*******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 1:39 AM EDT...
See tropical storm Laura and Marco sections below for the two currently active tropical cyclones in the Atlantic basin. See area of interest section below for the far eastern Atlantic tropical wave of low pressure being monitored for tropical cyclone formation. Elsewhere...models indicate another vigorous tropical wave worth watching for tropical development could emerge from western Africa in about three days.
Above is a more recent satellite image of tropical storms Laura and Marco...as well as the eastern Atlantic tropical wave of low pressure being monitored for development (area of interest #1) as the image in the above birdseye view chart is a few hours old.
TROPICAL STORM LAURA...Tropical depression thirteen in the last 24 hours has intensified into Tropical Storm Laura while firing stronger thunderstorm bursts toward the southwest side of its broad circulation...and the circulation had consolidated further southwest during this strengthening process. This has resulted in the forecast track points being shifted south and west...with the more west position moving Laura into the northeastern Caribbean Islands (northern Lesser Antilles...Virgin Islands...and soon Puerto Rico) earlier than expected. My track forecast has an increasing north tilt after 24 hours while following 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). The updated track takes Laura across multiple land interactions with the northern Caribbean Islands...including Puerto Rico...the Dominican Republic...Haiti...and then Cuba. After that point...what Laura does after it enters the Gulf of Mexico heavily depends on what Tropical Storm Marco does. For now I forecast by 120 hours that Marco will be making its way northwestward inland into eastern Texas...thus I forecast Laura by 120 hours to turn gradually bend more northward in track while gravitated toward the low pressure field of Marco.
Regarding strength...I am forced to significantly be lower than my previous forecast as the more southwest position of Laura places it closer to dirsupting upper vorticity being stretched into the eastern Caribbean and on a path to now interact with the northern Caribbean Islands instead of dodging them. While the upper-level winds will become much more favorable as the upper vorticity moves westward and away after 24 hours...with the western Atlantic upper ridge then overspreading Laura with a vast area of low shear and upper outflow...I do not show any strengthening through 72 hours from the anticipated land interactions. The reality is one of two things can happen...either Laura can regenerate a new center offshore of the northern Caribbean Islands and then rapidly strengthen under the favorable upper winds...or Laura may simply degenerate into a tropical wave from the land interactions. So my updated intensity forecast is of low confidence. After Laura moves into the Gulf of Mexico and away from land..I only cautiously raise the intensity of the tropical storm 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 whose upper outflow might impenge on Laura's western outflow. See bulletins at the home page of this site for an update on Laura's impact potential given this changed forecast situation.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Aug 21)...45 mph maximum sustained wind tropical storm centered over the northern Lesser Antilles at 16.8N-62.6W
IOH 24 Hr Forecast (1800Z Aug 22)...45 mph maximum sustained wind tropical storm centered between Puerto Rico and the Dominican Republic at 17.5N-68W
IOH 48 Hr Forecast (1800Z Aug 23)...45 mph maximum sustained wind tropical storm centered between northwest Haiti and Cuba at 19.5N-74W
IOH 72 Hr Forecast (1800Z Aug 24)...45 mph maximum sustained wind tropical storm centered over central Cuba at 21.5N-80W
IOH 96 Hr Forecast (1800Z Aug 25)...50 mph maximum susateind wind tropical storm centered in the eastern Gulf of Mexico at 25N-85.2W
IOH 120 Hr Forecast (1800Z Aug 26)...60 mph maximum sustained wind tropical storm centered in the Gulf of Mexico south of Louisiana at 27.5N-91W
TROPICAL DEPRESSION FOURTEEN (RECENTLY UPGRADED TO TROPICAL STORM MARCO)...Tropical depression fourteen moving into the western Caribbean Sea surprised me during the last 24 hours by not strengthening when it seemingly had a well organized structure with mutliple spiral cloud bands. Instead throughout the day on Friday it seemed to become elongated north to south...perhaps while developing a northern lobe of low pressure supported by the eastern divergence zone of the cut-off upper trough exteding into the Gulf of Mexico. Very recently an expanding circular thunderstorm complex within this northern lobe of low pressure has caused this system to strengthen into Tropical Storm Marco. It also appears this system is turning more northward sharply in track than previously anticipated while the northern lobe of the circulation appears to be becoming the dominant.
