*******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 SEPTEMBER 2 2020 5:32 PM EDT...
See Tropical Storm Nana and Tropical Depression Omar sections below for the two currently active tropical cyclones in the Atlantic basin. See area of interest section below for tropical low pressure areas being watched for development. Elsewhere...models generally agree that another vigorous tropical wave of low pressure capable of development will emerge from western Africa in about 4 days.
TROPICAL STORM NANA...Since special udpate #109A from early this morning...Nana stopped strengthening above 60 mph maximum sustained winds. This is due to the effects of northerly wind shear induced by the approach of upper vorticity to the northeast. One thing I am concerned with Nana is whether or not a zone of split flow upper divergence will setup between the upper vorticity to the northeast and Gulf of Mexico upper ridge to the north...if so this could enhance Nana's thunderstorms depsite the shear which may allow for a run at category 1 hurricane strength. Interests in Belize should be rushing preparations to completion as if a category 1 hurricane is approaching as it wouldn't take much more strengthening to make Nana a hurricane. I will note that the central pressure has climbed from 998 to 999 mb between 11 AM to 2 PM...so at this point I am not forecasting Nana to become a hurricane. My 24 hour track forecast point is already inland at the Belize/Guatemala border...so I show a low intensity by that point due to land interaction. My track forecast points are adjusted westward from my previous forecast as Nana is a bit further west of my previous forecast. In the model summary below...only the GFS suggests Nana will get dragged southward by the shearing upper northerly winds. Thus I continue to forecast Nana will continue steadily west at 17N latitude.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 2)...60 mph maximum sustained wind tropical storm centered north of Honduras at 17.1N-85.1W
IOH 24 Hr Forecast (1800Z Sep 3)...40 mph maximum sustained wind tropical storm centered on the northern Belize/Guatemala border at 17N-89.5W
IOH 48 Hr Forecast (1800Z Sep 4)...Remnant low pressure dissipating just south of the Bay of Campeche coast of Mexico at 17N-94W
TROPICAL STORM OMAR (RECENTLY DOWNGRADED TO TROPICAL DEPRESSION OMAR)...Tropical depression fifteen shortly after yesterday's birdseye view post strengthened into Tropical Storm Omar...the earliest fifteenth named storm on record in the Atlantic basin while beating Ophelia of 2005 which became a named storm on September 7...thus continuing the hyperactive pace of the 2020 Atlantic Hurricane Season. Omar has not been a photogenic tropical storm...thanks to its entanglement with a frontal low pressure to the east forming with the support of the shortwave upper trough that has been passing Omar to the north. Instead of continuing more northward around the west side of the northwest Atlantic surface ridge...Omar instead has been tracking more eastward toward the developing frontal low pressure...requiring me to once again shift my forecast track points southeastward. The more east position of Omar places the tropical storm under northwesterly shearing winds on the back side of the shortwave upper trough which have been blasting Omar's thunderstorms southeastward from its cloud swirl center...thus I have lowered my intensity forecast as well...especially as the National Hurricane Center just downgraded Omar back down to tropical depression status. The northerly shear will continue for the next 48 hours as Omar first track eastward and then northeastward while continuing to follow the frontal low pressure forming to the east...with the frontal low bending more northward in track while hitting the northwest side of the Atlantic subtropical ridge. By 72+ hours...Omar itself will continue more northward in track while it too hits the northwest side of the Atlantic subtropical ridge...and the shear direction should switch from northerly to southwesterly thanks to the approach of the upper trough currently over central Canada.
On a final note...I assume for the next 48 hours Omar will hang on to tropical depression status as it has been hanging on to tropical cyclone status despite the shear over the last 24 hours...but it wouldn't surprise me if the shear dissipates Omar early...or if a zone of upper convergence develops on the back side of the shortwave upper trough as we typically see with mid-latitude troughs which may also kill off Omar early. I then weaken Omar to a remnant low from the expected southwesterly shear and cooler waters by 72 hours.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 2)...35 mph maximum sustained wind tropical depression centered in the western Atlantic at 36.2N-67.4W
IOH 24 Hr Forecast (1800Z Sep 3)...35 mph maximum sustained wind tropical depression centered in the western Atlantic at 36N-60.5W
IOH 48 Hr Forecast (1800Z Sep 4)...35 mph maximum sustained wind tropical depression centered in the northwestern Atlantic at 37.5N-57.5W
IOH 72 Hr Forecast (1800Z Sep 5)...Remnant low pressure centered in the northwestern Atlantic at 41N-54.5W
