Leslie has remained the main story in the tropics, and was expected to be a hurricane by now, but in a previous post, i expressed a few concerns I had regarding the strengthening of Leslie, including dry air and Leslie’s large size. However, Leslie had managed to stack its circulations a few days ago, and strengthening took place as result, but as Leslie was trying to organize its core, it ingested some dry air into the circulation from the upper level low and anticyclone to its southwest, and the downward MJO helped to enhance the dry air surrounding Leslie, making it difficult for Leslie to get its act together. What also complicated matters for Leslie was the very slow movement over the waters of the central atlantic which are running at 80-82 degrees and with TCHP values of 50-60 kJ cm^-2, which are decent for a tropical cyclone, but the fact that Leslie has sat over the same areas for several days has led to upwelling and cooled the waters by 3-5 degrees. This should change as Leslie begins to move more quickly in response to the trough of low pressure now over the east coast, the same one that caused the severe weather today over the mid-atlantic and the northeast. Thankfully, Leslie will pass east of Bermuda, which will spare them the worst effects of Leslie, but they will still get heavy rain and periodic gusty winds. After that, Leslie will likely become extra tropical cyclone before hitting the eastern Canadian Maritimes, but before then Leslie may have a brief window to become a hurricane during the next 48 hours before wind shear increases and SST and TCHO values decrease.
We also have had Hurricane Michael in the atlantic, and was what many, including Joe Bastardi called a “waste of a name.” Of course they are all now very quiet because Michael has become the only major hurricane thus far this year, and has produced the most ACE of any storm this season. What has really helped Michael out was its small and compact size, partially due to its nontropical origins from a trough split over the central atlantic. Michael has been somewhat limited by some light to moderate wind shear and the naturally low TCHP and SSTs over the northeastern atlantic which are 80-81 degrees, and 10-15 kJ cm^-2 respectively. Overall, Michael is no threat to any land, and should weaken again soon, but Michael has proven to be a very tough storm, and has strengthened in the face of its small size and low SST and TCHP. Michael could get involved with Leslie and may merge with Leslie after they become extratropical northeast of the Canadian maritimes.
The other area of note in the atlantic is 91L which was the wave that I was monitoring a few days ago, and I had mentioned that it could become a significant tropical cyclone over the atlantic down the road. 91L currently is a 1011 millibar tropical low pressure, and is currently located near 14.5 North, 25 West, and generally moving just north of west at around 17-20 MPH. 91L is much farther north than most of the other invests this year, and if it strengthens at a steady rate, and with the troughs of low pressure naturally getting stronger later in the summer, it will be very difficult for this system to get very to the west, but of course it is a long ways out and a lot can happen from now until then. My thinking on 91L is it probably will not too much of a threat for the US, and will likely miss to the east, but Bermuda may need to watch this system for potential impacts.
Aside from the tropics, we are also talking about fall-like weather over the US because of the abnormally strong cold front that has swept through the US, and now it is so cold behind this frontal boundary, that there are Frost advisories across central and western North Dakota, and northern South Dakota. This first frost is a little earlier than normal, and usually the first frost/freeze is not until about the 4th week of September. If this is any indicator of what is to come, it may be a much colder than normal fall for many east of the rockies. In the longer ranges, it appears that there will be a storm system which will move up into the Hudson Bay, and will help to deepen a trough of low pressure over the eastern US around SEP 17-20 and could be the first freeze for many along and north of I-80 for areas east of the rockies. The last several consecutive model cycles have been steadily getting more aggressive with the cold with this system, and If this occurs, that frost/freeze will be ahead of schedule by 2-3 weeks for those areas.
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