top of page
Home: Text


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 

Home: Text
Home: Blog2
  • Writer's pictureNCHurricane2009


*******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 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 dot gov) for the latest watches/warnings and official forecasts on active tropical cyclones.**********

...WEDNESDAY MAY 27 2020 12:20 PM EDT...

Even though the Atlantic hurricane season does not start until June 1...continuing daily birdseye view posts on the Atlantic tropics due to newly-formed Tropical Storm Bertha that has made landfall in South well as two additional areas of interest for tropical development. See Bertha and area of interest sections below for additional details.

TROPICAL STORM BERTHA...Since yesterday afternoon...we have been monitoring a mid-level low pressure spin with thunderstorms located offshore of the southeastern United States that formed near 29N-79W. Through last evening...ASCAT passes suggested a surface rotation that was southwest of the mid-level one...and it appeared a tropical cyclone was not going to form. But warm Gulf stream well as low wind shear and upper anticyclonic outflow supplied by a longwave upper ridge that has persisted over eastern North America and the west edge of the Atlantic...favored the mid-level rotation...and just offshore of South Carolina the mid-level rotation suddenly strengthened enough to reach the surface and become Tropical Storm Bertha. In some sense one could also say the broad surface rotation has relocated northeastward while tightening beneath the mid-level rotation. Conditions appear quiet favorable for tropical development as Bertha deepened quickly from 1009 to 1004 mb and winds at the center climbed from 45 to 50 mph maximum sustained. But the current landfall with the South Carolina coast (between Georgetown and Charleston) will thankfully end further strengthening...gusty winds with some isolated wind damage/power outages are possible along this portion of the coast and just inland in the next few hours. Bertha continues north while embedded in deep-layered southerly flow between a western North America upper trough and associated broad surface frontal low pressure over the central United States...and a surface ridge to the east supported by the eastern convergence of the overhead upper ridge. But there is a westward angle in the track seen in radar as the surface ridge is quiet expansive while covering parts of the northeastern United States. Therefore my updated forecast track is adjusted westward. My updated track is also adjusted northward as the GFS model since yesterday has suggested what is now Bertha will accelerate rapidly northwestard and already reach the Ohio/Pennsylvania Border in 24 hours! Given the current radar animation and the National Hurricane Center official forecast which does not accelerate Bertha that northward adjustment in the forecast track is only slight. In the next 24 hours...a swath of heavy rainfall with possible flash flooding looks possible over northeastern South Carolina...central and northwestern North Carolina...western Virginia...and even as far inland as West Virginia given Bertha's brisk north-northwestward track.

****** forecast. Visit (hurricanes dot gov) for official forecast***********

0 Hr Position (1200Z May 27)...50 mph maximum sustained wind tropical storm centered on the South Carolina coast at 32.9N-79.7W

IOH 24 Hr Forecast (1200Z May 28)...Remnant low pressure centered over western Virginia at 37N-81W

AREA OF INTEREST #1...A shortwave upper trough continues diving southeastward across the western Atlantic while influenced by a longwave upper ridge located to the west over eastern North America. The eastern divergence zone of this upper trough is producing showers and thunderstorms in the open western Atlantic well to the southeast of Bermuda and northeast of the Lesser Antilles. Over the next 24 hours computer models remain in agreement that this upper trough will amplify into a cut-off upper vortex while continuing its southeastward dive...which will result in the gradual formation of a surface low pressure in the increased eastern divergence zone of the upper vortex over the next couple of days. The surface low pressure will form where water temperatures are currently running at 24 to 26 deg or just below the threshold for tropical development. However the forecast cut-off upper vortex is expected to be cold enough where it can help boost thunderstorm activity...therefore the forecast surface low pressure could acquire tropical characteristics and become a subtropical cyclone. I have extended short-term 0% odds of subtropical development now thorugh May 29 as the latest presentation of the upper vortex in the GFS model shows the vortex still being elongated and titled southwest-to-northeast such that wind shear may still be too high for any kind of tropical development through that timeframe. Peak odds of development are shown in my latest outlook to be at 72 hours when the upper vortex is shown to be more circular and the surface low pressure is shown in models to be more mature. I have also slightly raised those odds of development as the GFS has finally joined the Euro (ECMWF) in showing a more respectable surface low pressure developing. But my peaks odds of development are still on the low side at this time because the robust longwave upper ridge with its east convergence will maintain a strong surface ridge in the western Atlantic...perhaps strong enough to make it more difficult for a surface low pressure to take hold. Regarding track...the upper vortex and surface low are once again shown to be further east in the lastest I have adjusted my forecast track accordingly. The longwave upper ridge over eastern North America is expected to finally shift into the northwestern Atlantic...which will steer this system (upper vortex and surface low) northwestard and then northward during the forecast period. By 96 hours I drop the odds of development to a very small 5% as the track will take this system to water temperatures in the low 20s of deg C...and the upper vortex by that time will at the 200 mb layer be at 1215 dekameters above sea level (would like to see the upper vortex be at 1200 or less at those water temperatures to believe the upper vortex is cold enough to support thunderstorm activity). By 120 hours...development should no longer be possible with even cooler water temperatures...and as the system at the surface and upper levels should become abosrbed by what is now a vigorous upper trough and surface frontal system over western Canada.

