MY 2020 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #109
Updated: Sep 3
*******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.**********
...TUESDAY SEPTEMBER 1 2020 3:35 PM EDT...
See tropical depression fifteen and Tropical Storm Nana sections below for the two currently active tropical cyclones in the Atlantic basin. See area of interest #2 section below for the west Africa tropical wave of low pressure being monitored for development.
Elsewhere...the National Hurricane Center has re-introduced the central Atlantic tropical wave of low pressure now at 40W in their outlook as I was putting this birdseye view post together as it has seen some increase in thunderstorms and spin. Therefore it is not marked in the above birdseye view chart as an area of interest...but in my next post I will re-introduce it as an area of interest should its spin and thunderstorms persist or increase...or if it continues to remain in the National Hurricane Center outlook. See area of interest #2 section below for additional remarks on the wave at 40W.
AREA OF INTEREST #1 (RECENTLY UPGRADED TO TROPICAL STORM NANA)...The tropical wave of low pressure crossing through the central Caribbean Sea has continued become better organized with a well-defined spiral of thunderstorm bands as it passed south of Jamaica. Surface observations earlier in the morning had indicated the system had reached tropical storm force but did not yet have a fully closed surface circulation...so there was a breif window of time where this system was tagged by the National Hurricane Center as potential tropical cyclone sixteen to get tropical storm watches up for the north coast of Honduras and the coast of Belize early. Aircraft reconaissance around noon EDT found that the potential tropical cyclone had a fully closed circulation with 50 mph maximum sustained winds...thus it was upgraded promptly to Tropical Storm Nana. This tropical storm is the earliest fourteenth named storm on record in the Atlantic basin...beating Nate from 2005 which formed on September 5th...thus continuing the hyperactive pace of the 2020 Atlantic Hurricane Season!
My updated forecast track below is adjusted westward due to the current position of Nana. The tropical storm will remain in between a lobe of upper vorticity to the west currently over the western Caribbean and a much larger lobe to the northeast over the central Atlantic. The 1200Z GFS guidance today and yesterday has trended with a faster northwest motion for the western lobe of vorticity as it tracks toward the upper trough associated with the central Canadian frontal cyclone...and has insisted that a piece of upper vorticity from the lobe to the northeast will be slingshotted southwestward toward Nana by the strong Gulf of Mexico upper ridge. Putting this picture together along with the National Hurricane Center forecast which has a south bend in the track in the long range...I have adjusted my longer term forecast points southward as I believe Nana will be strong/tall enough to be dragged potentially southward by the piece of upper vorticity to approach from the northeast...and also the upper vorticity to the west now seems it will be too far northwest of Nana to drag it north. However my updated forecast track just has a straight west component in the long range instead of a southward bend as the 1200Z GFS insists by 72 hours that the piece of upper vorticity to the northeast will be dissipating while starved of higher latitude colder air.
Regarding strength...with 50 mph maximum sustained winds Nana is following my previous intensity forecast from birdseye view post #107...so I have mostly reverted back to that forecast. However unlike that forecast...I show minimal hurricane strength at landfall time for Belize/the Yucatan peninsula which is in agreement with the National Hurricane Center advisory package from 1 PM EDT. Even though Nana looks very well organized on satellite..none of the computer models in the model summary below show Nana developing...perhaps because of the piece of upper vorticity to approach from the northeast which could potentially disrupt the storm. So I think only cautiously strengthening Nana to minimal hurricane strength is a good idea for now given the signal from many of the models that refuse to strengthen this system. See bulletins at the home page of this site for the impact potential of Tropical Storm Nana given this current outlook.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 1)...50 mph maximum sustained wind tropical storm centered south of Jamaica at 16.6N-77.9W
IOH 24 Hr Forecast (1200Z Sep 2)...60 mph maximum sustained wind tropical storm centered southwest of the Cayman Islands at 17N-83W
IOH 48 Hr Forecast (1200Z Sep 3)...75 mph maximum sustained wind hurricane approaching landfall on the north coast of Belize at 17N-88W
IOH 72 Hr Forecast (1200Z Sep 4)...Remnant low pressure centered inland just south of the Bay of Campeche coast of Mexico at 17N-93W
TROPICAL DEPRESSION FIFTEEN...The tropical surface low pressure offshore of the southeastern United States that formed in thunderstorms just out ahead of a cold front strengthened into tropical depresion fifteen yesterday afternoon...but the depression has not strengthened further since that time while dealing with westerly vertical shear induced by a shortwave upper trough passing to the north that originated from the central Canada upper trough and passed over Illinois yesterday. Going forward...the shear will be relaxing as the shortwave moves away and the warm upper ridge in the Gulf of Mexico gets expanded into the northwest Atlantic thanks to the warm sector of the surface frontal cyclone approaching from cenral Canada. Therefore I forecast brisk strengthening to a moderate to strong tropical storm in the next 24 hours...then hold that strength steady through 72 hours as my updated forecast track takes this system along the north edge of the warm Gulf stream where cooler waters may dampen additional strengthening potential. After 72 hours the cold of the Canadian frontal cyclone should approach and absorb this system...and the forecast track takes this system into much cooler waters. Thus I forecast rapid weakening with absorption by the front at 96 hours. It should be noted this intensity forecast is higher than I had yesterday while I hold peak strength for longer...as the 1200Z GFS no longer has a second shortwave upper trough heading toward this system and shearing it (instead this second shortwave is now shown to curve northward around the west side of the upper ridge currently over eastern Canada). Previously this second shortwave trough was also expected to catch up to the first one and amplify into an upper vortex to the east of the depression...potentially producing a subtropical cyclone east of the depression in the GFS modeling. Now the first shortwave upper trough is shown in the GFS to be weaker to the east of the depression without the re-enforcement of the second...resulting in a weak/broad surface low instead of a subtropical cyclone...thus I have not added a new area of interest to the east of the depression.
