2012 Atlantic Hurricane Season Birdseye Discussion #101

By: NCHurricane2009 , 8:26 AM GMT on September 09, 2012

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...SEPTEMBER 9 2012...4:30 AM EDT...
Tropical Storm Leslie shows no signs of regaining hurricane strength. Leslie should pass east of Bermuda in the next 24 hours...but the wind field of the storm has grown large enough in the last 24 hours such that tropical storm conditions should soon clip Bermuda. A tropical storm warning is currently in effect for Bermuda. In addition...Leslie is bringing surf and rip currents to all northern Caribbean Islands...Bahamas...Bermuda...the east US shore...soon to spread into Atlantic Canada (Nova Scotia and Newfoundland). Direct impacts (high winds and rain) to Atlantic Canada appear most likely in eastern Newfoundland beginning late Tuesday. See Leslie special feature section for further details.

Michael has stopped weakening in the last 24 hours. It is expected to stay over open waters and potentially get absorbed by the east side of Leslie in the next 72 to 96 hours. See Michael special feature section for details on this strong hurricane.

The vigorous tropical wave that emerged from Africa in the previous discussion has become better organized...and has been upgraded to disturbance Invest 91-L. 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 0117Z-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 is beginning to accelerate northward as the next frontal system in the mid-latitude westerlies (paragraph P1) knocks out the blocking 1021 mb low-level ridge (paragraph P3) to the north. Leslie has accelerated a bit faster to the north than I previously predicted...and moreover the NHC has accelerated the predicted pace at which Leslie advances northward. Therefore...my new forecast track in Figure 1 is at (instead of behind) the NHC forecast track pace. My initial leftward bias with respect to NHC is because I still see enough paragraph P3 low-level ridging to the east of Leslie (in the 00Z GFS) for a straight northward track in the next 24 hrs. For the late part of the track forecast...I speculate that 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. However...because Leslie is now forecast to be further north when she meets the upper trough...the cool air will not be advected as far south...and hence the upper trough will not be as amplified. Therefore...my track and the NHC's has shifted to the right for the end of the forecast...but my forecast grows a leftward bias by that time as I think the NHC's forecast track underplays the northward hook.

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

Intensity-wise...the dry air ingested into the south half of the storm in the previous discussion has rotated into the east half and eroded the core...creating a large ragged eye feature. What's left of the t-storms is largely in the west half...so perhaps the western rain bands currently over Bermuda will soon diminish as the dry air continues to rotate around (although there is a new t-storm bubble that has just formed north of the center that could delay this). We saw with Isaac in discussion #90 how dry air eroded the core...and without a core the t-storm latent heat release is not focused at the center. Rather the t-storm latent heat release inflates the upper outflow over a large area...and hence surface pressure falls occur over a large area that has expanded the wind field size of Leslie in the last 24 hours. Because Isaac did not strengthen that much under such circumstances...and since the 00Z GFS still shows a strong southwesterly upper jet soon to develop in advance of the paragraph P1 frontal system's upper trough at a location just north Bermuda...my updated intensity forecast in Figure 1 no longer re-strengthens Leslie into a hurricane (75+ mph max winds) under the assumption that SW shear from the jet will soon start to affect Leslie. Even though she should be under SW shear and cross the 26 deg C isotherm (into cooler waters) late in the forecast...I show no weakening either...thanks to expected upper divergence from the paragraph P1 upper trough supporting Leslie (much like how a non-tropical system is supported). I agree with NHC on moving up the timeframe to non-tropical transition to 24 hrs earlier...because the now-faster northward track in Figure 1 takes Leslie to cooler waters sooner.

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. Previous impact statements in Figure 1 said Bermuda would get clipped by tropical storm conditions if the track shifted left. We can now expect Bermuda to get clipped by tropical storm conditions...not because of a shift in track...but because the wind field size expanded as explained in the previous paragraph. I progressively lean the impact swath towards the right of the storm track to represent SW shear Leslie is expected to soon encounter. Since she should be moving rapidly northeast by the end of the forecast...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.

Updated NHC track forecast is in Figure 2 below. Michael has deviated from the previous NHC track forecast by hooking more northward toward ex-Isaac. Intuitively this would seem to necessitate a rightward shift of the whole forecast...but the NHC has maintained their previous track from 24 hrs ago. A look at the 00Z GFS shows a quick strengthening of the paragraph P3 1021 mb low-level ridge to the NW of Michael...driven by the western upper convergence of the ex-Isaac upper shortwave...and driven by eastern upper convergence of the paragraph P5 C Atlc-to-Caribbean upper ridge (as that upper ridge itself gains great amplitude thanks to strong low-level warm air advection ahead of what should be non-tropical Leslie). With the GFS showing the 1021 mb ridge NW of Michael strengthening quickly...and with the NHC forecast in Figure 2 matching up pretty well with the forecast flow around this ridge...I currently agree with the NHC track forecast. Therefore...Michael should soon turn wesward around the south side of the ridge...then later hook and accelerate northward about the west side of the ridge and into the east side of Leslie (where he gets absorbed). Compared to my previous forecast...I gave Michael 12 more hours till absorption time (this is also 12 hours later than the 11 PM EDT NHC forecast in Figure 2)...because Michael is currently maintaining his strength longer than previously forecast.

