MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #115 (Weekend Edition)
*******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.**********
...SUNDAY SEPTEMBER 19 2021 12:45 AM EDT...
Satellite image as of 2050Z. Areas of interest circled in yellow are not mentioned in the NHC tropical weather outlook. Areas circled in a green dashed line are in the NHC 5-day tropical weather outlook. Areas circled in a solid green line are in the NHC 2-day tropical weather outlook:
NHC TAFB Surface Analysis 1200Z (Sept 18):
GFS Model Upper-Level Winds 1200Z (Sept 18):
The Atlantic tropics remain active with multiple
systems to watch:
**See Odette section below for an update on the tropical storm offshore of the northeastern US which has recently lost its tropical characteristics.
**See area of interest #1 section below for an update on the tropical wave of low pressure in the central tropical Atlantic that has recently been upgraded to Tropical Depression Sixteen… which could bring coastal sea swells to the northern Lesser Antilles by Monday and may angle toward Bermuda over the next few days.
**See area of interest #2 section below for an update on the tropical wave of low pressure in the eastern tropical Atlantic
Elsewhere… satellite imagery shows another tropical wave of low pressure with concentrated thunderstorms nearing the eastern tropical Atlantic from the west coast of Africa. Given the stout presence of eastern tropical Atlantic upper ridging with low shear and upper outflow… this tropical wave may soon become yet another area of interest.
TROPICAL STORM ODETTE (RECENTLY DOWNGRADED TO REMNANTS OF ODETTE)... As of 5 PM EDT… the NHC has declared Tropical Storm Odette… located offshore of the northeastern US coast… a non-tropical frontal cyclone as its thunderstorm mass has become well-separated from its swirl center due to ongoing westerly shear. Despite the shear… Odette has been able to slightly strengthen to 45 mph maximum sustained winds… probably an indication Odette is obtaining non-tropical support in the form of upper divergence ahead of the eastern Canada upper trough and southeastern US upper vorticity… both of which are approaching. The two approaching upper-level features are expected to merge into an amplifying upper trough. This amplification is expected as adjacent warm upper ridging to the west amplifies in the warm sector of the frontal system now over western North America. After the models were widely spread on the evolution of the upper trough yesterday… today they are beginning to converge on the upper trough amplifying into a cut-off upper vortex around 40N latitude and 45 to 50W longitude (with the exception of the GFS which is a northeastern outlier… but not nearly as far to the northeast as yesterday). My previous forecast track lies near the emerging model consensus… and so my updated one is the same except with a north and east adjustment to account for the current position of Odette.
The intensity forecast below ramps up through 48 vhours as the tremendous eastern divergence zone of the amplifying upper trough aids Odette. A south drift is assumed at days 3 to 5 as the upper vortex is pushed south by the adjacent amplified upper ridging to the west. During this time I forecast the intensity to taper down as Odette whirls beneath the center of the upper vortex where upper divergence is lacking… and assume the south drift toward warmer 26 deg C waters will allow Odette to re-acquire some tropical characteristics by days 4 and 5. Because of this forecast… I will continue forecasts on Odette in future updates even though it is no longer a tropical feature at present. Odette will bring coastal sea swells to the mid-Atlantic US… northeastern US and Atlantic Canada shorelines over the next few days.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 18)... 45 mph maximum sustained wind frontal cyclone centered at 39.1N-65.1W
IOH 24 Hr Forecast (1800Z Sep 19)... 60 mph maximum sustained frontal cyclone centered at 42N-57.5W
IOH 48 Hr Forecast (1800Z Sep 20)... 70 mph maximum sustained frontal cyclone centered at 43N-50W
IOH 72 Hr Forecast (1800Z Sep 21)... 65 mph maximum sustained frontal cyclone centered at 40.5N-49.5W
IOH 96 Hr Forecast (1800Z Sep 22)… 60 mph maximum sustained subtropical storm centered at 39.5N-49W
IOH 120 Hr Forecast (1800Z Sep 23)… 50 mph maximum sustained subtropical storm centered at 39N-49W
AREA OF INTEREST #1 (RECENTLY UPGRADED TO TROPICAL DEPRESSION SIXTEEN)… The tropical wave of low pressure in the central Atlantic has continued to organize with an increase in thunderstorms with its low pressure spin. ASCAT-B and ASCAT-C descending passes suggested an oblong circulation near 14N-51W with 30 knot (35 mph) max sustained winds on its northeast side… with this circulation located just south of the organizing thunderstorm activity due to southerly shear from the central Atlantic upper vorticity. Satellite animation at 1800Z however suggested a circulation more embedded in the thunderstorms near 16N-52W… perhaps indicating the ASCAT passes were a bit outdated or the north part of the oblong circulation seen in the passes had consolidated into a circular spin more coupled with the thunderstorms. The NHC as of 11 PM EDT has upgraded this system to tropical depression sixteen with a center fix at 16.4N-53.1W… however this center fix applies after 1800Z. Therefore for my forecasting purposes… I assumed the center at 1800Z was 16N-52W.
