BIRDSEYE VIEW POSTS

Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at IOHurricanes@outlook.com 

 
 
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MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #58

*******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 JULY 23 2021 12:30 PM EDT...

See area of interest sections below for multiple areas being monitored for tropical development along a cold front currently in the western Atlantic...which include:

(1) Area of interest #1...surface low pressure area with thunderstorms that formed this morning offshore of Florida and Georgia

(2) Area of interest #2...surfacce low pressure area forecast to form north of Bermuda.


AREA OF INTEREST #1...The strong upper ridge over central Canada is in the process of cutting off a large portion of the current northeast Canada upper trough into a string of upper vorticity that will become pushed southeast into the western Atlantic. The divergence zone of the upper trough has previously generated a surface frontal low pressure that has been moving across the coastal northeast US and southeastern Canada...with this frontal low pressure driving a surface cold front into the western Atlantic. Just ahead of the tail end of this cold front...it appears a surface low pressure spin with showers and thunderstorms on its southeast side has formed this morning in the waters offshore of Georgia and northeast Florida...centered near 31N-79W. This surface low pressure and its thunderstorms are being supported by divergence on the south side of the upper trough which will soon become the upper vorticity string as discussed above.


Because the upper vorticity string is cold core in nature...it is forecast to generally weaken due to widespread warm southerly flow occurring ahead of the large frontal low pressure system currently moving across Canada. However over the next 24 hours...the tail end of the upper vorticity string is forecast to still be present and shift southeast...and because the surface low pressure area of interest offshore of Florida and Georgia is being supported by the divergence ahead of this upper vorticity string my updated outlook below shows a southeast drift of the surface low in the short-term. I have low 30% odds for tropical cyclone formation in the short-term as the southeast-shifting upper vorticity may be disruptive to the organization of this system's thunderstorms. After 24 hours...as the atmosphere continues to warm ahead of the Canadian frontal low...a large warm upper anticyclonic ridge is expected to build across much of the United States while the cold core upper vorticity in the region gradually diminishes. Models have trended more north with the track of the Canadian frontal low such that it is no longer forecast to weaken the current surface ridge over the eastern US...instead the eastern US surface ridge will persist during the forecast period under the convergence zone of the southeast side of the upper anticyclonic ridge and northwest side of the gradually weakening upper vorticity. This results in a big change to the forecast track...which now turns this system west into northeast Florida and then possibly the northeast US Gulf coast along the Florida panhandle while steered west then west-northwest around the southwest side of the surface ridge. This scenario also means the surface low pressure area will be under a potential upper convergence zone between the southeast side of the upper anticyclonic ridge and northwest side of the weakening upper vorticity to the southeast. Tropical development will still be possible however if the thunderstorm activity persists and creates enough focused latent heat release to make upper outflow that combats the upper convergence regime...but with the potential for upper convergence my peak odds of development are not above 50% at this time. I slightly lower the odds of development to 40% by 72 hours due to land interaction...as this system tracks over the northern Florida peninsula and toward the Florida panhandle coast...followed by a drop to 0% odds of development by 96 hours once the forecast track bends north into the southeastern US.


In addition to possible sea swells and rip currents along the southeast US and northwestern Bahamas coastline this weekend...the updated forecast track also means a possiblity of sea swells and rip currents along the northeast US Gulf coast early next week. In addition...direct impacts including heavy rains and possible breezy or gusty winds are now possible across northern Florida late this weekend into early next week...interests here should now be watching this system if they are not already. The area of heavy rainfall associated with this system may spread as far north as southern Georgia and southern Alabama if the forecast track below holds.

******infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (1200Z Jul 24)...30% chance of tropical cyclone formation (offshore of the southeastern US and north of the western Bahamas near 29N-77W)

IOH 48 Hr Outlook (1200Z Jul 25)...50% chance of tropical cyclone formation (offshore of northeast Florida near 29N-80W)

IOH 72 Hr Outlook (1200Z Jul 26)...40% chance of tropical cyclone formation (northwest coast of the Florida peninsula near 29N-83W)

IOH 96 Hr Outlook (1200Z Jul 27)...0% chance of tropical cyclone formation (inland over the Florida/Alabama border near 30.8N-86W)


