*******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.**********
...TUESDAY AUGUST 29 2023 2:45 PM EDT...
This update... originally scheduled for the overnight... has been delayed into this afternoon due to a myriad of active tropical cyclones and areas of interest in addition to a busy past couple of days personally for me.
Three tropical cyclones are currently active in the Atlantic basin as follows:
(1) Major Hurricane Franklin is beginning to accelerate northeastward on a path to pass Bermuda just to the north and will produce wide-reaching coastal surf for Bermuda... the US east coast from the mid-Atlantic to the northeast shoreline... and Atlantic Canada (Nova Scotia and Newfoundland) over the next few days. See Franklin section below for more information.
(2) Idalia is now a briskly strengthening hurricane in the eastern Gulf of Mexico and will produce impacts to a large swath of the southeastern United States. The coastal storm surge and wind impacts to the northwestern part of the Florida peninsula and far eastern Florida panhandle are expected to be life-threatening. See Idalia section below for more information.
(3) The central Atlantic tropical low pressure is now tropical depression eleven... see TD Eleven section below for more information.
In addition... looking for possible development from other features as follows:
(1) See area of interest #27 section below for more information on a tropical low pressure area to materialize in the eastern Atlantic and over the Republic of Cabo Verde Islands as two tropical waves departing from Africa merge.
(3) See area of interest #28 section below for notes on another tropical wave of low pressure which has seen an increase in thunderstorm activity while moving across the southern Lesser Antilles... this wave may later bring disturbed weather to Jamaica... the Cayman Islands... Belize... and the Yucatan region of Mexico in the 3 to 5 day range.
Note the forecasts for each system below were completed at 0600Z or 1200Z earlier today... any changes that have occurred since then are noted in each system's section.
MAJOR HURRICANE FRANKLIN... As is often seen with compact tropical cyclones in a favorable environment (warm sea surface temps... upper outflow... and low shear)... Franklin was able to rapidly intensify and achieved a category 4 peak of 150 mph maximum sustained winds with 926 mb central surface pressure as of 11 PM EDT Monday. For instance a tight and well-defined surface low pressure center allows for surface convergence and thunderstorms to be concentrated... and compact tropical cyclones have a tight surface pressure gradient such that only a smaller drop in pressure is needed to increase the wind speed. The hurricane has begun its northward and northeastward turn in the flow ahead of an upper trough approaching from the Great Lakes region of North America... but at a further east position than my previous forecast... and so my updated one shown below is adjusted accordingly. Weakening is forecast going forward as stronger southwesterly flow associated with the incoming upper trough imparts disrupting wind shear. I forecast transition into a non-tropical frontal cyclone by 72 hours as Franklin merges with the cold front associated with the upper trough and becomes supported by the upper divergence on the trough’s east side.
Regarding impact to land areas:
(1) Bermuda will be subjected to notable coastal surf by tomorrow and Thursday as the hurricane passes to the north. As of this writing a tropical storm warning is in effect in case a southward deviation in the track occurs which would allow Bermuda to see gusty winds from Franklin's south side during this timeframe.
(2) The mid-Atlantic and northeastern US coast... as well as the shores of Atlanic Canada (Nova Scotia and Newfoundland)... will see coastal surf over the next 48 hours.
Update as of 11 AM EDT... Franklin has weakened to 130 mph maximum sustained winds while satellite imagery indicates an eyewall replacement cycle is underway.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0600Z Aug 29)... 145 mph maximum sustained wind major hurricane centered over the western Atlantic at 29.8N-71W
IOH 24 Hr Forecast (0600Z Aug 30)... 125 mph maximum sustained wind major hurricane centered west of Bermuda at 32N-69W
IOH 48 Hr Forecast (0600Z Aug 31)... 100 mph maximum sustained wind hurricane centered just north-northeast of Bermuda at 35.5N-63W
IOH 72 Hr Forecast (0600Z Sep 1)... Hurricane-force remnant frontal cyclone centered south-southeast of Newfoundland at 41.5N-51W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 AM EDT**************************
5-day Position (0600Z Sep 3)... 60 mph maximum sustained wind frontal cyclone centered in the northeast Atlantic at 59.1N-27.5W
HURRICANE IDALIA... Idalia has arrived into the southeastern Gulf of Mexico after crossing the western tip of Cuba while being pulled northward by a lengthy upper trough spanning the Great Lakes region of North America to the northern Gulf of Mexico. The thunderstorm activity has pivoted from being lopsided south-of-center to now east-of-center due to Idalia's position relative to the trough... with the trough now restricting the storm's western outflow. However the thunderstorm core is covering the center enough to strengthen Idalia... and as of 5 AM EDT Idalia is now a category 1 hurricane with 75 mph maximum sustained winds... and as of 2 PM EDT Idalia has now reached 90 mph maximum sustained winds. Noting that during the forecast period the north part of the lengthy upper trough continues east toward Hurricane Franklin while the south part stays behind while pinned in place by an amplifying warm core western US upper ridge induced by warm southerly flow ahead of a western US frontal system (the western US surface frontal system will materialize over the next 24 hours while induced by the divergence zone of the current northeastern Pacific upper vortex slated to make landfall). Shear imparted by the southern fragment of the upper trough is expected to lessen as the western US upper ridge will be amplified enough to cause the southern upper trough fragment to slightly retrograde westward and more out of the way... and Idalia is expected to intensify briskly before it makes landfall in the southeastern United States through the northwestern Florida peninsula or far eastern Florida panhandle. Even though Idalia is behind the previous intensity forecast... it is making up for it now by its recent brisk strenghtening rate and so my peak intensity forecast is the same as in special update #64B. Also noting I have nudged the pre-landfall track of Idalia westward due to the storm's current position relative to the prior forecast.
