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By: NCHurricane2009 , 11:41 PM GMT on August 26, 2012
...AUGUST 26 2012...7:40 PM EDT...
This is the most important birdseye discussion I have released all season due to the ominous potential that Isaac brings to the US Gulf shores later this week. Tropical Storm Isaac has been lashing south Florida and the Florida Keys...and weather in these areas will begin improving on Monday. In the next couple of days....I believe Isaac has a high chance of becoming a major hurricane...perhaps one of super intensity. The forecast models and NHC track have shifted westward into the Mississippi-Louisana area...and I agree with an even more westerly solution headed specifically for Louisiana. For all US Gulf coast residents...please obey mandatory evacuation orders in all storm surge prone areas that may be issued in the coming hours or days. In storm surge prone New Orleans...please be mindful that you may need to evacuate in the next day or day-and-half...so begin preparations for evacuation in the event the worse case scenario starts to materialize. Latest watches/warnings can be found on www.nhc.noaa.gov Please pay attention to local and national media for any mandatory evacuation orders.
Remnants of Joyce dissipate at the surface levels...and therefore is no longer expected to regenerate into a tropical cyclone. Info on remnant of Joyce in paragraph P7.
Tropical wave currently in the eastern tropical Atlantic...Invest 97-L... has slowed its organization...but still expected to become the next Atlantic tropical cyclone. See second special feature section below for details.
Yet another tropical wave is emerging from western Africa...and is already upgraded to 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 1200Z, and the 1327Z-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...computer models are split in two camps. Models like the Euro (Figure 3) suggest that the surface ridge that builds behind the paragraph P1 upper trough will be too weak to push Isaac westward toward Louisiana...while the opposite is true of the GFS (Figure 2). For instance compare the expanse of the sea-level pressure orange shading across the central US in the 00Z Aug 28 points of both models.
Unfortunately due to objective evidence I see...I am going to lean toward the GFS solution...which is more threatening to Louisiana...including storm-surge-prone New Orleans. This means quiet a westward shift from my previous forecast (the NHC has also been shifting their track westward thru the last 24 hrs). Evidence for the longer-term westerly solution is that Isaac has been tracking more south and west compared to previous short-term track solutions by both the NHC and mine. The more southerly position means that Isaac is going to get pulled less-poleward by the surface ridge weakness ahead of paragraph P1 upper trough. The more westerly position means he is less likely to get trapped in the arpmit between the paragraph P4 ridge to the east and blocking ridge that build behind the paragraph P1 upper trough. This means he is more likely to be steered west by the blocking ridge. I also think this blocking surface ridge is going to be as strong (or stronger) than what GFS shows...especially if Isaac becomes a strong hurricane whose upper anticyclone adds resistance to the northwesterly flow behind the paragraph P1 upper trough. The added resistance would increase the upper convergence...hence strengthening the blocking surface ridge. Lastly...the GFS seems to have a good handle on the paragraph P1 upper trough timing & evolution...as what it forecasts for 00Z tonight (00Z Aug 27) is matching current 200 mb wind barbs...which shows fragment of the upper trough becoming cut-off near E New Mexico-W Texas...while the remainder of the fragment is moving into SW Hudson Bay.
After 5 PM Sunday (i.e. anytime now)...the upsteam shortwave upper ridge (between the paragraph P1 and P2 upper troughs) is still shown by GFS passing to the north of Isaac...and with the paragraph P2 surface ridging firmly in place to the north...I lean my track more leftwards (relative to NHC) between 5 PM Sun and 5 AM Mon. After 5 AM Monday is when the low-level ridge weakness from the paragraph P1 upper trough begins passing to the north of Isaac (even in the case of the westward-biased GFS)...so I bend my track gradually NW after that time.
Around Tuesday is when I expect the blocking low-level ridge beneath the back side of the paragraph P1 upper trough to slow and influence the track of Isaac. I bend the track further west from the NHC by early Wednesday as I think the low-level ridge will strengthen as Isaac and its upper anticyclone reach significant intensity (see two paragraphs ago for theory on how the strong upper anticyclone does this). By late Wednesday and beyond...I expect the next frontal system will dive southeast as shown by the models...which would knock out the blocking low-level ridge to the north and coax Isaac into a more north and eventually more northeasterly track well inland.
Warnings remain spread across across the Florida Keys...parts of south Florida and the Florida panhandle. Advisories are also spread across the Missisippi...Alabama...and Lousiana coasts...including the New Orleans Louisiana area which is most vulnerable to storm surge effects of a strong hurricane. Please obey mandatory evacuation orders in all storm surge prone areas along the US Gulf coast. In storm surge prone New Orleans...please be mindful that you may need to evacuate in the next day or day-and-half...so begin preparations for evacuation in the event the worse case scenario starts to materialize. 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 afternoon
Figure 2: GFS model output this afternoon. Top four panes are sea-level pressure in mb...and bottom two panes are 300 mb (upper-level) vorticity and wind direction.
