*******Note that forecast 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 OCTOBER 8 2023 3:30 PM EDT...
Continuing to watch a deep-layer cyclone northwest of the Azores that could acquire tropical characteristics in the days ahead... see area of interest #43 section below for more details. See area of interest #44 section below for information on a tropical wave of low pressure in the eastern tropical Atlantic that continues to become better organized and is at high risk of tropical cyclone formation... any tropical cyclone that develops here is likely to move northwestward into the open central Atlantic in the long range.
For the Gulf of Mexico... within the next four days a tropical disurbance is likely to develop in the southwestern Gulf with the disturbance and heavy rainfall likely spreading northeastward toward southeastern Louisiana... southern Mississippi... southern Alabama... southern Georgia... and northern Florida by the end of the 4-day window:
(1) At the surface a cold front has arrived to the southwestern Gulf of Mexico while driven by the amplified upper trough over eastern North America. In addition a nearby eastern Pacific tropical low pressure is at risk of tropical cyclone formation and has been upgraded to potential tropical cyclone sixteen-E (refer to the official NHC eastern Pacific outlook at hurricanes.gov)... any eastern Pacific tropical cyclone that develops is likely to lift northward across southeastern Mexico and toward the southwestern Gulf of Mexico thanks to an upper trough that pivots into western North America from the Pacific. As such the current southwest Gulf surface front and remnants of the eastern Pacific feature would provide the surface low pressure field/spin to trigger a southwestern Gulf tropical disturbance.
(2) Southward cold air transport on the west side of the current eastern North America frontal cyclone will cause the cold core eastern North America upper trough to amplify into a southward-shifting vortex... and combined with the arrival of the second upper trough into western North America the current Mexico upper ridge will be forced southeastward into the northwestern Caribbean. Divergence on the northwest side of the Caribbean upper ridge will be aiding in the development of a southwestern Gulf tropical disturbance.
(3) Any disturbance that does develop in the southwestern Gulf will be accelerating northeastward toward the above-mentioned land areas of the US Gulf coastal region thanks to deep-layer southwesterly flow on the northwest side of the Caribbean upper ridge and out ahead of the western North America upper trough and surface frontal low pressure it will be generating. The strength of the upper southwesterly winds will likely mean excess wind shear for tropical cyclone formation... and the tropical disturbance could quickly transition into a non-tropical one should the cold front from western North America quickly arrive and overspread the disturbance. Therefore not declaring a tropical area of interest for the Gulf of Mexico in this update.
AREA OF INTEREST #43... The surface frontal cyclone northwest of the Azores has whirled southwestward to a position near the center of its parent upper vortex to make a deep-layer cyclone. The deep-layer cyclone is undergoing a gradual decay phase as the surface frontal cyclone slowly weakens below the core of the upper vortex where upper divergence is lacking. The deep-layer cyclone is forecast to continue a southwestward drift over the next 24 hours under the influence of warm core upper ridging to the west. By 48 hours the latest model runs indicate that a north fragment of the current eastern North American upper trough skips over the north side of the upper ridging and drags the deep-layer cyclone eastward... with the upper trough fragment then leaving behind the deep-layer cyclone at 72 hours. However the next high-latitude upper trough digs in from Greenland and drags the deep-layer cyclone northeastward after 72 hours.
