paragraph P4.

P2...Cut-off upper vorticity remains south of Tropical Cyclone Michael in the open Atlantic...and has elongated into a north-south upper trough squeezed between the Caribbean-to-C Atlc upper ridge cell and upper ridge cell toward Africa (both cells mentioned in paragraph P5). This upper trough features one upper vortex just south of Michael...and another upper vortex just east of the Lesser Antilles. Elsewhere...cut-off upper trough over the Canary Islands has been absorbed by paragraph P3 upper trough.

P3...Remnant surface low of Isaac is moving eastward across the Atlantic high seas and is currently positioned NE of the Azores. Eastern divergence of its shortwave upper trough continues supporting the system. This shortwave upper trough's western convergence supports a greater-than-1020 mb ridge SSE of Greenland. The paragraph P5 C Atlc-to-Caribbean upper ridge should soon gain great amplitude (thanks to strong low-level warm air advection ahead of what should be non-tropical Leslie). As this occurs...the ex-Isaac shortwave upper trough will become a cut-off upper vortex that dives southward around the east side of the amplified upper ridge...which means ex-Isaac itself will soon dive southward to the east of the Azores.

P4...Atlantic surface ridge has been extended into the northern Caribbean Islands as it merges with what was a Gulf low-level ridge in paragraph P1. The entire surface ridge is now supported by SE convergence of lengthy paragraph P5 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.

P5...Upper ridging across the tropical Atlantic persists. Northern Gulf of Mexico upper ridge (paragraph P1) has merged with the Caribbean to Central Atlantic upper ridge cell in last 24 hrs. Embedded upper vorticity in relatively lower pressures persists in the western Caribbean. The Caribbean to Central Atlantic upper ridge has been stretched into the north Atlantic by low-level warm air advection ahead of the paragraph P1 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.

P6...Surface trough west of the Cape Verde Islands in the previous discussion (now midway between the Cape Verdes and Lesser Antilles) is still suprisingly a separate feature as it becomes absorbed by the west side of tropical wave Invest 91-L. At any time now...I expect this surface trough to be removed from the NHC TAFB maps.

P7...Due to a westward-moving cluster of t-storms...satellite imagery suggests the next tropical wave has just rolled off of Africa (see lower-right corner of above atmo chart).

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