MY 2021 ATLANTIC HURRICANE SEASON BIRDSEYE VIEW POST #109 (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 9:55 PM EDT...
Satellite image as of 2300Z. 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:
GFS Model Upper-Level Winds 1200Z:
The mid-September climatological peak of the Atlantic Hurricane Season is showing … with the formation of Tropical Storm Nicholas in the western Gulf of Mexico and four other areas of interest being monitored for future development. Each system is detailed in its own section below.
TROPICAL STORM NICHOLAS… As of 11 AM EDT earlier today… aircraft reconnaissance found the large tropical disturbance now in the western Gulf of Mexico had intensified into the fourteenth tropical storm of the 2021 Atlantic Hurricane Season… Nicholas. The closed circulation is toward the south part of the sprawling activity in the western Gulf. In other words most the thunderstorms are being pulled north of the circulation with light southerly shear continuing… as the upper ridging expanding into the region in the wake of what was the upper vorticity in the northwest Gulf has not yet fully overspread Nicholas. Despite the shear… the thunderstorm bands on the north side of Nicholas are becoming well-organized… so once the shear relaxes once the upper ridging better covers Nicholas brisk strengthening in the next 24 hours is on the table… especially with the warm 30 deg C water… and this is what I continue to forecast. My forecast track in the short-term is nudged northwest from the storm’s current position… and a northward track is expected while Nicholas gets funneled in the flow between the current southeast US surface ridge and an approaching frontal system from western North America. As Nicholas approaches the Texas coast (24 to 48 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 upper ridging. But I do show enough strengthening to push this system into hurricane strength considering that by 24 hours this system could very well be just below hurricane force… and only a small margin of additional strengthening after that time would make Nicholas a hurricane.
Models are continuing to show Nicholas slowing down while sliding northeast across coastal Texas. This is expected to occur as the tropical storm strengthens and becomes tall enough to couple to the upper westerly flow which would push Nicholas against the southeast US surface ridge instead of letting Nicholas 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 Nicholas coupled to the upper westerly flow. My updated longer-term forecast track is thus updated to show a slower and more northeast angle in track. After 72 hours… I assume Nicholas will weaken from land interaction which will finally allow it to become shallower and decouple from the upper westerlies… allowing the system to glide northward around the west side of the southeastern US surface ridge.
With these forecast updates:
**The potential for tropical storm conditions and heavy rainfall along the Texas and Mexico coast south of the Corpus Christi/Matagorda Bay area appears low due to the forecast track and upper-level shearing winds keeping the strongest thunderstorms offshore. However coastal sea swells in the area are likely… especially further north where Nicholas is forecast to strengthen.
**Preparations for tropical storm to hurricane force conditions from Corpus Christi/Matagorda Bay all the way toward the Texas/Louisiana border should be underway… impacts expected by Tuesday. Preparations should include relocating further inland if you live in an area prone to coastal storm surge. Heavy rainfall with flash flooding potential is a concern in this region… and also inland over eastern Texas and western Louisiana… especially as Nicholas is expected to slow down as discussed above which will prolong the rainfall and potentially make widespread and life threatening flash flooding. The rainfall flooding could be similar to what has been seen in this region in recent storms such as Harvey 2017 and Imelda 2019. Be mindful of things that will keep you safe from floodwater… such as avoiding driving into a water-covered roadway to prevent your vehicle from getting stuck which could result in drowning.
**Southwestern Louisiana may see tropical storm conditions by Wednesday… especially if the northeast track keeps Nicholas close to shore which would allow it to maintain strength.
******Infohurricanes.com forecast. Visit hurricanes.gov (hurricanes dot gov) for official forecast***********
0 Hr Position (1800Z Sep 12)… 40 mph maximum sustained wind tropical storm centered in the southwestern Gulf of Mexico at 21.7N-95.5W
IOH 24 Hr Forecast (1800Z Sep 13)… 70 mph maximum sustained wind tropical storm centered just offshore of the Texas/Mexico border at 25.5N-96.5W
IOH 48 Hr Forecast (1800Z Sep 14)... 80 mph maximum sustained wind hurricane centered on the Texas coast between Corpus Christi and Galveston at 28.8N-95.5W
IOH 72 Hr Forecast (1800Z Sep 15)… 35 mph maximum sustained wind tropical depression centered on the southern Texas/Louisiana border at 30.5N-94W
IOH 96 Hr Forecast (1800Z Sep 16)… Remnant low centered on the Louisiana/Arkansas border at 32.7N-93W
AREA OF INTEREST #1... The tropical wave of low pressure in the eastern tropical Atlantic has passed through the Republic of Cabo Verde Islands without producing notable weather over the islands… while dry Saharan air and also wind shear induced by upper vorticity deposited by the northeast Atlantic upper trough has precluded development. This is my final statement on this wave on this blog as it is no longer expected to develop.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Sep 13)… 0% chance of tropical cyclone formation (eastern tropical Atlantic near 19N-29W)
AREA OF INTEREST #2… The surface trough of low pressure approaching the northern Lesser Antilles is becoming entangled in a large area of showers and thunderstorms north and east of the northeastern Caribbean Islands being generated by 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 middle 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 will pull Nicholas northbound. In short… the upper vortex will be moving westward in tandem with the surface trough of interest over the short-term… and as it continues to fade the upper air pattern will become more anticyclonic with lower shear and upper outflow by 3+ days.
