*******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 JANUARY 24 2023 2:30 AM EDT...
Frontal surface low pressure area in the eastern Atlantic becoming better defined... monitoring this feature for acquisition of tropical characteristics within the next 72 hours. See area of interest #1 section below for more details.
As done on this site starting last year... I will be sequentially numbering up areas of interest for possible Atlantic tropical development throughout the year... resetting back to #1 at the start of each year. This scheme is so that each area of interest retains a numeric identity from update to update... which reduces confusion when simultaneous areas of interest begin and end when tropical activity increases particularly during the Atlanitc Hurricane Season. I have not counted the short-lived northwest Atlantic tropical event on January 16 as #1 as that system was expected to quickly shed its tropical characteristics and not become identified as a subtropical or tropical cyclone by the National Hurricane Center... instead I am identifying the area of interest discussed below as #1 for this year (refer to special update #0A available on the home page of this site for a report on the January 16th event).
AREA OF INTEREST #1... A series of upper troughs are currently pivoting across North America and around a polar cold core upper vortex that has settled over northeastern Canada within the last few days. Over the next few days... the eastern divergence zones of each North American upper trough is expected to produce a series of strengthening winter-time surface frontal cyclones that shift northeastward across the eastern United States. Warm southerly flow ahead of each frontal cyclone is expected to in turn amplify the current western Atlantic warm core upper ridge... this process is in fact already underway with the warm southerly flow ahead of the strengthening frontal cyclone that has quickly moved into Atlantic Canada from the eastern US. Amplification of the western Atlantic upper ridge which will in turn cause upper troughs downstream of (northeast of) the upper ridge to amplify into eastern/central Atlantic cut-off upper vortices. The first such cut-off upper vortex is already materializing in the open eastern Atlantic as the south end of the current north Atlantic upper trough has already amplified in response to the amplifying western Atlantic upper ridge. The increasing eastern divergence zone of the amplifying upper trough was producing a new eastern Atlantic surface frontal low pressure initially in the vicinity of 40N latitude by 1200Z this past Monday... which has quickly reformed and become better defined much further south toward 33N latitude over the last 12 hours as the upper divergence maximum has quickly shifted south with the ongoing amplification of the upper trough. Over the next 24 hours the upper trough is expected to finish amplifying into a cut-off upper vortex... with the surface frontal low whirling into a position beneath the upper vortex as typically seen with a post-mature frontal low. Because sea surface temperatures in the region are running in the low-20 deg C range and the upper air temps of the upper vortex are forecast to be cold enough for instability at these water temperatures (200 mb heights forecast to be well below 1200 dekameters... at around 1180 to 1185 dekameters)... will be monitoring the surface frontal low for an increase in core thunderstorm activity and possible acquisition of tropical characteristics (tropical meaning the surface low pressure becomes aided by the warm core outflow generated by thunderstorm latent heat release).
The short-term forecast track in the outlook below is based on the position of the initial upper vortex mentioned in the prior paragraph in the latest model runs as the surface frontal low pressure of interest is expected to whirl into a position below the vortex. This has resulted in a southward shift in the forecast track due to the latest model data. For the middle of the 5-day forecat period... the current upper trough over the northeastern US is expected to evolve into a second cut-off upper vortex to approach from the northwest once it moves upstream of (northeast of) the amplifying western Atlantic upper ridge... with the initial and second cut-off upper vortices eventualy merging. Noting that the latest models have also trended further south with the merged upper vortex. For the end of the 5-day forecast period... the western Atlantic and current northeast Atlantic upper ridges are forecast to merge to the north... resulting in a longer-term west-southwest track of the surface low in the steering flow on the south side of the merged ridge. Because the upper layer of the steering merged deep-layer ridge will tend to be southwest of the surface layer... the upper vortex is expected to stay to the east while the surface low moves faster to the west... eventually placing the surface low on the northwest quadrant of the upper vortex. At this position the upper vortex will also aid in the south angle of the surface low's track while upper convergence in this quadrant of the vortex causes a long-range weakening trend of the surface low.
I have raised odds of subtropical cyclone formation to 20% for the short-term as the more reliable GFS and ECMWF models have trended with more initial seperation between the approaching northeastern US upper trough and currently-forming eastern Atlantic cut-off upper vortex. This will prevent the divergence zone of the approaching upper trough from expanding (broadening) the surface low pressure into a system without a well-defined center. And the currently well-defined center of the surface low pressure is showing signs of increasing shower activity. I have kept lower 10% odds of subtropical cyclone formation for the 48+ hour window as the approaching upper trough/second upper vortex eventually nears and likely broadens this system with its divergence zone. I drop odds of subtropical development to 0% by 96 hours as the surface low pressure weakens while encountering upper convergence as noted at the end of the prior paragraph.
IOH 24 Hr Outlook (0000Z Jan 25)... 20% chance of subtropical cyclone formation (eastern Atlantic near 30N-36W)
IOH 48 Hr Outlook (0000Z Jan 26)... 10% chance of subtropical cyclone formation (eastern Atlantic near 29N-37W)
IOH 72 Hr Outlook (0000Z Jan 27)... 10% chance of subtropical cyclone formation (central Atlantic near 28N-40W)
IOH 96 Hr Outlook (0000Z Jan 28)... 0% chance of subtropical cyclone formation (central Atlantic near 27.5N-45W)
...COMPUTER MODEL SUMMARY...
Source...Florida State University Experimental Forecast Tropical Cyclone Genesis Potential Fields (http://moe.met.fsu.edu/tcgengifs/)
1200Z (Jan 23) CMC Model Run...
**For area of interest #1... north-south elongated frontal low located at 39.5N-36.5W at 0 hours dives south while developing a southern center near 26.2N-35W through 24 hours... through 48 hours frontal low evolves into a more circular but broad subtropical low near 26.5N-34W... broad subtropical low swings west-southwest and weakens through 120 hours while reaching 20.5N-47.5W
1200Z (Jan 23) ECMWF Model Run...
**For area of interest #1... north-south elongated frontal low located at 39.8N-36W at 0 hours dives south while developing a southern center near 32.5N-35W through 18 hours... through 30 hours frontal low evolves into a more circular but broad subtropical low near 29N-36W... subtropical low develops a better defined center near 27N-36W at 42 hours... through 66 hours subtropical low shifts southwest while beginning to weaken and loses defined center while passing near 25N-37.5W... subtropical low continues west and weakens further to a surface trough near 25N-46W by 120 hours.
0000Z (Jan 24) GFS Model Run...
**For area of interest #1... north-south elongated frontal low located at 33N-34W at 0 hours evolves into a more circular but broad subtropical low near 30N-37W through 18 hours... subtropical low develops a better defined center near 28.8N-39W at 24 hours... through 45 hours subtropical low shifts south while beginning to weaken and loses defined center while passing near 23.5N-37W... subtropical low subsequently shifts west and weakens further to a surface trough near 25N-46W by 120 hours.
1800Z (Jan 23) NAVGEM Model Run...
**For area of interest #1... north-south elongated frontal low located at 35N-34.5W at 0 hours evolves into a more circular but broad subtropical low near 29.5N-35W through 18 hours... broad subtropical low swings west-southwest and weakens to a surface trough through 120 hours while reaching 25N-48W