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Tropical Storm Fiona: Challenging Forecast Ahead

Tomer Burg • 15 September 2022 • Current Weather

Post Highlights

Tropical Storm Fiona, the 6th named storm of this largely inactive Atlantic hurricane season, developed into a tropical cyclone yesterday in the open Atlantic waters. Unlike previous hurricanes Danielle and Earl, Fiona is expected to directly affect large swaths of land with heavy rain, flooding and gusty winds incoming to some of the Caribbean islands. Afterwards, Fiona's forecast becomes highly uncertain, both track and intensity wise, and it is too early to make any definitive calls regarding whether it will directly affect the United States and if so, then where.

This forecast post is generally oriented at the public, analyzing the latest observations with Tropical Storm Fiona, why the forecast is so uncertain, and some key points to keep in mind over the next few days as we continue to monitor Fiona.

Visible satellite centered over Fiona this afternoon, with aircraft reconnaissance observations in the colored line, and other annotations added for illustrative purposes.

Tropical Storm Fiona is currently in its 2nd day of being a tropical cyclone, after having had advisories initiated yesterday by the National Hurricane Center. While Fiona quickly intensified with maximum sustained winds of about 60 mph, it has a long road ahead if it is to become a hurricane (74 mph or higher) in the near term.

The image above shows visible satellite imagery from the GOES-16 satellite earlier this afternoon, when an aircraft reconnaissance flight from the U.S. Air Force flew into Fiona to observe the inner structure of the storm. You may be wondering – why do we need planes to fly into tropical cyclones when we have plenty of ways to observe tropical cyclones, largely from multiple satellites? While this is true, these are not in-situ observations, meaning they are not being recorded directly within the storm. For storms that are far away from land, intensity is estimated using satellite data, though these estimates can be somewhat inaccurate at times. The hurricane hunters provide much more detailed observations of the storm's inner structure, which not only assist forecasters at the National Hurricane Center, but are also entered into weather forecast models which can help improve the model's representation of the storm's structure, and accordingly improve its forecast.

Looking at the image above, we see that Fiona is a disorganized storm, with an exposed low-level circulation displaced west of the deep convection to its east. While maximum sustained winds are above 50-60 mph, these are not uniform around the storm – notice how winds are much weaker south of Fiona's center than to the north. Along with the overall disorganized structure, this suggests Fiona is still far from becoming a hurricane at the moment.

Water vapor satellite imagery, with vertical wind shear (knots) in contours. Image from CIMSS.

Looking at the CIMSS analysis, we can see why Fiona remains so disorganized. Vertical wind shear, meaning the difference in wind between the upper levels and lower levels of the atmosphere, is quite strong at about 20 knots. As the direction of the wind shear is from the west, this means that the deeper convection, which extends higher up in the atmosphere, is displaced to the east of the low-level center, which is not as noticeably affected by the stronger upper-level westerly wind.

Stronger wind shear (generally 20 knots or higher) is typically detrimental to tropical cyclone intensification, especially after a tropical cyclone has formed. This is not always the case – some tropical cyclones do intensify in spite of deep-layer vertical wind shear, which is a more complex subject and is still an area of ongoing research. For this case, however, the wind shear is aiding to prevent more substantial intensification in the short term.

ECMWF model forecast total rainfall (inches) over the next 6 days.

In the short term, Fiona is forecast to continue tracking westward, reaching Puerto Rico on Sunday and the Dominican Republic on Monday. While the intensity forecast is somewhat uncertain and depends on how vertical wind shear affects Fiona, the track forecast is fairly high confidence through Monday as Fiona remains embedded in an easterly steering flow.

Strong wind gusts and rough seas will be a hazard with Fiona, though regardless of how strong the maximum sustained winds are, the biggest hazards are expected to be heavy rain and flooding. Some of the largest flooding disasters from tropical cyclones have been from storms with fairly weak winds – for example, Tropical Storm Erika (2015), despite only peaking with 50 mph sustained wind, produced devastating flooding in Dominica. Accordingly, the image above from the ECMWF model shows the potential for over 6-10 inches of rain in Puerto Rico and the Dominican Republic. Given the mountainous terrain in these islands, mudslides will be a risk as well. If you are in the path of this storm, please continue to monitor offical forecasts for updates on local impacts.

Forecast track density for a 139-member multi-model ensemble.

The longer-term forecast becomes much more uncertain for Tropical Storm Fiona. In cases of large uncertainty, looking at a single deterministic model (e.g., GFS), or even a single ensemble suite (e.g., GEFS) does not fully capture the range of possible solutions, and looking at multiple model systems can help us better visualize the uncertainty.

