MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #34
*******Note that forecasts and outlooks in this post are NOT the official forecast from the National Hurricane Center (NHC). They are my own detailed views on the Atlantic tropics based on current observations and latest computer model runs. As such do not make decisions based on my posts...consult news media...watches and warnings from your local weather office...and any evacuation orders issued by local governments to make the most informed and best decisions. Visit the NHC website hurricanes.gov (hurricanes dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********
...MONDAY JUNE 28 2021 9:34 PM EDT...
See Tropical Storm Danny section below for an update on the tropical storm which quickly formed from the surface trough of low pressure that was moving across the west Atlantic. See area of interest #1 section below for an update on the vigorous tropical wave of low pressure now traversing the central tropical Atlantic.
Elsewhere...a tropical disturbance has not been able to form over the Gulf of Mexico as the upper vortex that has been moving westward in tandem with Danny is already in the northeastern Gulf…and unlike what was previously forecasted this vortex will tend to suppress upper outflow potential over the Gulf.
And finally…another tropical wave of low pressure is in the eastern tropical Atlantic to the east of area of interest #1…with a large but disorganized field of showers and thunderstorms. Various models runs over the last couple of days suggest some development of this wave…for example refer to today’s 1800Z GFS and 1200Z NAVGEM which developed this wave instead of area of interest #1. Therefore this tropical wave may also emerge as an area of interest for tropical development in the coming days.
TROPICAL STORM DANNY…Satellite image of Danny at landfall…image taken at 2316Z:
The surface trough of low pressure in the western Atlantic that was offshore of Georgia and South Carolina has accomplished a lot over the last 24 hours…becoming a cloud swirl surface low in the early morning hours…tropical depression four by 11 AM EDT…then Tropical Storm Danny by 3:05 PM EDT. Danny has formed over warm Gulf Stream waters and underneath supportive divergent easterly flow between a northeast US upper ridge cell and a small upper vortex that has moved into northeast Florida. Because the upper vortex is nearby and the upper flow is linear over Danny…the strongest thunderstorms have been pushed to the west of the center of rotation. At 5 PM EDT and just before landfall…Danny squeezed out 45 mph maximum sustained winds while its central pressure dropped further to 1009 mb. As of 8 PM EDT…Danny’s center made landfall near Beaufort South Carolina while falling to 40 mph maximum sustained winds…and is expected to continue west-northwest across Georgia while steered by the southwest side of the Atlantic surface ridge. The upper atmospheric setup along the inland forecast track is tropical in that the only method to sustain a surface low pressure is for thunderstorms to release enough latent heat of condensation to strengthen the upper outflow of the southwest quadrant of the northeast US warm upper ridge cell…and like with most landfalling tropical cyclones there is not enough moisture over land to do this. Therefore Danny is expected to weaken to a dissipating surface trough of low pressure in the next 24 hours. In other words with a lack of an upper divergence zone from a cold core upper trough or upper vortex…we are not expecting a repeat of Claudette from last week where a tropical system is maintained over land through non-tropical means (note the upper vortex over northeast Florida is too far south of Danny’s forecast track to aid Danny over land).
Based on the latest Doppler radar…the heaviest rain remains west of the center due to the upper winds blowing over the storm as discussed in the previous paragraph. This area of heavy rainfall may produce isolated flash flooding tonight over eastern Georgia…at a location west of Savannah and along the US Highway 1 corridor as far west as Macon…and as far north as the South Carolina/Georgia border. Use caution when driving in heavy rain…do not drive your car on a water-covered roadway to avoid getting your vehicle stuck which could cause you to drown. Gusty winds on the South Carolina coast south of Edisto Beach should fade soon as Danny weakens tonight…only very isolated damage is possible.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Jun 28)...45 mph maximum sustained wind tropical storm centered just offshore of southern South Carolina near 32.3N-80.1W
IOH 24 Hr Forecast (1800Z Jun 29)...Dissipating surface trough located over northwest Georgia near 34N-84.5W
AREA OF INTEREST #1…The vigorous tropical wave of low pressure that was in the eastern tropical Atlantic is now accelerating westward across the central tropical Atlantic. The thunderstorm core and spin at the north apex of the wave appears to have been negatively affected by the dry Saharan air in the region while largely dissipating (albeit some small clumps of thunderstorms have developed as of 0000Z on infrared satellite)…with the spin also becoming less defined. As a result I have dropped my peak 5-day odds of development to 25%. Other than dry air…conditions for development are otherwise favorable due to warm waters along the forecast track (26+ deg C)…coupled with low shear and upper outflow beneath the tropical upper ridge in the region. The cold core upper vorticity in the western Atlantic will continuously shrink while cut-off from high-latitude cold air…therefore wind shear will remain low even as the tropical wave crosses the eastern and central Caribbean by days 3 to 5.
