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By: NCHurricane2009 , 8:39 AM GMT on September 08, 2012
...SEPTEMBER 8 2012...4:45 AM EDT...
Hurricane Leslie has weakened to a tropical storm the last 24 hours while still lurking north of the Lesser Antilles and south of Bermuda. It is forecast to regain hurricane strength and gradually accelerate northward across the west Atlantic. Leslie should pass east of Bermuda. 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). Direct impacts (high winds and rain) to Atlantic Canada appear most likely in eastern Newfoundland on Wednesday. See Leslie special feature section for further details.
Michael is a slowly-weakening hurricane...his max winds now weakening at the rate of about 5 mph every 24 hours. It is expected to stay over open waters and potentially get absorbed by the east side of Leslie in the next 96 hours. See Michael special feature section for details on this strong hurricane.
A tropical disturbance southeast of Hurricane Michael (in the eastern tropical Atlantic) has been dropped as a special feature on this blog. See paragraph P7 in tropical belt discussion for details on this disturbance.
A vigorous tropical wave has emerged from Africa...newly-introduced into the National Hurricane Center tropical weather outlook. Computer model support has been persistent with this wave...and coupled with the fact it is entering favorable upper winds...it is now introduced as a special feature on this blog. See 3rd special feature section below for details.
I still do not expect development from tropical disturbance Invest 90-L in the northern Gulf of Mexico...and it is receiving a reduced probability in the latest NHC tropical weather outlook. This system is currently mentioned in paragraph P3 in the mid-latitudes discussion below.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 0000Z, and the 0122Z-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.
...THERMODYNAMICS BIRDSEYE CHART...
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).
...SPECIAL FEATURE...TROPICAL STORM LESLIE...
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 1017 mb low-level ridge (paragraph P3) to the north. Both my and the NHC's updated forecast track in Figure 1 represent a continuation of the previous.
I am slower than the NHC from get-go and throughout...because Leslie has so far followed my previous forecast quiet well which had a slower progression to the north. My leftward bias toward the latter part of the forecast is because I still see enough paragraph P2 and P3 low-level ridging to the east of Leslie (in the 00Z GFS) for a straight northward track between 11 PM Sat (24 hrs) and 11 PM Sun (48 hrs). 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. The 00Z GFS also supports this idea...and so I show such a hook toward the end of my track forecast. My 120 hr position is a bit north of the GFS...which shows an interesting blocking low-level ridge to the north of Leslie that causes GFS to slow her northward progress. From studying the upper wind forecast from GFS...it appears this blocking low-level ridge is the result of upper southerly flow of the paragraph P1 upper trough (amplified by Leslie) converging heavily with an upper westerly jet stream to the northeast.
Figure 1: My forecast for Tropical Storm Leslie created 2 AM EDT this morning.
Intensity-wise...in the last 24 hrs Leslie has weakened from a 75 mph max wind hurricane to a 65 mph wind tropical storm. With grand anticyclonic outflow over Leslie...coupled outflow enhancement into the upper vortex east of Florida (paragraph P5) and outflow enhancement into an upper vortex dropped off by paragraph P2 upper trough to her east...there is a recipe for strengthening...so what caused the weakening? The above thermo chart suggests that upper convergence on the SE half of paragraph P5 C Atlc-to-Caribbean upper ridge has intensified some of the paragraph P4 dry air...and in turn Leslie has ingested this dry air into her south half. Moreover...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 the dry air and any cooler waters she has upwelled....I re- strengthen her to 75 mph max winds in the next 24 hrs. Beyond 24 hrs...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. I agree with NHC on a transition to non-tropical on day 5...as she becomes entirely supported by the eastern 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.
...SPECIAL FEATURE...HURRICANE MICHAEL...
Updated NHC track forecast is in Figure 2 below. Michael has finally turned NW while becoming steered by the paragraph P2 deep-layered ridge coming in from the NW as the NHC predicted. Since he has almost exactly followed the previous NHC track forecast...and the current NHC track forecast is a continuation of the previous...I see no reason to disagree with that track forecast. Later in the forecast track...he should accelerate northward about the west side of the deep-layered ridge and into the east side Of Leslie (where I agree with NHC that he will get absorbed by 96 hours).
Figure 2: My forecast for Hurricane Michael created at 2 AM EDT this morning.
