Western Pacific Tropical Analysis: July 19, 2017 – Subtropic Shenanigans

After an exceptionally quiet start to the 2017 Pacific Typhoon Season, the Tropical Western Pacific is about to enter a more active period, and some of this activity appears like it will becoming from some unlikely sources. Three invests are currently declared across the basin, and all have at least some shot at earning a name. From west to east, invests 95W and 97W are currently located over the eastern waters of the Subtropical Western Pacific, and hail from non-tropical origins. Invest 95W, which is also a JMA tropical depression, probably has the most immediate chance at development in the basin, but the future of 97W looks particularly interesting. Invest 96W resides in the South China Sea and has a chance to follow in Talas’s footsteps. The remnants of a convective complex off the southeastern Kyushu coast may also become tagged as an invest at some point. The monsoon trough also returns to the Philippine Sea this week, and the potential for development here beyond the five day period appears higher than average.

***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.***

Of all the development prospects, Tropical Depression/Invest 95W is presently the furthest along. Per JMA as of 18Z July 19, the system was positioned at 26.5ºN, 161.2ºE and moving with only a slow westward drift. JMA has begun to issue advisories on Tropical Depression 95W, meaning that they are expecting to upgrade it to a tropical storm within 24 hours. Should their current forecast verify, it would be named “Noru.”

95W is one of two invests currently designated that have non-tropical origins (the other is 97W a little to the east). Unlike with 97W though, the low level vorticity isn’t now just working its way down to the surface and has been around for a few days. The most recent ASCAT pass of 95W revealed a fairly organized but not quite closed circulation with winds of 25-30 kt just east of the center. Considering the improved convective evolution and development of upper level outflow since the time of that pass, it’s not hard to imagine that 95W could end up being named as the JMA is currently forecasting. In fact, I would be surprised if 95W is not named at this point. Regardless, 95W will be in no hurry moving to the WNW and then NW as it rounds the southwestern edge of a blocking mid-latitude ridge.

Intensity guidance is not particularly aggressive with 95W, and the only member that brings it above the 64 kt typhoon threshold is the HWRF. Considering the abnormally warm waters that currently reside in the Subtropical Western Pacific, moist surrounding airmass, and sheltered location from stronger upper level winds below the blocking ridge, such a forecast doesn’t seem outlandish. However, I’d be more inclined to think a 50-60 kt peak intensity is more likely. Favorable conditions will only last about 96 hours at most before 95W arrives in the hostile exit region of a splitting jet rounding the top of the subtropical ridge centered over China.

Back in the South China Sea, a disturbance located near the eastern end of a weak monsoon trough has been designated as Invest 96W. As of 18Z July 19, the ATCF position for 96W was at 13.7ºN, 116.4ºE, or just west of the Philippines. 96W has been characterized by intermittent and disorganized bursting convection and does not appear to be particularly well marked at the time. 96W is currently experiencing a decently favorable setup reminiscent of the recent Tropical Storm Talas. In fact, 96W is expected to follow a very similar path to Talas. However, 96W’s initial organization is not on par with Talas initially. This difference in organization may prevent 96W from becoming a classifiable tropical storm. With about 72 hours before the system is expected to move inland near the China/Vietnam border, it wouldn’t catch me off-guard to see 96W develop. However, the poorer initial organization coupled with a slightly weaker belt of monsoon southwesterlies make me believe that no development is the most likely outcome.

The last and probably most interesting of the three currently designated invests is 97W. Like 95W, 97W currently resides in the waters of the Subtropical Western Pacific. As of 18Z July 19, the ATCF position for 97W was at 24.1ºN, 178.6ºE. This places 97W to the east of 95W and very near the boundary of the basin with the Central Pacific. 97W is an upper level low that has begun to work its way down to the surface.

Invest 97W has become somewhat of the guidance’s darling as it meanders northwest and then west-northwest across the Subtropical Western Pacific. After fully working its way down to the surface and becoming warm core, the majority of the last few guidance suites have blown up 97W into an intense system over the Subtropical Western Pacific. This is a fairly unusual solution; intense systems are fairly rare in the Pacific subtropics. Most of the intensification appears to occur after the system has moved to the northwest some, into approximately the area vacated by 95W. As previously mentioned, it is an area of sufficiently warm sea surface temperatures, abundant moisture, and low shear, so strengthening shouldn’t be totally unexpected. However, climatology does not favor some of the sub-950 mb solutions seen in recent guidance.

