2012 Atlantic Hurricane Season Birdseye Discussion #84A (Special Update)
...AUGUST 22 2012...3:30 AM EDT...
This special update is written to provide a more updated Isaac forecast graphic and discussion. Discussion #84 was released with a graphic that was 12 hours older than the discussion itself. Paragraph P numbers below refer to paragraph numbers in the mid-latitude and tropical belt sections of discussion #84.
This special update also includes a statement about tropical wave Invest 96-L...located to the east of Tropical Storm Isaac.
Return to full discussion #84 for an update on the rest of the Atlantic tropics.
...UPDATE ON TROPICAL STORM ISAAC...
Tropical Storm Isaac continues churning toward the Lesser Antilles...Puerto Rico...and Virgin Islands. Conditions will detereorate in this areas over the next several hours...where preparations should be underway. Preparations in the Lesser Antilles will need to near completion sooner as Isaac will strike them first later today. As of 2 AM EDT (an hour before the release time of this discussion)...Isaac was centered at 15.6N-56.4W...or 345 miles east of Guadeloupe in the Lesser Antilles...with maximum sustained winds at the center of 40 mph. My updated forecast versus the NHC's is shown in Figure 1 below.
Track-wise for Isaac...I am in a conundrum at this hour. The latest GFS model runs (as well as the NHC) have been gradually shifting a bit northward...following closer to an older forecast I did in discussion #82A (where I gave more credence to paragraph P6 upper vortex to the west steering the system as Isaac became stronger/taller). In discussion #84...I had been going with a further south solution that was earlier shown by NHC (where I gave less credence to the paragraph P6 upper vortex...because Isaac was weaker/shallower than the #82A solution showed...and after some thought I concluded anyway that a more intense Isaac would have its upper outflow push the paragraph P6 upper vortex more bullishly westward and out of the way).
I still prefer the more southward discussion #84 solution...so all of my track forecast in Figure 1 below is a copy-paste of the track forecast in discussion #84. One preference to that solution is that since its formation...Isaac's deepest t-storm clouds have taken a SW bias in satellite pictures...and we have seen that storms in their formative stages (like Isaac) like to regenerate their centers a bit closer to the deepest t-storm clouds. A second preference to that solution is that it still makes sense to me that an aggressively intensfying tropical cyclone's upper anticyclone will push the paragraph P6 upper vortex out of the way. A third preference is that Isaac's is currently weaker/shallower when compared to what discussion #82A showed by now...so I would rather underplay the paragraph P6 upper vortex steering influence (hence underplay the more north #82A solution).
Longer-term (days 3 to 5)...The Atlantic low-level ridge (paragraph P4) still has no weaknesses far south enough to influence Isaac. So logic says the NW turn shown should then be caused by an upper-level weakness that a very strong/very tall Isaac can feel. By days 3 to 5...models show an upper-level ridge weakness caused by the large-scale paragraph P1 upper trough moving into the W Atlantic...featuring a narrow extension across the Carolinas to Gulf of Mexico. Having my track further south/further west by those days is also consistent with my thinking of an aggressively-strengthening Isaac whose upper anticyclonic outflow pushes out/de-amplifies the paragraph P1 upper trough (a less amplified upper trough means that strong/vertically tall Isaac won't feel as much poleward tug from the upper trough).
The track forecast for days 4 and 5 is threatening to Hispaniola (Haiti and the Dominican Republic)...Jamaica...and Cuba. The next watches or warnings are likely to be raised in these areas...so these areas should get ready to prepare when advisories are raised.
Beyond day 5...the track forecast is suggestive of a Florida threat...so interests in Florida should monitor the progress of Isaac carefully. Upstream of the paragraph P1 large-scale upper trough...the models show a shortwave upper ridge followed by the next major mid-latitude upper trough immediately after. If Isaac is as far NW as shown by day 5...the next mid-latitude upper trough's low-level ridge weakness is far south enough to steer even the weakest/shallowest of tropical cyclones poleward...so a poleward turn is imminent at some point regardless of Isaac's strength or vertical depth. If Isaac remains tall/vertically deep...I interpret the brief shortwave upper ridge would nudge him more west beyond day 5. Therefore...solutions after Florida range from a threat in the east Gulf of Mexico (if the shortwave upper ridge nudges the track west)...or alternatively a track that goes up the US east coast. I prefer the east Gulf solution at this time...in part due to the theory a couple of paragraphs ago that the paragraph P1 upper trough is going to be de-amplified by a strong Isaac upper anticyclonic outflow.
Figure 1: My forecast for Tropical Storm Isaac this early morning.
Intensity-wise...Isaac's ramp up keeps taking longer than I think...due to the dry air in paragraph P4. As old discussion #82A said...I believe outside of dry air...Isaac has almost unlimited strengthening potential with warm waters and a good upper atmosphere. Discussion #82A also suggested upper outflow enhacement from....
(1) Paragraph P6 upper vortex to its west.
(2) A second upper vortex that should form to its east. Indeed paragraph P5 in discussion #84 confirms this second upper vortex is forming.
Even better for Isaac...outflow enhacement to the north is going to come from the large-scale paragraph P1 upper trough as it moves into the Atlantic by days 3 to 5. That is why I still show the same bullish strengthening rate in Isaac that I showed in discussion #84...as I believe Isaac's ramp up is due anytime now (despite the persistent dry air intrusions that have been stopping this so far). Later on...I numb the intensity of Isaac...giving consideration to land interaction.
Impact swath in Figure 1 begins with the shape/size of the tropical storm wind field shown in the NHC advisory as of this writing. Then...I imagine the tropical storm wind field growing into a generically-sized major hurricane. With today's skill...it is almost impossible to forecast the true size fluctuations that occur in a tropical cyclone...so the swath size could easily change in the next 5 days. The swath location could also undergo shifts in the next days if there is a good shift in forecast track.
...UPDATE ON EASTERN ATLANTIC TROPICAL WAVE INVEST 96-L...
The National Hurricane Center at 2 AM EDT has increased the 48-hour tropical cyclone formation probability of this system to 90%...which probably means any time now they will start advisories on yet another Atlantic tropical cyclone. If this becomes a named tropical storm before 5 PM EDT today...we will beat Tropical Storm Jose of 2005 as the earliest tenth Atlantic tropical storm on record. The mention of breaking (or tying) this record was brought up during the intro section of discussion #83.
In general...I expect this to gradually recurve NW over the next days if it becomes an intensifying tropical cyclone. My reasoning for this is found in the Invest 96-L special feature section of discussion #84.