2012 Atlantic Hurricane Season Birdseye Discussion #99

By: NCHurricane2009 , 8:59 AM GMT on September 07, 2012

...SEPTEMBER 7 2012...5:00 AM EDT...
Hurricane Leslie refuses to strengthen during the last 24 hours while still lurking north of the Lesser Antilles and south of Bermuda...forecast to gradually accelerate northward across the west Atlantic. There has been a rightward shift in the forecast track...and therefore the threat to Bermuda has been reduced. However...a tropical storm watch is in effect for Bermuda and should be acted upon. 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 (Nova Scotia and Newfoundland). Due to the rightward shift in forecast track...direct impacts (high winds and rain) to Atlantic Canada appear most likely in eastern Newfoundland sometime after Tuesday. See Leslie special feature section for further details.

In the last 24 hours...Michael exploded into the season's first major hurricane with 115 mph max sustained winds. His max winds are now weakening at the rate of about 5 mph every 6 hours. It is expected to stay over open waters and potentially gets absorbed by the east side of Leslie just after 5 days. See Michael special feature section for details on this strong hurricane.

A tropical disturbance southeast of Hurricane Michael (in the eastern tropical Atlantic) is currently disorganized and beginning to track northwest into less favorable upper winds. The t-storm activity associated with the disturbance is less intense. If these trends continue...this will be the last blog post in which I make this system a special feature. See third special feature section below for details on this disturbance.

I still do not expect development from tropical disturbance Invest 90-L in the northern Gulf of Mexico...even though it is still mentioned in this morning's NHC tropical weather outlook. Therefore I 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 0000Z, and the 0120Z-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 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 1020 mb deep-layered ridge (paragraph P2) to the north. My and the NHC's updated forecast track in Figure 1 represents quiet a rightward shift from the previous. A search through NHC advisories in last 24 hours reveals the computer models shifted to the right...and pulled Leslie more slowly northward...but why? The only thing the NHC advisories mentioned that changed in the models was the amplitude of the paragraph P1 upper trough. It is claimed this upper trough has less amplitude than what the models showed before...probably due to better sampling of the upper trough by weather stations over Canada.

As paragraph P3 (mid-latitudes discussion) mentions...ex-Isaac (remnant of Isaac) has also gone thru some changes in last 24 hours. Its shortwave upper trough is dissipating...but it is explained that it is expected to develop another shortwave upper trough. That means we are still expecting western upper convergence from some sort of ex-Isaac upper shortwave...which means we still expect a transient low-level ridge (supported by that upper convergence) to build north of Leslie.

Previously...we said the paragraph P1 frontal system would come roaring in...causing Leslie to accelerate northward...with a wiggle to the left caused by the transient low-level ridge. With the paragraph P1 system now less amplified than thought before...it will take longer for this system to knock out the transient low-level ridge. Rather...the transient low-level ridge is going to be stronger...persist longer...hence acting as a blocking feature that now elongates the amount of time Leslie remains stalled. Based on the new impressiveness of the transient low-level ridge in 00Z GFS...I hardly move Leslie northward in the next 24 hrs (just as it has done in the past 24 hrs)...causing me to be slower than the NHC's track from the get-go and throughout. 00Z GFS shows some of this low-level ridge remaining by 48 hrs...just enough for me to wiggle the track ever-so-slightly left by 11 PM Sat. Note that Leslie accelerates northward after 24 hrs...as the paragraph P1 frontal system finally comes roaring in. My leftward bias toward the latter part of the forecast is because I still see enough low-level ridging to the east of Leslie (in the 00Z GFS) for a straight northward track between 11 PM Sat (48 hrs) and 11 PM Sun (72 hrs).

Just beyond 120 hrs (not shown in Figure 1)...we previosuly speculated 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 northward hook is not likely to be as pronounced as we thought before...because the paragraph P1 upper trough is not as amplified to begin with as we previously thought.

Figure 1: My forecast for Hurricane Leslie created 2 AM EDT this morning.

