Over the Tropical Western Pacific, the most potent monsoon trough setup of the year thus far has allowed Typhoon Lan to take center stage. Lan is a particularly large, sprawling system, and with favorable background conditions, is a system with a very high intensity ceiling. How much of this potential becomes realized depends largely on notoriously difficult to forecast inner core dynamics. Lan will move to the north, slowly at first but with a gradual increase in speed, and then to the northeast into the mid-latitudes. Prior to becoming a likely significant extratropical cyclone, Lan is expected to impact Japan with either a pass just offshore or direct landfall in about five days. An additional system or two may accompany Lan, but the impacts from Lan’s vast circulation should keep any additional systems in check concerning intensity.
***NOTE: While I would consider myself well-learned in meteorology, I am still a student with more to learn before becoming a degreed meteorologist. This forecast is not from an official source and should not be treated as such. For official information, please refer to your local weather agency.***
The 2017 Pacific Typhoon Season has largely been accompanied with a very weak or absent monsoon trough, so the season is unsurprisingly running well below seasonal activity. However, a high amplitude upward pulse from Madden Julian has ejected east from the Maritime Continent and overspread the western Pacific. Accompanying it has been a very impressive surge in monsoon southwesterlies extending into the Philippine Sea. As mentioned in the post’s lead paragraph, this is by far the most robust monsoon trough setup so far this year. Taking shape at the center of the Philippine Sea on the monsoon trough is Typhoon Lan. As of 03Z October 18, Typhoon Lan was located at 12.1ºN, 132.5ºW. Both JMA and JTWC assessed maximum sustained winds of 65 kt in their latest advisory packages.
As can be seen in the loop above, Lan is not the most visually striking typhoon. That is because it has yet to put together a respectable inner core, despite very favorable conditions. To me, this appears to be partly because of Lan’s size. Lan originally developed as a more normal sized system initially, but an additional surge of monsoon southwesterlies from the parent monsoon trough wrapped around and into the circulation, greatly expanding the system’s size. Lan featured an impressive CDO 12 or so hours ago and looked to be in the midst of core building, but the convection over the center has since collapsed. Conditions will remain particularly favorable for the next 72 hours prior to Lan’s passage of 20ºN, but considering the current state of the core (seen in a METOP-B pass from 0054Z October 18), it will probably take at least 24 hours for Lan to build a core and take advantage of the favorable conditions. Once a core is established, intensification is likely to be of the rapid variety. Most intensity guidance take Lan to category 4 intensity in 48 hours. Some guidance like the HWRF is probably consolidating the core too quickly, but forecasting a category 4 sometime in the next 48-72 hours does not appear unreasonable to me.
Lan will be moving north into a slight weakness in zonal subtropical ridge. Poleward outflow from Lan may actually help enhance the subtropical ridging to the east, which in turn will help draw Lan to the north with a gradual increase in speed. Lan should then begin to take on an easterly component to accompany the present northward motion once it begins to round the axis of the eastern ridge near 25ºN in 3-4 days. It is around this time when Lan will begin to interact with the right entrance region of a standing jet streak. The jet streak may temporarily increase upper divergence, but it will also begin shearing Lan as well as introducing dry air aloft into the circulation. The approach towards the mid-latitude flow in which the jet streak is embedded within will result in Lan’s acceleration towards the northeast. Lan is expected to pass near or just clip Honshu as a weakening, but still possibly significant system in slightly over 5 days. Lan could bring the potential for strong winds and impressive wave action along the coast, but rainfall may be the biggest threat to Japan from Lan. Due to Lan interacting with the right entrance region of the jet streak, heavy rain may extend well ahead of the main system.
Due to the track largely parallel with Japan, only a small difference in trajectory will exist between a direct strike and an offshore pass. At the moment, I am favoring a direct strike. Most guidance members are leaning towards the left of the two options. Probably the highest regarded member of guidance keeping Lan offshore is the HWRF. However, as mentioned earlier, the HWRF is probably consolidating Lan’s core too quickly, in turn ramping up Lan earlier than is likely. This may be impacting downstream depictions of subtropical riding and the jet streak, in turn steering the system further to the east. It’s a solution worth watching, but more the reliable guidance members remain to the west, landfalling in Honshu. Keep in mind that all the normal caveats for a 5+ day track forecast apply here. A Honshu encounter is just beyond JMA’s and JTWC’s 5 day forecasts, but an extrapolation from their 5 day forecast points appear to favor a direct strike as well.
Due in part to a busy schedule balancing college courses and a part time job, this is regrettably my first entry in over a month. I’m not sure I can keep a regular blog schedule like I was able to over the summer, but I will try to post when I can. Thank you all for reading! Until the next entry is up, updates will come in this entry’s comments section.