BIRDSEYE VIEW POSTS

Since 2012 on the Weather Underground (www.wunderground.com) 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 via Disqus on Weather Underground at www.wunderground.com/cat6. You can see my Disqus feed at this link for my latest comments. Feel free to reply to me with your disqus account or e-mail at IOHurricanes@outlook.com 

 
 
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MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #96

*******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 AUGUST 19 2020 2:45 PM EDT...

See area of interest sections below for multiple tropical waves of low pressure being monitored for development. Elsewhere...the southern portion of the large upper trough over eastern North America is expected to become cut-off in the Gulf of Mexico due to an amplifying upper ridge over western North America. Even though the disturbance mentioned in area of interest #1 is now more likely to stay further south and not dominate the eastern Gulf of Mexico...the chances of this cut-off upper trough triggering a subtropical or tropical disturbance in the eastern Gulf of Mexico with its eastern divergence zone are still low as the upper trough is likely to weaken while remaining cut-off.


AREA OF INTEREST #1...The tropical wave featuring a weak low pressure spin moving across the south-central Caribbean Sea earlier this morning had banding features on the north side of its circulation...which has since devolved into circular thunderstorm bursts. The tropical wave is also further west than I previously forecasted...therefore my forecast track points are adjusted westward. Given the devolved appearance I have delayed the timeframe of tropical cyclone formation by about 24 hours. These initial adjustments in track and intensity have longer term consequences such that I have made a notable downward adjustment in the intensity forecast and notable westward shift in the track forecast in the longer range due to the following observations noted in the next paragraph.


This system will be heavily influenced by the south fracture of the large eastern North America upper trough which will be cut-off in the Gulf of Mexico by an amplified western North America upper ridge. The evolution of the western North America upper ridge and also the northwest Atlantic upper ridge (supported by warm air advection ahead of the current front along the United States east coast) will also be big players. Initially the cut-off upper trough's eastern divergence zone will drop surface pressures along the tail end of the United States east coast front...located in the northern Gulf of Mexico...with this surface low pressure field allowing this system to bend north enough in track by 48 hours such that it does not make landfall in Nicaragua and Honduras. After 48 hours...the future track of this sytem heavily depends on the strength of this system. This is because the Gulf of Mexico surface low pressure field is expected to weaken as the cut-off upper trough begins to weaken while starved of higher latitude colder air...with the Atlantic surface subtropical ridge instead building westward into the southeastern United States due to the southeastern convergence zone of the northwestern Atlantic upper ridge. The surface ridging over the southeastern United States will strengthen further at 120+ hours as frontal systems over southern Canada driven by shortwave upper troughs will waft the warm air associated with the western North America upper ridge toward the Great Lakes region...with the southeastern convergence zone of the Great Lakes nose of the upper ridge responsbile for strengthening the southeastern United States surface ridge. This means a weaker/shallower system would be steered more westward by the building southeastern United States surface ridge...tracking directly into wind shear generated by the south side of the Gulf of Mexico cut-off upper trough and also across the land of the Yucatan peninsula...thus staying weak and continuing a more westward track...which appears to be the idea shown in the 0600Z GFS model run. Meanwhile the CMC and NAVGEM show this system rapidly develop in the western Caribbean Sea/southern Gulf of Mexico...becoming strong/tall enough to be turned more northward by the cut-off upper trough on a track that keeps this system away from the shear on the south side of the upper trough and reduces land interaction with the Yucatan...re-enforcing the strength of this system for a continued more north track. Given that this system has not gotten better organized in the last 24 hours and is tracking faster to the west...I am trending away from the stronger/more north track solution and toward the weaker/more west track solution.


After 120 hours...the Great Lakes to western North America upper ridge will push the cut-off upper trough in the Gulf of Mexico into Texas and stretch it as far west as northwestern Mexico...leaving the possibilty of reduced wind shear and more rapid strengthening after 120 hours even with my weaker/more west track solution shown for the next 120 hours. Whether the track begins to bend more north or not in the long range with this possible strengthening will depend on exactly how far west the cut-off upper trough moves...a further east position having potential to bend the strong/taller system more north...a further west position being too far away to influence the track. Given the more west track...the potential of direct impacts (rainfall/gusty winds) to Jamaica...the Cayman Islands...and western Cuba has reduced...and has increased for Nicaragua...Honduras...Belize...and the Yucatan region of southeastern Mexico.