The forecast track is due west within the deep-layered easterly flow on the south sides of the Atlantic surface ridge and tropical upper ridge…followed by an increasing north angle in the track by days 3 to 5 as the tropical wave rounds the southwest side of the Atlantic surface ridge. Due to the current position of the tropical wave…I have once adjusted my forecast track points westward. The updated forecast track is also faster than the previous as the wave has remained weak and less amplified…and lower amplitude waves in the atmosphere tend to move faster. The forecast track now brings this system to the Lesser Antilles by Wednesday afternoon and evening…and then toward Haiti and Jamaica by day 5. The chances for the tropical wave to cross the Lesser Antilles as a tropical cyclone have dropped due to the recent disorganization of the wave…however it is still possible for the wave to take advantage of the favorable factors mentioned in the previous paragraph and deliver heavy rains and gusty winds. How the wave affects Haiti and Jamaica is unclear at this time…after all it is also possible the wave simply weakens too much from the dry Saharan air in the short-term and is unable to respond to the forecast favorable conditions in the Caribbean.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 29)...0% chance of tropical cyclone formation (central tropical Atlantic near 13N-51W)
IOH 48 Hr Outlook (1800Z Jun 30)…5% chance of tropical cyclone formation (just northeast of Barbados near 13.5N-58W)
IOH 72 Hr Outlook (1800Z Jul 1)…15% chance of tropical cyclone formation (eastern Caribbean Sea near 14.5N-64.5W)
IOH 96 Hr Outlook (1800Z Jul 2)…25% chance of tropical cyclone formation (just south of the southwestern Dominican Republic near 17N-71.2W)
IOH 120 Hr Outlook (1800Z Jul 3)…25% chance of tropical cyclone formation (between Jamaica and eastern Cuba near 18.8N-77.5W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...
**For Tropical Storm Danny...weakens quickly to a remnant low which dissipates over central Georgia at 30 hours.
**For area of interest #1…crosses the Lesser Antilles as an amplified tropical wave at 54 hours…compact tropical cyclone formation shown at 15.5N-66W at 72 hours…weakens to a remnant low just south of Jamaica at 102 hours…remnants located just south of the Cayman Islands at 120 hours.
1200Z ECMWF Model Run...
**For Tropical Storm Danny...weakens quickly to a remnant low which dissipates over central Georgia by 24 hours.
**For area of interest #1…crosses the Lesser Antilles as an amplified tropical wave just after 48 hours…no tropical cyclone formation shown
1800Z GFS Model Run...
**For Tropical Storm Danny...weakens quickly to a remnant low which dissipates over northwest Georgia at 21 hours.
**For area of interest #1…crosses the Lesser Antilles as a weak tropical wave at 48 hours…no tropical cyclone formation shown
**For the tropical wave currently at 24W longitude…a tropical low pressure spin is shown to develop at 45 hours near 8N-40.5W…tropical cyclone formation shown east of Trinidad near 11N-59W at 96 hours…crosses the southern Lesser Antilles at 102 hours…intensifies into a compact hurricane while passing just north of Aruba…Bonaire…and Curacao through 120 hours.
1200Z NAVGEM Model Run...
**For Tropical Storm Danny...weakens quickly to a surface trough while making landfall on coastal Georgia at 12 hours
**For area of interest #1…crosses the southern Lesser Antilles as a tropical low pressure spin at 60 hours…weakens back to a tropical wave while passing just south of Haiti at 96 hours…no tropical cyclone formation shown
**For the tropical wave currently at 24W longitude…develops into a tropical low pressure near 8.5N-35W at 42 hours…tropical cyclone formation shown near 12N-54W at 114 hours…tropical cyclone located east of Barbados at 120 hours.