The plot above shows a 139-member multi-model ensemble, combining ensemble members from the EPS (51 members), GEFS (31 members), UKMET (36 members) and CMC (21 members). Notice that while the short-term forecast up to Hispaniola is fairly confident, forecast confidence sharply decreases afterwards, with a bifurcation in tracks with some members showing a quick recurve well east of the US, and others tracking Fiona into the Gulf of Mexico. All in all, the spread in possible solutions ranges from Mexico to Newfoundland!

1800 UTC GEFS 500-hPa geopotential height spread and ensemble surface low positions.

This uncertainty is indeed very large – but it should be emphasized not all forecasts are equal. Some storms have higher confidence forecast tracks for various reasons, while others are inherently less predictable. Fiona falls into the latter category – looking at the plot above, we see that when the GEFS ensemble shows Fiona near the Bahamas, a high amplitude ridge is stationed over the US Plains, with a trough over the Northeast US. While not visible here, the trough axis also extends farther down closer to the Bahamas.

Let's discuss the range of all possible solutions to examine just why the forecast is so uncertain. If Fiona quickly recurves as some of the ensemble members show, it will interact with the upper-level trough near the Bahamas resulting in a track north. Afterwards, however, note that there's large ensemble spread with the Northeast US trough. Should Fiona recurve, a deeper trough will quickly sweep it well offshore, while a weaker trough may subsequently allow ridging to rebuild north of Fiona, resulting in a northwesterly track towards the U.S. East Coast.

In the scenario that Fiona fails to recurve and remains weak in the short-medium term, it will likely continue to track west-northwest. At that point, it may end up anywhere from the Yucatan Peninsula to Florida. From there, a large range of outcomes is possible, from Fiona dissipating, to intensifying in the Gulf, to recurving northeast back into the Atlantic, depending on Fiona's evolution as well as that of the Plains ridge.

Trend in GFS forecast tracks for Fiona over the last several runs. Image courtesy of Brian Tang's website.

The large uncertainty certainly makes the longer-term forecast for Fiona a challenge, though we do have some hints that can help narrow down the range of possible solutions. Earlier forecasts by models such as the GFS & ECMWF depicted Fiona quickly intensifying into a hurricane and recurving northward well east of the United States, posing no risk to land except Bermuda. However, this picture has been shifting in recent hours, and we can refer back to the short-term observations to get a sense of why this is the case.

The image above shows the trend in the last few GFS run forecast tracks for Fiona, from Brian Tang's Website. Notice how more recent runs have not only been more west in the longer term, but also more south in the short term. As Fiona has struggled to quickly intensify and continues to be impacted by strong westerly wind shear, prospects of a quick recurve out to sea have decreased as well, with models trending towards a weaker storm farther west.

EPS ensemble correlation matrix for Fiona's forecast. Read below for an explanation of this plot.

The plot above shows a correlation matrix using the EPS 51-member ensemble for Fiona. What this plot does is for each forecast hour, it correlates the x-axis variable (in this case, minimum pressure or MSLP) to the y-axis variable (longitude). We see many negative (blue) statistically significant values, indicating that for almost every forecast hour, a stronger Fiona correlates with a farther east track (i.e., recurve), while a weaker Fiona correlates with a farther west track (i.e., closer to the latest GFS run). In other words, the more Fiona continues to struggle intensifying, the more likely it is to fail to quickly recurve out to sea and instead continue west as a weak system in terms of wind speed, but one that is still capable of heavy rainfall and flooding.

Comparison of different ensemble means, valid early on September 23rd.

Comparing different ensemble groups, we can see that there's evident clusters in the longer term. The CMC ensembles take a weakened Fiona west into the Gulf of Mexico, the GEFS quickly recurve a strong Fiona well offshore, while the EPS & UKMET ensembles largely split the difference in the middle.

Given the reasoning mentioned earlier, the quick recurve scenario appears to be somewhat less likely. This would suggest Fiona may have a closer approach to the Southeast US and Cuba than the multi-model ensemble mean shows, though just how close Fiona gets remains uncertain. Additionally, in this scenario, Fiona would likely remain weak at least initially, should it still be classifiable as a tropical cyclone, though as discussed earlier, even a storm with weak winds is still capable of heavy rainfall and flooding.

To summarize the key points from this analysis, and the upcoming forecast for Tropical Storm Fiona:

In the short-term, heavy rainfall capable of flooding and mudslides is possible in Puerto Rico and Hispaniola. Readers in affected areas should continue to monitor official forecasts for updates on local impacts.
Fiona's forecast track is very uncertain in the longer range, though a quick recurve offshore appears to be becoming less likely as of this evening.
It is too soon to say if Fiona will directly affect the United States, and if so then where. At the moment, the East Coast into the Gulf Coast should continue to monitor Fiona over the coming days, but it's too early for definitive statements, and be aware of posts on social media that purport to know where and what impacts Fiona will have in the US.

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