Figure 3: ECMWF (Euro) model output this afternoon. Four panes are sea-level pressure in mb.
Intensity-wise...Isaac has been sputtering a bit due to southerly shear and dry air ingestion from paragraph P6 upper vortex just to his southwest. However...I urge this maybe about to change...as Isaac has been close to developing a tight core and eye on radar images from the Florida Keys. As we approach sunset...visible satellite imagery shows an increase of t-storms over the center and ominous hot-tower bursts near the banding-type eye location. His upper anticyclonic outflow appears to be energized from the latent heat release of this t-storm activity...which is squashing out the unfavorable paragraph P6 upper vortex. Given the warm Gulf waters of 30 deg C and improving upper outflow...I expect a high chance of a rapid intensification episode...so my intensity forecast in Figure 1 for Monday (which makes him a high-end category 1 hurricane at 5 PM EDT) maybe too conservative. By 5 AM Tuesday...I have him a major hurricane (115+ mph max winds). The reason I show an incredible intensification to 145 mph max winds (category 4) by 11 AM Tuesday is because I believe Isaac's northwesterly track would have distanced himself from the unfavorable paragraph P6 upper vortex...making conditions even more favorable for Isaac. As he enters less oceanic heat content (below 30 deg C) in the northern Gulf...and as he receives more land interaction...I expect Isaac to begin weakening for a category 3 landfall...which is what I show in Figure 1. After that...I show a generic weakening trend as he tracks further inland Thursday and Friday.
Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 11 PM EDT NHC advisory....growth during the forecast rapid intensification...then declination in size to represent landfall and weakening.
...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 97-L...
Eastern Atlantic tropical wave is now west of the Cape Verde Islands. In the last 24 hrs...its cloud pattern has stopped becoming better organized about its surface low pressure spin...perhaps due to southeasterly shear on the south side of the paragraph P5 upper ridge and east side of inverted upper trough to the west of the disturbance (inverted upper trough also mentioned in paragraph P5). However...I still expect a 100% chance of this becoming a tropical cyclone (depression or storm) by sometime on Monday August 27...if not Tuesday August 28.
The combination of the inverted upper trough west of the disturbance (paragraph P5) and Atlantic deep-layered ridge north of the disturbance (paragraph P4) appear to be steering this system on a general WNW heading. Model consensus on www.wunderground.com/tropical still suggests that the large scale ridge weakness to the west of this system should recurve the track northward then NE into the open seas. Large scale ridge weakness is a combination of paragraph P1 and P2 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...Next upper trough in the mid-latitude westerlies has entered the picture from the upper-left of the above charts. Upper divergence east of this upper trough continues supports a surface frontal system across the central US that curls into a vigorous cyclone over central Canada. Upper convergence behind this upper trough supports a surface ridge entering the upper-left corner of the above atmo chart with a 1020 mb center.
P2...Main portion of upper trough over the eastern US/Canada has entered the Atlantic high seas southeast of Greenland in last 48 hours...leaving behind a NW Atlantic shortwave...and leaving behind upper vorticity from the Carolinas to the eastern Gulf that is merged with paragraph P6 upper vortex. Surface frontal cyclone supported by divergence from this upper trough...now located SE of Greenland...is intensifying on its way to Europe. 1017 mb frontal low over SE Virginia...supported by divergence from the upper vorticity over the Carolinas....has weakened to 1020 mb. Aforementioned NW Atlantic shortwave's divergence supports a frontal low north of Bermuda that weakened to 1020 mb. Warm air advection ahead of this surface frontal activity still supports north Atlantic upper ridge. Meanwhile...upper convergence on the back side of this upper troughing has been supporting a 1020 mb surface ridge across the eastern US.
P3...Cut-off upper vortex west of the Canary Islands persists.
P4...Atlantic surface ridge with 1023 to 1025 mb centers persists. The north Atlantic upper ridge (paragraph P2) has become stacked directly above....creating a deep-layered ridge axis. Deep-layered easterly flow on the south side is helping to waft Africa desert dry air westward across the Atlantic tropics. Surface ridge supported by northwesterly flow (on back side of paragraph P2 Atlc high seas upper trough) converging with westerly flow (on north side of upper ridge axis).
...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. The remainder of the upper ridge is toward west Africa. A sprawling upper vortex persists in relatively lower pressures east of Isaac's upper anticyclone. 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.
P6...Upper vortex in the western Caribbean Sea persists...and is merging with fragment of paragraph P2 upper trough (extending across the eastern Gulf of Mexico to Carolinas).
P7...Remnant surface trough of Joyce has dissipated SE of Bermuda. Split flow divergence between the north Atlantic upper ridge (paragraph P2) and adjacent upper vortex (paragraph P5) supports t-storm activity in the area.
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