AREA OF INTEREST #2...As noted in the above area of interest #1 section...a large portion of the current northeast Canada upper trough is in the process of becoming cut-off into an upper vorticity string that will become pushed southeast into the western Atlantic. The north portion of this upper vorticity string will still behave as an upper trough...and the models have been forecasting another surface low pressure area to form along the same cold front that has helped trigger area of interest #1...but at a location more offshore from the eastern US and to the north of Bermuda while supported by the eastern divergence zone of the upper trough feature. Because the cold core upper trough feature is forecast to weaken under the warm sector of the current approaching Canadian frontal low pressure system...this could result in reduced southwesterly shear on the east side of the upper trough...with the fast northeast track of the surface low pressure area also helping to reduce the shearing effect of the southwesterly upper flow. Moreover the latest GFS and ECMWF models...usually the more reliable of the global models...show a compact circular circulation forming at some point within the surface low pressure area...which could be an indication of a tropical thunderstorm core. Therefore I have declared the portion of the cold front currently northwest of Bermuda as another area of interest...but also have low 15% odds of development until signs of organized thunderstorm activity develop along this area of the cold front and/or forecast surface low pressure area. Note that my odds are slighlty above the 10% mark as the GFS and ECMWF have been suggesting a tropical thunderstorm core as noted above. Regarding the latest suite of model runs...the GFS and ECMWF are showing a singular area of low pressure...while the less reliable CMC and NAVGEM models show a stretched low pressure area with two centers. My forecast positions in the outlook below are weighted toward the GFS and ECMWF singular center...but are slightly south of those solutions in case the CMC and NAVGEM are correct and the southwestern of the two centers develops thunderstorm activity. By 72 hours...I drop odds of development to 0% as the surface low pressure area moves northeast into cooler water.

******infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (1200Z Jul 24)...0% chance of tropical cyclone formation (north of Bermuda near 35N-65W)

IOH 48 Hr Outlook (1200Z Jul 25)...15% chance of tropical cyclone formation (northwest Atlantic near 38.5N-55W)

IOH 72 Hr Outlook (1200Z Jul 26)...0% chance of tropical cyclone formation (northwest Atlantic near 44N-48W)


...COMPUTER MODEL SUMMARY...

Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)


0000Z CMC Model Run...

**For area of interest #1...defined surface low pressure forms near 30N-78W at 30 hours...drifts west under influence of eastern US surface ridge and makes landfall over the northeast coast of Florida at 66 hours...weakens to an inland surface trough over norhteast Florida by 78 hours.

**For area of interest #2...defined surface low pressure forms north of Bermuda near 35N-64W at 54 hours...while entangled with another frontal low pressure that forms to the northeast the surface low becomes dragged east-northeast and reaches cooler waters near 41N-45W by 108 hours.


0000Z ECMWF Model Run...

**For area of interest #1...defined surface low pressure forms near 30.5N-78.5W at 24 hours...moves west-southwest under influence of eastern US surface ridge and makes landfall over east-central Florida at 72 hours.

**For area of interest #2...defined surface low pressure forms north of Bermuda near 35N-64W at 48 hours...at 72 hours a circular compact low pressure (possible signature of tropical system) suggested near 42N-52.5W while moving northeast into cooler water at 72 hours...transitions to non-tropical frontal low over cooler water by 96 hours near 49N-46.5W


0600Z GFS Model Run...

**For area of interest #1...weak low pressure area defined just offshore of South Carolina and Georgia and just ahead of cold front at 0 hours...drifts southeast further offshore and becomes better defined near 30.2N-78.5W at 18 hours...drifts west-southwest under eastern US surface ridge and toward the east-central Flroida coast through 48 hours while weakening to a surface trough...

**For area of interest #2...defined surface low pressure forms north-northeast of Bermuda near 35.5N-63.5W at 39 hours...at 51 hours a circular compact low pressure (possible signature of tropical system) suggested near 39.5N-56W...transitions to non-tropical frontal low over cooler water by 78 hours near 45N-46W.


0600Z NAVGEM Model Run...

**For area of interest #1...weak low pressure area becomes defined just offshore of Georgia and just ahead of cold front at 6 hours...drifts south through 42 hours under the influence of the eastern US surface ridge while becoming better defined at 29.5N-77W...drifts west-northwest while rounding the southwest side of the surface ridge while making landfall on the Georgia coast at 84 hours.

**For area of interest #2...defined surface low pressure forms north-northeast of Bermuda near 36N-62.5W at 30 hours...elongates then splits into two surface low pressure areas by 60 hours (one near 39N-54.5W and a second one near 44N-48W)...by 78 hours the northern of the two low pressure areas becoems the dominant and over cooler water near 49.5N-41W while evolving as a non-tropical frontal low pressure.

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