After landfall... Idalia should transition into a less tropical feature supported by the eastern divergence zone of the southern upper trough fragment... therefore this system is expected to only gradually weaken despite land interaction. By day 3... the non-tropical remnants are likely to turn eastward and offshore from the Carolina coast in the westerly flow between the northern and southern upper trough fragments... and with the westerly flow being neutral (no convergence but also lacking supportive divergence) the remnants are likely to also weaken. Regeneration back to a tropical system in the 3+ day window would also be challenged by the shear caused by the upper westerlies. Noting the computer model forecast track guidance has shifted southward due to the blocking effect of a surface ridge that builds over the Great Lakes underneath the western convergence zone of the northern upper trough fragment that heads toward Hurricane Franklin... and my updated long-range forecast track is shifted in that direction. However even with a further south position to Idalia's offshore remnants... shear will take a northwesterly direction and be difficult to escape by 5+ days as what is currently the northeastern Pacific upper vortex skirts across North America as an amplified upper trough whose southwest side eventually reaches Idalia's environment.
Regarding impact to land areas:
(1) For western Cuba... satellite imagery shows trailing bands of heavy rainfall traversing the region... additional rainfall accumulations are possible today. Coastal surf will gradually decrease as Idalia pulls northward and away.
(2) Interests across the Florida Keys and southern half of the Florida peninsula should have completed preparations for Idalia by now (for heavy rain... coastal surf... and gusty winds) as the storm is upon them... while interests across the northern Florida peninsula... eastern panhandle of Florida... and southeastern Georgia have a few more hours to prepare. Due to the current intensity forecast... the wind and coastal storm surge threat for the northwestern part of the Florida peninsula and far eastern part of the Florida panhandle has increased to life threatening levels... evacuate if ordered or recommended to do so by news media and your local government. Notable wind damage will likely spread inland across the northeastern part of the peninsula and southeastern Georgia.
(3) I recommend interests across South Carolina as well as eastern and central North Carolina to continue preparing for gusty winds... coastal surf... and heavy rainfall to arrive by Thursday.
(4) With the south shift in the long-range track forecast... the potential for gusty winds across southeastern Virginia and southeastern Maryland have ended. However heavy rainfall and coastal surf remain likely by Thursday.
