Tropical Storm Nangka

[Somerset Squall]

Invest 92W has rapidly consolidated east of the Philippines over the last 12 hours and thus the fourth tropical depression of the 2009 Pacific typhoon season has formed. Initial intensity is 25kts. Deep convection is persisting over the LLC and banding is improving. 04W is in low shear and warm waters which should allow strengthening. 04W will move clockwise around a large subtropical ridge to the north- initially a northwesterly motion through the Philippines and then north towards Taiwan. JTWC forecast 04W to become a typhoon prior to landfall in Luzon, which is expected to occur in 48hrs time. Significant weakening is then expected due to land interaction, but some re-strengthening could occur over the warm waters of the South China sea before 04W visits Taiwan. Both countries need to closely watch this system.

[Cookie]

things are picking up again

a few hours old this image below but it looks good

[Somerset Squall]

Great images there Cookie. They show the really intense core of convection over this very healthy looking system currently.

04W has continued to organise, and is now a 35kt tropical storm named Nangka. Nangka is currently moving slowly westwards, further west than predicted as the subtropical ridge (STR) to the north extended further west, blocking a northward curve. However, Nangka should begin to move northwestwards then northwards around the western periphery of the STR soon. The Philippines are in danger from severe flooding, as Nangka is forecast to slowly move northwards along the entire island chain, and then over Luzon. Nangka itself may intensify a little more as low shear persists, waters remain warm, and land interaction is minimal (due to small islands). Luzon however, is a greater landmass which will act to weaken Nangka beyond 36hrs. JTWC are expecting a peak of 50kts before this occurs. Nangka will probably emerge off the northwest coast of Luzon at around 72hrs where some re-intensification may occur over the warm waters of the South China sea. However, if the track shifts eastwards at all, Nangka will spend more time over Luzon and may weaken more than forecast. I thin this is unlikely due to the westward shift the STR has already taken. Extratropical transistion is expected near Taiwan as shear increases and sea temperatures cool off. By this point, Nangka will probably acclerate northeastwards along the northwestern and northern periphery of the STR into the open, cold waters of the north Pacific and complete extratropical transition.

[Somerset Squall]

Nangka has strengthened this evening and intensity is now at 45kts. Nangka has continued to move westwards as the stubborn ridge to the north has also extended a little further west, delaying the northward turn. This means that Nangka may move west of Luzon, therefore less weakening is likely before the storm moves towards Taiwan. In fact, as there are dual outflow channels and shear is low, Nangka may become a typhoon to the west of Luzon before moving into the higher shear near Taiwan. It is also possible that Nangka won't reach Taiwan at all if the subtropical ridge to the north keeps shifting west.

[Cookie]

very heavy rain with this system.

36 hr rainfall totals above

[Cookie]

4 deaths from this sadly http://mmweather.ning.com/profiles/blogs/4...t-in-twister-in

[Somerset Squall]

Nangka has weakened slightly to 40kts due to land interaction and moderate shear. However, looking at satellite imagery it seems Nangka is finally emerging into the South China Sea. Waters are warm here, and Nangka will continue to feel the benefits of good outflow which will offset the moderate shear to allow some steady intensification over the next day or so. If shear eases, some aggressive strengthening is possible- though shear currently is expected to remain a hindering factor. Nangka will push northwards along the western periphery of the subtropical steering ridge which has steered Nangka for it's entire lifetime so far. Eventually, Nangka will accelerate northeastwards near Taiwan as the storm reaches the northern periphery of the ridge. At this point, shear will increase substantially which will begin a quick weakening trend, at around 72hrs. Though JTWC doesn't officially forecast Nangka to become a typhoon in the South China Sea, I think it certainly is possible.

[Cookie]

moved into open waters for now

[Somerset Squall]

Indeed it has Cookie. However, shear has increased sooner than expected and this will cap any further intensity gains. Intensity remains at 40kts. The LLC was partially exposed earlier though a small lobe of convection has flared over the centre in the last few hours. Most of the convection is lumped south of the centre, displaced by the shear. Nangka's track has continued to shift westwards due to the westwards motion of the subtropical steering ridge controlling the system. Nangka is now moving north-northwestwards but much later than expected which now makes landfall in Southern China the more likely option, rather than grazing Taiwan.

[Cookie]

[Somerset Squall]

High shear is finally overwhelming Nangka. Intensity has decreased to 30kts and thus Nangka is now a tropical depression. Nangka isn't far from landfall, which should occur later tonight/tomorrow morning. The system will then rapidly dissipate inland within the next 24hrs. The system will bring heavy rains to areas east of Hong Kong but it shouldn't be anything too serious I would imagine.

[Cookie]

last warning issued

Last warning by JWTC

WTPN32 PGTW 262100

MSGID/GENADMIN/NAVMARFCSTCEN PEARL HARBOR HI/JTWC//

SUBJ/TROPICAL CYCLONE WARNING//

RMKS/

1. TROPICAL DEPRESSION 04W (NANGKA) WARNING NR 018

01 ACTIVE TROPICAL CYCLONE IN NORTHWESTPAC

MAX SUSTAINED WINDS BASED ON ONE-MINUTE AVERAGE

WIND RADII VALID OVER OPEN WATER ONLY

---

WARNING POSITION:

261800Z --- NEAR 22.9N 114.4E

MOVEMENT PAST SIX HOURS - 325 DEGREES AT 08 KTS

POSITION ACCURATE TO WITHIN 060 NM

POSITION BASED ON CENTER LOCATED BY SATELLITE

PRESENT WIND DISTRIBUTION:

MAX SUSTAINED WINDS - 025 KT, GUSTS 035 KT

WIND RADII VALID OVER OPEN WATER ONLY

DISSIPATED AS A SIGNIFICANT TROPICAL CYCLONE OVER LAND

REPEAT POSIT: 22.9N 114.4E

---

FORECASTS:

12 HRS, VALID AT:

270600Z --- 24.5N 113.6E

MAX SUSTAINED WINDS - 015 KT, GUSTS 025 KT

WIND RADII VALID OVER OPEN WATER ONLY

DISSIPATED AS A SIGNIFICANT TROPICAL CYCLONE OVER LAND

---

REMARKS:

262100Z POSITION NEAR 23.3N 114.2E.

TROPICAL DEPRESSION (TD) 04W (NANGKA) LOCATED APPROXIMATELY 40 NM

NORTH-NORTHEAST OF HONG KONG, CHINA HAS TRACKED NORTHWESTWARD AT 08

KNOTS OVER THE PAST SIX HOURS. TD 04W HAS MADE LANDFALL EAST OF HONG

KONG AND WILL CONTINUE TO TRACK INLAND, WITH FURTHER WEAKENING OVER

LAND. THIS IS THE FINAL WARNING ON THIS SYSTEM BY THE JOINT TYPHOON

WARNING CENTER (NAVMARFCSTCEN). THE SYSTEM WILL BE CLOSELY MONITORED

FOR SIGNS OF REGENERATION.//