Regarding the track forecast...the depression's track is curving more eastward rather than northward around the northwest Atlantic surface ridge...perhaps as the shortwave upper trough passing to the north is dropping surafce pressures south of the surface ridge and to the east of the depression...giving the depression a pocket of low pressure to travel toward. Thus my updated track forecast is adjusted southward...keeping the system over warmer waters longer which is another reason for my higher intensity forecast. I accelerate the forward speed of the depression after 24 hours as the blocking northwest Atlantic surface ridge shifts eastward with the parent western convergence zone of the north Atlantic upper trough helping to generate it. By 72+ hours the depression will run into the more prominent Atlantic subtropical surafce ridge...so I bend the track more northward by that timeframe.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Sep 1)...35 mph maximum sustained wind tropical depression centered offshore of the southeastern United States at 34.7N-73.1W
IOH 24 Hr Forecast (1200Z Sep 2)...60 mph maximum sustained wind tropical storm centered in the western Atlantic at 36N-71W
IOH 48 Hr Forecast (1200Z Sep 3)...60 mph maximum sustained wind tropical storm centered in the northwestern Atlantic at 37.5N-66W
IOH 72 Hr Forecast (1200Z Sep 4)...60 mph maximum sustained wind tropical storm centered in the northwestern Atlantic at 40N-61W
IOH 96 Hr Forecast (1200Z Sep 5)...Absorbed by cold front while passing south of Newfoundland at 44N-59W
AREA OF INTEREST #2...A tropical wave of low pressure that was over interior western Africa is now approaching the west coast of Africa and eastern tropical Atlantic while producing a circular thunderstorm mass west of Mali. To the west of this wave...curved thunderstorm bands south of the Republic of Cabo Verde Islands have increased...perhaps a low pressure left behind by the tropical wave currently at 27W? In addition the National Hurricane Center has just re-introduced the tropical wave now at 40W in their tropical weather outlook as it has shown some increase in spin and thunderstorms. Today's modeling seems to agree that the wave at 40W and this wave will orbit each other in a fujiwhara type of interaction...keeping the wave at 40W 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 of the two waves will develop...with the GFS and NAVGEM preferring the wave at 40W...and the CMC and ECMWF preferring this tropical wave.
My updated forecast track for this wave in the outlook below has a more northward angle as it appears likely to pivot more northward while attempting to absorb what appears to be a low pressure feature south of the Republic of Cabo Verde Islands and then possibly absorb or interact with the wave at 40W. I have raised my peak 5-day odds of tropical cyclone formation from 25% yesterday to 50% today as there is plenty of thunderstorm activity across this wave and the low pressure feature south of the islands...and also the dry saharan air layer today appears to be less prevalent than it has been over the last several days.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Sep 2)...0% chance of tropical cyclone formation (east-southeast of the Republic of Cabo Verde Islands near 15N-20W)
IOH 48 Hr Outlook (1200Z Sep 3)...20% chance of tropical cyclone formation (over the southwestern Republic of Cabo Verde Islands near 15.5N-25W)
IOH 72 Hr Outlook (1200Z Sep 4)...30% chance of tropical cyclone formation (tropical Atlantic near 16N-30W)
IOH 96 Hr Outlook (1200Z Sep 5)...40% chance of tropical cyclone formation (tropical Atlantic near 16.5N-35W)
IOH 120 Hr Outlook (1200Z Sep 6)...50% chance of tropical cyclone formation (tropical Atlantic near 17N-40W)
...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 (recently upgraded to Tropical Storm Nana)...no tropical cyclone formation shown (area of interest poorly initialized at 0 hours). For tropical depression fifteen...loses identity offshore of Nova Scotia in advance of frontal cyclone approaching from central Canada in 84 hours. For area of interest #2...tropical cyclone formation suggested just offshore of western Africa near 12N-19W in 48 hours...passes over the Republic of Cabo Verde Islands in 84 hours...located at 17.5N-35W at 120 hours.
0000Z ECMWF Model Run...For area of interest #1 (recently upgraded to Tropical Storm Nana)...organizes into tropical low pressure offshore of Nicaragua and Honduras in 48 hours...dissipates after landfall by 72 hours (area of interest poorly initialized at 0 hours). For tropical depression fifteen...weakens to a surface trough near 38N-55W in 96 hours. For area of interest #2...organizes into a tropical low pressure over the Republic of Cabo Verde Islands in 72 hours...tropical cyclone formation suggested by 96 and 120 hours (located at 18.5N-38.5W at 120 hours).
1200Z GFS Model Run...For area of interest #1 (recently upgraded to Tropical Storm Nana)...low pressure area dissipates by 18 hours. For tropical depression fifteen...opens to a surface trough near 37.5N-64W in 66 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...tropical wave near 40W nearly stationary for a few days while performing a fujiwhara interaction with area of interest #2...tropical cyclone formation suggested from this wave near 13N-40W by 144 hours.
0600Z NAVGEM Model Run... For area of interest #1 (recently upgraded to Tropical Storm Nana)...no tropical cyclone formation suggested (area of interest poorly initialized at 0 hours). For tropical depression fifteen...loses its identity ahead of large frontal cyclone approaching from central Canada while located offshore of Newfoundland in 90 hours. For area of interest #2...no tropical cyclone formation shown. Elsewhere...tropical wave near 40W drifts eastward toward area of interest #2 in a fujiwhara type interaction...organizes into a tropical low pressure near 11.5N-33W in 60 hours...located at 14N-29W in 120 hours.