MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #23
Updated: Jun 19
*******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.**********
...THURSDAY JUNE 17 2021 10:22 PM EDT...
See area of interest #1 section below for Gulf of Mexico tropical development expected over the next 24 hours. See area of interest #2 section below for notes on the current area of disturbed weather offshore of the southeastern United States…which has a small chance to evolve into a subtropical cyclone between Bermuda and Atlantic Canada tomorrow evening.
AREA OF INTEREST #1 (POTENTIAL TROPICAL CYCLONE THREE)…The following is a more recent satellite image as of this writing…taken at 0211Z:
The broad tropical low pressure currently over the southern Gulf of Mexico continues to be supported by the eastern divergence zone of the eastern US upper trough that has recently amplified southward into the western Gulf of Mexico (the upper trough has been amplifying due to adjacent amplification of central US warm upper ridging…associated with the current central Canada frontal system's warm sector). Organized thunderstorm bands have continued to setup on the east side of the circulation….meanwhile on the west side activity is suppressed by the lack of divergence directly below the upper trough axis. The latent heat release of the thunderstorms is bolstering NW Caribbean warm core upper ridging…and in turn the outflow of the upper ridge is also aiding to gradually strengthen this system. The mix of support from the divergence zone of the upper trough (non-tropical mechanism) and the outflow mechanism (tropical mechanism) makes it arguable that this system is subtropical…or it could be argued there is enough warm core upper outflow that this system is fully tropical…it remains to be seen what the NHC will do as far as classification when this system is likely upgraded to a cyclone.
As of 5 PM EDT…the NHC has declared this system “potential tropical cyclone three” as it is very likely to intensify further into a cyclone with tropical characteristics…in order to officially begin issuing tropical storm warnings from coastal southeast Louisiana to coastal Mississippi & Alabama as such conditions are expected by late tomorrow and into Saturday. My updated forecast track is nudged westward due to the current central position of this system. Due to the size of the upper divergence zone on the east side of the upper trough...expecting a large/broad cyclone with a lax surface pressure gradient...and in addition to the upper trough limiting thunderstorms on the west side of the center...I only forecast a peak of 50 mph maximum sustained winds at the present time. The upper trough is forecast to soon leave behind upper vorticity around the NW US Gulf coast region…and by Saturday morning the upper trough currently over central Canada will be passing over the Great Lakes region while potentially stretching the upper vorticity into a SW to NE elongation as it tries to grab the vorticity. Due to this elongation...westerly wind shear over this system could increase just before landfall time…thus some weakening is possible as this system makes landfall.
In the longer term and after landfall….things get more complex as seen for example by the contrast between the GFS and ECMWF. The solution championed by the ECMWF is for the current northeast Pacific upper trough (offshore of Alaska and western Canada) to merge with the NW US Gulf coast vorticity into an amplified upper trough that strengthens this system into a gusty inland frontal cyclone over the southeast US. The solution championed by the GFS is for a piece of the northeast Pacific trough to quickly merge with the current central Canada upper trough…with the remainder of the northeast Pacific upper trough arriving too late to merge with the NW US Gulf vorticity and enhance this system. Given the possibility shown in the ECMWF and other models…on the home page bulletins of this site I have added statements for the potential for gusty winds over the east-central US by early next week. Given latest GFS solution of the NW US Gulf coast vorticity possibly not merging with the NE Pacific trough…that could leave the vorticity to weaken and lose amplitude and thus move faster to the east…thus my forecast track in future updates may need to be adjusted eastward in the long term should the GFS scenario indeed play out (the remnant low will be coupled to/supported by the divergence zone of the upper vorticity in the GFS scenario).
