*******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.**********
...WEDNESDAY JUNE 23 2021 11:09 PM EDT...
See area of interest sections below for multiple areas being monitored for tropical development in the Atlantic basin. Elsewhere…the tail end of the cold front being in the western Atlantic will likely leave behind a surface trough of low pressure offshore of the Carolinas over the next day or so…to be supported by the divergence zone of an upper trough fragment to also be left behind over the southeast coastal US. If the surface trough develops enough thunderstorm activity and resultant latent heat release…it could increase warm upper ridging aloft that would reduce the shearing effects of the upper trough fragment…and the 1200Z GFS and NAVGEM models hinted at this. However thus far there is a lack of widespread thunderstorm activity to declare a new area of interest at this time.
AREA OF INTEREST #1…The National Hurricane Center (NHC) continues to monitor a tropical wave of low pressure in the central Atlantic for further development. Earlier today the tropical wave appeared to be spread out west-to-east…while featuring one western cluster of showers over the southern Lesser Antilles and another to the east-northeast near 55W longitude. While the western cluster dissipated this evening…the eastern cluster appears to have rotation (near 12.5N-56W) upon which a quickly forming thunderstorm flare up has recently developed. This thunderstorm complex is probably being aided by the southeastern divergence zone of the upper vortex nearing from the north. This upper vortex is expected to shear this tropical wave and prevent it from developing much…however if the thunderstorm flare up releases enough latent heat it may build enough warm upper ridging to ward off the worst of the shear for a few more hours. Therefore I agree with the NHC’s short-term odds of development of 10% for tonight. By 48 hours…I trim the odds of development downward to 0% as the tropical wave moves into the Caribbean and directly into the westerly shearing winds on the south side of the upper vortex. Regardless of tropical cyclone formation or not…the wave may bring enhanced rainfall to the central and southern Lesser Antilles if the wave’s thunderstorms remain enhanced by the divergence zone of the upper vortex when the wave arrives to the islands late tomorrow or early Friday.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 24)…10% chance of tropical cyclone formation (just north of Barbados near 13.5N-59.5W)
IOH 48 Hr Outlook (1800Z Jun 25)…0% chance of tropical cyclone formation (eastern Caribbean Sea near 14N-62.5W)
AREA OF INTEREST #2…The upper vortex currently in the far North Atlantic is expected to move south toward the Azores under the influence of the amplified upper ridge over the west Atlantic. A new surface frontal low is expected to form in the eastern divergence zone of the upper vortex over the next day or so. Although water temps around the Azores are in the low 20 deg C range…the upper vortex may be cold enough to support thunderstorms and acquisition of tropical characteristics for the surface frontal low. However I have lowered odds of subtropical development to 5% for two reasons:
(1) The 200 mb height of the upper vortex is 1200 dekameters…would like to see a colder upper vortex with a height of below 1200 dekameters for more confidence in subtropical development at these water temps.
(2) Models have trended with a slightly more north track for this system…toward slightly cooler water.
Forecast positions in the Outlook below are based on the positions of the forecast cut-off upper vortex over the next few days from today’s 1200Z GFS model run. By 96 hours I trim off development odds down to 0% as the surface frontal low by then will have whirled directly below the upper vortex center where there is a lack of divergence…which will cause it to gradually weaken by that time if it does not develop before then. Regardless of subtropical development or not…if the surface frontal low becomes strong enough gusty winds and coastal sea swells will be possible for the Azores within the next three days.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 24)…0% chance of subtropical cyclone formation (northeast Atlantic near 44N-26W)
IOH 48 Hr Outlook (1800Z Jun 25)…5% chance of subtropical cyclone formation (just northwest of the western Azores near 40N-30W)
IOH 72 Hr Outlook (1800Z Jun 26)…5% chance of subtropical cyclone formation (just northwest of the western Azores near 40N-30W)
IOH 96 Hr Outlook (1800Z Jun 27)…0% chance of subtropical cyclone formation (northeast Atlantic 40.5N-35W)
AREA OF INTEREST #3…While putting this post together this evening…a few changes have occurred with the African tropical wave of low pressure that the models have been forecasting to develop in the eastern tropical Atlantic:
(1) The NHC introduced this tropical wave into their 5-day outlook today…and after the above chart was generated introduced it into their 2-day outlook as of 8 PM EDT (this is why in the above chart the wave is marked as an NHC 5-day outlook feature when it is now a 2-day outlook feature)
(2) The wave is further southwest than I previously assessed yesterday and earlier today…already on the west coast of Africa with a spin near 6N-12.5W while featuring organized thunderstorms bands to the southwest of the spin that are supported by the southern outflow of the upper ridge in the region. The forecast track points are adjusted south and west in my updated outlook below.
