MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #24
*******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.**********
...FRIDAY JUNE 18 2021 9:42 PM EDT...
See area of interest #1 section below for an update on the Gulf of Mexico vigorous tropical disturbance that will soon make landfall over the central US Gulf coastal zone region. See area of interest #2 section below for notes on the area of disturbed weather that was offshore of the southeastern United States but is now north-northeast of Bermuda.
AREA OF INTEREST #1 (POTENTIAL TROPICAL CYCLONE THREE)...The broad tropical low pressure currently over the northern Gulf of Mexico continues to be supported by the eastern divergence zone of upper vorticity left behind on the northwest US Gulf coast. The north end of the broad circulation has become considerably better organized and stronger (while now packing 45 mph maximum sustained winds) at a location just offshore of SE Louisiana and Mississippi where the eastern thunderstorm bands have become stronger and concentrated. The consolidation of this system towards the north has reduced the amount of time to landfall which will now occur tonight instead of tomorrow…therefore preparations for coastal storm surge and gusty potentially damaging winds in the tropical storm warned area (coastal SE Louisiana…coastal Mississippi…coastal Alabama) should have been completed by now. This system has not received a name or tropical storm designation by the NHC as the broad circulation has not developed a defined surface center…but instead multiple surface swirls. For example between 2200Z and 0000Z a swirl was seen ejecting southwestward away from SE Louisiana…in the opposite direction of the deep-layer southerly steering flow on the east side of the NW US Gulf coast upper vorticity that has been moving this system north. That means the swirl is probably not the main center of rotation…but is instead a secondary surface vortex orbiting the west side of the main broad surface rotation which is continuing north into SE Louisiana. My forecast below assumes this system will develop a defined consolidated center and tropical storm status around landfall time tonight…and then weaken to an inland tropical depression over southern Mississippi by 24 hours from now. But even if it does not gain the scientific definition of a tropical storm before landfall…this will NOT lessen the impacts from this system as the thunderstorm bands on the east side are already producing tropical storm force winds and heavy rainfall with flash flooding potential!
After landfall...things get more complex as seen for example by the ongoing contrast between the GFS and ECMWF. The solution championed by the ECMWF (and also by the CMC and NAVGEM as of late) is for the current upper trough making landfall on the west Canada coast to merge with another upper trough currently over NW Canada and also the NW US Gulf coast upper vorticity into an amplified upper trough that strengthens this system into a gusty inland frontal cyclone over the southeast US. The GFS shows the upper troughing from Canada being a little to slow to merge with the NW US Gulf coast upper vorticity…resulting in the cold core upper vorticity instead dissipating in the warm sector of the Canadian frontal low to be generated by the upper troughing…and because the inland remnants of this system would be supported by the divergence of the vorticity the remnants quickly dissipate in the GFS solution. However given the ongoing alternate solution in the other models…on the home page bulletins of this site I have continued statements for the potential for gusty winds over the east-central US by early next week.
