BIRDSEYE VIEW POSTS

Since 2012 on the now retired Weather Underground blogs, I have been posting annotated "birdseye view" charts of the Atlantic basin, with a detailed explanation and forecasting that references the chart. From there you may know me as "NCHurricane2009." While I now do these "birdseye view" posts here, I will continue to do comments at Yale Climate Connections via Disqus where the former Weather Underground community has moved to. Feel free to reply to me there, at my Disqus feed at this link, or via e-mail at IOHurricanes@outlook.com 

 
 
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MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #108 (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 12 2021 3:45 AM EDT...

Satellite image as of 2320Z. 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 11):

GFS Model Upper-Level Winds 1200Z (Sept 11):

It sure feels like the peak of the Atlantic Hurricane Season… see area of interest sections below for several areas being monitored for future tropical development in the Atlantic in the wake of Larry.


REMNANTS OF LARRY... As of 11 AM EDT on Saturday… Hurricane Larry finally lost tropical characteristics between Newfoundland and Greenland… becoming a powerful frontal cyclone supported by the divergence zone of an amplified upper trough. The same upper trough has also aided in the intensification of a frontal cyclone that recently exited northeastern Canada. The most recent WPC (Weather Prediction Center) surface analyses show that the frontal cyclone has absorbed Larry’s remnant cyclone (https://www.wpc.ncep.noaa.gov/html/sfc2.shtml). Therefore the absorbing cyclone will become responsible for possible strong winds over southeast Greenland tomorrow… as well as ongoing coastal sea swells that will affect the Atlantic Canada… southeast Greenland… and Iceland shores in the coming days. As Larry is no longer an entity… this is my final statement on Larry on this blog.


AREA OF INTEREST #1... The tropical wave of low pressure that was over far Western Africa is now in the eastern tropical Atlantic moving into the Republic of Cabo Verde Islands with a more consolidated low pressure swirl. The swirl has struggled to maintain thunderstorms due to dry Saharan air… however heavy rains and gusty winds in the islands cannot be ruled out… especially if the swirl develops stronger thunderstorms in the overnight hours. The north angle in this system’s westward track is due to the surface ridge weakness associated with area of interest #5. In fact this system is positioned a little further north of my previous forecast… the updated one below is adjusted accordingly. Once this system moves past the ridge weakness associated with area of interest #5… I forecast a more west track by 48 to 72 hours. The upper trough of area of interest #5 is depositing upper vorticity to the north of this system...which will weaken to an upper vortex off to the northwest by 96+ hours as the vorticity remains cut-off from high-latitude cold air. The increased north angle in track by 96+ hours is to account for the possiblity of this system developing a strong/tall enough structure that can be pulled by the forecast upper vortex.


Due to the aforementioned dry saharan air... and northward-adjusted track which keeps this system closer to westerly shearing winds associated with the upper vorticity… my peak odds of development are lowered to 30% (the NHC outlook odds are lowered to 50%… and most recently as of 2 AM EDT the odds are lowered further to 30% as well). Lowering the odds also makes sense as all of the global models have now stopped showing this system developing. I drop odds of development to 10% by day 5 as this system moves even closer to the upper vorticity where shear is higher.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (0000Z Sep 13)… 30% chance of tropical cyclone formation (west of the Republic of Cabo Verde Islands near 17.5N-28W)

IOH 48 Hr Outlook (0000Z Sep 14)… 30% chance of tropical cyclone formation (eastern tropical Atlantic near 18.5N-32.5W)

IOH 72 Hr Outlook (0000Z Sep 15)… 30% chance of tropical cyclone formation (eastern tropical Atlantic near 18.8N-37.5W)

IOH 96 Hr Outlook (0000Z Sep 16)… 30% chance of tropical cyclone formation (central tropical Atlantic near 20.5N-42W)

IOH 120 Hr Outlook (0000Z Sep 17)… 10% chance of tropical cyclone formation (central tropical Atlantic near 22N-46W)


AREA OF INTEREST #2... Satellite image of quickly organizing tropical disturbance in the Gulf of Mexico as of 0526Z:

The large tropical disturbance across the Gulf of Mexico and parts of southeastern Mexico… consisting of a surface tropical wave and pre-existing surface trough of low pressure to the south (same trough that birthed Mindy a few days ago)… stalled surface front to the north… and widespread thunderstorm squalls supported by a large region of upper divergence on the east side of western Gulf upper vorticity… has over the last 24 hours shown an increasingly defined rotation toward the south in the eastern Bay of Campeche. There have been multiple occasions of thunderstorm bands organizing with this rotation… in particular with the latest satellite frames (the above frame from 0526Z is a good example)… therefore tropical cyclone formation appears imminent and I have issued a tropical cyclone formation forecast as outlined below.