Intensity-wise...in the last 24 hrs Leslie has not strengthened at all (maintaining 75 mph max winds). With 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 strengthening right now. However...the NHC cites other resources at their disposal which finds that her quasi-stationary motion has upwelled cooler waters...with temps as low as 26 deg C right below the center. Note that this 26 deg C minimum is not shown in the above thermo chart..because the chart uses a different resource than what the NHC is citing. Since we expect her to accelerate northward away from any cooler waters she has upwelled....I strengthen her to 85 mph max winds by Saturday (the time she begins accelerating northward). By day 2 and beyond...GFS still shows a strong southwesterly upper jet developing in advance of the paragraph P1 frontal system's upper trough at a location just north Bermuda. So I am forced to not strengthen Leslie further as she approaches Bermuda's latitude....because of anticipated SW shear from the jet. Even though she should be under SW shear and cross the 26 deg C isotherm (into cooler waters) by day 5...I show no weakening below 75 mph max winds...thanks to expected upper divergence from the paragraph P1 upper trough supporting Leslie. It is conceivable that just after day 5...she could easily become a strong non-tropical low of category 1 hurricane force...entirely supported by divergence of the upper trough.

Impact swath in Figure 1 is initialized on the 11 PM EDT NHC tropical storm wind radius...which I no longer inflate in size due to the now-expected meager 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 SW 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. These impact statements have been modified due to the changes in forecast in last 24 hours.

My updated track forecast is in Figure 2 below. Michael has been dragged NE by the frontal cyclone in paragraph P2...which has caused the NHC to slightly adjust their forecast even more NE in the last 24 hrs. He is also quasi-stationary with a wobbling eye based on infrared satellite this morning...which means the immediate NW track shown in the NHC forecast has not yet begun. Because of his more NE than expected track...I think he is going to be more trapped in the armpit between the paragraph P2 deep-layered ridge coming in from the NW and paragraph P4 low-level ridge to the east. Furthermore...I cannot imagine a NW track beginning with the quick passage of ex-Isaac to the north thru the next 48 hrs...which would briefly weaken the paragraph P2 deep-layered ridge that would push him NW. Therefore...I show Michael drifting northward thru 48 hrs...ultimately forcing me to be slower and to the right-of-NHC thru the forecast period. After 48 hrs...00Z GFS shows the paragraph P2 and P4 low-level ridges melding together to the ENE of Michael...which would cause him to accelerate northward about the west side of the melded ridge and into the east side Of Leslie (where he will get absorbed just after 120 hrs...or just after 11 PM Tue).

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

Intensity-wise...I am assuming that Micheal's vertical warm core structure (surface spin and upper outflow) should have gained some height during his rapid intensification episode...though I don't see evidence of this in the above atmo chart 200 mb wind barbs...which shows an upper vortex over and south of Michael instead of upper anticyclonic outflow over Michael (source of this upper vortex mentioned in paragraph P2). With my assumption (again which I have no evidence of)...this means he is more sensitive to any northerly shear that would be delivered by Leslie's upper outflow.

Since the rapid intensification...Michael has weakened by 5 mph for every 6 hours. The eye is shrinking on infrared. As we saw in compact Hurricane Kirk recently...the eye quickly disappeared just after shrinkage (as if it underwent an eye wall replacement cycle). Kirk also weakened quickly after the eye disappeared. Assuming Michael is like Kirk at this phase...I would at least prefer to maintain the observed weakening rate thru 24 hrs...which makes me below the 11 PM EDT NHC intensity guidance from the get-go. I am also assuming that a quasi-stationary Michael should soon upwell cooler waters like we are now seeing with Leslie...another reason to weaken Michael.

The weakening rate is flattened after 24 hrs...my assumption being that...

(1) He is east enough of Leslie's outflow to dodge northerly shear
(2) Whatever cooler waters he upwells is still warm enough for a minimal (75 mph) hurricane.