Update as of 11 AM EDT... due to Idalia's recent strengthening rate the NHC forecast now calls for a 125 mph maximum sustained wind major hurricane before landfall.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (0600Z Aug 29)... 70 mph maximum sustained wind tropical storm centered just north of western Cuba at 22.6N-85W
IOH 24 Hr Forecast (0600Z Aug 30)... 120 mph maximum sustained wind hurricane centered just offshore of the far eastern Florida panhandle at 29N-84W
IOH 48 Hr Forecast (0600Z Aug 31)... Frontal cyclone centered over southeastern North Carolina at 34N-78W
******National Hurricane Center (hurricanes.gov) official forecast as of 5 AM EDT*************************
Peak Strength (0600Z Aug 30)... 120 mph maximum sustained wind major hurricane centered just offshore of the far eastern Florida panhandle at 27.8N-84.3W
5-day position (0600Z Sep 3)... 45 mph maximum sustained wind tropical storm centered between Bermuda and the US east coast at 32.5N-70.5W
AREA OF INTEREST #23 (RECENTLY UPGRADED TO TROPICAL DEPRESSION ELEVEN)... The tropical low pressure in the open central Atlantic that has been curving northwest around the southwest periphery of the Atlantic surface ridge has stalled while blocked by another surface ridge traversing eastward across the northwest Atlantic. From its stalled position... it has interacted with the eastern divergence zone of an amplified upper trough fragment which has allowed it to build concentrated thunderstorms. Due to the amplified nature of the upper trough fragment the shear has not been too bad... and as a result the tropical low has recently strengthened into tropical depression eleven. The forecast track calls for a slow northward drift through 48 hours as the blocking surface ridge gradually becomes replaced by the southerly flow ahead of Franklin. After that time an acceleration to the northeast is anticipated as the southwesterly flow in the environment of this system increases as what will be the remnant frontal cyclone of Franklin passes to the north. Over the next 48 hours the upper wind profile looks better for this system as the upper trough fragment currently imparting light westerly shear gets pushed southward and away by an upper anticyclone with low shear and upper outflow arriving from the west (this is the anticyclonic outflow currently on top of Hurricane Franklin which will get displaced away from Franklin when the hurricane experiences shear). As a result I forecast strengthening of this system into a high-end tropical storm early in the forecast period. By 72+ hours the upper trough that transitions Franklin into a frontal cyclone will overspread this system with southwesterly shear... however the shear may be mitigated by this system's northeastward acceleration which is in alignment with the shear vector such that I do not show weakening from 48 to 72 hours. As the cold front of Franklin's to-be remnant frontal cyclone overspreads this system... I forecast transition of this system into a remnant frontal cyclone by 96 hours which will be supported by the divergence on the east side of the upper trough. Transition into a less tropical system will also be induced by passage into waters below 26 deg C.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1200Z Aug 29)... 35 mph maximum sustained wind tropical depression centered in the central Atlantic at 27.9N-51.4W
IOH 24 Hr Forecast (1200Z Aug 30)... 50 mph maximum sustained wind tropical storm centered in the central Atlantic at 30N-51.5W
IOH 48 Hr Forecast (1200Z Aug 31)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 32N-51.5W
IOH 72 Hr Forecast (1200Z Sep 1)... 70 mph maximum sustained wind tropical storm centered in the central Atlantic at 37.5N-47.5W
IOH 96 Hr Forecast (1200Z Sep 2)... Frontal cyclone centered west-northwest of the Azores at 42.5N-37.5W
******National Hurricane Center (hurricanes.gov) official forecast as of 11 AM EDT**************************
Peak Strength (0000Z Aug 30)... 40 mph maximum sustained wind tropical storm centered at 28.3N-51.6W
Loss of tropical cyclone status (1200Z Sep 1)... 30 mph maximum sustained wind remnant low centered in the central Atlantic at 35N-51W
AREA OF INTEREST #27... The large tropical wave of low pressure from western Africa has recently marched into the eastern tropical Atlantic and as of 1200Z is producing a center of rotation midway between the Republic of Cabo Verde Islands and Senegal near 15.5N-20W. It appears this tropical wave is becoming entangled with another inland wave to the southeast based on the latest satellite imagery... therefore for the next 48 hours the forecast track in the outlook below is slowed while the two waves merge into a large tropical low pressure area. By 72 hours a few factors will contribute to an increase in the north angle of the track as follows:
(1) The current east Atlantic upper vortex is expected to shift south as upper vorticity being pushed by amplifying warm core North Atlantic upper ridging (the upper ridging is forecast due to the warm surface southerly flow ahead of Franklin as it transitions into a strong non-tropical frontal cyclone). The eastern divergence zone of this upper vorticity will contribute to an Atlantic surface ridge weakness to the northwest which this system will be moving toward. If this system becomes sufficiently strong/tall... it will also be dragged north by upper southerly flow ahead of the upper vorticity.
(2) In the short-term the surface ridge weakness to the northwest will also be re-enforced by the presence of Hurricane Franklin and what is now Tropical Depression Eleven.
(3) In the longer-term... the surface ridge weakness to the northwest will be maintained by what is expected to be the remnants of Idalia offshore of southeastern US and an additional frontal low to the east of ex-Idalia that will materialize under the support of the current northeast Pacific upper vortex once it moves into the northwest Atlantic by day 5.
