2012 Atlantic Hurricane Season Birdseye Discussion #72

By: NCHurricane2009 , 8:37 AM GMT on August 06, 2012

...AUGUST 6 2012...4:30 AM EDT...
Ernesto and Florence each are much lesser threats than they appeared to be previously. Ernesto has passed south of Jamaica and Cayman Islands without tropical storm impacts reaching the islands...but threat of tropical storm conditions shifts to southern Yucatan/Belize/NE Guatemala region...as well as to northern Honduras. See special feature sections for more details on Ernesto and Florence.

This discussion has taken a long time to get out due to experimental surface and 200 mb (upper-level) outlooks shown in atmospheric birdseye chart section. I may not continue such a product because of how much time it takes me to create them. My updated forecasts for Ernesto and Florence...as well as some other features...are heavily dependent on these outlooks. If these forecasts do not work...another reason for me to stop or adjust the methods used in these outlooks.


This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1200Z, and the 1330Z-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.

5-day surface outlook created at 1200Z. The streamlines represent lower-level atmospheric flow...ONLY drawn at the well-defined perimeter of surface ridge flows. Cyclonic flow is assumed to be occurring between the surface ridge flows. 0-hour flow is based on HPC's North America map wind barbs. Flow beyond that is based on GFS model wind barb display for its surface to 10-meter high forecast...shown on NOAA's MAG page.

5-day 200-mb (upper-level) outlook created at 1200Z. The streamlines represent upper-level atmospheric flow...ONLY drawn at the well-defined perimeter of upper-level trough/vortex flows. Anticyclonic flow is assumed to be occuring between the upper-level trough/vortex flows. 0-hour flow is based on 200-mb wind barbs in above atmospheric birdseye chart. Flow beyond that is based on GFS model wind barb display for its 200 mb forecast...shown on NOAA's MAG page.


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).

Ernesto has been accelerating rapidly west across the Caribbean Sea...in strong low-level trade wind flow driven by wind going from the Atlantic surface ridge (paragraph P5) and into the ITCZ to his south. For a tropical cyclone that is not well-established like Ernesto...rapid westward motion is not good for a few reasons:
(1) A cyclonic circulation that moves quickly westward has winds accelerate on its north side...but winds that are slowed on its south side. If the cyclonic circulation is not yet well-established...the south side of circulation may open and the system no longer qualify as a tropical cyclone.
(2) Rapid westward motion in deep tropics is sympomatic of strong low-level inflow into ITCZ...so the tropical cyclone is competing for low-level inflow against the ITCZ and hence may not build as much t-storm activity necessary for development.
(3) The force of Ernesto's upper anticyclone had been pushing the E Caribbean inverted upper trough into central Caribbean (paragraph P6)...and the ventilation provided by the upper anticyclone had been producing t-storms protecting Ernesto from dry air (see paragraph P5 for source of dry air). With the rapid acceleration to the west...Ernesto's center left behind the favorable upper anticyclone...sliding right into the dry air and beneath the unfavorable inverted upper trough.

The Gulf portion of Atlantic surface ridge (paragraph P5) has weakened due to low pressure field of ex-Invest 91L (paragrpah P7) and cold front of mid-latitude system in paragraph P1. This surface ridge weakness has caused Ernesto to finally slow down...allowing its trailing t-storm complex to catch up. A very recent check of upper winds reveals Ernesto is still battling inverted upper trough directly overhead...and above 200-mb outlook suggests this inverted upper trough won't be punched out by Ernesto's t-storm latent heat release till 24 hrs or a little later...so I forecast no strengthening for next 24 hrs in Figure 1. After 24 hrs...the above 200 mb outlook shows Ernesto's growing upper anticyclone pushing Gulf upper vortex (paragraph P4) westward...with possible grand anticyclonic outflow in between Gulf upper vortex to the west and paragraph P3 mid-oceanic upper vorticity encroaching from the east. With great outflow and warm waters...the only inhibiting factor for strengthening after 24 hrs would be land interaction...so my intensity forecast in Figure 1 shows a decent strengthening rate while Ernesto is over water...and a weakening rate when Ernesto is over land.

Low-level steering solution (using above 5-day surface outlook).......Current ex-91L/paragraph P1 cold front surface ridge weakness shows as an inverted E Gulf of Mexico trough wedged between central US ridge (paragraph P1) and Atlantic ridge (paragraph P5). This inverted trough grows into SE US from 0 hr to 72 hr...its growth spurt in 72 hr due to W Canada impulse (paragraph P1) swinging in by this time. Inverted trough grows into a complete SW-NE ridge weakness thru 96 and 120 hr...but south of that weakness the Atlantic ridge is shown bulging west from 96 to 120 hr. From surface perpsective...inverted trough suggests steady WNW steering thru 72 hrs...then collapse in steering in SW-NE ridge weakness by 96 hrs (as Atlantic ridge tries to push Ernesto north while central US ridge blocks this)...then perhaps a WNW nudge by 120 hrs as Atlantic ridge bulges west as stated above.

