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By: NCHurricane2009 , 9:04 AM GMT on August 28, 2012
...AUGUST 28 2012...5:00 AM EDT...
Thankfully as Tropical Storm Isaac approaches the US Gulf coast...he did not rapidly intensify...nor intensify into a hurricane. Despite this...the large-sized wind field of Isaac is capable of stirring a large amount of water...and therefore the storm surge threat still exists. For all US Gulf coast residents...mandatory evacuation orders should have been obeyed in storm surge prone areas. Latest watches/warnings can be found on www.nhc.noaa.gov
Tropical wave currently in the eastern tropical Atlantic...Invest 97-L... has stopped organizing due to unfavorable upper winds. See second special feature section below for details.
Yet another tropical wave has emerged from western Africa...and continues to be a special feature on this blog. See 3rd special feature section below for details.
...ATMOSPHERIC FEATURES BIRDSEYE CHART...
This chart is generated based on surface analysis from the National Hurricane Center TAFB at 1800Z, and the 1924Z-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 ISAAC...
My latest forecast versus the NHC's is shown in Figure 1 below.
Track-wise for Isaac...my previous forecast was done at 5 PM EDT Aug 26 (full discussion #89)....in which I leaned toward the westward-biased GFS model due to some stated observations and theories...and also due to GFS model accurately forecasting the atmosphere. To make that judgement... basically I looked at how one of the GFS runs that day was predicting the surface and upper-level states of the atmosphere for 00Z that evening...and indeed it proved correct as we hit 00Z that evening. Even right now..that same GFS run from yesterday (in Figure 2 below) is still proving correct with near-perfection...for instance compare the 00Z Aug 28 frame of surface pressures with tonight's actual 00Z Aug 28 HPC surface map shown in that same figure. Meanwhile...the Euro model (which had a rightward bias in track) under-predicted the intensity of the central US blocking surface ridge currently building (see paragraph P1 for what is causing this surface ridge). To see this...you can compare the Figure 2 HPC map below (showing the actual isobar contours of that ridge) with yesterday's 00Z Aug 28 Euro output in Figure 3 of discussion #89.
As stated in special update #89A...Isaac has been tracking NW midway between my leftward biased track in discussion #89 and the NHC's rightward biased track at the time of discussion #89. The NHC has adjusted leftward to that current NW track...and I suppose it is appropriate for me to meet in the middle and join the current NHC forecast and current NW track. Therefore in the short-term...I have no difference to the NHC forecast track as seen in Figure 1 below. The current NW track is caused by surface ridge weakness of surface front mentioned in paragraph P1. On this track...we remain highly confident of a landfall over or very near Buras, Louisiana later this afternoon.
Longer-term...the NHC is last 24 hrs has adjusted their inland track more leftwards...basically placing it where I placed it in my previous track forecast (Figure 1 discussion #89). Therefore I also do not disagree with the longer-term inland track that the NHC currently shows...as seen in Figure 1 below. The details of the longer term track forecast make sense. We see a slow down later this afternoon and Wednesday consistent with the building of the central US blocking ridge shown in the accurate GFS run of Figure 2 below (cause of central US ridge in paragraph P1). Then later we see a northward then northeastward acceleration as the blocking ridge gets knocked out ahead of next frontal system diving southeast in the mid-latitudes. This next frontal system is shown by the last (lower-right) frame of the accurate GFS model output in Figure 2 below...via the blue shading in the upper-left of that frame representing the low surface pressures of that frontal system. This next frontal system is not yet in the scope of the above birdseye charts...nor mentioned in the mid-latitude discussion below.
Warnings remain spread across across US Gulf coast from Louisiana to ...parts of Florida panhandle...including all of the Alabama and Mississippi coastline in between. Mandatory evacuation orders in all storm surge prone areas along the US Gulf coast should have been obeyed at this point in time. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.
