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By: NCHurricane2009 , 5:24 AM GMT on August 26, 2012
...AUGUST 26 2012...1:30 AM EDT...
Tropical Storm Isaac en route to south Florida and the Florida Keys at this hour. Later this week....in addition to wind damage...Isaac could cause a significant rainfall and flood event in NW Florida...W Georgia...and Alabama if it meanders as forecast. There are also computer models that suggest an intense hurricane...and also a more westerly track threatening Mississippi and SE Louisiana. Although I do not agree with an intense hurricane scenario...nor the Mississippi/Lousiana scenario...residents in these areas should also keep abreast of Isaac. See Isaac special feature section for details.
Interests in Bermuda should continue monitoring the remnants of Joyce as the upper winds have become a bit more favorable for her to regenerate. Due to her potential for regeneration...I am keeping the remnants as a special feature on this blog. See Joyce special feature section for details.
Tropical wave currently in the eastern tropical Atlantic...Invest 97-L... continues to become better organized. See third 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 1925Z-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...initially he followed my more northward track shown in discussion #87...then he veered westward and hugged the north coast of Cuba such that the NHC track was the more accurate. I attribute this to attraction towards 1004 mb Cayman Islands low mentioned in paragraph P6. I may have also overplayed the interation with Carolinas vorticity and 1017 mb low mentioned in paragraphs P2 and P6. Because he has hugged the north coast of Cuba...my forecast track in Figure 1 is an overall leftward shift of my previous...but in the very short term is still a hair to the right of the NHC's due to the way his track is peeling off the north coast of Cuba.
After 5 PM Sunday...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 bend my track more leftwards between 5 PM Sun and 5 AM Mon. Immediately after 5 AM Monday is when the low-level ridge weakness from the paragraph P1 upper trough is to the NW of Isaac...so I bend my track back NW after that time. The more northward bend on Tuesday is due to the west lobe of paragraph P4 surface ridge influencing the steering.
In the longer term...the paragraph P1 upper trough is expected to pass north of Isaac...whose western upper convergence creates a blocking low-level ridge that slows him quiet a bit as he approaches the east US Gulf coast. I believe this blocking low-level ridge to the north...paragraph P4 low-level ridge to the east...and east-west frontal surface trough separating the two ridges...are all going to create conflicting steering for a very slow Isaac track by day 4. I lean the track rightwards on day 4 thinking the frontal surface trough might try to drag Isaac thru the narrow gap between the low-level ridges. By day 5...GFS shows the blocking ridge shifting eastward ahead of the next frontal system...and hence stacking with the paragraph P4 surface ridge. The combined ridge makes a southeasterly flow that I think would push Isaac NW by day 5.
Warnings remain spread across across much of Cuba...the Bahamas...the Florida Keys...and Florida. Latest watches/warnings can be found on www.nhc.noaa.gov
Figure 1: My forecast for Tropical Storm Isaac this morning
Thru the forecast period...Isaac should maintain his favorable upper anticyclonic outflow...so its a matter of how much exposure to water and the condition of his core in determining his future intensity. Due to the earlier land interactions with Haiti and Cuba..his core initially looked poor on infrared satellite...but is now improving with a recent t-storm burst over and NE of the center.
I show Isaac strengthening into a modest category 1 hurricane before reaching the Florida Keys tomorrow afternoon...at a similar rate shown in my previous forecast since that forecast has done quiet well in the last 24 hours (and given that his core is improving as noted above). My Gulf of Mexico intensity forecast is more bullish than previous to account for less land interaction with south Florida than I previously thought...but matches the NHC's peak of 105 mph max winds. I begin weakening him on Tuesday as I think his slower track will begin upwelling cooler waters in the NE corner of the Gulf. By 5 days...he should be inland...so I made a guess of a generic weakening rate for days 4 and 5.