The forecast positions in the outlook below are initially based on the location of the eastern divergence zone of the upper vortex. By days 4 and 5 the frontal system that turns Nicholas 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 and northward into. So far the upper vorticity is not forecast to be amplified enough to pull this system into the southeast US in the long range… that is assuming that this system becomes strong/tall enough to be steered by upper winds in the first place.
Although the NHC has this system at 50% odds of development… I currently have a lower peak of 30% odds due to all models not showing development at any given time thus far. In addition by day 5 the forecast southeast US upper vorticity may shear this system. I plan to raise odds 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 the next 24 hours. Coastal sea swells could reach the Bahamas and southeast US coast by days 4 and 5 if this system goes on to develop.
IOH 24 Hr Outlook (1800Z Sep 13)… 0% chance of tropical cyclone formation (just north of Puerto Rico near 20N-65.5W)
IOH 48 Hr Outlook (1800Z Sep 14)… 10% chance of tropical cyclone formation (north of the Dominican Republic and east of the eastern Bahamas near 21.5N-70W)
IOH 72 Hr Outlook (1800Z Sep 15)… 20% chance of tropical cyclone formation (northeast of the Bahamas near 25N-72.5W)
IOH 96 Hr Outlook (1800Z Sep 16)… 30% chance of tropical cyclone formation (offshore of the southeastern US near 30N-74W)
IOH 120 Hr Outlook (1800Z Sep 17)… 30% chance of tropical cyclone formation (southeast of Cape Hatteras North Carolina near 34N-74W)
AREA OF INTEREST #3… Computer models continue to suggest that the western Africa tropical wave of low pressure… currently passing 10W longitude… will emerge into the eastern tropical Atlantic by 24 hours… and eventually develop in the eastern or central tropical Atlantic afterwards. Unlike area of Interest (AOI) #1… this system is expected to not have a north bend in track caused by AOI #4 as AOI #4 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 45% 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 has 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.
IOH 24 Hr Outlook (1800Z Sep 13)… 0% chance of tropical cyclone formation (west coast of Africa near 11N-15W)
IOH 48 Hr Outlook (1800Z Sep 14)… 0% chance of tropical cyclone formation (southeast of the Republic of Cabo Verde Islands near 11.5N-20W)
IOH 72 Hr Outlook (1800Z Sep 15)… 15% chance of tropical cyclone formation (south-southwest of the Republic of Cabo Verde Islands near 12N-25W)
IOH 96 Hr Outlook (1800Z Sep 16)… 35% chance of tropical cyclone formation (eastern tropical Atlantic near 12.5N-30W)
IOH 120 Hr Outlook (1800Z Sep 17)… 45% chance of tropical cyclone formation (eastern tropical Atlantic near 13N-35W)
AREA OF INTEREST #4… The northeast Atlantic surface frontal low and upper trough continue to be monitored for subtropical development as the upper air temps of the upper trough are sufficiently cold (200 mb height below 1200 dekameters) for thunderstorm generation despite water temps in the low 20s of deg C. So far the shower and thunderstorm activity has not shown signs of organizing into a tropical core. I have very low 5% odds of development as the upper trough will soon warm to above 1200 dekameters in height (200 mb height) while remaining cut-off from high-latitude cold air… which will cause the atmosphere to become increasingly stable considering water temps are mild. By 48 hours the odds of development are 0% as the atmosphere becomes all the more stable. Forecast is for an East-northeast drift into Portugal by Tuesday due to the incoming flow ahead of the upper trough associated with the current North Atlantic frontal cyclone. 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 predict.
IOH 24 Hr Outlook (1800Z Sep 13)… 5% chance of subtropical cyclone formation (offshore of southern Portugal near 37.5N-16W)
IOH 48 Hr Outlook (1800Z Sep 14)… 0% chance of subtropical cyclone formation (just offshore of southern Portugal near 38.5N-10W)