Tropical Storm Alberto Forms off of South Carolina
- 2 commentsTropical Storm Alberto has formed off of the Carolina coast, fulfilling the concern we’ve had for a warm-core low to develop over the Gulf Stream beneath the upper-level trough split that has become cut-off near the SE U.S. coast. Alberto currently has tropical storm force winds of 45mph. Moderate convection has been sustained for about 18 hours now, mostly weighted in the northwest quadrant. The southeast quad is devoid of convection due to a lack of strong inflow and wind shear from the jetstream which is just to the south. Directly over the center of the storm, however, wind shear is weaker due to the elongated upper low becoming partially stacked with the surface low, something that some of the big-hitting models did not see happening a few days ago, but was still a concern based on some of their biases, which were discussed in the previous post. A stacked low has allowed convection to warm the mid-upper atmosphere and transition the low into a warm-core system that can now be considered a tropical cyclone. Upper-level anticyclonic outflow to the north of the system is further evidence of its warm-core transition.
Alberto will be meandering within weak steering currents off the SE U.S. coast for the next day or so before really going anywhere due to the blocking ridge over New England which is preventing any movement northward. Later tomorrow and Monday Alberto should begin moving northward or northeastward towards the North Carolina coast, likely giving it a close brush or a brief landfall around Monday before curving out to sea east of the mid-Atlantic coast. Given that most of the weather is weighted on the northwest side of the storm, residents along the North and South Carolina coasts should expect heavy showers and the possibility of tropical storm force winds even if the track keeps Alberto a bit offshore. Alberto is tiny, so the heaviest impacts may be localized to a rather small swath.
Alberto does have some problems to face over the next couple of days that will likely limit intensification. A low to the northeast and a new low forecasted to develop to the southeast coming up from the Bahamas will probably steal low-level inflow from the eastern side, making it difficult to develop thunderstorms there. The northern low moving towards the mid-Atlantic coast will also start shoving dry continental air down at Alberto’s northwestern side, which will likely start choking the system a bit, causing it to weaken when it begins moving northward. Sea surface temperatures are also only around 26C in Alberto’s section of the Gulf Stream, and drop off by several degrees close to the coastline. While this can support a tropical storm and even a minimal hurricane in this thermodynamic environment, the other conditions involved make me believe that Alberto likely can’t exceed a 60mph tropical storm, and a peak intensity between 50mph and 60mph is probable. Peak intensity should occur before any possible landfall, and Alberto will likely be weakening as it recurves.
Overall, Alberto is not a particularly significant or dangerous threat, but could bring tropical storm conditions to the North Carolina coastline as it brushes by or makes landfall there around Monday. This kind of situation was expected with this type of early-season setup, and the pattern delivered. After Alberto, the northwest Caribbean and the Gulf of Mexico will have to be watched for the next focus of disturbed weather later next week and beyond.
We shall see what happens!
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Possible Hybrid Development off the Carolinas this weekend; West Caribbean Turns Wetter Along with Florida, Bahamas
- 3 commentsThe evolution of a complex pattern this week will create an opportunity for a hybrid low to develop off the SE U.S. coast, and at the very least a continued wet pattern from Cuba, Florida, and the Bahamas up to the Carolinas. An upper low moving across the southern U.S. over the next few days will be splitting off from the westerlies and becoming stuck near the SE U.S. coastline by Friday and into this weekend. An old frontal boundary has already stalled out east of Florida and extending down towards the Yucatan Peninsula, sparking showers across the entire area. This trough is tapping into a stream of tropical moisture originating in the eastern Pacific, where Tropical Storm Aletta recently formed and another monsoonal low is developing south of Mexico. As the upper low stalls this weekend, a hybrid, subtropical low may form underneath of it as it gets trapped south of a blocking ridge over New England for a few days.