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

0 Hr Position (1200Z Aug 19)...Tropical low pressure centered offshore of the northwestern Venezuela coast at 14N-72W

IOH 24 Hr Forecast (1200Z Aug 20)...Tropical low pressure centered in the central Caribbean Sea at 14.5N-78W

IOH 48 Hr Forecast (1200Z Aug 21)...40 mph maximum sustained wind tropical storm centered just offshore of the eastern Honduras coast at 16N-84W

IOH 72 Hr Forecast (1200Z Aug 22)...50 mph maximum sustained wind tropical storm centered on the east Yucatan coast at the Belize/Mexico border at 17.5N-88W

IOH 96 Hr Forecast (1200Z Aug 23)...35 mph maximum sustained wind tropical depression centered on the northwest coast of the Yucatan peninsula at 20.8N-90W

IOH 120 Hr Forecast (1200Z Aug 24)...50 mph maximum sustained wind tropical storm centered in the western Gulf of Mexico at 22N-94W


AREA OF INTEREST #2...This tropical low pressure currently entering the central tropical Atlantic from the eastern Atlantic appears to be merging with the tropical wave just to the east-northeast...thus taking on an elongated southwest-to-northeast structure which seems to be verified by ASCAT-B and ASCAT-C ascending passes of surface winds(https://manati.star.nesdis.noaa.gov/datasets/ASCATCData.php). The southwestern lobe of the elongated low pressure seems to be the dominant while featuring multiple organized thunderstorm bands and central bursts...while the northeastern lobe seems to have lost activity from dry saharan air. The southwestern lobe seems be lifting northwestward rather than going more directly west perhaps while it absorbs the northeastern lobe...so although I have shifted my forecast points southward due to the dominance of the southwestern lobe...I still forecast a more northward angle in track in the next 24 hours...followed by a more westward track in the middle of the forecast period around south side of the Atlantic subtropical surface ridge after this system finishing absorbing the tropical wave to the east-northeast. By the end of the forecast period...I tilt the track a little more northward while following the forecast contour of the southwestern side of the Atlantic subtropical ridge...which will be increasingly building westward toward the southeastern United States (see area of interest #1 section for more details on the evolution of the surface subtropical ridge).


Regarding strength...the models have trended downward while delaying the timeframe of forecast tropical cyclone formation...or in the case of the 0000Z ECMWF model run no tropical cyclone formation is forecasted at all. While I still forecast tropical cyclone formation in the next 24 hours due to the current organization...I have nudged my intensity forecast downward to take into account some of the negative factors that might affect this system and are perhaps being considered by the models. One of those is the dry saharan air layer lurking just to the north...which seems to have more potential to affect this system with the loss of thunderstorm activity to the northeast. Another factor is the large size of this system which will need some time to consolidate before it has the ability to strengthen more briskly. A third factor is the cut-off upper vortex currently in the central Atlantic. Specifically some of this upper vorticity...which has potential to disrupt the western outflow of this system...will become stretched southwestward toward the eastern Caribbean region by the robust northwestern Atlantic upper ridge to persist in warm southerly flow ahead of the frontal zone currently exiting the eastern United States coast. By 96 hours the 0600Z GFS shows the northwest Atlantic upper ridge and tropical upper ridge axis bridging over the vicnity of Puerto Rico...which might place this system in a more favorable upper outflow environment...so I forecast some brisk strengthening into a strong tropical storm. By 120 hours...I forecast land interaction with Puerto Rico and the Dominican Republic will induce some weakening.


Due to the current outlook for this system...interests in the Lesser Antilles...Virgin Islands...Puerto Rico...the Dominican Republic...and Haiti are advised to watch this system carefully over the next few days.

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

0 Hr Position (1200Z Aug 19)...Tropical low pressure centered at 12.5N-44.5W

IOH 24 Hr Outlook (1200Z Aug 20)...35 mph maximum sustained wind tropical depression centered at 14N-49W

IOH 48 Hr Outlook (1200Z Aug 21)...45 mph maximum sustained wind tropical storm centered at 15N-55W

IOH 72 Hr Outlook (1200Z Aug 22)...50 mph maximum sustained wind tropical storm centered over the northern Lesser Antilles at 16N-61W

IOH 96 Hr Outlook (1200Z Aug 23)...65 mph maximum sustained wind tropical storm centered over the southeastern Puerto Rico coast at 18N-66W

IOH 120 Hr Outlook (1200Z Aug 24)...60 mph maximum sustained wind tropical storm centered over the northwestern Dominican Republic coast at 19.8N-71W


