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 #99

Updated: Aug 23

*******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.**********


...SATURDAY AUGUST 22 2020 7:03 PM EDT...

See tropical storm Laura and Marco sections below for the two currently active tropical cyclones in the Atlantic basin. See area of interest sections below for tropical waves of low pressure being monitored for tropical cyclone formation.


TROPICAL STORM LAURA...Laura is putting up a fight this afternoon whiel firing tremendous thunderstorm bursts as it moves across the northastern Caribbean islands (Virgin Islands...northern Lesser Antilles...Puerto Rico...and shortly the Dominican Republic and Haiti). Therefore expect heavy rainfall with flash flooding potential across all of these islands over the next 24 hours. My track forecast is unchanged from my previous discussion...showing an increasing north tilt to commence soon while Laura follows the forecast countour of the southwestern side of the Atlantic subtropical ridge...which will has and will continue to build increasigly westward toward the southeasern United States (see Tropical Storm Marco section below for more details on the expected evolution of the surface subtropical ridge). This track will take Laura across Cuba and then the southeastern Gulf of Mexico after it passes over Haiti. What Laura does as it enters the Gulf of Mexico heavily depends on what Tropical Storm Marco does. For now I forecast by 96 hours that Marco will be making its way northwestward inland into the southern Louisiana/southeast Texas region...thus I forecast Laura by 96+ hours to bend more northward in track while gravitated toward the low pressure field of Marco.


Regarding strength...despite Laura's currently impressive thunderstorm mass I have not raised my intensity forecast from my previous update due to land interaction expected with Haiti and the Dominican Republic...followed by Cuba. It should be noted that the upper-level winds are becoming more favorable for Laura to develop as the upper vorticity that was over the eastern Caribbean and impinging on Laura's western outflow has moved westward and away into the southern Caribbean...which is allowing the western Atlantic upper ridge to overspread Laura with a vast area of low shear and upper outflow. So the reality is one of two things can happen...either Laura can regenerate a new center offshore of Haiti...the Dominican Republic...or Cuba and then rapidly strengthen under the favorable upper winds...or Laura may simply degenerate into a tropical wave from the land interactions. So my intensity forecast is of low confidence. After Laura moves into the Gulf of Mexico and away from Cuba..I only cautiously raise the intensity of the tropical storm into a category 1 hurricane as it moves toward Louisiana as its possible Laura may have to spend some time recovering from land interaction...and also because I lean toward Marco becoming the stronger tropical cyclone in the short-term whose upper outflow might impenge on Laura's western outflow. With Marco also expected to pass by or over southern Louisiana...it is possible Marco leaves behind upwelled cooler waters that might also dampen Laura's intensity. See bulletins at the home page of this site for an update on Laura's impact potential given this current outlook.

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

0 Hr position (1200Z Aug 22)...50 mph maximum sustained wind tropical storm centered between Puerto Rico and the Dominican Republic at 17.9N-67.5W

IOH 24 Hr Forecast (1200Z Aug 23)...45 mph maximum sustained wind tropical storm centered between northwest Haiti and Cuba at 19.5N-74W

IOH 48 Hr Forecast (1200Z Aug 24)...45 mph maximum sustained wind tropical storm centered over central Cuba at 21.5N-80W

IOH 72 Hr Forecast (1200Z Aug 25)...50 mph maximum sustained wind tropical storm centered in the eastern Gulf of Mexico at 25N-85.2W

IOH 96 Hr Forecast (1200Z Aug 26)...60 mph maximum sustained wind tropical storm centered in the Gulf of Mexico south of Louisiana at 27.5N-91W

IOH 120 Hr Forecast (1200Z Aug 27)...85 mph maximum sustained wind hurricane centered just inland and over southwestern Louisiana/southeast Texas border at 30.5N-94W


