Early Activity Continues for Atlantic Hurricane Season 2011
May 26 Morning,
Introduction
Early this week, we saw the formation of yet another early-season Atlantic tropical disturbance (Invest 92L). This is already the second system that has gained attention in the National Hurricane Center Tropical Weather Outlook before June 1. The first system was 91L about one month ago. Hurricane season in the Atlantic doesn't officially begin until June, and lasts through November 30. These months demarcate when a great majority of Atlantic tropical (and subtropical) cyclones occur.
A Pattern in these Early Disturbances
For meteorological interest, I have posted a synoptic history of 92L below. Comparing 91L at this link and 92L below seems to indicate a common thread for how this year's early systems are forming. In both cases, 91L and 92L form from what I would like to call for now a "subtropical jet reflection" of mid-latitude upper troughs. In the eastern divergent side of an upper trough is commonly a band of clouds. These "subtropical jet reflections" I note are a SEPERATE band of clouds further southeast of the upper trough cloudiness. This seperate band of clouds seems to be associated with a secondary jet in the subtropical latitudes that flows parallel to the south side of a corresponding mid-latitude upper trough.
How these secondary subtropical jets (or so-called "reflections") form in advance of the mid-latitude upper troughs is a bit mysterious to me, and they seem to coincide with the passage of an upper trough to the north. For instance in the previous blog post of 91L, I have a sequence of satellite images prior to 91L's formation (Figure 1 of the previous post). The subtropical jet reflection preceding 91L doesn't show up until upper troughs 2 and 3 pass by to the north on April 11 and 12. In the satellite sequence shown in Figure 1 of this post, the subtropical jet reflection preceding 92L develops when a massive and slow-moving upper low in the western Atlantic passes by to the north on May 9 through 15.
Second, an upper low has to spin up within the subtropical jet to trigger the disturbance. In Figure 1 of the previous post, upper troughs 5 and 6 dig into the subtropical jet, and leave behind upper low 8 just prior to 91L's genesis. In Figure 1 below, an upper low spins up along the subtropical jet on May 23 (the day of 92L's formation), apparently due to an upper trough from the northwest digging in.
These upper lows have high upper divergence in their northeast quadrant with respect to the subtropical jet. In that quadrant, the flow branches between easterly flow going into the upper low and the mainstream westerly flow of the subtropical jet, and such divergence causes the genesis of a surface low that becomes our disturbance.
Synopsis of 92L
Similar to disturbance Invest 91-L (see this link), this disturbance appears traceable to a persistent southwesterly subtropical upper-level jet developing out ahead of mid-latitude upper troughs. As shown by the GOES 12 satellite imagery sequence in Figure 1 [1], an impressive mid-latitude upper low dominated the western Atlantic Ocean in mid-May with a subtropical southwesterly jet developing to its southeast. The upper low exited the western Atlantic on May 15 to 17 with divergence from a trailing upper trough supporting continued cloudiness along the subtropical jet. A second upper low vortex with an upper trough entered the western Atlantic Ocean from the northeastern United States on the 17th to the 20th, with the cloudiness along the subtropical jet becoming concentrated toward the divergence associated with the second system. The rapid genesis of this disturbance on May 23 appears to coincide with this second upper disturbance digging into the subtropical jet, which apparently led to the formation of a vigorous upper low along the subtropical jet at a location well east-southeast of Bermuda. Strong divergence northeast of the upper low center also led to the development of a surface low that was monitored for subtropical cyclone formation due to the high organization of the associated storm activity.
The surface low became stationary while trapped between low-level ridges to the northeast and northwest. Simultaneously, the parent upper low and organized storm activity moved away to the east and northeast. This exposed the surface low to shearing upper westerlies in the wake of the upper low beginning on May 24, and the system was no longer monitored for development.
Figure 1: Above is a sequence of GOES (Geostationary) 12 infrared satellite images [1] leading up to the formation of disturbance Invest 92-L (blue is upper air analysis and red is surface analysis). Red Ls are analyzed surface lows. Blue dashed lines are analyzed upper troughs, and blue Ls are analyzed upper lows.
[1] Index of ftp://eclipse.ncdc.noaa.gov/pub/isccp/b1/.D2790P/i mages/2011. GOES12 Satellite Imagery Archives under Folders 129 to 143.
Figure 2: Colorized infrared satellite image of the subtropical disturbance taken during the early morning of May 24
Invest 93L over the next days?
I have been tracking yet another "subtropical jet reflection" that I am suspecting could be yet another early season disturbance in the making, this reflection occuring out ahead of the mid-latitude upper low/trough that produced the infamous Joplin, Missouri Tornado earlier this week. I encircled the area of cloudiness in Figure 3 that I suspect (and am predicting, albeit while going out on a limb) could evolve into Invest 93L over the next days, located just south of Bermuda tracking due east. This reflection first showed up as a seperate band of clouds on May 23 over the Bahamas, to the southeast of the upper low/trough that dropped the Joplin tornado. The reflection has been traveling east with the longitudinal location of that upper trough/low.
Figure 3: Tropical Atlantic analysis for May 26, 2011. Blue Ls are analyzed upper lows, blue dashed lines are upper troughs, and other features are highlighted in red.
This "reflection" has also become associated with a weak upper trough axis that has been located due south of the main upper trough/low that produced the Joplin tornado. Note that all upper troughs in Figure 3 are marked with blue dashed lines.
I am not sure if this weak upper trough is directly a southern fracture of the Joplin upper-level system, or if this weak upper trough is a manifestation of the Joplin upper-level system digging toward the direction of its subtropical jet reflection. Either way, divergence on the east side of this weak upper trough has been producing the cloudiness for this suspect area that may become 93L. The cloudiness has become just slightly better organized with a comma shape over the last several hours. However, the cloudiness is currently not very strong, and there is no surface trough/low with this system as of yet in the surface analysis provided by the National Hurricane Center.
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