Upper-level steering soliuton (using above 5-day 200 mb outlook).......Paragraph P4 states that the only reason upper vortex in Gulf will move out of the way is due to a strengthening Ernesto's upper anticyclone. If the upper vortex did not move out of the way...it would shear apart Ernesto and keep him shallow...so I can't imagine a scneario at this point where a vertically-coupled/strong Ernesto would be steered by this upper vortex. Therefore...I keep this upper vortex a non-factor in Ernesto's upper-level steering solution. This leaves SW US upper ridge (paragraph P1 & P2) and upper trough (paragraph P1)...but the paragraph P1 upper trough amplfies southward and erodes SW US upper ridge thru 72 hrs (almost to suggest gradual collapse in upper steering solution thru 72 hrs). The 200 mb 5-day outlook hints the SW US upper ridge recovers and noses in from the west by 96 and 120 hrs...which would help in a westward upper-level steering solution.

In Figure 1...since the above upper-level steering solution is collapsed thru first 72 hrs...I will be using the first 72 hrs worth of above lower-level steering solution regardless of how strong Ernesto gets. Between 72 hrs and 96 hrs (11 PM Wed to 11 PM Thu)...am forced to still use low-level steering soltuion as Ernesto should be weaker/shallow from Yucatan landfall. Again...the low-level solution at 96 hrs is a weak one...while Atlantic ridge tries to push Ernesto north while central US ridge blocks this. Ernesto should be re-strengthening between 96 hrs and 120 hrs...allowing me to blend in more of the 120 hr upper-level steering solution (low-level solution at that time is westward bulging Alantic ridge giving WNW nudge...upper-level solution at that time is SW US upper ridge nosing in from the west and providing a more westward push).

Figure 1: Forecast for Tropical Storm Ernesto this morning.

Alas...after reaching a peak strength of 60 mph winds...Florence has weakened quiet a bit by couhing on dry air (mentioned in paragraph P5). As noted in discussion #71 and #71A...this dry air threat to Florence always loomed...but it has been finally realized.

The weakening has caused Florence to become more shallow...hence causing her to turn westward while losing touch with upper-level ridge weakness (paragraph P3) and becoming more steered by low-level ridge (paragraph P5). Previous forecast adjustment in discussion #71A was suggestive of WNW heading all the way after Florence unexpectedly had strengthened...the theory being that she would always feel the upper-level ridge weakness because of her refusal at the time to weaken from dry air. In hindsight...even if she had maintained her strength/vertical depth...she still would have turned more westward anyway as...
(1) Convergence behind British Isles/W Europe upper trough strengthened the low-level ridge to her north (paragraph P5).
(2) Comparison of yesterday's and today's atmo birdseye chart shows a westward shift in upper-level ridge weakness (paragraph P3 feature).

My forecast in Figure 2 is now aggressive in dissipating Florence into a tropical depression then remnant low in next 24 hrs. Above 5-day 200 mb outlook suggests semi-permanence of mid-ocean upper vorticity (paragraph P3)...so by 48 hrs Florence's current westward pace would begin to bring her to southwesterly vertical shear from this upper vorticity...which is why I do not forecast her remnant low to regenerate into a tropical cyclone. By 96 to 120 hrs (days 4 and 5)...above 200 mb outlook begins breaking up the upper vorticity into one vortex over Bahamas/W Caribbean...and another vortex to the NE...with a col of more favorable upper winds in between (E of Bahamas). This col would roughly be where I expect Florence's remnant low by 5-days...but I think it too early to speculate on re-generation of the remnant low this far out.

My remnant low track forecast in Figure 2 is rather far south of the NHC's...as I take a strict approach using the above 5-day surface outlook to predict her steering. This strict approach assumes the remnant will be too shallow to experience any gravitation toward the mid-oceanic upper vorticity shown in the 200 mb 5-day outlook. From 48 to 120 hrs...the surface outlook shows a surface ridge weakness propagating from Great Lakes to the eastern US and west edge of Atlantic...associated with what is now a second impulse of paragraph P1 frontal system currently over W Canada. I finally bend the track of remnant low more northward by 120 hours in response to this weakness...placing it E of the Bahamas by that time.

Figure 2: Tropical Storm Florence forecast

P1...Frontal system and upper trough from W Canada has crossed the central US and central Canada in last 24 hours. Vigorous frontal surface cyclone supported by E divergence of upper trough has moved from Manitoba to S Hudson Bay while deepening from 999 to 996 mb in last 24 hrs. Upper convergence on back side of upper trough supports central US 1025 mb surface ridge. Low-level warm air advection ahead supports W Atlantic upper ridge...while SW US upper ridge persists behind this upper trough. Another shortwave upper trough and frontal cyclone over W Canada will soon dive SE and re-enforce this upper trough.