Now is the time to prepare for coastal storm surge and gusty winds with some damage potential if you are in the tropical storm warning zone along coastal southeast Louisiana…coastal Alabama…and coastal Mississippi. Preparations should be completed by tomorrow afternoon as conditions are forecast to already deteriorate by tomorrow night given the current large size of this system. Due to this size…this system could also bring coastal sea swells to the Florida panhandle coast. Heavy rainfall with flash flooding potential is possible at and beyond the tropical storm warning zone and across the southeast and east-central US thru early next week…see the home page bulletins of this site for more details.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Jun 17)...30 mph maximum sustained broad tropical low centered over the Gulf of Mexico at 22.9N-92.4W
IOH 24 Hr Forecast (1800Z Jun 18)...50 mph maximum sustained wind tropical storm centered over the Gulf of Mexico at 27.5N-92W
IOH 48 Hr Forecast (1800Z Jun 19)...30 mph maximum sustained wind tropical depression centered over the Mississippi/Louisiana border at 31N-91.5W
IOH 72 Hr Forecast (1800Z Jun 20)...Remnant frontal low centered over northwestern Alabama at 34.5N-87.5W
AREA OF INTEREST #2…Linear thunderstorm bands in the western Atlantic are associated with a surface cold front supported by the divergence zone of the amplified upper trough over the eastern US. In response to amplifying upper ridging over the central US (associated with the current central frontal system's warm sector)...the upper trough over the eastern US is forecast to amplify further into two possible vortices...one over the open western Atlantic to the north of Bermuda...and another over the northwestern US Gulf coast region to be entangled with area of interest #1. The increasing divergence zone of the amplifying upper vorticity that enters and crosses the western Atlantic may cause the activity currently in the western Atlantic to evolve into a surface low pressure that shifts east to the waters north of Bermuda...and moreover the surface low could briefly acquire tropical characteristics late tomorrow despite moving east past the warm Gulf stream and into slightly cooler 24 deg C waters. However I have low odds of subtropical cyclone development…at 10%…as the temperatures of the upper vorticity may not be quiet cold enough for the development of instability and thunderstorms (the 200 mb height of the vorticity is forecast to be 1209 dekameters for tomorrow night...would like to see 1200 dekameters or less for more confidence in subtropical or tropical development). However what may offset the slightly cooler water and slightly too warm upper vorticity is the amount of upper divergence forecast to occur on the east side of the upper vorticity which could allow for thunderstorm generation anyway. I also have low odds of development as models are still mixed on the exact evolution of the upper vorticity…which affects the size and location of the surface low.
I drop odds of development back down to 0% by Saturday (48 hours) as the upper vorticity warms while remaining cut-off from high latitude cold air...which will make thermodynamic conditions less favorable for thunderstorms.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 18)…10% chance of subtropical cyclone formation (northeast of Bermuda near 36N-62.5W)
IOH 48 Hr Outlook (1800Z Jun 19)…0% chance of subtropical cyclone formation (northwestern Atlantic near 38N-58W)
...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 area of interest #1...weak and broad surface low makes landfall over south-central Louisiana at 54 hours without forming into a tropical cyclone….remnant low center arrives to the coast at the NC/VA border at 96 hours….re-strengthens as a non-tropical frontal cyclone offshore of Nova Scotia by 120 hours.
**For area of interest #2…surface low becomes defined near 40.5N-56W at 48 hours...weakens to a surface trough offshore of SE Newfoundland at 72 hours.
0000Z ECMWF Model Run…
**For area of interest #1...tropical cyclone formation shown by 48 hours offshore of SW Louisiana near 27.5N-93W…after landfall maintains strength while transitioning to a non-tropical frontal cyclone…frontal cyclone center arrives to the coast at the NC/VA border at 120 hours.
**For area of interest #2…weak and broad surface low becomes defined near 37.5N-61.5W at 48 hours…weakens to a surface trough offshore of Atlantic Canada between 72 and 96 hours
1200Z GFS Model Run...
**For area of interest #1...surface low becomes stretched SW-NE without a defined center by 15 hours…NE lobe forms a tight center that becomes a tropical cyclone by 24 hours just south of SE Louisiana…makes landfall over SE Louisiana at 42 hours…remnant low dissipates over the Georgia/Tennessee border at 78 hours.
**For area of interest #2….weak and broad surface low becomes defined NE of Bermuda near 34.5N-62.5W at 33 hours…weakens to a surface trough south of Newfoundland at 69 hours.
1200Z NAVGEM Model Run...
**For area of interest #1...tropical cyclone formation shown near 26.5N-94.5W at 36 hours….shifts NNE and makes landfall over south-central Louisiana at 54 hours…remnants gradually re-intensify across the southeast US as a frontal cyclone…with the cyclone center arriving to the Maryland/Delaware border at 114 hours.
**For area of interest #2…an elongated surface low becomes defined near 37.5N-58W at 42 hours…weakens to a surface trough offshore of SE Newfoundland at 60 hours.