The models are currently divided fairly evenly on their support for this event, with the UKMET and CMC leading the way and showing a tight low on their most recent runs, but the ECMWF and GFS are more timid with their solutions. The key difference seems to lie in the positioning of the upper low. The UKMET and CMC take the upper low closer to the coastline or out over the water, allowing the low shear environment to envelope the surface low and facilitate warm-core development. The ECMWF and GFS, on the other hand, keep the upper low inland and do not allow warm-core processes to intensify the surface low, though the ECMWF joins the UKMET and CMC in showing a weakening system as it moves inland, suggesting at least a partially warm-core low that would weaken over land. It will be interesting to see if the GFS is too far inland with the upper low because of an overdone forecast for tropical development in the western Caribbean, which would increase the resistance from the upper ridge southeast of Florida, thereby blocking the upper low from moving out over the Gulf Stream.
Despite the overbearing GFS, a festering low may start developing in the western Caribbean this weekend as well, and into next week may at least make the region look interesting after the southeast hybrid low has left the scene. With the MJO pulse currently reaching its maximum amplitude over the eastern Pacific and Caribbean, a system of interest cannot be ruled out, especially with the aforementioned classic early-season development pattern of low heights and old frontal boundaries south of the United States. This kind of a setup consisting of one threat to the north of the subtropical jetstream, and then a second threat to the south of the jetstream later, of more truly tropical origin, is a common progression of things in the pre-season or early hurricane season. We don’t always get development from either, but it sets up at least the opportunity.
Overall, at this time I don’t expect a particularly significant system to develop off the SE U.S. coast, however, the pattern favors some kind of mischief to take place as all of this tropical moisture made available by the MJO interacts with old surface troughs lying around in the region and the cut-off upper low that will be stalling nearby. Regardless, I expect the pattern will turn even wetter for the Carolinas this weekend as at least a surface trough laden with tropical moisture becomes blocked by the New England high and makes a move for the coast as it gets pulled underneath the upper low. Florida and the Bahamas may also get in on some more rain. Later next week, Florida and the Bahamas may get yet another shot of tropical rains as something gets drawn out of the western Caribbean, possibly a weak development threat.
We shall see what happens!
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Watching for Tropical Stirrings in the Caribbean May 20th – June 5th
- 2 commentsThe time of year has come to start watching for our first tropical system to form in the Atlantic. The period from May 20th through June 5th is starting to look favorable for monsoonal activity to invade the western Caribbean and move north or northeast into the eastern Gulf of Mexico or the Bahamas. Convection should be enhanced in the area by a Madden-Julian Oscillation (MJO) pulse coming eastward into the eastern Pacific and western Caribbean in a couple of weeks. The model MJO forecasts below show the GFS being the most bullish, which is not surprising.
In fact, the GFS showed this happening 3 weeks ago. I don’t mean that it saw this event 3 weeks in advance. I mean that saw it happening way too early, and that is a model failing. Indeed, the GFS has been showing a tropical storm in the western Caribbean on almost every run for 3 weeks now in the western Caribbean, which until now has made no sense. However, it is worth mentioning now, because the pattern is more supportive for this to occur. A slower MJO progression as shown by the European models is called for, indicating that at some point during the May 20th-June 5th period we should see a pickup in tropical thunderstorm activity in the Caribbean that could lead to an early storm. This is this morning’s 12z GFS run’s depiction of the storm at Day 11, which may still be a bit too early:
Additionally, the upper pattern around this time is looking suspicious for tropical activity. The GFS ensembles have the polar jet retreating into southern Canada, with general ridging over the central and eastern United States. However, 500mb heights consistently sag south and east of the U.S., with a weak trough of some kind hanging back and reaching into the NW Caribbean, a pattern indicative of upward motion occurring in the tropics, and a trough becoming involved in the monsoonal circulation in the early season is a good way to get a quick tropical storm that gets drawn out of the Caribbean.
Now this is still nearly 2 weeks out, so details will be scarce until the time draws nearer, but if we do get some kind of a development, expect it to be drawn out of the western Caribbean northward into either the eastern Gulf of Mexico or across Cuba into Florida or the Bahamas, but likely being sheared to the east as it goes, typical of early-season systems. If anything the pattern should turn wet for these areas, hopefully bringing needed tropical rains to Florida. It is also possible that the eastern Pacific will see activity and possibly their first storm before the Atlantic has a chance to see any action. The time has come to start monitoring the tropics again.
We shall see what happens!
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