P2...Longwave upper trough regime continues across Atlantic high seas. W Atlantic and SW US upper ridges used to flank this system...but now have been moved to paragraph P1. Deep-layered low and upper trough continue to gradually push into the British Isles and western Europe (upper-right corner of above atmo birdseye chart). Frontal depression (now 1000 mb) and upper trough S of Greenland persists. Upper convergence behind this upper trough supports a surface ridge over E Newfoundland at 1024 mb. According to above 5-day 200 mb (upper-level) outlook...upper trough S of Greenland should merge with mid-ocean upper vorticity (paragraph P3) and British Isles/west Europe upper trough thru next 96 hours...the merger's upper divergence supporting SE-diving surface cyclone over and north of Azores as suggested by flow in upper-right corner of above surface 5-day outlook. I expect this SE-diving surface cyclone to be associated with aformentioned 1000 mb cyclone currently south of Greenland.

P3...Mid-ocean upper vortex persists. It continues to have a westward upper trough extension thru waters south of Bermuda....the extension locked between upper anticyclonic outflow of Ernesto to the south and W Atlantic upper ridge (paragraphs P1 and P2) to the north.

P4...Cut-off upper vorticity over west tip of Cuba has retrograded west into Gulf of Mexico. According to above 200-mb (upper-level) outlook...this upper vorticity will continue to retrograde west into Texas/Mexico by 72 hours. Considering this same outlook thru 72 hrs shows upper trough in paragraph P1 diving south and eroding SW US upper ridge (paragraphs P1 and P2)...it is not the SW US upper ridge that would drive this upper vortex into Texas/Mexico...but the upper anticyclone of Ernesto. With that said...if Ernesto turns out to be weaker than expected...then upper vortex may not retrograde as fast as shown in outlook. By 96 and 120 hours out...the outlook hints at SW US upper ridge re-flaring behind paragraph P1 upper trough...so the upper vortex retrogression by that time would be assisted if Ernesto's upper anticyclone fails to do the job.

P5...Atlantic surface ridge of 1023 to 1027 mb centers is supported by a few upper convergent sources while stretching from SE US to NW Africa....including convergence SE of the of the SW US and west Atlantic upper ridges (paragraphs P1 and P2)...and convergence behind British Isles/W Europe upper trough (paragraph P2) that has strengthened this ridge. South side of this surface ridge is helping to waft Africa desert dry air westward across the Atlantic tropics. However...this dry air is reduced due to moisture from Tropical Storm Ernesto...Tropical Storm Florence...and tropical wave emerging from Africa (paragraph P9). Within this surface ridge...mid-ocean surface trough is still supported by upper divergence between mid-ocean upper vortex in paragraph P3 and W Atlantic upper ridge (paragraphs P1 and P2). This mid-ocean surface trough is further SW and located now just SE of Bermuda while moving with this upper divergence maximum. Decaying front extending from British Isles/W Europe cyclone (paragraph P2) also cuts into this surface ridge.

P6...Upper ridge continues to cover all of the Atlantic tropics. T-storm latent heat release from Tropical Storm Ernesto...Tropical Storm Florence...and tropical wave emerging from Africa (paragraph P9)...continue to locally inflated this upper ridge into cells of upper anticyclonic outflow...with upper troughs forming between these cells. Inverted upper trough over E Caribbean has been pushed into central Caribbean by Ernesto's upper anticyclone...but this inverted upper trough did disrupt Ernesto (see Ernesto special feature section above for details). The upper trough WSW of the Cape Verde Islands is now E of the Lesser Antilles...and should merge with the mid-ocean upper vorticity mentioned in paragraph P3 when observing the above 200 mb 5-day outlook. Another inverted upper trough has formed SW of the Cape Verde Islands in between Florence's anticyclone and paragraph P9 tropical wave anticyclone...but after 24 hrs I dissipate this inverted upper trough in the above 5-day 200 mb outlook with Florence expected to diminish.

P7...Tropical wave fragment was over north Florida earlier at 1200Z...formerly tropical disturbance Invest 91-L. It was producing intermittent t-storms supported by split flow upper divergence between upper vorticity in paragraph P3 and upper vortex in paragraph P4.

P8...Tropical WSW of Cape Verde Islands in previous discussion is approaching Lesser Antilles. It continues struggling while ingesting dry air described in paragraph P5. Upper trough E of the Lesser Antilles (paragraph P6) is also suppressing this tropical wave.

P9...Based on satellite imagery...next tropical wave (with currently disorganized t-storms) is emerging from Africa into the eastern tropical Atlantic.

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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1. GeorgiaStormz
10:10 PM GMT on August 06, 2012
thx nch09
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