Figure 1: My forecast for Tropical Storm Isaac this morning
Figure 2: GFS model output from Aug 26 2012 afternoon. Top four panes are sea-level pressure in mb. The bottom image is an HPC map from 00Z Aug 28 2012...which proves the near perfect accuracy of the above Aug 26 2012 GFS model output.
Intensity-wise...Isaac has not strengthened that much in the last 36 hours. Part of my reasoning for the bullish intensity forecast in my previous discussion #89 (which now obviously has big error) was that I believed Isaac's current northwesterly track would have distanced himself from the unfavorable paragraph P6 upper vortex that induced southerly shear...and was a source of adjacent dry air. Indeed the southerly shear has relaxed as he distanced himself...but some of this dry air is still entraining into the circulation (see above thermo chart...where a slot of dry air starts in the southwest Gulf near the upper vortex...curling into a narrow band reaching into the north half of Isaac's core). I think this dry air band is why Isaac's satellite presentation is impressive on his south half...but not so much the case in his north half. What is more puzzling is that on several occasions...it seemed Isaac was forming a banding-type eye and tight core as if he was about to rapidly intensify...but I think this dry air allowed the core to collapse several times such that he never really re-obtained a tight core since he left Haiti and Cuba days ago. Without a tight core...the t-storm latent heat release is not centrally concentrated...but rather spread out throughout the storm. In turn...this means Isaac's upper anticyclone is inflated by latent heat release over a wide area...and when that upper anticyclone ventilates over a wide area...surface pressures fall over a wide area rather than just at the center...creating a more lax pressure gradient (and hence less wind) for the same central pressure value. This might be why despite a low central pressure of 977 mb (as of 4 AM CDT) characteristic of a hurricane...the max sustained winds remain only at 70 mph (just under hurricane force). This wide area of surface pressure falls means Isaac's wind field (although weaker than normal for that central pressure) has a large diameter. With these thoughts...I only forecast a meager intensification to 75 mph max winds (minimal hurricane) by landfall later this afternoon...then dissipation to an inland remnant low by Thursday afternoon. This is less aggressive than what the NHC shows (NHC 10 PM CDT forecast showed a peak of 90 mph max winds before landfall...and shows a peak of 80 mph max winds per hot-off-the-press 4 AM CDT forecast).
Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 10 PM CDT NHC advisory...later declining in size to represent weakening due to landfall. Some highlights to note about the impact statements of Figure 1. Impact statement (c) highlights the storm surge threat...which is going to be more significant than usual for a tropical storm/minimal hurricane....as the creation of the large tropical storm wind radius stirs up a large amount of water...so all mandatory evacuation orders along the US Gulf coast should have been obeyed despite the less-than-expected intensification of the storm. Impact statement (b) makes a point about an emerging flood risk when the storm slows and associated heavy rains persist over the same area. Impact statement (b) also emphasizes that heavy rains can be expected in squalls outside of the shown impact swath. For example...Palm Beach on the east Florida coast saw a persistent squall of heavy rain for several hours that created unexpected flooding...despite having been outside of my drawn impact swaths for the last couple of forecasts. I expect these far reaching squalls...in part due to the large size of Isaac...to be biased to the east...thanks to the 1020 mb frontal low ejecting from SE Virginia in paragraph P2...and thanks to the incoming central US frontal system in paragraph P1 progged to spread the moisture eastward as the front itself sweeps eastward. According to the latest HPC forecasts...the worst of the rains in these eastern squalls should gradually lift northward from Florida...and into Georgia and South Carolina (and maybe into southern North Carolina).
...SPECIAL FEATURE...ATLANTIC TROPICAL WAVE INVEST 97-L...
Tropical wave west of the Cape Verde Islands is now midway between the Cape Verde Islands and Lesser Antilles. In the last 24 hrs...its cloud pattern has stopped becoming better organized about its surface low pressure spin...due to southerly shear on the east side of paragraph P5 upper vortex located adjacent to this feature. However...with the paragraph P5 upper ridging surrounding the area (which would enhance the upper outflow and reduce the shear if the adjacent upper vortex dissipated)...I do not yet want to discount potential for tropical cyclone formation.