Regarding odds of subtropical cyclone formation... the forecast track has this system arriving to 20 to 22 deg C water temps by 24 hours. Also the overhead upper vortex is expected to remain rather cold over the next four days to support instability and possible development of core thunderstorms and tropical characteristics (200 mb heights of the upper vortex are currently at around 1175 dekameters... only warming to 1185 dekameters through day 4 as the upper vortex remains isolated from fresh cold air injections... and looking for 1200 dekameters or less for tropical development in the 20 to 22 deg C sea surface temperature range). Because the core of the surface cyclone has not yet developed much shower and thunderstorm activity... I am not raising peak odds of subtropical cyclone formation above 30% in this update cycle... and reserve those peak odds for when the cyclone sees its warmest water temps. Odds of subtropical development will be adjusted accordingly in future updates when we see how the cyclone actually reacts when it arrives to the 20 to 22 deg C water temps. Between 48 and 72 hours the forecast track still keeps this system over 20 deg C water waters... however I begin to lower odds of subtropical development from the peak as the surface layer of the cyclone will have decayed below the lack of divergence at the upper vortex core as described in the prior paragraph if it has not undergone subtropical development by then. By 120 hours the upper-layer of the cyclone will have merged with the high-latitude upper trough that moves in from Greenland... with the merged upper trough leaving behind the surface layer of the cyclone and causing the surface cyclone's dissipation with its trailing western convergence zone. This is when I drop subtropical development odds to 0%.
Regardless of whether or not the deep-layer cyclone acquires tropical characteristics... expect a prolonged period of coastal surf for the Azores over the next three days. Coastal surf will likely decrease after day 3 as the surface layer of the cyclone decays.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Oct 9)... 30% chance of subtropical cyclone formation (northeast Atlantic near 42.5N-37.5W)
IOH 48 Hr Outlook (1200Z Oct 10)... 30% chance of subtropical cyclone formation (northeast Atlantic near 42.5N-35W)
IOH 72 Hr Outlook (1200Z Oct 11)... 20% chance of subtropical cyclone formation (northeast Atlantic near 42.5N-31W)
IOH 96 Hr Outlook (1200Z Oct 12)... 10% chance of subtropical cyclone formation (northeast Atlantic near 43N-27.5W)
IOH 120 Hr Outlook (1200Z Oct 13)... 0% chance of subtropical cyclone formmation (northeast Atlantic near 45N-20W)
******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************
Not in the official outlook
AREA OF INTEREST #44... The tropical wave of low pressure in the far eastern tropical Atlantic has continued to become better organized underneath the low shear and outflow of the tropical upper ridge in the region. With these favorable conditions to last another 48 hours... I have begun a tropical cyclone formation forecast as outlined below with a specific track and intensity projection. Although this system has been moving sluggishly toward the west thanks to the surface ridge weakness caused by area of interst (AOI) #43... its westward speed will soon pick up as the surface ridge over Europe will be expanded southwestward into the eastern Atlantic as the south side of the upper vortex associated with AOI #43 has evolved into an upper trough whose western convergence zone will initiate the southwestward expansion of the surface ridge. The eastern Atlantic surface ridge then remains intact for some time as the outflow over this system clashes with the outflow of the central Atlantic mid-latitude upper ridge to create a zone of upper convergence. As such a steady westward track around the surface ridge is shown through 48 hours. After that time an increase in the north angle of the westward track is expected due to (1) some energy from the current eastern North America upper trough which shifts toward the central Atlantic and creates a central Atlantic surface ridge weakness that attracts this system... (2) the central Atlantic mid-latitude upper ridge remains strong enough to break off the south part of the upper vortex of AOI #43 into another eastern Atlantic upper trough by 48+ hours... and assuming this system becomes strong/tall enough to couple with upper winds it will be dragged northwestward by this upper trough.