Even if the GFS scenario plays out…I see potential for the remnants to hang on as a tropical low through 72 hours as the warm sector of the forecast Canadian frontal low could amplify the NW Caribbean warm upper ridge northward and over the remnants…with the outflow of the upper ridging potentially supporting the remnants in this kind of a scenario. Thus there is some potential for the remnants to acquire tropical cyclone status along the coastal Carolinas around 72 hours (Monday)…so I have extended the forecast track of this system for some time in order to prepare for probability outlooks on tropical cyclone formation after this system weakens inland…by my next full update on the Atlantic tropics. By 96 hours (Tuesday)…if the remnants do indeed re-generate as a tropical cyclone…loss of tropical characteristics would occur over cooler waters offshore of the NE US and as the cold front associated with the Canadian upper trough overspreads the area. However divergence on the east side of the upper trough would help the remnants through its eastern divergence zone even as tropical characteristics are lost. So in addition to gusty winds over parts of the east-central US…the potentially strengthening tropical or non-tropical remnants of the system could produce coastal sea swells…rip currents…and rough seas along the east-central US…northeast US…and Nova Scotia coastal region Monday and Tuesday. Gusty winds may even reach Nova Scotia by Wednesday depending on the exact track of the potentially strengthening remnant system.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Jun 18)...45 mph maximum sustained wind tropical low centered offshore of southeastern Louisiana at 27.9N-91.2W
IOH 24 Hr Forecast (1800Z Jun 19)...30 mph maximum sustained wind tropical depression centered over southern Mississippi at 31N-90W
IOH 48 Hr Forecast (1800Z Jun 20)...Remnant low centered over Georgia at 33N-84W
IOH 72 Hr Forecast (1800Z Jun 21)…Remnant low centered over Cape Fear North Carolina at 34N-78W
IOH 96 Hr Forecast (1800Z Jun 22)…Remnant non-tropical frontal low centered offshore of Massachusetts at 40.5N-69.5W
AREA OF INTEREST #2...A weak frontal low has formed along the western Atlantic cold front…supported by the divergence on the east side of the upper vorticity currently in the NW Atlantic. The frontal low is currently north-northeast of Bermuda near 36N-62.5W with some weak shower and thunderstorm activity just north of the center aided by the somewhat cold upper air temps of the upper vorticity and also by the aforementioned upper divergence. I currently assign a very low 5% odds for subtropical depression formation in the next 12 hours…before the upper vorticity warms while remaining cut-off from high latitude cold air...which will make thermodynamic conditions less favorable for thunderstorm generation as this system is over lukewarm waters below 26 deg C. If this system does not acquire subtropical status…this will be my final statement on this system on this blog.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 12 Hr Outlook (0600Z Jun 19)…5% chance of subtropical cyclone formation (northwestern Atlantic near 37N-60W)
IOH 48 Hr Outlook (1800Z Jun 19)...0% chance of subtropical cyclone formation (northwestern Atlantic near 38N-57.5W)
...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 area of interest #1...weak and broad surface low makes landfall over south-central Louisiana at 18 hours without forming into a tropical cyclone...remnant low center arrives to the North Carolina Outer Banks at 72 hours...re-strengthens as a non-tropical frontal cyclone offshore of Massachusetts by 96 hours...opens to a surface trough while making landfall in Nova Scotia at 108 hours.
**For area of interest #2...surface frontal low dissipates near 36N-62.5W in 18 hours.
1200Z ECMWF Model Run...
**For area of interest #1...tropical cyclone formation shown between 0 and 24 hours hours offshore of SE Louisiana...makes landfall over SE Louisiana by 24 hours...after landfall maintains strength while transitioning to a non-tropical frontal cyclone...frontal cyclone center arrives to the Outer Banks of North Carolina at 72 hours...opens to a surface trough in the vicinity of Nova Scotia between 96 and 120 hours.
**For area of interest #2...surface low arrives to 39.5N-57.5W at 24 hours...weakens to a surface trough offshore of SE Newfoundland at 48 hours.
1200Z GFS Model Run...
**For area of interest #1...weak and broad surface low makes landfall over south-central Louisiana at 21 hours without forming into a tropical cyclone...remnant low dissipates over the northern Alabama/Mississippi border at 48 hours
**For area of interest #2...weakens to a surface trough offshore of SE Newfoundland at 48 hours
1200Z NAVGEM Model Run...
**For area of interest #1...tropical cyclone formation shown near 27.5N-92.8W at 6 hours...makes landfall over south-central Louisiana at 24 hours...remnants gradually re-intensify across the southeast US as a frontal cyclone with the cyclone center arriving to the NC/VA border at 78 hours...frontal cyclone makes landfall over western Nova Scotia at 108 hours...frontal cyclone whirls northward across Labrador and eastern Quebec through 120 hours.
**For area of interest #2...surface frontal low dissipates near 37.5N-62.5W in 18 hours.