TROPICAL STORM MARCO...Luckily for the northeastern Yucatan peninsula of southeastern Mexico...the cut-off upper trough in the Gulf of Mexico has pushed Marco's heaviest weather to the east of its center...and the center has tracked more north and less west while becoming incrasingly strong and tall enough to be steered by the southerly flow on the east side of the upper trough. However for the western tip of Cuba...heavy rains and gusty winds are likely occuring from the eastern rain bands of Marco as the tropical storm enters the Gulf of Mexico from the Yucatan channel waters between Cuba and Mexico. The trpoical storm has briskly strengthened to 65 mph maximum sustained winds overnight while finally developing a tight core of thunderstorms...but has not been able to stregnthen further into a hurricane thus far...likely the effects of southwestelry shear induced by the upper trough.


Going forward...Marco will be heavily influenced by the cut-off upper trough...as well as the evolution of the western North America upper ridge and also the west Atlantic upper ridge (supported by warm air advection ahead of the current front over the southeastern United States). The cut-off upper trough is expected to gradually weaken while starved of higher latitude colder air...with the Atlantic surface subtropical ridge building westward into the southeastern United States due to reduced upper divergence on the east side of the weakening upper trough and also due to the southeastern convergence zone of the western Atlantic upper ridge. The surface ridging over the southeastern United States will strengthen further during the forecast period 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. The Great Lakes nose of the upper ridge will also help push the cut-off upper trough westward out of the Gulf of Mexico and into the south-central United States during the forecast period.


With the outlook mentioned in the previous paragraph...I expect Marco to continue northwestward...a combination of the west push of the surface ridge building toward the southeastern United States and north pull of the cut-off upper trough. With the northeastward adjusted forecast track (due to Marco's track continuing to stay more north and less west)...I now expect Marco to move into southern Louisiana a bit sooner than my previous forecast. As Marco nears Louisiana...I expect some west bend in track as Marco runs into the strengthening southeastern United states surface ridge...and also as the cut-off upper trough responsible for dragging Marco northward gets pushed westward to the point it has less influence on Marco's track. My track forecast is bent back northward again by 96 hours as Marco rounds the west side of the steering southeastern United States surface ridge...and also catches up to the upper trough which may attepmt to drag Marco northward again. However my 96 hour forecast point is the same as yesterday instead of being also adjusted northeastward...as the 1200Z GFS shows the surface ridge being a bit stronger which would tend to keep Marco more toward the southwest. Regarding intensity...I have raised my intensity forecast as Marco is already stronger than my previous forecast. The forecast shows gradual strengthening initially due to the current wind shear...followed by a ramp up in the intensification rate as the upper trough shifts westward and away which will allow for a reduction in shear with time. See bulletins at the home page of this site for an update on Marco's impact potential given this current outlook.

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

0 Hr Position (1200Z Aug 22)...65 mph maximum sustained wind tropical storm centered just offshore of the northeastern Yucatan peninsula at 21.4N-85.5W

IOH 24 Hr Forecast (1200Z Aug 23)...80 mph maximum sustained wind hurricane centered over the central Gulf of Mexico at 25.5N-88W

IOH 48 Hr Forecast (1200Z Aug 24)...100 mph maximum sustained wind hurricane centered just south of southeastern Louisiana at 28.5N-91W

IOH 72 Hr Forecast (1200Z Aug 25)...85 mph maximum sustained wind hurricane centered just inland and over southwestern Louisiana/southeast Texas border at 30.5N-94W

IOH 96 Hr Forecast (1200Z Aug 26)...Remnant low pressure centered over northeast Texas at 33N-96W


AREA OF INTEREST #1...The once vigorous tropical wave of low pressure over the eastern tropical Atlantic has succumbed to dry sahran air as both its northeastern and southwestern thundertorm complexes have dissipated. Therefore tropical cyclone formation from this feature is no longer anticipated...this is my final statement on this feature on this blog unless the tropical wave later redevelops thunderstorm activity in an atmospheric environment that remains favorable for tropical development.