As the cool core western Gulf upper vorticity weakens while remaining cut-off from high-latitude cold air... it will become replaced by expanding upper ridging from the south that will provide a lower shear and upper outflow environment more conducive to development. Combined with warm 30 deg C waters… I forecast brisk intensification between 24 and 48 hours… once the current rotation in the Bay of Campeche organizes into a tropical depression. Although my forecast track is initially adjusted south to account for the current position of the rotation… the later part of the forecast is basically unchanged as this system will be funneled northward in the flow between the current southeast US surface ridge and an approaching frontal system from western North America. As this system approaches the Texas coast (48 to 72 hours)… I show a notably slower intensification rate as the northward track takes this system into possible westerly shear on the north side of the forecast upper ridging… but I do show enough strengthening to push this system into hurricane strength as the upper ridging in some of the GFS solutions expands enough to keep some of the shear at bay further to the north.


Although I forecast this system to continue north inland after landfall on the Texas coast… some models prefer nearly stalling this system near the coast at 72+ hours. This would occur if this system strengthens and becomes tall enough to couple to the upper westerly flow which would push this system against the southeast US surface ridge instead of letting it flow freely around the ridge. The stalled scenario would be made worse if the stall occurs just offshore of the Texas coast which would slow the weakening rate and keep this system coupled to the upper westerly flow. My forecast assumes for now that the upper ridging will expand enough to keep the strongest upper westerly flow northward over land… allowing this system to make landfall and weaken over land so it avoids coupling with the upper flow.


With these forecast updates:

**The potential for tropical storm to hurricane force conditions along much of the Texas coast by Tuesday is increasing. Much of the Texas coast could see impacts due to the projected path taking this system parallel to a long section of the coast. I recommend now is the time to gather storm prep supplies early… and think about preparation plans such as relocating further inland if you live in an area prone to coastal storm surge. Heavy rainfall with flash flooding potential is a concern along coastal and eastern Texas and possibly western Louisiana… especially if this system slows down toward landfall time as discussed above.

**Tropical Storm conditions and heavy flash flooding rains are possible in Veracruz and Tamaulipas by Monday if the forecast track undergoes a westward shift.

**Additional heavy rains with flash flood potential are possible in the western Yucatan and southeastern Mexico provinces over the next several hours.

******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********

0 Hr Position (0000Z Sep 12)… Tropical low pressure centered in the eastern Bay of

Campeche at 19.5N-92.5W

IOH 24 Hr Forecast (0000Z Sep 13)… 35 mph maximum sustained wind tropical depression centered in the southwestern Gulf of Mexico at 21N-95W

IOH 48 Hr Forecast (0000Z Sep 14)… 70 mph maximum sustained wind tropical storm centered offshore of northeastern Mexico at 25N-96W

IOH 72 Hr Forecast (1800Z Sep 14)... 80 mph maximum sustained wind hurricane making landfall just east of Corpus Christi Texas at 28.5N-96W

IOH 96 Hr Forecast (1800Z Sep 15)... Remnant low centered over northeastern Texas at 32N-96W


AREA OF INTEREST #3... A tropical wave of low pressure in the central Atlantic has already crossed the Lesser Antilles… but has left behind its surface low pressure swirl to the east of the islands which has over the last day become less defined and weakened into a surface trough. The trough has some disorganized thunderstorm activity on its north side… with stronger activity closer to the northern Lesser Antilles developing in the eastern divergence zone of nearby upper vorticity to the northwest. This upper vorticity… while remaining cut-off from high-latitude cold air… is forecast to weaken to an upper vortex while being pushed westward by the central Atlantic upper ridge… and models have shown possible development of the aforementioned surface trough while it becomes enhanced by the eastern divergence zone of the upper vortex . Towards the end of the 5-day forecast period the upper vortex is slated to be near the Bahamas while pushed further west by upper ridging to be associated with the warm sector of the frontal system that pulls area of interest #2 northbound. In short… the upper vortex will be moving westward in tandem with the surface trough of interest over the next five days… and as it continues to fade the upper air pattern will become more anticyclonic with lower shear and upper outflow by days 4 and 5.


The forecast positions in the outlook below are based on the location of the eastern divergence zone of the upper vortex… and are also nudged north from my previous outlook due to the current more north position of the surface trough. By days 4 and 5 the frontal system that turns area of interest #2 northward will leave behind upper vorticity over the southeast US whose eastern divergence zone would create a narrow surface ridge weakness that this system would track northwest toward. So far the upper vorticity is not forecast to be amplified enough to pull this system into the southeast US in the longer range… that is assuming that this system becomes strong/tall enough by day 5 to be steered by upper winds in the first place.