AREA OF INTEREST #3...A strong tropical wave of low pressure over western Africa has seen an outbreak of strong thunderstorms last night...and is now being monitored for tropical cyclone formation as it moves into the eastern tropical Atlantic. In fact the 0600Z GFS and 0000Z ECMWF model runs suggested there is some potential for this tropical wave to develop (also yesterday's 0000Z ECMWF had a similar suggestion)...and the tropical wave has been added to the National Hurricane Center tropical weather oulook as of early this morning. As the tropical wave approaches the west African coast...it apperas on colorized infrared satellite imagery to have a curved band of thunderstorms on its southwest side...and another such band well inland to the east. With this banding as a clue...I am estimating the lowest pressure of the tropical wave is in the vicinity of 11N-13W...and in the outlook below I assume a steady westward track across the eastern tropical Atlantic under the influence of the steering Atlantic subtropical ridge from there. Even though this tropical wave will stay below the axis of tropical upper ridging prevailing across the Atlantic...which will provide a low shear and upper outflow environment conducive to development...it appears the northeasterly flow on the northwest side of the tropical wave is driving a surge of dry saharan air southwestward into the future path of this wave...thus I only slowly raise the odds of tropical cyclone formation to 20% during the forecast period.

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

IOH 24 Hr Outlook (1200Z Aug 20)...0% chance of tropical cyclone formation (offshore of western Africa near 11N-17.5W)

IOH 48 Hr Outlook (1200Z Aug 21)...5% chance of tropical cyclone formation (south-southeast of the Republic of Cabo Verde Islands near 12N-22.5W)

IOH 72 Hr Outlook (1200Z Aug 22)...10% chance of tropical cyclone formation (southwest of the Republic of Cabo Verde Islands near 12.5N-27.5W)

IOH 96 Hr Outlook (1200Z Aug 23)...15% chance of tropical cyclone formation (tropical Atlantic near 13N-32.5W)

IOH 120 Hr Outlook (1200Z Aug 24)...20% chance of tropical cyclone formation (tropical Atlantic near 13.5N-37.5W)


...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...tropical cyclone formation suggested just east of the Nicaragua/Honduras Border in 54 hours...gradual strengthening as a compact tropical cyclone that makes landfall on the east Yucatan Peninsula coast of southeastern Mexico in 90 hours...emerges into the Gulf of Mexico from the northwest Yucatan peninsula in 108 hours while weaker/broader from the landfall...rapid intensification of tropical cyclone suggested in the western Gulf of Mexico near 23N-94W in 126 hours. For area of interest #2...shown to cross the Lesser Antilles and northeastern Caribbean Sea as a south-southwest to north-northeast elongated tropical wave in 72 hours...north end of tropical wave organizes into a tropical low pressure center over southeast Puerto Rico in 84 hours...tropical low moves across southern Haiti and southern Dominican Republic in 108 hours...tropical low moves over eastern Cuba by 126 hours...tropical cyclone formation suggested on north coast of central Cuba by 132 hours. For area of interest #3...no tropical cyclone formation shown.


0000Z ECMWF Model Run...For area of interest #1...no tropical cyclone formation shown. For area of interest #2...no tropical cyclone formation shown. For area of interest #3...tropical wave emerges from western Africa into the eastern tropical Atlantic in 48 hours...tropical cyclone formation suggested near 17.8N-39W in 120 hours.


0600Z GFS Model Run...For area of interest #1...tropical wave organizes into a tropical low pressure on east coast of Belize in 96 hours...tropical low emerges into the Bay of Campeche in 120 hours. For area of interest #2...tropical cyclone formation suggested east of the northern Lesser Antilles near 17.5N-59W in 60 hours...weakens back into a tropical wave over Puerto Rico in 78 hours. For area of interest #3...tropical wave emerges into eastern tropical Atlantc in 36 hours...organizes into a tropical low pressure spin near 15N-39W in 126 hours.


0600Z NAVGEM Model Run...For area of interest #1...tropical cyclone formation suggested on the northeast coast of the Yucatan peninsula in 102 hours...curves northward across central Gulf of Mexico and toward southeastern Louisiana in 132 hours while strengthening. For area of interest #2...tropical cyclone formation suggested north-northeast of the Virgin Islands near 20N-64W in 84 hours...moves into the eastern Bahamas as a slowly strengthening tropical cyclone in 120 hours. For area of interest #3...no tropical cyclone formation shown.

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