Next Update on THURSDAY MORNING SEP 1ST - UNLESS CONDITIONS WARRANT AN EARLIER UPDATE 
TROPICAL CYCLONE ‘HERMINE’ TO MAKE LANDFALL BIG BEND – THREAT ENTIRE EAST COAST
Based on satellite imagery & ship reports, Tropical Depression #9 has almost certainly intensified into a Tropical Storm with 40Kt sustained surface winds. The quasi-stationary cyclone appears to be centered near 24˚N/88˚W with an estimated pressure near 999mb. (There has not been a RECON report in over 15 hrs – and no explanation from NHC or the Hurricane Hunters; though having 3 A/C and crews out in Hawaii has no doubt negatively impacted the ability to fly continuous RECONS in the ATL. Presumably, unless there is another unreported issue, RECONs should resume later today.)
The storm has become much better organized over the last 12 hrs with much improved anti-cyclonic outflow above the cyclone in all quadrants (though the strongest outflow remains in SE to SW sector) and wind shear has fallen to ~15Kts, a typical threshold for strengthening of a tropical cyclone.
There is now excellent agreement on the storm track for the first 48-72 hrs, with landfall likely to be between Apalachicola and Port Leon, Florida THU night. Intensity forecasts at the time of landfall range from a moderate tropical storm intensity of 55Kts to a minimal Hurricane with 70Kt winds (60Kts seem most likely) – but more importantly will be the storm surge due to the prolonged period southerly winds across a long fetch of Gulf waters along and east of the storm track. Even a borderline hurricane can create storm surges of 4 to 6 feet in the Big Bend region.
The cyclone is then expected to travel along GA to NC coastline on FRI and then continue generally northeastward. The GFS runs have led the EURO model in keeping the storm close to the coast, even though the ECMWF based track has shifted westward as well, it remains well off the coast compared to the GFS. NHC has shifted the course westward – but remains well to the right of the model consensus (keeping the storm well off the mid and north Atlantic coast). It should go without saying that interests along the east coast need to closely monitor this system.
I’ll have another update THURSDAY MORNING, SEP 1ST - UNLESS CONDITIONS WARRANT AN EARLIER UPDATE
INVEST 92L SOUTH OF CAPE VERDES HEADING WESTWARDThe strong tropical wave/Low (INVEST 92L) that came off the West African coast yesterday continues moving westward, albeit at a slower forward motion of ~12 Kts. In addition, the large circulation field has resulted in a much stronger surge dust-laden air associated with the SAL (Saharan Air Layer), and this has effectively killed off all significant convection. In addition, the next upstream Tropical Wave still over Africa is westbound at 20Kts and is ‘catching up’ with the lead system south of the Cape Verdes. The close proximity of this second wave and the strong surge of the SAL MAY be responsible for the global models suddenly ‘backing off’ from developing the system into a hurricane in a week or so. Regardless, the system still has the potential to develop into a significant storm during the next 5-10 days that could become a threat to the US IN 10-14 DAYS.
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Fig 1: Overview of Tropical Atlantic BasinHurricane GASTON is weakening and heading out to sea while very small TD#8 well east of the mid-Atlantic states also heads out sea. TD#9 (likely TS HERMINE) is becoming better organized in the GOM. Meanwhile, the strong Tropical Wave/Low that’s INVEST 92L near and south of the Cape Verdes is moving westward. It has brought a strong dust laden surge of the SAL which has knocked down all convection. But the large and well defined circulation still has a very significant chance of development during the week ahead as it heads westward.
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Fig 2: SURFACE REPORTS AND COLOR ENHANCED IR IMAGERY Ship reports TD#9 having reached tropical storm intensity – as does the development of a circular, CDO above the developing storm.
Fig 3: 85Ghz MICROWAVE IMAGE The overnight Microwave image shows the development of deep convective bands to the east and southeast of the center, with indications of banding around the LLC.
Fig 4: Water Vapor Imagery Loop The overnight water vapor imagery shows continually improving high level outflow and an overall cloud signature of a tropical storm strength cyclone.
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Fig 5: Upper Level Winds and Shear Analysis Satellite derived wind vectors show a MUCH improved outflow pattern over the storm, with anti-cyclonic flow centering itself over the developing cyclone which will promote intensification. In addition, wind shear has dropped off to near 15Kts, but is NOT expected to drop much lower – and this implies a slow intensification rate.
Fig 6: SPECIALIZED EUMETSAT IMAGERY OF INVEST 92L The above specialized imagery highlights the dust laden air (Pinkish shading) associated with a stronger than anticipated surge of the SAL ahead of INVEST 92L which is westbound at 12Kts – along with strong convection over west Africa associated with the next upstream Tropical Wave that is westbound at 20Kts.
Fig 7: EARLY MODEL RUNS The latest Track forecasts are tightly clustered and in reasonable agreement with the latest (12Z) GFS model run – and bring the storm onshore near or just east of Apalachicola, FL tomorrow night as a moderately strong Tropical Storm – with an outside chance it could reach minimal Hurricane Intensity. The storm track has again shifted further west – in good agreement with the GFS and trends.
Fig 8: OFFICIAL NAVY FORECAST FOR TD#9/HERMINE The latest available NAVY forecast chart (based on NHC input) has shifted westward – calling for the cyclone to move along the SE US coast over the weekend. However, based on the excellent continuity of the GFS, the track is likely even further west than the last NHC forecast. (The fuchsia colored arrows indicate MY own outlook for the track of this system which is based on the GFS model run. In fact, the 12Z run has the storm even closer to the NJ coast than I’ve shown.)
✭ NEXT TROPICAL WX UPDATE TOMORROW MORNING – SEP 1ST ✭
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