AREA OF INTEREST #1...A tropical wave of low pressure that was over interior western Africa is now exiting the west coast of Africa and entering the eastern tropical Atlantic while merging with a low pressure area near 12.5N-22.5W. I speculate that this low pressure area was left behind by the tropical wave now at 35W longitude. The merger between these two systems is creating a large area of curved shower and thunderstorm bands to the south of the Republic of Cabo Verde Islands. My outlook in the next 24 hours assumes this disturbance will generally be stationary near 12.5N-22.5W while the tropical wave absorbs the low pressure area. After that time...the models agree that the tropical low pressure near 13N-37W...area of interest #2...and this wave will orbit each other in a fujiwhara type of interaction...keeping area of interest #2 generally stationary for a few days while this system tracks more northward in its westward track. The disagreement in the modeling has more to do with which feature will develop...with the GFS preferring area of interest #2...and the CMC...ECMWF...and NAVGEM preferring this tropical wave. Albiet the ECMWF develops both this wave and area of interest #2...but assumes this system will dominate over area of interest #2 once both are tropical cyclones. My forecast track in the outlook below between 48 to 72 hours bends the track of this system northward as it undergoes a fujiwhara interaction with area of interest #2...followed by a more westward track resuming after the intraction is complete. I retain peak 5-day odds of tropical cyclone formation at 50%...and have short-term odds at a very low 10% through 72 hours to give time for this large broad disturbance to potentially absorb area of interest #2...and then consolidate after the potential absorption.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 3)...10% chance of tropical cyclone formation (south-southeast of the Republic of Cabo Verde Islands near 12.5N-22.5W)
IOH 48 Hr Outlook (1800Z Sep 4)...10% chance of tropical cyclone formation (southwest of the Republic of Cabo Verde Islands near 13N-27.5W)
IOH 72 Hr Outlook (1800Z Sep 5)...10% chance of tropical cyclone formation (tropical Atlantic near 14.5N-32.5W)
IOH 96 Hr Outlook (1800Z Sep 6)...30% chance of tropical cyclone formation (tropical Atlantic near 15N-37.5W)
IOH 120 Hr Outlook (1800Z Sep 7)...50% chance of tropical cyclone formation (tropical Atlantic near 15.5N-42.5W)
AREA OF INTEREST #2...A tropical low pressure spin has become increasingly defined near 13N-37W...and I speculate it has been left behind by the tropical wave now crossing 45W longitude. This feature remains in the National Hurricane Center tropical weather outlook since yesterday afternoon while bursts of thunderstorms continue to occur on the northwest side of the low pressrue swirl. The thunderstorms are being sheared off to the northwest quadrant due to easterly upper winds on the south side of the tropical upper ridge in the region...normally shear does not occur here during the peak of the hurricane season in September as tropical systems tend to move westward and keep up with the upper easterly winds. Instead this low pressure swirl is generally stationary and drifting east-northeastward while gravitated toward the much larger low pressure system mentioned in area of interest #1. Because this system is the smaller of the two disturbances and is getting sheared by its generally stationary motion...I have only low 10% odds of tropical cyclone formation while assuming the larger disturbance in area of interest #1 will absorb it before it has a chance to develop.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 3)...10% chance of tropical cyclone formation (tropical Atlantic near 13N-36.5W)
IOH 48 Hr Outlook (1800Z Sep 4)...10% chance of tropical cyclone formation (tropical Atlantic near 13.5N-36W)
IOH 72 Hr Outlook (1800Z Sep 5)...0% chance of tropical cyclone formation (absorbed by area of interest #1 near 13.5N-35.5W)
...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 Nana...weakens while approaching landfall in northern Belize over the next 24 hours...dissipates shortly after 24 hours. For Tropical Storm Omar...opens to a surface trough near 37.5N-57.5W in 72 hours. For area of interest #1...absorbs area of interest #2 while passing near 17.5N-36W in 84 hours...tropical cyclone formation also suggested by 84 hours...located at 16N-41.5W in 120 hours. For area of interest #2...generally stationary near 11N-37.5W till it is absorbed by area of interest #1 in 84 hours.
1200Z ECMWF Model Run...For Tropical Storm Nana...makes landfall in northern Belize by 24 hours...dissipates shortly thereafter. For Tropical Storm Omar...weakens to a surface trough near 37N-58W in 72 hours. For area of interest #1...passes north of area of interest #2 thorugh 96 hours...tropical cyclone formation suggested near 17.5N-47.5W in 120 hours. For area of interest #2...generally stationary near 11N-37.5W through 72 hours...organizes into a very compact tropical cyclone near 17.5N-40W by 144 hours. Elsewhere..tropical wave of low pressure emerges from western Africa by 96 hours...tropical cyclone formation suggested just east of the Republic of Cabo Verde Islands near 17N-20.5W in 120 hours.
1200Z GFS Model Run...For Tropical Storm Nana...makes landfall in southern Belize in 24 hours...dissipates shortly thereafter. For Tropical Storm Omar...opens to a surface trough near 40N-54W in 84 hours. For area of interest #1...no tropical cyclone formation shown. For area of interest #2...tropical cyclone formation suggested near 12.5N-40W in 150 hours. Elsewhere...tropical wave of low pressure emerges from western Africa by 96 hours...tropical cyclone formation suggested just east of the Republic of Cabo Verde Islands near 16.5N-21W in 114 hours.
1200Z NAVGEM Model Run... For Tropical Storm Nana...makes landfall in northern Belize by 18 hours...loses identity over western Guatemala/Mexico border in 36 hours. For Tropical Storm Omar...opens to a surface trough near 39N-54W in 60 hours. For area of interest #1...tropical cyclone formation suggested near 17.5N-30W in 90 hours...located at 15N-34W in 120 hours. For area of interest #2...drifts east-northeastward toward area of interest #1 and becomes absorbed by area of interest #1 by 84 hours. Elsewhere...tropical wave currently at 59W organizes into a tropical low pressure just south of western Cuba in 126 hours...tropical cyclone formation suggested in long range as this system moves northwestward into the Gulf of Mexico.