Going forward...Marco will be heavily influenced by the cut-off upper trough in the Gulf of Mexico...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 later in 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 consolidating toward the north side of its elongated circulation)...I now expect Marco to pass just northeast of instead of directly over the northeastern Yucatan peninsula while it moves northwestward in the short term. I then expect the track to bend increasingly westward in the middle of the forecast period as Marco gets wedged toward 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 by 120 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.
Regarding intensity...short term influences that may hamper the strengthening is the history of this system lacking a tight core of thunderstorms and southwesterly shear induced by the cut-off upper trough. My intensity forecast thus shows slow strengthening initially...followed by a gradual ramp up in the intensification rate as the upper trough shifts westward and away which will allow for a reduction in shear with time...and perhaps as this system establishes a tight core. There is one caveat though to this intensity forecast...it appears very recently a tight core may already be developing as the circular thunderstorm complex that has intensified this system into Tropical Storm Marco appears to be becoming better organized...and so this system may strengthen more rapidly than I show in my intensity forecast below. See bulletins at the home page of this site for an update on Marco's impact potential given this updated forecast which has some changes compared to yesterday.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Aug 21)...40 mph maximum sustained wind tropical depression centered in the western Caribbean Sea at 18.7N-84.9W
IOH 24 Hr Forecast (1800Z Aug 22)...50 mph maximum sustained wind tropical storm centered just offshore of the northeastern Yucatan peninsula at 21N-86W
IOH 48 Hr Forecast (1800Z Aug 23)...65 mph maximum sustained wind tropical storm centered over the central Gulf of Mexico at 25N-89W
IOH 72 Hr Forecast (1800Z Aug 24)...90 mph maximum sustained wind hurricane centered south of Louisiana at 28N-92W
IOH 96 Hr Forecast (1800Z Aug 25)...90 mph maximum sustained wind hurricane centered inland just north of Galveston Bay Texas at 30N-95W
IOH 120 Hr Forecast (1800Z Aug 26)...30 mph maximum sustained wind tropical depression centered over northeast Texas at 33N-96W
AREA OF INTEREST #1...The vigorous tropical wave of low pressure over the far eastern Atlantic is less organized than I previously anticipated. The vigorous thunderstorm cluster toward 10N latitude associated with this wave that I have been tracking turns out to be only the southwestern extent of the tropical wave...with a northeastern thunderstorm complex that has only recently emerged from the west coast of Africa and headed toward the vicinity of the Republic of Cabo Verde Islands also being a part of this tropical wave. Putting these two thunderstorm complexes together...this tropical wave likely has a southwest-to-northeast elongated circulation instead of a circular one. Thus I have lowered my 5-day peak odds of tropical cyclone formation from 70% issued in my special update last night to now 50%. Forecast positions in the outlook below is based on initializing this system at 10N-25W...a location just northeast of the southwestern thunderstorm complex as I assume the elongated circulation may consolidate toward this complex as it appears to be the more persistent and vigorous one. I have an initial more northward angle in track however for the next 24 hours...assuming the southwestern lobe of the circulation will be tugged toward the northeastern lobe while absorbing it...followed by a more westward track around the Atlantic subtropical ridge resuming after the absorption is complete. Upper-level winds are favorable for this tropical wave to develop with time...as it 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. The 1200Z GFS did show some of the current central Atlantic upper vorticity getting pushed southeastward toward the Atlantic tropics in the vicnity of 40W in 96 hours (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 as I expect it will be past 40W longitude by 96+ hours.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Aug 22)...10% chance of tropical cyclone formation (tropical Atlantic near 11.5N-30W)
IOH 48 Hr Outlook (1800Z Aug 23)...30% chance of tropical cyclone formation (tropical Atlantic near 12N-35W
IOH 72 Hr Outlook (1800Z Aug 24)...40% chance of tropical cyclone formation (tropical Atlantic near 12.5N-40W)
IOH 96 Hr Outlook (1800Z Aug 25)...50% chance of tropical cyclone formation (tropical Atlantic near 13N-45W)
IOH 120 Hr Outlook (1800Z Aug 25)...50% chance of tropical cyclone formation (tropical Atlantic near 13.5N-50W)
...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...moves across the northern Lesser Antilles...Virgin Islands...Puerto Rico...Domincain Republic...and Haiti as a weak tropical cyclone/tropical wave through 36 hours...moves across Cuba from 54 to 72 hours while becoming better organized...strengthens into a hurricane while moving northwestward across the eastern Gulf of Mexico and toward southeastern Louisiana by 108 hours...located over Mississippi in 120 hours while moving northward as a weakening inland tropical cyclone. For tropical depression fourteen...no tropical cyclone shown (fourteen initialized poorly at 0 hours in model run). For area of interest #1...no tropical cyclone formation shown. Elsewhere...large tropical wave exits western Africa in 66 hours...organizes into a large broad tropical low pressure over the Republic of Cabo Verde Islands and waters to the southwest in 90 hours...tropical low pressure located at 12.5N-36W in 126 hours.