While modeled conditions currently look favorable for 97W over the next week, it won’t take much to introduce some hostilities. 97W is currently dancing around near the subtropical jet stream. The subtropical jet stream is currently expected to weaken and move east-southeastwards away from 97W. However, should it exert its influence on 97W longer than expected, the more intense solutions would fail to come to fruition. Alternatively, the splitting jet expected to impact 95W could extend eastwards and bring upper shear to 97W or the blocking mid-latitude ridge could introduce dry air subsidence trough subsidence that 97W could then entrain. The point is it takes absolutely perfect conditions to be maintained for an appreciable time period to get a strong system in the subtropics, and I’m not entirely sold that is currently the case. At the moment, I am expecting 97W to achieve a maximum intensity of 75-85 kt in about 5 days. However, at this point, this is an extremely low confidence forecast. The one thing that is almost for certain is that 97W should make for an entertaining system to track after an exceptionally quiet start to the 2017 Pacific Typhoon Season.

One more system that is not currently designated but deserves a mention currently lies off the southeast coast of the Japanese island of Kyushu. The system originates from a Mesoscale Convective Complex that moved off the coast about 24 hours ago and appears to be gaining some organization. However, the window for this system to develop is rather small. Strong upper level northeasterly winds are expected to begin raking over the system in about 36 hours, undoing any organization that occurs up to that point. Brief development isn’t an impossibility, but it is also not expected at this time. It wouldn’t surprise me to see this system tagged as Invest 98W though.

In addition to all the current systems in various states of development, the monsoon trough is expected to extend back to the east and into the Philippine Sea within the next five days. It is around that time that guidance is beginning to pick up on an emerging system in the Philippine Sea. It’s still too early to forecast an intensity for a system that has yet to even form an initial disturbance, but I do believe that this potential system’s ceiling is the highest yet observed the season. Whether that potential is realized is yet to be seen, but confidence in development is considerably higher than average. This possible system is most likely to track northwest or north-northwest into a break in subtropical ridging, in the general direction of Taiwan and the southern Ryukyus.

My next entry will likely arrive this weekend, but with packing and moving in my near future, it is not a 100% certainty. Until the next entry arrives, analyses and updates in forecast philosophy will arrive in the comments section.

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4 thoughts on “Western Pacific Tropical Analysis: July 19, 2017 – Subtropic Shenanigans”

  1. Lots going on right now. TD 95W just got upgraded to a tropical storm by JMA, receiving the name “Noru.” 97W is now also a depression that is likely to receive a name very soon. The next name up for grabs is “Kulap.” Guidance is actually beginning to favor Noru as the dominant storm between the two. More interaction between the two also appears likely, and this binary interaction may help keep Noru anchored in the more favorable conditions which currently surround it. Track, and by extension, intensity forecasting remains a nightmare with these two. The 12Z ECMWF solution was particularly entertaining, showing a wicked Fujiwhara between the two until they merge into one storm.

    96W is also overachieving. Although it is currently not classified as a tropical depression by either JMA or JTWC, the system has clearly organized into a very small tropical cyclone. Hopefully the agencies pick up on this development soon.

    Invest 98W has also been designated east of Luzon. Some guidance is spinning up a quick system from this Invest in advance of the monsoon trough system expected to begin consolidating late this weekend/early next week.

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  2. Update: 97W became TS Kulap, 96W is now TD 08W, 98W is TD 10W, the system beginning to consolidate on the monsoon trough has been tagged as Invest 99W, and the old convective complex is now Invest 90W. Things are going nuts. New post coming tomorrow.

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  3. No update today unfortunately. I’m in the process of moving into a new place, and I’ve gotten caught up in that. The SSD loops are acting funny today too, so instead of trying to force an update today, I’ll just wait until tomorrow.

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