The combination of the adjacent upper vortex west of the disturbance (paragraph P5) and Atlantic ridge north of the disturbance (paragraph P4) appear to be steering this system on a general WNW heading...albeit a turn to the west is possible if it rounds the north side of the adjacent upper vortex. I still agree that a large-scale ridge weakenss from the west will recurve this system northward then northeast into the open Atlantic. With the paragraph P2 system passing by to the north...it now appears this weakness would be a combination of paragraph P1 upper troughs/surface features shooting eastward into and across the Atlantic.
...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE EMERGING FROM AFRICA...
Yet another vigorous tropical wave is emerging from Africa into the Atlantic tropics based on satellite imagery. Its cloud pattern is signatory of easterly vertical shear on the south side of paragraph P5 upper ridge. However...as seen with the recent developments of Isaac...Joyce...and Invest 97-L...t-storm latent heat release from a tropical wave can locally inflate the upper ridge into an upper anticyclonic center that enhances the outflow and reduces the shear. Given the recent string of tropical wave development and computer model support this wave has...I am already upgrading it to a special feature on this blog...on the presumption it will become yet another Atlantic tropical cyclone. Computer model runs suggest that this system will not get swept up in the ridge weakness that 97-L will...and therefore show a more westerly track with this system than 97-L.
P1...Upper trough in the mid-latitude westerlies has entered the central US and central Canada. Upper divergence east of this upper trough continues supports a surface frontal system across the central US that curls into a vigorous cyclone moving into NE Canada (marked with 988 mb center in above atmo chart). Upper convergence behind this upper trough supports a surface ridge building into the central US...with multiple centers above 1020 mb becoming marked in the area.
P2...Main portion of upper trough over the eastern US/Canada has entered the Atlantic high seas southeast of Greenland in last 72 hours...leaving behind a NW Atlantic shortwave...and leaving behind upper trough over Carolinas that is now along the NE US shore. Surface frontal cyclone supported by divergence from this upper trough...now located SE of Greenland...is heading toward Europe...with a second frontal cyclone seen on satellite trailing behind. 1020 mb frontal low over SE Virginia...supported by divergence from the NE US shore upper trough....has moved into the NW Atlantic NW of Bermuda. Aforementioned NW Atlantic shortwave has merged with parent high seas upper trough...with the associated 1020 mb surface low deepening to 1018 mb while moving into the north-central Atlantic. Warm air advection ahead of this surface frontal activity still supports north Atlantic upper ridge....but this upper ridge has merged with tropical upper ridging in paragraph P5.
P3...Cut-off upper vortex west of the Canary Islands has dissipated.
P4...Atlantic surface ridge with 1022 to 1025 mb centers persists. Easterly flow on the south side of this ridge (in conjunction with easterly flow on south side of paragraph P5 upper ridge) is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge is supported by northwesterly flow (on back side of paragraph P2 Atlc high seas upper trough) converging with westerly flow (on north side of paragraph P5 upper ridge).
...TROPICAL BELT DISCUSSION...
P5...Upper ridge across the tropical Atlantic persists. T-storm latent heat release from Isaac has caused this upper ridge to concentrate into an anticyclonic center over the storm and across the Gulf of Mexico/west Caribbean. The remainder of the upper ridge is in east half of the Atlantic. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone...currently located in the eastern Caribbean. Tropical wave Invest 97-L continues to locally inflate the upper ridge with its t-storm latent heat release...resulting in a large scale inverted upper trough west of the disturbance. In last 24 hrs...this inverted upper trough has shrunken into an upper vortex adjacent to 97-L.
P6...Upper vortex in the western Caribbean Sea has been pushed into the Bay of Campeche by the sprawling upper anticyclone over Tropical Storm Isaac.
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