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

IOH 24 Hr Outlook (1200Z Aug 23)...0% chance of tropical cyclone formation (tropical Atlantic near 12N-32.5W)


AREA OF INTEREST #2...Another tropical wave of low pressure...currently located over inland western Africa in the vicnity of southwestern Mali...has produced curved bands of thunderstorms on its west side. Previous days of computer model runs...including today...agree that this tropical wave has potential to become better organized into a low pressure spin just to the southwest of the Republic of Cabo Verde Islands in the timeframe that is now about 72 hours away. Outside of the dry saharan air...conditions in the eastern tropical Atlantic will be favorable for this tropical wave to develop as a tropical upper ridge axis with low shear and upper outflow will prevail in the region. However my peak 5-day odds of tropical cyclone formation are only 10% at the present time as dry saharan air has recently hampered development in the eastern tropical Atlantic. The 1200Z GFS shows some of the current central Atlantic upper vorticity getting pushed southeastward toward the Atlantic tropics in the vicnity of 40W later in the forecast period (as the current west Atlantic upper ridge expands into the north Atlantic due to warm air advection ahead of a variety of fronts to emerge from Canada...with the expanded ridge pushing on the upper vorticity). However based on the forecast positions below...I do not expect this upper vorticity to negatively affect this tropical wave's development during the 120-hour forecast period as it will be east of 40W longitude during this period.

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

IOH 24 Hr Outlook (1200Z Aug 23)...0% chance of tropical cyclone formation (western Africa near 15N-14W)

IOH 48 Hr Outlook (1200Z Aug 24)...0% chance of tropical cyclone formation (just offshore of western Africa near 15N-19W)

IOH 72 Hr Outlook (1200Z Aug 25)...10% chance of tropical cyclone formation (southwestern Republic of Cabo Verde Islands near 15N-24W)

IOH 96 Hr Outlook (1200Z Aug 26)...10% chance of tropical cyclone formation (tropical Atlantic near 15N-29W)

IOH 120 Hr Outlook (1200Z Aug 27)...10% chance of tropical cyclone formation (tropical Atlantic near 15N-34W)


...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 Tropical Storm Laura...no strengthening shown till 60 hours when Laura enters Gulf of Mexico from Cuba...curves northward into the Florida panhandle by 84 hours in a Fujiwhara interaction with Marco while strengthening into a strong tropical cyclone. For Tropical Storm Marco...makes landfall over southeast Louisiana as a strong tropical cyclone in 60 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...large tropical wave organizes into a large broad tropical low pressure just southwest the Republic of Cabo Verde Islands in 72 hours.


1200Z ECMWF Model Run...For Tropical Storm Laura...rapid strengthening shown along northwest Cuba coast in 48 hours...makes landfall as an intense tropical cyclone over southeast Louisiana in 96 hours. For Tropical Storm Marco... makes landfall as an intense compact tropical cyclone in southeast Louisiana in 48 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...large tropical wave organizes into a large broad tropical low pressure just southwest the Republic of Cabo Verde Islands in 72 hours.


1200Z GFS Model Run...For Tropical Storm Laura...no strengthening shown till 60 hours when Laura enters Gulf of Mexico from Cuba...makes landfall over south-central Louisiana as an intense tropical cyclone in 108 hours. For Tropical Storm Marco...makes landfall as an intense tropical cyclone over southeastern Louisiana at 54 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...large tropical wave organizes into a large broad tropical low pressure just southwest the Republic of Cabo Verde Islands in 72 hours.


1200Z NAVGEM Model Run... For Tropical Storm Laura...no strengthening shown till 60 hours when Laura enters Gulf of Mexico from Cuba...makes landfall over southeastern Louisiana as an intense tropical cyclone in 96 hours. For Tropical Storm Marco...loses its identity in northwest Gulf of Mexico in 66 hours as Laura dominates to the southeast. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...large tropical wave organizes into a large broad tropical low pressure just southwest the Republic of Cabo Verde Islands in 84 hours.

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