Western Pacific Tropical Analysis: July 13, 2017

The Tropical Western Pacific remains rather quiet at the moment, but at least two area will have a non-zero chance for tropical development in the next few days. The first area is currently located in the subtropical waters south of Japan has been tagged as Invest 93W. The window for 93W is brief though, and I do not expect it to develop. The second area is only just becoming discernible in the South China Sea as a modest surge in the Southwest Monsoon begins to transpire. While I do believe that development chances are higher with this emerging disturbance than they are with 93W, development prospects are still not especially good. Should it develop, the system’s ceiling will remain very low. No other areas are expected to emerge with development prospects over the next seven days.

***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.***

As mentioned my previous entry, a piece of energy became orphaned from its parent system from the mid-latitudes. This system has lingered in the subtropical waters south of Japan near Chichijima for the past couple of days. In that time it has gained enough organization to become tagged as Invest 93W. As of 18Z July 12, 93W was positioned at 25.1*N, 140.8*E. An RGB composite visible loop of 93W is provided below

As it stands right now, 93W is probably near the zenith of its organization. From this point onwards, it will likely begin a losing battle with westerly shear and dry air (both seen in the July 13 00Z Chichijima sounding). An upper level low is located to the system’s north is the primary reason for these increasingly hostile shear, and the dry air is in constant supply due to upper level convergence/subsidence on the system’s northwest flank. These two factors will bring 93W to its demise over the next 2 or so days as it continues to meander the subtropical waters in the vicinity of Chichijima.

Further to the southwest in the Tropical Western Pacific, a modest surge in the Southwest Monsoon is beginning to ramp up across the South China Sea. This surge is amplifying convergence near the tip of the low level jet, leading to increased convective activity. Over the next few days, surface pressure falls from this thunderstorm activity should help to spawn a broad and weak circulation. The 12Z EMCWF’s depiction of this evolution is provided below.

The main question with this potential system is will it have enough time to tighten its broad monsoon circulation enough to be classified before moving inland over southeastern Asia in about four days. At the moment, I am leaning a little more towards it staying too broad, giving it about a 40% chance at classification. Even if it does earn classification, any system that develops will remain weak due to the time constrains mentioned above.

Aside from these two prospects, the Tropical Western Pacific will remain uncharacteristically hostile for at least the next week. At the low levels, there is very little hint of any monsoon southwesterlies anywhere east of the Philippines. Without these southwesterlies needed to maintain the monsoon trough, any disturbances along the Intertropical Convergence Zone have been weak and unable to develop. Aloft, 500 mb heights remain higher than normal, indicating continued upper level subsidence. An anticyclonic wave break in the mid-latitude Western Pacific has also helped to reinforce the Tropical Upper Tropospheric Trough, which is currently covering an abnormally large area across the heart of the basin. Both the 500 mb normalized height anomalies and 355 K Potential Vorticity from the July 12 18Z GFS is displayed below.

If no tropical development occurs by this weekend, 2017 year to date Accumulated Cyclone Energy (ACE) will fall below that of 2010, making 2017 one of the three least active seasons up to that point since 1970.

My next entry will be posted this weekend (Probably Sunday). Until the next entry is posted, analyses and updates in forecast philosophy will arrive in the comments section.

4 thoughts on “Western Pacific Tropical Analysis: July 13, 2017”

  1. You can see the low level jet associated with the SW monsoon beginning to ramp up on the July 13 00Z sounding from Ho Chi Minh. Everything from just above the surface to just above 700 mb is from the west at 20 kt or greater. Expect these west winds to increase in magnitude some over the next couple of days.

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