Although the NHC has introduced this system into their outlook product and already kept odds of development at 40%… I currently have a slightly lower peak of 30% odds due to all models not showing development at any given time thus far. These odds would be raised if this system gets better organized and/or if all/most models start to agree on developing this system. It is possible this system brings heavy rains to the northern Lesser Antilles… Virgin Islands… and Puerto Rico in about 2 days if it sees an increase in thunderstorms with the aid of the upper vorticity as described above. Coastal sea swells could reach the Bahamas and southeast US coast by day 5 if this system goes on to develop.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (0000Z Sep 13)… 0% chance of tropical cyclone formation (just east of the northern Lesser Antilles near 17.5N-60W)

IOH 48 Hr Outlook (0000Z Sep 14)… 5% chance of tropical cyclone formation (just north of Puerto Rico near 20N-65.5W)

IOH 72 Hr Outlook (0000Z Sep 15)… 15% chance of tropical cyclone formation (north of the Dominican Republic and east of the eastern Bahamas near 21.5N-70W)

IOH 96 Hr Outlook (0000Z Sep 16)… 25% chance of tropical cyclone formation (northeast of the Bahamas near 25N-72.5W)

IOH 120 Hr Outlook (0000Z Sep 17)… 30% chance of tropical cyclone formation (offshore of the southeastern US near 30N-74W)


AREA OF INTEREST #4… Computer models are converging on another western Africa tropical wave of low pressure emerging from the coast by 48 hours and developing in the eastern tropical Atlantic within the next five days. Satellite imagery shows over Africa a thunderstorm complex at 10W and another at 0W… so I assume the wave of interest is in between at 5W… which also fits the bill for a wave that exits Africa in 48 hours using a typical 5W longitude per day forward speed that tropical waves travel at. Unlike area of Interest (AOI) #1… this system is expected to not have a north bend in track caused by AOI #5 as AOI #5 will have moved into Portugal and dissipated by the time this system emerges from Africa. While a more south track would keep this wave further away from dry Saharan air… dry Saharan air surges can sometimes reach south… and this tropical wave is not well-organized yet. Therefore I park my development odds by day 5 at a cautious 40% for the time being instead of the NHC’s current 60%… as we were in this situation a few days ago with AOI #1 where we had model support and high NHC development odds and now dry Saharan air put a damper on the development of AOI #1. However if the organization of this wave improves and it wards off dry Saharan air… then I certainly will raise my odds of development in future updates.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (0000Z Sep 13)… 0% chance of tropical cyclone formation (western Africa near 11N-10W)

IOH 48 Hr Outlook (0000Z Sep 14)… 0% chance of tropical cyclone formation (west coast of Africa near 11N-15W)

IOH 72 Hr Outlook (0000Z Sep 15)… 5% chance of tropical cyclone formation (southeast of the Republic of Cabo Verde Islands near 11.5N-20W)

IOH 96 Hr Outlook (0000Z Sep 16)… 20% chance of tropical cyclone formation (south-southwest of the Republic of Cabo Verde Islands near 12N-25W)

IOH 120 Hr Outlook (0000Z Sep 17)… 40% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-30W)


AREA OF INTEREST #5… The upper trough associated with a frontal low that has been moving across the north and northeastern Atlantic has been amplifying southward as the adjacent warm Central Atlantic upper ridge amplified due to Hurricane Larry’s latent heat release and also due to the warm sector of the rapidly intensifying frontal cyclone that absorbed Larry. The southward amplification of the upper trough has brought cold upper air over mild northeast Atlantic waters… which could allow for thunderstorm generation with the surface frontal low (located at 42N-19.5W) as it begins its post-mature decay phase beneath the upper trough… and as a result some tropical characteristics may develop with the surface frontal low.


While I do agree that the thermal profile is typical for subtropical development (an overhead cold core upper vortex embedded in the upper trough whose 200 mb height is below 1200 dekameters… and surface waters at low 20 deg C range)… The surface frontal low appears broad and was not generating a defined center with organized thunderstorms… and so I did not think of this system as an area of interest. However the NHC has introduced it in their outlook product… and so it’s now an area of interest at this point regardless. Due to the aforementioned observation of a broad frontal low with no tight center… and none of the models predicting a tropical core forming… I have low 10% odds of development… which is lower than the NHC outlook of 20%. My odds are trimmed down with time as the upper trough warms to above 1200 dekameters in height (200 mb height) by 48+ hours while remaining cut-off from high-latitude cold air… which will cause the atmosphere to become increasingly stable considering water temps are mild.


Forecast track is initially southeast under the continued influence of the amplified central Atlantic upper ridge… then an East-northeast drift into Portugal by Tuesday due to the incoming flow ahead of the upper trough associated with the powerful cyclone that absorbed Larry. Impacts to southern Portugal are likely to be insignificant as the frontal low is in its post-mature decay phase where it continually weakens below the upper trough axis where there is a lack of divergence aloft… that is assuming that the frontal low does not develop a tropical core which no models currently predict at this time.

******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********

IOH 24 Hr Outlook (0000Z Sep 13)… 10% chance of subtropical cyclone formation (northeastern Atlantic near 37.5N-18W)

IOH 48 Hr Outlook (0000Z Sep 14)… 5% chance of subtropical cyclone formation (offshore of southern Portugal near 37.5N-15W)

IOH 72 Hr Outlook (0000Z Sep 14)… 0% chance of subtropical cyclone formation (just offshore of southern Portugal near 38.5N-10W)

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