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

Intensity-wise...Michael has essentially stopped weakening...becoming a steady-state category 2 hurricane of 100 mph max winds. With the above forecast track...for the next 48 hours he will be over the same water temps he is currently over...so I maintain his current strength thru 48 hrs. 200 mb wind barbs in the above atmo chart suggest that Michael may have finally become tall like a typical tropical cyclone...with 200 mb upper anticyclonic flow present over and east of the storm. So perhaps he is now more sensitive to any northerly shear that Leslie's upper outflow may deliver. Whether he is or is not sensitive to this northerly shear...I argue that soon...Leslie's upper outflow will get sheared eastward by the paragraph P1 mid-latitude system...which displaces it over and north of Michael such that northerly shear is avoided. In fact...with the outflow being over Michael...this may actual ventilate and help him after 48 hrs hrs! However...he will have made a rapid northward acceleration into cool waters. So after 48 hrs...I show a weakening rate that I think is a good compromise between unfavorable cool waters and favorable upper outflow.

Impact swath in Figure 2 is initialized based on the small tropical storm wind radius shown by NHC at 11 PM EDT...which I only slightly shrink in size on the presumption Michael remains a compact tropical cyclone that slowly weakens.

A strong tropical wave with t-storms has emerged from Africa in the previous discussion...currently passing over the Cape Verde Islands with a 1009 mb low pressure center whose t-storms have become better organized. The t-storms are 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.

Early computer model runs suggest that a tropical cyclone that forms from this wave would first track westward about the south side of paragraph P4 ridge...followed by a northward recurvature into the open Atlantic while drawn into the large-scale ridge weakness induced by what should be the large and powerful extratropical (non-tropical) remnants of Leslie and its cold front. 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 get more backing from jet stream upper troughs that get more intense from mid-September and forward. However...I will concede that because Leslie will not amplify the paragraph P1 upper trough as much as previously thought (see Leslie special feature section)...the chance that this system does not recurve northward into the associated ridge weakness has increased.

P1...Upper trough in mid-latitude westerlies has intensified a strong surface frontal cyclone from 996 to 986 mb in last 24 hrs as the frontal cyclone pushes east from S Hudson Bay and into eastern Canada. The associated surface frontal zone is stretched across the eastern coast of the US...northern Gulf of Mexico...and US-Mexico border. Western upper convergence of the upper trough supports building 1024 to 1022 mb surface ridge over the western US. Relatively warmer air south of the surface front supports the southern US upper ridge. Upper convergence on the SE half of this upper ridge supports a Gulf of Mexico surface ridge extending to a 1015 mb center over the Bahamas...albeit the surface ridge is collapsing due to the incoming frontal zone.

P2...Upper trough SE of Greenland in the previous discussion is now approaching western Europe...and in the last 72 hours has left behind an upper vortex currently just south of Hurricane Michael. It has re-absorbed the Azores upper vortex it left behind in the previous discussion. The upper trough's eastern divergence supports a surface frontal cyclone now pushing into Europe from the British Isles. Upper convergence behind this upper trough used to support a low-level ridge SE of Atlantic Canada...which is now becoming supported by upper convergence behind ex-Isaac's shortwave upper trough (paragraph P3).

P3...Remnant surface low of Isaac is moving eastward across the Atlantic high seas. Eastern divergence of its shortwave upper trough continues supporting the system. This shortwave upper trough's western convergence supports a 1021 mb ridge currently SE of Atlantic Canada (this ridge formerly supported by convergent west side of paragraph P2 upper trough). Remnant surface trough of disturbance Invest 90-L in the northern Gulf of Mexico should become absorbed by cold front in paragraph P1 in next 24 hrs.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to surface cyclone mentioned in paragraph P2. What is left is in the eastern Atlantic at 1023 to 1024 mb...a portion of which is supported by western convergence of paragraph P2 upper trough. 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 ahead of cold front in paragraph P1. Embedded upper vorticity in relatively lower pressures south of this upper ridge persists...located offshore of the SE US and the western Caribbean. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched into the north Atlantic by low-level warm air advection ahead of the remnant of Isaac (paragraph P3). Embedded Canary Islands upper trough has been absorbed by paragraph P2 upper trough. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures south of the paragraph P2 upper trough.

P6...Surface 1012 mb low west of the Cape Verde Islands in the previous discussion has weakened to a surface trough to the west of tropical wave Invest 91-L in the last 48 hours. It has some small t-storm bursts...but is currently under southerly shear as it gradually approaches upper vortex south of Michael mentioned in paragraph P2.

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