MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #22
Updated: Jun 19, 2021
*******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.**********
...WEDNESDAY JUNE 16 2021 11:15 PM EDT...
Satellite image as of 0250Z. Areas of interest circled in yellow are not mentioned in the NHC tropical weather outlook. Areas circled in a green dashed line are in the NHC 5-day tropical weather outlook. Areas circled in a solid green line are in the NHC 2-day tropical weather outlook:
NHC TAFB Surface Analysis 1800Z:
GFS Model Upper-Level Winds 1800Z:
See area of interest #1 section below for Gulf of Mexico tropical development expected over the next couple of days. See area of interest #2 section below for notes on the current area of disturbed weather offshore of the southeastern United States. which could evolve into a subtropical cyclone between Bermuda and Atlantic Canada on Friday.
REMNANTS OF BILL...In the last 24 hours…Bill lost tropical characteristics while moving over cool waters offshore of Atlantic Canada and while being supported by non-tropical means by the eastern divergence zone of the incoming upper trough over the eastern US & Canada. The upper trough is also supporting a much larger frontal low to the west in which Bill’s remnant is forecast to lose its identity in…as the remnant crosses southeastern Newfoundland. This is my final statement on Bill on this blog as it is no longer a tropical cyclone.
AREA OF INTEREST #1...The broad tropical low pressure that has been parked over southeastern Mexico is in the process of regenerating northeastward into the Gulf of Mexico…while transitioning into a tropical system aided 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 western US warm upper ridging…associated with the current western North America frontal system's warm sector). A surface trough of low pressure has setup in the southern Gulf of Mexico around 91.5W with numerous thunderstorm bands on the east side of the surface trough. Meanwhile activity to the west of the surface trough is suppressed by the lack of divergence directly below the upper trough axis…typical for a tropical system enhanced by an upper trough…therefore expecting this kind of lopsided thunderstorm structure until this system makes landfall on the US Gulf coast on Saturday.
At this time...my forecast track leans east of the latest model consensus...more toward Louisiana and Mississippi...where the upper divergence maximum of the upper trough has set up. Albeit the model consensus has shifted eastward over the last day…so I have more confidence in a more east position. Perhaps the more west solutions have a fragment of the amplified upper trough cut-off into a vortex...with the vortex whirling this system more west. So I have once again nudged my updated forecast track westward as we are not that far out from this event beginning and some model runs still have a westward solution with a landfall at the TX/LA border. The 1800Z GFS has a different take on a westward solution…instead of whirling this system west toward the core of a cut-off upper vortex…it has the surface rotation elongate north-south due the size/shape of the upper trough’s divergence region…with the north end becoming a tropical cyclone swung west by the remaining south lobe of the surface circulation.
Due to the size of the upper divergence zone on the east side of the upper trough...expecting a large/broad tropical 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. By day 2…the upper trough is forecast to leave behind upper vorticity around the NW US Gulf coast region. By day 3...the upper trough currently over western 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 by day 3…thus some weakening is possible as this system makes landfall. By day 4 and beyond...this system could transition into a gusty non-tropical frontal cylcone that moves across the southeastern and east-central US...as an upper trough currently over the north Pacific ends up potentially merging with the NW US Gulf coast vorticity into an amplified upper trough that supports the surface remnants of this system. It is also possible the two upper features don't merge and the surface remnants of this system dissipates quickly after landfall. Models remain mixed between both scenarios...and given that one of the players is still currently far away in the north Pacific it is hard to know if this system will become an impactful remnant frontal cyclone over the inland eastern US at this time.
Confidence is increasing on a Saturday coastal storm surge and wind event anywhere from coastal southeast Texas to Louisiana & Mississippi. Gather storm preparation supplies early to avoid last minute crowding...espeically for those not yet vaccinated against the COVID-19 virus. Unvaccianated people who live in an area prone to coastal storm surge in this region whould also come up with a plan to relocate to a hotel or family/friend residence further inland...as a public storm shelter may not be as ideal for controlling your exposure to COVID-19. The circulation of this system is expected to be large in size...therefore this system could bring coastal sea swells to the Alabama and Florida panhandle coast as well this weekend.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Jun 16)...Broad tropical low pressure centered offshore of west coast of Yucatan peninsula at 20.5N-91.5W
IOH 24 Hr Forecast (1800Z Jun 17)...35 mph maximum sustained wind tropical depression centered over the Gulf of Mexico at 23N-91.5W
IOH 48 Hr Forecast (1800Z Jun 18)...50 mph maximum sustained wind tropical storm centered over the Gulf of Mexico at 27.5N-91.5W
IOH 72 Hr Forecast (1800Z Jun 19)...30 mph maximum sustained wind tropical depression centered over the Mississippi/Louisiana border at 31N-91.5W
IOH 96 Hr Forecast (1800Z Jun 20)...Remnant frontal low centered over northwestern Alabama at 34.5N-87.5W
AREA OF INTEREST #2…Linear thunderstorm bands offshore of the Carolinas are in association with a new surface frontal low supported by the divergence zone of the amplified upper trough over the eastern US. In response to amplifying upper ridging over the western US (associated with the current western North America 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 offshore of the Carolinas evolution into a surface low pressure that shifts east to the waters north of Bermuda...and moreover the surface low could briefly acquire tropical characteristics on Friday despite moving east past the warm Gulf stream and into slightly cooler 24 deg C waters. However I have lowered my odds of subtropical cyclone development to 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 on Friday...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 lowered the odds of development as models are mixed on the exact evolution of the upper vorticity…which affects the size and location of the surface low (for example the NAVGEM has a similar forecast track to the Outlook below…bur with a large/broad circulation whereas the 1800Z GFS had a tight/small and potentially subtropical circulation further north aided by the narrow ribbon of warm Gulf Stream waters near 38N latitude).
I drop odds of development back down to 0% by Saturday (72 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 17)…0% chance of subtropical cyclone formation (western Atlantic near 34N-70W)
IOH 48 Hr Outlook (1800Z Jun 18)…10% chance of subtropical cyclone formation (northeast of Bermuda near 36N-62.5W)
IOH 72 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 72 hours without forming into a tropical cyclone….remnants gradually re-intensify across the southeast US as a frontal cyclone…with the cyclone center arriving to the coast at the NC/VA border at 120 hours.
**For area of interest #2…weak surface low becomes defined near 39N-56W at 72 hours...weakens to a surface trough offshore of SE Newfoundland at 96 hours.
0000Z ECMWF Model Run...Not available at above-mentioned source
1800Z GFS Model Run...
**For area of interest #1...surface low becomes stretched north-south without a defined center….at 63 hours a tight center forms in the north lobe of the circulation and becomes a tropical cyclone offshore of the Texas coast between Galveston and Matagorda Bay…makes landfall on the TX/LA border at 75 hours…remnant low dissipates over the SE US at 99 hours.
**For area of interest #2…well-defined surface low forms near 38.5N-60W at 36 hours…weakens to a surface trough offshore of SE Newfoundland at 72 hours.
1200Z NAVGEM Model Run...
**For area of interest #1...tropical cyclone formation shown near 25N-94W at 48 hours....makes landfall on the TX/LA border at 78 hours…remnants gradually re-intensify across the southeast US as a frontal cyclone…with the cyclone center arriving to the NC/TN border at 126 hours.
**For area of interest #2…a large surface low becomes defined north of Bermuda at 48 hours near 35N-64W…weakens to a surface trough near 38N-60W at 84 hours.