2012 Atlantic Hurricane Season Birdseye Discussion #71A (Special Update)
...AUGUST 4 2012...7:00 PM EDT...
This special update serves to address recent developments in Tropical Storm Ernesto...newly-developed Tropical Storm Florence...and tropical disturbance Invest 91-L moving from the Bahamas into Florida. Ernesto and Florence each have special feature sections in full discussion #71...while 91-L was discussed in paragraph P7 of discussion #71.
Concerning Tropical Storm Ernesto...he weakened from 60 to 50 mph shortly after discussion #71 was released...which baffled the National Hurricane Center (and me as well) considering his impressive upper outflow structure and favorable conditions. He has re-strengthened to 60 mph winds this afternoon...but the short-lived weakening has made Ernesto behind my intensity forecast in discussion #71. Given the favorable outlook on Ernesto in discussion #71...I expect him to catch up...so I will not change my intensity forecast...which currently passes his center south of Jamaica as a high-end category 1 hurricane and has him make landfall as a category 3 across the Yucatan. Trackwise...Ernesto is a bit behind schedule...but more importantly for Jamaica a bit more north. This means my discusison #71 impact swath (based on current tropical storm wind radius from NHC which I dilate in time to represent a strengthening storm) is more likely to cover all of Jamaica instead of the island's south half. Ernesto being slightly behind schedule means Jamaica impacts arriving tomorow evening instead of tomorrow late afternoon. All of Jamaica is now under a tropical storm warning...so all interests in Jamaica should be preparing.
Concerning Tropical Storm Florence...she has strengthened more than forecast in discussion #71. The small storm canopy once biased to the SW side of center has become very symmetric over the center. Like any tropical cyclone south of a tropical upper ridge axis...Florence has been trying to reduce easterly vertical shear and increase symmetry in its upper anticyclonic outflow by trying to locally pump up the upper ridge with t-storm latent heat release...and she appears to be successful as evidenced by the new symmetry in the storm canopy. It looks as though her low-level circulation has shrunken while tightening with this small storm canopy...which maybe why her north side has avoided adverse effects of dry air ingestion and water temps at/below 26 deg C. My updated thinking on track is now straight WNW as she has been...because she will now be strong/deep-layered enough to always feel the upper-level ridge weakness (caused by mid-ocean upper vortex) given that she does not weaken from dry air ingestion and/or cooler waters to the north (any significant weakening means that she will turn west while losing "feel" of upper-level ridge weakness). My updated intensity forecast is listed below...showing strengthening...then flattening due to possible dry air effects...then some more strengthening as I am still gambling on warmer waters/symmetric upper outflow allowing Florence to win against dry air.
5 PM Sat Aug 04 2012...50 mph...initial
5 AM Sun Aug 05 2012...60 mph...12 hr
5 PM Sun Aug 05 2012...65 mph...24 hr
5 AM Mon Aug 05 2012...65 mph...36 hr
5 AM Tue Aug 06 2012...65 mph...60 hr
5 AM Wed Aug 07 2012...75 mph...84 hr
5 AM Thu Aug 08 2012...75 mph...108 hr
Some final notes about Florence. Because I now think she will continue straight WNW (instead of stair-stepping W due to weakening shown in previous forecast)...this may place her closer to mid-ocean upper vortex...in which case she could get sheared by it later on. Will re-assess computer model upper wind forecasts by my next full discussion. Also...impact swath shown in discussion #71 Figure 2 needs to be more symmetric about storm track in short-term to account for increased symmetry in storm canopy...but can be left at same initial size based on 5 PM EDT NHC tropical storm wind radius and radius of storm canopy on satellite.
Concerning tropical wave fragment Invest 91-L...it has developed a surface 1016 mb low pressure spin and organizing t-storms in its east half...but is about to make landfall in Florida. Given high surface pressures from Atlantic surface ridge (paragraph P5...discussion #71) and land interaction...I still am not expecting tropical cyclone formation. Westward retrograding of upper vortex (paragraph P4...discusison #71) into Gulf of Mexico is still on pace with GFS model's forecast from this morning...which should mean this disturbance gets suppressed by the upper vortex as it continues WNW into the NE Gulf of Mexico...leaving behind the favorable outflow of upper anticyclonic cell to its east that had allowed its east half to organize. Because of surface ridge expected to build behind next upper trough (paragraph P1...discussion #71)...this disturbance should then continue WNW into Louisiana...and from there it would act to enhance shower and t-storm activity along tail end of upper trough's associated surface cold front.
Some final notes on Invest 91-L. If the upper vortex retrogrades west faster than expected for any reason...then 91-L would stay in touch with favorable upper anticyclonic cell to its east and tropical development would need to be re-assessed.