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

IOH 24 Hr Outlook (1200Z May 28)...0% chance of subtropical cyclone formation (western Atlantic near 28N-55W)

IOH 48 Hr Outlook (1200Z May 29)...0% chance of subtropical cyclone formation (western Atlantic near 28N-57W)

IOH 72 Hr Outlook (1200Z May 30)...20% chance of subtropical cyclone formation (western Atlnatic near 29N-57.5W)

IOH 96 Hr Outlook (1200Z May 31)...5% chance of subtropical cyclone formation (western Atlantic near 35N-57.5W)

IOH 120 Hr Outlook (1200Z Jun 1)...0% chance of subtropical cyclone formation (western Atlantic south of Newfoundland near 41N-55W)

AREA OF INTEREST #2...The upper air pattern in the eastern the southwest of Central America and south of southeastern becoming increasingly favorable for unsettled weather as a widespread upper ridge with outflow expands in the wake of the northward-retreating upper trough in the Gulf of Mexico. The tropical wave currently at 105W may have also contributed to kicking up thunderstorm activity in the region (this same tropical wave may have helped generate the pre-Tropical Storm Bertha disturbance in the northwestern Caribbean on May 24th...see birdseye view post #10). computer models have long been eastern Pacific tropical cyclone is likely to form in the next few days. Meanwhile on the Atlantic side...specifically the Central America-Yucatan Peninusla-Western Caribbean region...the favorable upper ridge is likely to expand northward into the area as the upper vortex currently consolidating over the central United States shifts westward and away under the influence of a longwave upper ridge to build over western North America at 72+ hours. Once the upper ridge arrives to the Atlantic is likely to increase thunderstorms and drop surface pressures that draw the expected eastern Pacific tropical cyclone northeastward toward the Atlantic side. Therefore it is possible that the eastern Pacific tropical cyclone crosses Central America and becomes an Atlantic tropical cyclone...or alternatively the eastern Pacific tropical cyclone would dissipate over Central America and a second seperate low pressure area at risk of becoming its own tropical cyclone develops on the Atlantic side. Given that multiple computer models (particularly the NAVGEM...GFS...and CMC) have been insisting on these solutions...I have begun another area of interest over Honduras and the western Caribbean Sea. I only slowly raise odds of development at 96+ hours as any surface low pressure area that forms here will be large/broad due to the large size of the upper ridge...thus likely taking time to develop into an Atlantic-side tropical cyclone if this were to occur.

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

IOH 24 Hr Outlook (1200Z May 28)...0% chance of tropical cyclone formation (Honduras near 15N-87W)

IOH 48 Hr Outlook (1200Z May 29)...0% chance of tropical cyclone formation (Honduras near 15N-87W)

IOH 72 Hr Outlook (1200Z May 30)...0% chance of tropical cyclone formation (Honduras near 15N-87W)

IOH 96 Hr Outlook (1200Z May 31)...5% chance of tropical cyclone formation (north coast of Honduras near 16N-87W)

IOH 120 Hr Outlook (1200Z Jun 1)...10% chance of tropical cyclone formation (western Caribbean Sea near 16.5N-87W)

10 views0 comments


bottom of page