2012 Atlantic Hurricane Season Birdseye Discussion #82A (Special Update)
...AUGUST 19 2012...3:05 AM EDT...
Later this week...a major hurricane is possible for the northeastern Caribbean region from what is now tropical wave Invest 94-L.
Short-term outlook for Invest 94-L from full discussion #82 (paragraph P numbers are associated with full discussion #82):
Strong tropical wave SE of the Cape Verde Islands 48 hrs ago is now SW of the Cape Verde Islands...still showing impressive t-storm activity and now showing cyclonic turning in those clouds. Within the paragraph P5 upper ridge...t-storm latent heat release has been attempting to generate a warm core upper anticyclone aloft....which is reducing the easterly shear on the south side of that upper ridge. As a result...tropical cyclone formation is more likely...and I am predicting a 100% chance of a tropical depression or tropical storm out of this system sometime Sunday (i.e. later today).
My longer-term outlook for Invest 94-L based on 00Z GFS model run of surface and 200 mb (upper-level) dynamics (paragraph P numbers are associated with full discussion #82):
My outlook is summarized in Figure 1 below. Intensity-wise...one impediment against 94L would be the central Atlantic upper vortex (paragraph P6) thereatening to shear the system...but the GFS model has this upper vortex retrograde westward and out of the way thanks to sprawling upper anticyclone over Invest 94L (assuming 94L becomes a strong tropical cyclone) and thanks to W Atlc upper ridge forecast to grow strong in warm air advection ahead of vigorous frontal system mentioned in paragraph P1. GFS also spins up another upper vortex to the east of 94L that tries to chase 94L. I think this second upper vortex is due to relatively lower pressures east of 94L's sprawling upper anticyclone (assuming 94L grows into a strong tropical cyclone)...and perhaps due to some of the shortwave upper vorticity associated with dissipating surface cyclone SE of Newfoundland mentioned in paragraph P2 (the GFS at the surface shows some of this surface cyclone's vorticity swing southeastward to the NE of 94L...which is why I state this).
All-in-all...with the GFS showing a sprawling upper anticyclone persisting with 94L through the forecast period...I expect that both of these upper vortex centers will not shear 94L...but if anything enhance the upper outflow east and west of 94L...and therefore I see almost unlimited strengthening potential in the next 5 days. My intensity forecast in Figure 1 reflects this...bringing 94L to a category 3 115 mph max wind major hurricane by day 4 (5 AM Thu) in the NE Caribbean area...then weakening 94L to a regular hurricane as it crosses the unfavorable mountainous terrain of Hispaniola (Haiti and the Dominican Republic) by day 5 (5 AM Fri). My intensification rate in the short-term starts off a bit slower...representing the gradual development we are used to during tropical cyclone formation...and also due to some potential for 94L to ingest some of the dry air to the northwest (paragraph P4) at it initially spins up into a tropical cyclone.
Track-wise...at the surface...low-level ridging (paragraph P4) is firmly established to the north of 94L thru the forecast period...with all low-level ridge weaknesses shown to be too far north to have any effect of 94L's steering. Therefore...the only thing to pull 94L northward in the next 5 days would be some low-latitude upper vorticity...that is if 94L becomes a strong/vertically tall tropical cyclone capable of "feeling" such upper vorticity. Based on the 00Z GFS model run...thru 96 and 120 hrs...there is upper vorticity lingering over the Bahamas that retrogrades into Florida and E Gulf of Mexico associated with what is now the paragraph P6 upper vortex. The base of the paragraph P1 upper trough is shown lingering over the Carolinas and SW Atlantic by that time as well...so that is why my 96 hr and 120 hr forecast positions are bent more northward...to reflect a strong/vertically tall tropical cyclone "feeling" all this upper vorticity.
Other things to note about my track forecast in Figure 1. If for some reason 94L remains a weak/shallow feature by 96 and 120 hrs...my track forecast will have a northward bias...as the low-level Atlantic ridge (paragraph P4) will be firmly in place to instead steer it straight west thru that time. In addition...my longitudinal positions for 94L are a bit ahead of where the 00Z GFS generally places 94L...as the initial 7-hourly motion shown in Figure 1 suggests to me that GFS was carrying this system a bit too slowly to the west.
Figure 1: My best guess track and intensity forecast for tropical wave Invest 94-L this early morning. TS = Tropical Storm, H = Hurricane, MH = Major Hurricane
Return to full discussion #82 for an assessment on the rest of the Atlantic tropics.