For the next 96 hours I forecast a north angle in the west track of this system. It does make sense to have a north angle due to a myriad of factors including Odette’s surface ridge weakness… as the western end of the Atlantic surface ridge will be weakened further with the approach of the frontal system that is currently over western North America by day 4… and upper southwesterly flow ahead of the central Atlantic upper vorticity as this system is becoming stronger/taller to be guided by upper winds as it has now evolved into a tropical cyclone. Even though this upper vorticity is slated to fade away while cut-off from high-latitude cold air… the upper trough/vortex to be tied with Odette could leave behind upper vorticity in the west Atlantic by day 3 that would prolong the upper southwesterly flow. A direct turn to the north is shown by day 5 once Odette and incoming North American frontal system combine to nearly collapse the west end of Atlantic surface ridging. Regarding forecast track adjustments… I have only adjusted the short-term forecast north due to the current center fix of the depression… but have kept the medium range forecast track the same as Odette is expected to be further east around 50W longitude which should keep the surface ridging in the west Atlantic stronger to prevent further north positions in track in the medium term.
My intensity forecast is a continuity of my previous… and assumes this system could reach 50 mph max sustained winds before the shear worsens as this system moves closer to the central Atlantic upper vorticity. Even though this shear may be prolonged by upper vorticity deposited by the upper trough/vortex to be tied to Odette… the GFS does hint that by days 4 and 5 this round of upper vorticity will also weaken while remaining cut-off from high-latitude cold air. As a result I suggest possible additional strengthening on day 5.
Given the current forecast track... interests in the northern Lesser Antilles will likely see coastal sea swells by Monday. Interests in Bermuda should be aware of this system as Odette may help to pull the track northeast in the direction of Bermuda after day 5.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 18)... 35 mph maximum sustained wind tropical depression centered at 16N-52W
IOH 24 Hr Forecast (1800Z Sep 19)... 45 mph maximum sustained wind tropical storm centered at 18N-56W
IOH 48 Hr Forecast (1800Z Sep 20)... 50 mph maximum sustained wind tropical storm centered northeast of the northern Lesser Antilles at 20N-60W
IOH 72 Hr Forecast (1800Z Sep 21)... 50 mph maximum sustained wind tropical storm centered north of the northern Lesser Antilles at 21.5N-63W
IOH 96 Hr Forecast (1800Z Sep 22)… 50 mph maximum sustained wind tropical storm centered east-northeast of the eastern Bahamas at 23.5N-67.5W
IOH 120 Hr Forecast (1800Z Sep 23)… 60 mph maximum sustained wind tropical storm centered at 25N-68.5W
AREA OF INTEREST #2… The vigorous tropical wave of low pressure in the eastern Atlantic has continued to become organized as outer thunderstorm bands and central bursts fill the rotating circulation. Therefore I have raised my peak development odds for this wave to 90% (the NHC has also recently increased development odds to 70%).
Due to the current position of the center of spin which passed 11N-27W earlier this evening… my updated forecast track is adjusted east. The forecast track is for a gradually increasing north angle toward the surface ridge weakness being induced by Odette… which will settle in the vicinity of 50W in the coming days as Odette nearly stalls. Upper vorticity that has been left behind in the North Atlantic by the current northeast Atlantic upper trough is forecast to near this system by 3 days. The latest GFS sweeps this upper vorticity to the east and away due to the push of the upper vortex to be tied to Odette. However given that the GFS is an eastern outlier with Odette… I think it may be overdoing the east shift of the upper vorticity… thus I think shear from this upper vorticity may still affect this system. Thus I slightly lower development odds in the 3 to 5 day window.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 19)... 90% chance of tropical cyclone formation (eastern tropical Atlantic near 12N-31W)
IOH 48 Hr Outlook (1800Z Sep 20)... 90% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-34W)
IOH 72 Hr Outlook (1800Z Sep 21)... 80% chance of tropical cyclone formation (eastern tropical Atlantic near 14.5N-38W)
IOH 96 Hr Outlook (1800Z Sep 22)… 80% chance of tropical cyclone formation (central tropical Atlantic near 16.5N-41W)
IOH 120 Hr Outlook (1800Z Sep 22)… 80% chance of tropical cyclone formation (central tropical Atlantic near 18.5N-44W)