Regarding intensity... I forecast intensification through 48 hours as this system remains in favorable low shear and outflow below the tropical upper ridge in the region. After that time the forecast track takes this system into increased southwesterly shear under the east side of the upper trough that materializes at 48+ hours... followed by passage directly below the axis of the upper trough where a lack of divergence would oppress this system. I still forecast some strengthening between 48 and 72 hours as the shear is still on the lighter side... followed by weakening to a tropical depression from 72 to 120 hours from the inevitable increase in shear and then the oppressive environment directly below the upper trough axis. Assuming this system survives the oppressive environment below the upper trough axis... it could go on to linger as a tropical cyclone below the more favorable central Atlantic mid-latitude upper ridge for some time after day 5.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Oct 8)... Tropical low centered in the eastern tropical Atlantic at 7.5N-20.5W
IOH 24 Hr Forecast (1200Z Oct 9)... 35 mph maximum sustained wind tropical depression centered in the eastern tropical Atlantic at 8N-24.5W
IOH 48 Hr Forecast (1200Z Oct 10)... 50 mph maximum sustained wind tropical storm centered in the eastern tropical Atlantic at 9N-29.5W
IOH 72 Hr Forecast (1200Z Oct 11)... 60 mph maximum sustained wind tropical storm centered in the eastern tropical Atlantic at 10.5N-32.5W
IOH 96 Hr Forecast (1200Z Oct 12)... 45 mph maximum sustained wind tropical storm centered in the eastern tropical Atlantic at 13.5N-35W
IOH 120 Hr Forecast (1200Z Oct 13)... 35 mph maximum sustained wind tropical depression centered in the eastern tropical Atlantic at 15.5N-37.5W
******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT***************************
Formation chance through 48 hours... 40%
Formation chance through 7 days... 80%
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields(http://moe.met.fsu.edu/tcgengifs/).
0000Z (Oct 8) CMC Model Run...
**For area of interest #43... frontal cyclone swings south in a cyclonic loop and reaches 41.5N-34W at 48 hours while potentially acquiring tropical characteristics... subsequently weakens to a frontal low that drifts east to 41N-30.5W through 120 hours
**For area of interest #44... tropical wave organizes into a tropical low near 10.5N-24.5W at 36 hours... tropical low reaches 12.5N-38.8W at 120 hours.
**Through 72 hours upper ridge over Mexico shifts east into western Caribbean under influence of current eastern North American upper trough that evolves into an upper vortex and next upper trough that moves into western North America from the Pacific... tropical low develops in the southwestern Gulf of Mexico and just offshore of northern Veracruz at 84 hours... tropical low moves north-northeastward into southeastern Louisiana through 120 hours.
0000Z (Oct 8) ECMWF Model Run...
**For area of interest #43... frontal cyclone swings south in a cyclonic loop and reaches 41.5N-35W at 42 hours while potentially acquiring tropical characteristics... subsequently weakens to a frontal low that drifts northeast to 43.5N-26W through 114 hours where it becomes absorbed by another new frontal low to the northeast
**For area of interest #44... tropical wave organizes into a tropical low near 14.5N-35.5W at 120 hours
0600Z (Oct 8) GFS Model Run...
**For area of Interest #43... frontal cyclone swings south in a cyclonic loop and reaches 41.5N-34.5W at 45 hours while potentially acquiring tropical characteristics... frontal cyclone weakens to a frontal low while moving northeastward to 44.5N-27.5W through 120 hours where it then dissipates shortly thereafter
**For area of interest #44... compact tropical cyclone formation suggested near 9N-29W at 69 hours... tropical cyclone moves northwest to 14.5N-35W at 120 hours
**Through 72 hours upper trough energy that ejects into the central Atlantic from the eastern North America upper vortex produces a frontal low near 35.5N-53W... upper trough remains amplified enough while continuing east across central Atlantic such that frontal low intensifies into a frontal cyclone with potential core tropical characteristics through 120 hours near 35N-41W
0600Z (Oct 8) NAVGEM Model Run...
**For Area of Interest #43... frontal cyclone swings south in a cyclonic loop and reaches 42N-34.5W at 42 hours while potentially acquiring tropical characteristics... frontal cyclone subsequently weakens to an elongated frontal low that moves east to 41.5N-26W through 120 hours
**For area of interest #44... tropical cyclone formation suggested near 10.2N-25W at 42 hours... tropical cyclone moves northwest while becoming a hurricane and reaches 15.5N-34W at 120 hours
**Tropical wave emerges from west coast of Africa at 108 hours and organizes into a tropical low near 10.5N-20.5W at 138 hours... tropical low reaches 12.5N-26W at 168 hours