1200Z ECMWF Model Run...For Tropical Storm Laura...passes over the northern Lesser Antilles...Virgin Islands...and just north of Puerto Rico in 24 hours with no strengthening shown...passes over Cuba from 48 to 72 hours and becomes better organized over the northwest coast of Cuba...strengthens into a compact hurricane while moving northwestward across eastern Gulf of Mexico and toward southeast Louisiana through 120 hours. For tropical depression fourteen...passes over the northeast Yucatan peninsula between 24 and 48 hours with no additional strengthening...intensifies into a compact stronger trpoical cyclone while moving northwestward toward the south Texas coast through 96 hours...fujiwhara interaction with Laura bends the track more westward into inland south Texas where system weakens rapidly after landfall by 120 hours. For area of interest #1...no tropical cyclone formation shown. Elsewhere...large tropical wave exits western Africa in 72 hours...organizes into a large broad tropical low pressure southwest of the Republic of Cabo Verde Islands in 120 hours.
1200Z GFS Model Run...For Tropical Storm Laura...passes over the northern Lesser Antilles...Virgin Islands...and over Puerto Rico in the next 24 hours with no strengthening shown...moves along north coast of Cuba from 54 to 72 hours with no strengthening shown...passes over Florida Keys with no strengthening shown at 78 hours...located south of Louisiana in 126 hours while remaining weak. For tropical depression fourteen...passes over the Yucatan peninsula from 42 to 54 hours as a disorganized low pressure...becomes better organized just northwest of the peninsula in 66 hours...due to fujiwahara interaction with Laura track is bent more westward into Texas/Mexico border in 102 hours...dissipated inland over the Texas/Mexico border in 120 hours. For area of interest #1...no tropical cyclone formation shown.
1200Z NAVGEM Model Run...For Tropical Storm Laura...passes over the northern Lesser Antilles...Virgin Islands...and over Puerto Rico in the next 24 hours with no strengthening shown...begins to pass between the Bahamas and Cuba in 48 hours...gradual strengthening shown while moving across Florida Keys in 72 hours...fujiwhara interaction with tropical depression fourteen to the west causes Laura to turn north across eastern Gulf of Mexico through 90 hours...makes landafll in Florida panhandle in 96 hours...remnant low over northeastern Georgia in 120 hours. For tropical depression fourteen...begins to strengthen and organize over the northeastern Yucatan peninsula in 42 hours...rapidly intensifies into a strong hurricane in central Gulf of Mexico in 72 hours...makes landfall over south-central Louisiana as an intense hurricane in 96 hours...begins weakening inland over northern Louisiana in 120 hours. For area of interest #1...no tropical cyclone formation shown. Elsewhere...large tropical wave exits western Africa in 72 hours...organizes into a large broad tropical low pressure west-southwest of the Republic of Cabo Verde Islands in 108 hours...tropical low located at 14.5N-30W in 120 hours.
Comments