2012 Atlantic Hurricane Season Birdseye Discussion #102

By: NCHurricane2009 , 8:33 AM GMT on September 10, 2012

...SEPTEMBER 10 2012...4:30 AM EDT...
Tropical Storm Leslie still showing no signs of regaining hurricane strength...although the National Hurricane Center forecasts it will before it strikes eastern Newfoundland early Tuesday. Tropical storm and hurricane watches have been raised for eastern Newfoundland...see www.nhc.noaa.gov for latest watches and warnings. As a large non-tropical (extratropical) gale...it will affect the Atlantic high seas shipping lanes after striking Newfoundland. See Leslie special feature section for further details.

Michael has started weakening again in the last 24 hours. 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 to 72 hours. See Michael special feature section for details on this hurricane.

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 1800Z, and the 1929Z-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 north-northeastward ahead of the next frontal system in the mid-latitude westerlies (paragraph P1). For the short-term...Leslie in the last 24 hrs has been better-following the NHC forecast (as opposed to mine) while bending rightward (I had predicted a straight north track for the previous 24 hrs). In the longer-term...I previously speculated 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. This effect caused me to have a leftwared bias to NHC's forecast late in the period...but now this is no longer the case as the NHC forecast has aligned with that solution in the last 24 hrs. Therefore...there is now no disagreement between my and the NHC's track forecast as shown in Figure 1. The NHC forecast continues to quicken the pace at which Leslie accelerates...which now means that Leslie should arrive to eastern Newfoundland early Tuesday instead of late Tuesday as previously mentioned.

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

Intensity-wise...Leslie seems to have mixed out the dry air mentioned in the previous two discussions....but their remains a large ragged eye signatory of the earlier dry air ingestion. However...she has weakened further from 65 to 60 mph max winds in last 24 hrs. The NHC forecasts Leslie to regain hurricane strength (75 mph max winds) before it strikes eastern Newfoundland...because of the rapid northeastward acceleration is predicted to 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). I prefer to keep Leslie at the same intensity she is now thru the forecast period...as a balance between the aforementioned favorable acceleration effect...the unfavorable shearing southwesterly upper jet in advance of the paragraph P1 frontal system's upper trough...favorable upper divergence from the east side of the paragraph P1 upper trough....and unfavorable cooler waters she is about to enter. Because the updated NHC forecast track is even faster...she should hit cooler waters even sooner...so I prefer to roll back the time of transition to non-tropical by 12 hours from the previous...but the NHC keeps the transition time the same from the previous.

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 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.

Updated NHC track forecast is in Figure 2 below. Michael in the last 24 hours has very-well followed the previous NHC forecast track as he turned westward (and even a bit south of west) while following the flow around the south side of the paragraph P3 low-level ridge...and the NHC has not made any adjustments to their track forecast for the next 24 hrs. I agree with the NHC's solution for the next 24 hrs...with that solution matching up pretty well with the forecast flow around this ridge in the 00Z GFS model. This means not long after 24 hrs...Michael should hook and accelerate northward about the west side of the ridge. After the northward hook...it is the longer-term NHC solution that has changed from 24 hrs ago...because Leslie is forecast to track faster to the NE than previously to the degree that Michael should now experience SW flow on the SE half of Leslie instead of getting absorbed by the E side of Leslie. This SW flow has forced to make the longer-term NHC solution more to the right than the previous...and I agree with this thinking. I also agree with the NHC solution of making Michael non-tropical by 48 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 has resumed weakening...now at a rate of 15 mph max winds every 24 hrs. The NHC has diagnosed the weakening due to northerly shear from Leslie's upper outflow. I disagree because once again Michael appears shallower than a typical tropical cyclone (hence less sensitive to vertical shear)...with its upper anticyclonic outflow disappearing from the 200 mb wind barbs in the above atmo chart...and becoming replaced by nearby upper vortex racing northward (vortex mentioned in paragraph P2). A typical tall tropical cyclone would have lost its t-storms as an upper vortex moves over...but Michael right now has a good ring of t-storms around its eye as if its whole vertical structure (surface spin and supportive upper outflow) is tucked shallow below that upper vortex. Even if I am wrong and Michael is tall enough to be sensitive to the northerly vertical 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 eliminated. In fact...with the outflow being over Michael...this may actual ventilate and help him between 24 and 48 hrs! But because I cannot deny that Michael has weakened...my intensity forecast for the next 24 hrs is based on extrapolating his current weakening rate...and my intensity forecast between 24 and 48 hrs I think is a good compromise between aforementioned favorable upper outflow and unfavorable cool waters. My intensity forecast in Figure 2 is now in 100% agreement with the NHC's 11 PM EDT forecast...so there is no difference between my and NHC's intensity forecast at this time.

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.

Strong tropical wave with t-storms passing over the Cape Verde Islands in the previous discussion is now west of the islands. It still features a 1009 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 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.

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 72 hrs for complex reasons. In my previous statement on this tropical wave...I stated that Leslie being forecast to move faster north than thought before would decrease the chances of this system recurving northward into non-tropical Leslie's associated ridge weakness...especially when considering what is currently a strong central US ridge (paragraph P1) building in right behind non-tropical Leslie. But what prevents the models steering this tropical wave west under that 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 72 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...Upper trough in mid-latitude westerlies has intensified a strong surface frontal cyclone from 986 to 977 mb in last 24 hrs as the frontal cyclone pushes onto the east coast of Canada. The associated surface frontal zone is stretched across the west Atlantic just offshore of the US...northern Gulf of Mexico...and Texas-Mexico border. Western upper convergence of the upper trough supports a large area of dry air and a building 1024 mb surface ridge over the central US. Relatively warmer air south of the surface front supports the southern US upper ridge...which has been pushed into the northern Gulf of Mexico in the last 24 hours as the front sags south. Upper convergence on the SE half of this upper ridge supports a southern Gulf of Mexico surface ridge extending eastward into the tropical Atlantic...and narrowly has joined the surface ridge in paragraph P4.

P2...Upper trough approaching western Europe in the previous discussion has exited the picture while moving into Europe. The cut-off upper vortex it left behind south of Hurricane Michael days ago is surging northward while trying to link with the paragraph P3 upper trough. It also leaves behind a cut-off upper trough over the Canary Islands. Associated surface cold front was strewn across the NE Atlantic as of 1800Z TAFB last night.

P3...Remnant surface low of Isaac is moving eastward across the Atlantic high seas and passing north 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 that has moved from SE of Atlantic Canada to now east of Atlantic Canada. 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 dive southward to the east of the Azores.

P4...Atlantic surface ridge has been eroded out of the western Atlantic thanks to the ex-Isaac cyclone (paragraph P3) and NE Atlantic cold front (paragraph P2). What is left is in the eastern Atlantic at 1023 mb...a portion of which is supported by western convergence of paragraph P2 Canary Islands 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 has been pushed into the northern Gulf of Mexico (paragraph P1). Embedded upper vorticity in relatively lower pressures south of this upper ridge persists...located in 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 paragraph P1 system. Like a mid-latitude upper ridge wave...eastern convergence of this Caribbean-to-C Atlc upper ridge supports a large area of dry air southwest of Tropical Storm Leslie. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures south of the Canary Islands upper trough mentioned in paragraph P2.

P6...Surface trough west of the Cape Verde Islands in the previous discussion is no longer a separate feature as it becomes absorbed by the west side of tropical wave Invest 91-L.

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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