Intensity-wise...his weakening rate has flattened to only 5 mph every 24 hours (during my previous discussion it was 5 mph every 6 hours). With the above forecast track...he will be south of the 26 deg C sea-surface-temp isotherm thru 72 hrs...so I weaken him on the current observed rate thru 72 hrs with the assumption that the waters are warm enough to support at least a hurricane (75+ mph) in the next 72 hrs. Michael has been behaving as a shallower-than-usual tropical cyclone...so I also assume Michael is not tall enough to be sensitive to northerly shear from Leslie's upper outflow. Even if he was...I argue that soon...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 avoided. In fact...with the outflow being over Michael...this may actual ventilate and help him between 72 and 96 hrs hrs! However...he will have made a rapid northward acceleration into cool waters. So between 72 and 96 hrs...I show the same weakening rate as I did in my previous forecast for that timeframe...which I think is a good compromise between unfavorable cool waters and favorable upper outflow. Michael should be absorbed by Leslie by 96 hrs.
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 weakens.
...SPECIAL FEATURE...TROPICAL WAVE FROM AFRICA...
A strong tropical wave with t-storms has emerged from Africa...currently located SE of the Cape Verde Islands. It has had computer model support over the last days...is entering a moist air environment in the wake of the disturbance in paragraph P7...and should leverage favorable upper outflow induced by upper ridge portion located toward Africa (paragraph P5). Therefore...I do not currently see a reason why this will not develop into the next Atlantic tropical cyclone. Early model runs suggest a tropical cyclone first tracking 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. Generally...mid-latitude systems create the ridge weaknesses for the storms to recurve northward.
P1...Upper trough in mid-latitude westerlies remains concentrated into an upper vortex stacked above a strong 996 mb 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 eastern half of the United States and curls into this frontal cyclone. Western upper convergence of the upper trough supports building 1028 to 1018 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 extending to a pair of 1016 mb centers offshore of the SE US.
P2...Upper trough south of Greenland in the previous discussion is now SE of Greenland...and in the last 48 hours has deposited a new upper vortex over and just south of Hurricane Michael....and another upper vortex over the Azores. The upper trough's eastern divergence supports a surface frontal cyclone now located SE of Greenland and evaluated at less-than-1000 mb. Upper convergence behind this upper trough used to support a low-level ridge south of Atlantic Canada...which is now tucked under upper ridging (created by warm air advection ahead of paragraph P1 system)...resulting in a 1018 mb deep-layered ridge currently SE of Atlantic Canada. Upper convergence behind this upper trough also supports a 1015 mb low-level ridge midway between Canada and Greenland.
P3...Remnant surface low of Isaac is entering the Atlantic high seas from Atlantic Canada. Ex-Isaac is getting support from eastern divergence of the paragraph P1 upper trough...and its local cool air advection has created a new supporting shortwave upper trough. This new shortwave upper trough's western convergence supports a 1017 mb ridge offshore of the NE US. Split flow divergence between the west side of upper vortex east of Florida (paragraph P5) and southern US upper ridge (paragraph P1) supports a 1009 mb low (tropical disturbance Invest 90-L). All of its t-storms have dissipated in what is dry air induced by upper convergence on the SE half of southern US upper ridge.
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 supported by western convergence of Canary Islands upper trough mentioned in paragraph P5. 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.
...TROPICAL BELT DISCUSSION...
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 east of Florida. Anticyclonic upper ridge in the Caribbean to central Atlantic (partially pumped up by the outflow of Leslie) has been stretched across the NE Atlantic and into west Europe by low-level warm air advection ahead of the paragraph P2 mid-latitude system. Embedded Canary Islands upper vortex has weakened into an upper trough in last 24 hrs. Remainder of the upper ridging is located toward the west coast of Africa in relatively higher pressures SE of the Canary Islands upper upper trough.
P6...1012 mb low SE of Tropical Storm Leslie and ENE of the Lesser Antilles in the previous discussion has been absorbed by the outer low pressure field of Leslie.
P7...A surface 1012 mb low west of the Cape Verde Islands was a special feature on this blog in previous discussion #99 (see Eastern Atlantic Tropical Disturbance special feature section in that post). It is no longer a special feature on this blog as its t-storms have decreased...but not sure why (the above thermo birdseye chart shows the system is embedded in a moist air mass...the above atmo chart shows it a surface trough below a favorable upper ridge portion located toward Africa...this upper ridge mentioned in paragraph P5).
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