Regarding odds of tropical cyclone formation... I agree with the NHC on a peak of 50% as it is uncertain if this system will degenerate in the shorter-term due to dry Saharan air lurking to the west and potentially excessive broad structure with no defined maximum of surface convergence and concentrated thunderstorms (the broadening is expected as the two tropical waves mentioned in the prior paragraph merge). These short-term factors are why I assign odds below the peak for the first 48 hours... but I do acknowledge the currently well-defined rotation between Senegal and the Cabo Verde Islands by at least keeping 30% odds for this period. If this disturbance goes on to survive these short-term factors... the disturbance may deal with some disruptive southerly shear ahead of the aforementioned east Atlantic upper vorticity in the 72 to 96 hour window. Upper air conditions become optimal for development by 120 hours (day 5) as the upper vorticity begins to retrograde westward and away under the influence of central Atlantic warm core upper ridging that materializes in the warm sector of the aforementioned northwest Atlantic frontal low forecast to develop east of ex-Idalia. Thus the peak odds of development in the outlook below are reserved for day 5.
Noting the Republic of Cabo Verde Islands may see heavy rain and gusty winds from this system over the next 48 hours regardless of tropical cyclone formation or not.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Aug 30)... 30% chance of tropical cyclone formation (southern Republic of Cabo Verde Islands near 15N-23W)
IOH 48 Hr Outlook (1200Z Aug 31)... 30% chance of tropical cyclone formation (just west of the southern Republic of Cabo Verde Islands near 15N-26W)
IOH 72 Hr Outlook (1200Z Sep 1)... 40% chance of tropical cyclone formation (eastern tropical Atlantic near 17.5N-30W)
IOH 96 Hr Outlook (1200Z Sep 2)... 40% chance of tropical cyclone formation (eastern tropical Atlantic near 20N-33W)
IOH 120 Hr Outlook (1200Z Sep 3)... 50% chance of tropical cyclone formation (eastern Atlantic near 22N-37.5W)
******National Hurricane Center (hurricanes.gov) official outlook as of 8 AM EDT*****************************
Formation chance through 48 hours...20%
Formation chance through 7 days (168 hours)...50%
AREA OF INTEREST #28...The tropical wave of low pressure that has been heading toward the southern Lesser Antilles from the central Atlantic was not very active over the last day or so while passing through a suppressing inverted upper trough. The tropical wave has quickly developed increased and somewhat organized thunderstorm activity while now moving across the southern islands while finding outflow on the southeast side of a western Atlantic upper anticyclone. I have increased my 5-day peak odds of tropical cyclone formation to a low 20% due to the notable increase in the wave's activity... the odds are still on the low side as computer model support agreeing with development has been lacking. Also the wave may contend with some easterly shear as it is on the south side of the upper anticyclone instead of directly underneath it. Regarding the forecast track... although the southeastern convergence zone of the western Atlantic upper anticyclone will produce a weak surface ridge to the north of the Caribbean... and to the south of Idalia and Franklin as those two storms lift northward and away... this wave will tend to be toward the west extent of the surface ridge. Therefore the forecast track below has some northward angle in the westward track through 72 hours. After that time... the lengthy upper trough that is currently steering Idalia breaks into northern and southern fractures (see Idalia section above for more info on that)... with the back convergent side of both upper trough fractures building an oblong surface ridge from eastern North America to the western Atlantic. The forecast track in the outlook below bends more westward after 72 hours as this system bumps into this oblong surface ridge.
Interests across Jamaica... the Cayman Islands... the Yucatan peninsula of Mexico... and Belize should be aware of this tropical wave as it potentially enters the region while remaining vigorous in the 3 to 5 day window. Even without tropical cyclone formation... the vigorous wave could produce heavy rainfall and gusty winds.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1200Z Aug 30)... 5% chance of tropical cyclone formation (southeastern Caribbean Sea near 12.5N-65W)
IOH 48 Hr Outlook (1200Z Aug 31)... 10% chance of tropical cyclone formation (central Caribbean Sea near 15N-71W)
IOH 72 Hr Outlook (1200Z Sep 1)... 15% chance of tropical cyclone formation (just south-southeast of Jamaica near 17N-77W)
IOH 96 Hr Outlook (1200Z Sep 2)... 20% chance of tropical cyclone formation (south-southwest of the Cayman Islands near 18N-82W)
IOH 120 Hr Outlook (1200Z Sep 3)... 20% chance of tropical cyclone formation (just offshore of the Belize/Mexico border at 18N-87W)
******National Hurricane Center (hurricanes.gov) official outlook as of 2 PM EDT*****************************
Not in the official outlook
...COMPUTER MODEL SUMMARY...
Not completed in this update due to busy day personally.
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