Impact swath in Figure 1 begins with the shape/size of the tropical storm wind radius shown at 11 PM EDT NHC advisory. Towards the end of 5 days...I make my impact swath more representative of what I think the "heavy rain radius" will be (the wind radius by that time should be much smaller and less important due to Isaac's weakening from landfall). I did this to emphasize the potential of a flood problem looming for north Florida...east Alabama...and Georgia if this storm meanders as shown.
I would like to thank fellow Wunderground bloggers for posting several computer model runs and other Isaac info throughout the day on Dr. Jeff Master's Blog. I have added a tornado statement to my graphic in Figure 1 due to bloggers referring to tornado forecasts issued from The Weather Channel's Dr. Greg Forbes. There were also a few computer models posted that suggested a much stronger hurricane in addition to more westerly tracks than shown here that threaten Misissippi and SE Louisiana. Although I do not agree with these scenarios...residents along the central Gulf coast should also keep abreast of Isaac given the westward shift in many of the models.
...SPECIAL FEATURE...REMNANTS OF JOYCE...
This section makes reference to the previous Joyce forecast in Figure 2 of discussion #86. Click on this link to view that forecast.
Joyce remnant has weakened further into a surface trough. Remnant is beginning to escape unfavorable southerly vertical shear from east side of upper vortex. This upper vortex persists due to relatively lower pressures east of Isaac's upper outflow (see paragraph P5). As stated in the Joyce section of previous discussion #87...after 5 AM Sunday (i.e. very shortly)...I predicted she would be under more favorable upper winds beneath the north Atlantic upper ridge (paragraph P2). During 1800Z...split flow divergence between the north Atlantic upper ridge and the upper vortex supported a transient t-storm burst the north side of the remnant Joyce trough. Later on...northerly shear from Isaac's outflow...and-or a shearing upper westerly jet delivered by paragraph P1 upper trough...could limit potential for regeneration.
Her remnant remains generally on-track with what was shown in discussion #86...and therefore Bermuda should continue monitoring the remnants of this system in case she regenerates.
...SPECIAL FEATURE...EASTERN ATLANTIC TROPICAL WAVE INVEST 97-L...
Eastern Atlantic tropical wave is now west of the Cape Verde Islands. Its cloud pattern continues getting better organized about its surface low pressure spin...but still a little suggestive of easterly shear on the south side of the paragraph P5 upper ridge. However we are seeing that its t-storm latent heat release is locally inflating this upper ridge into an upper anticyclone overhead that makes the upper outflow more symmetric...which in turn is reducing the easterly vertical shear. Given the rate of organization taking place...I currently expect a 100% chance this will be a tropical cyclone (depression or storm) by sometime on Monday August 27.
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 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 Isaac and Joyce both tracking more poleward...in conjunction with paragraph P1 and P2 upper troughs/surface features shooting eastward into and across the Atlantic.
P1...The 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 diffuse 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 1016 mb center.
P2...Main portion of upper trough over the eastern US/Canada has entered the Atlantic high seas south of Greenland...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...remains a diffuse 1006 mb area. 1017 mb frontal low has moved northward along North Carolina coast and into SE Virginia in last 24 hrs...supported by divergence from the upper vorticity over the Carolinas. Aformentioned NW Atlantic shortwave's divergence supports a new 1019 mb frontal low north of Bermuda. 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 surface ridging...with multiple 1027 to 1020 mb centers across the eastern US and SE Canada.
P3...Upper trough over Atlantic high seas has exited the picture. It leaves behind a cut-off upper vortex west of the Canary Islands...and a surface frontal zone still extending into the Azores.
P4...Atlantic surface ridge with 1022 to 1023 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.
...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 now extends to SE Mexico and northern Central America...and is merging with fragment of paragraph P2 upper trough (extending across the eastern Gulf of Mexico to Carolinas). Split flow upper divergence between Isaac's upper anticyclone (paragraph P5) and this upper vorticity has resulted in the formation of a 1004 mb surface low near the Cayman Islands. It is possible that a tropical wave that had been west of Isaac (paragraph P9 disc #85...paragraph P8 disc #86) contributed to this 1004 mb low.
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