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Public advisoryEdit

TROPICAL STORM CHAN-HOM PUBLIC ADVISORY NUMBER 15

WIKI-HURRICANES FORECASTING CENTER

6:00 PM JST FRI JUL 03 2015

...CHAN-HOM RAPIDLY WEAKENING AS FAST AS IT DEEPENED YESTERDAY...

SUMMARY OF 6:00 PM JST...09:00 UTC...INFORMATION

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LOCATION...10.5N 148.3E

CENTER POSITION CONFIDENCE: HIGH

ABOUT 335 MI...540 KM SE OF HAGATNA, GUAM

MAXIMUM SUSTAINED WINDS...45 KT...50 MPH...80 KM/H

MINIMUM BAROMETRIC PRESSURE...999 MB...29.50 INCHES

PRESENT MOVEMENT...NEARLY STATIONARY

DISCUSSION AND 48-HOUR OUTLOOK

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At 6:00 pm JST, the center of Tropical Storm Chan-Hom was situated near 10.3N, 148.5E, or about 335 miles (540 km) southeast of Hagatna, Guam. Maximum sustained winds were 45 knots (50 mph, 80 km/h), with higher gusts. The minimum barometric pressure was 988 millibars (hPa; 29.18 inHg), and the cyclone is nearly stationary. Although weakening has taken place over the past few hours and some fluctuations in intensity are possible over the next day or so, environmental conditions are expected to become more favorable for significant strengthening over the coming days. Residents living in the Mariana Islands should take precautions to protect life and property now.

NEXT ADVISORY

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Next complete advisory at 12:00 am JST.

$$

Forecaster YE

DiscussionEdit

TROPICAL STORM CHAN-HOM DISCUSSION NUMBER 14

WIKI-HURRICANES FORECASTING CENTER

6:00 PM JST FRI JUL 03 2015

As fast as Chan-hom deepened yesterday afternoon, the storm has degenerated such as quickly. The well-defined low-level circulation has rapidly decoupled far away from the mass of the deep convection, which it itself is not only losing organization, cloud tops there are warming and the overall mass is less circular than ever. A microwave passes from this morning showed a structure typically seen in a post-tropical remnant low. A 4z estimate yielded a Dt of 2.0/30 knts, while a 3z estimate from SAB supported a FT of T3.0./45 knts. Given how fast the decoupling took place and how it started several hours ago, the intensity has been cut to 45 knts, on the high end of the estimates.

The recent weakening trend is not unexpected as both the HWRF and GFS, in my eyes, two of the three best computer models, showed some sort of weakening and I haven't been very keen on intensifying this storm till the longer ranges. As such, the overall philosophy remains unchanged. UW-CIMSS shear maps indicate that Chan-hom continues to undergo northeasterly wind shear in the range of 20-35 knots. This shear appears to be originating from an upper-level low a short distance to the cyclone's north and east that has recently amplified over the last several hours quite a bit, hence the rapid weakening trend noted. However, over the coming days, this feature is expected to drift northwesterly while simultaneously weaken. This should allow for anticyclonic flow aloft to develop and lower shear atop Tropical Storm Chan-hom. The tricky portion of the forecast enters the picture when deciphering exactly when these unfavorable upper-level winds will relent. As mentioned above, recent runs of the GFS and HWRF suggest that Chan-hom will maintain intensity or even weaken further over the next 24 hours. Given that I can't see the storm presentation getting any worse, the 12 hour and 24 hour points maintain the storm's intensity. Thereafter, the GFS and HWRF level it off, if not, slow slight re-intensification. Assuming that the shear relaxes by then, conditions should be favorable for re-strengthening. However, since I think this storm will take its sweet time building an inner core, especially given the current exposed nature of the system, only slow deepening is shown.

Once the storm clears the Marianas, a more rapid intensification phase could begin, and I expect the storm to finally close off an inner core in between day 2 and day 3. Then, the fun begins, and rapid intensification is possible. While I could see this deepening as much as 55-60 knts in a day, I will assume that their will some dry air intrusions from time to time that prevent explosive (but stil) intensification. By day 5, Chan-hom should nearing its peak intensity as shown by the GFS. The new intensity forecast is more detailed then the previous one, and is only on the high end of the guidance and closely follows the GFS/HWRF. The new forecast s lower in the short-term, but higher in the long-term.

Although the cyclone has not moved much today, a mid-level disturbance to the Chan-hom's north, which was not forecast by most global models, created a weakness in the subtropical ridge to the system's north to form. This break will persist for another 24 hours or so, and a general northward track is expected during that time. After that time, a series of shortwave troughs across Japan and surrounding regions is expected to erode the western edge of the subtropical ridge and allow Chan-hom to track generally northwestward. The new forecast track is a little north of the previous, and is now on the north end or in the middle of the guidance. On its current projected path, Chan-hom is expected to cross the southern and central Mariana Islands as a typhoon, and residents there should be taking precautions accordingly.

INIT 03/0900Z 10.5N 148.3E 45 KT 50 MPH

12H 03/1800Z 10.7N 147.6E 45 KT 50 MPH

24H 04/0600Z 11.4N 146.3E 45 KT 50 MPH

36H 04/1800Z 13.2N 145.4E 50 KT 60 MPH

48H 05/0600Z 14.2N 144.2E 55 KT 65 MPH

72H 06/0600Z 17.4N 141.2E 75 KT 85 MPH

96H 07/0600Z 20.4N 137.2E 105 KT 120 MPH

120H 08/0600Z 23.9N 132.5E 135 KT 155 MPH

$$

Forecaster YE

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