As far as forecasting is concerned…one would think that climatology would mean that this wave would not develop (i.e. it is unusual to see development in this part of the Atlantic in June due to dry Saharan air…shear…water temps in the eastern Atlantic not yet being that warm). However it appears we are in a situation defying climatology as the presence of dry Saharan air is currently weak….the tropics wave’s updated forecast track will keep it over 26+ deg C water…and the tropical upper ridge in the region will remain in place and keep wind shear low. Moreover the usually conservative ECMWF model today was bullish on this system…suggesting a compact hurricane within the next few days. Therefore I have raised my odds of development to be 40%…slightly above the NHC outlook as of this writing. It will be an interesting few days to see if this wave actually goes on to develop and defy climatology…stay tuned.
******Infohurricanes.com outlook. Visit hurricanes.gov (hurricanes dot gov) for official outlook***********
IOH 24 Hr Outlook (1800Z Jun 24)…5% chance of tropical cyclone formation (eastern Atlantic near 7.5N-17.5W)
IOH 48 Hr Outlook (1800Z Jun 25)…20% chance of tropical cyclone formation (eastern Atlantic near 8.5N-21W)
IOH 72 Hr Outlook (1800Z Jun 26)…40% chance of tropical cyclone formation (eastern tropical Atlantic near 9N-26W)
IOH 96 Hr Outlook (1800Z Jun 27)…40% chance of tropical cyclone formation (eastern tropical Atlantic near 9.5N-31W)
IOH 120 Hr Outlook (1800Z Jun 28)…40% chance of tropical cyclone formation (eastern tropical Atlantic near 10N-36W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z CMC Model Run...
**For area of interest #1...no tropical cyclone formation shown
**For area of interest #2...surface frontal low arrives just west of the Azores by 60 hours….drifts east into the Azores by 84 hours…cyclonically loops to a position west of the Azores through 120 hours.
**For area of interest #3…tropical wave reaches the eastern Atlantic near 10.5N-19W by 36 hours…loses its closed surface spin while reaching 39W at 114 hours.
1200Z ECMWF Model Run...
**For area of interest #1...no tropical cyclone formation shown
**For area of interest #2...surface frontal low arrives just northwest of the Azores by 48 hours….cyclonically loops eastward into the Azores by 72 hours and then west-northwest to 40.5N-36W by 120 hours
**For area of interest #3…tropical wave arrives to the west African coast at 24 hours…tropical cyclone formation shown near 9.5N-19.5W at 48 hours…strengthens into a compact hurricane by 72 hours while passing 10.5N-25W…arrives to 13.5N-41W at 120 hours.
1200Z GFS Model Run...
**For area of interest #1...no tropical cyclone formation shown
**For area of interest #2...surface frontal low arrives just northwest of the Azores by 54 hours and remains stationary through 72 hours…drifts westward to 39.5N-35.5W by 120 hours.
**For area of interest #3…tropical wave reaches the eastern Atlantic near 9.5N-19.5W by 54 hours…loses closed spin near 26W at 78 hours.
**Surface low left behind by cold front at a location offshore of the US mid-Atlantic coast at 36N-72.5W at 24 hours…drifts north into Long Island New York by 51 hours.
1200Z NAVGEM Model Run...
**For area of interest #1...no tropical cyclone formation shown
**For area of interest #2...surface frontal low arrives just northwest of the Azores by 54 hours….makes two small cyclonic loops at this location thru 120 hours
**For area of interest #3…tropical wave reaches the eastern Atlantic near 12.5N-20W by 54 hours…no tropical cyclone formation shown
**Surface trough left behind by cold front at a location offshore of the US mid-Atlantic coast by 12 hours…trough dissipates by 48 hours.
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