I weaken Michael a touch more by 11 PM Mon...when I think he gets a taste of Leslie's northerly shear. Quickly after 11 PM Mon (after 96 hrs)...Leslie's upper outflow gets sheared eastward by the paragraph P1 mid-latitude system...which displaces it over and north of Michael such that northerly shear is gone between 96 and 120 hrs. In fact...with the outflow being over Michael...this may actual ventilate and help him between 96 and 120 hrs! However...he will have made a rapid northward acceleration into cool waters. So between 96 and 120 hrs...I show slow weakening as a compromise between unfavorable cool waters and favorable upper outflow. Not long after 120 hrs...Micheal should be absorbed by Leslie.

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 shrink in size on the presumption Michael remains a compact tropical cyclone that weakens.

T-storms persist beneath upper outflow of the upper ridge located toward Africa (mentioned in paragraph P5). Within these t-storms...a surface 1012 mb low persists west of the Cape Verde Islands and tracking slowly NW in the last 24 hours (NW track caused by weakening of paragraph P4 surface ridge thanks to paragraph P2 mid-latitude system and Hurricane Michael). 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 tropical wave mentioned in paragraph P6...or a surface low that has spun up along the ITCZ due to pressure falls generated by the upper outflow.

The t-storms have decreased in last 24 hours...but not sure why (the above thermo birdseye chart shows the system is embedded in a moist air mass). If the NW track of the surface low continues...it will soon leave the favorable upper ridge and into a less favorable environment of westerly vertical shear. Therefore...this maybe the last time I give this disturbance a special feature section on this blog.

P1...Next upper trough in mid-latitude westerlies has concentrated into an upper vortex stacked above a strong frontal cyclone currently over southern Hudson Bay...due to locally strong cool air advection on the back side of the cyclone. The associated surface frontal zone is stretched across the northern United States and curls into this frontal cyclone. Western upper convergence of the upper trough supports building 1022 to 1015 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...Upper trough in the NW Atlantic has merged with southern Greenland shortwave upper trough mentioned in the previous discussion. The upper trough has deposited a new upper vortex over and just south of Hurricane Michael. The merged upper trough's eastern divergence supports a surface frontal cyclone now located SE of Greenland and evaluated at less-than-1004 mb. Upper convergence behind this merged upper trough used to support a 1020 mb low-level ridge south of Atlantic Canada...which is now tucked under an upper ridge center (created by warm air advection ahead of paragraph P1 system)...resulting in a 1020 mb deep-layered ridge center. Upper convergence behind this upper trough also supports a 1021 mb low-level ridge midway between Canada and Greenland.

P3...Remnant surface low of Isaac is centered over Atlantic Canada. Its new supporting shortwave upper trough has been pushed SE and is dissipating...thanks to a building upper ridge caused by warm air advection in advance of the paragraph P1 system. With the shortwave upper trough dissipating...ex-Isaac is now getting support from eastern divergence of the paragraph P1 upper trough...and its local cool air advection is expected by models to break off a chunk of this upper trough into its own supporting shortwave. Meanwhile...upper convergence behind the dissipating shortwave used to support a surface ridge over the eastern Great Lakes that is now over the SE US (at 1014 to 1016 mb). Split flow divergence between the west side of Florida upper vortex (paragraph P5) and southern US upper ridge (paragraph P1) supports a 1010 mb low (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 Louisiana while the t-storms are blown southwestward by northerly flow west of the 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 48 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 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 the t-storms are not that impressive.

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 1021 mb ridge 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 the east coast of 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. What's left of upper vorticity that was near Michael is concentrated into a Canary Islands upper vortex...whose NE split flow divergence supports nearby 1014 mb surface low. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of the Canary Islands upper vortex.

P6...A 1012 mb low has been added SE of Hurricane Leslie and ENE of the Lesser Antilles in the tropical Atlantic. This surface low may be the tropical wave that was cancelled in yesterday's NHC TAFB maps (see eastern tropical Atlantic disturbance special feature section of previous discussion #98). The surface low is embedded in paragraph P4 dry air...and may turn NW and get absorbed into quasi-stationary Hurricane Leslie.

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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