Super typhoon Usagi moves over Taiwan
Friday, September 20, 2013, 6:28 PM -
Super typhoon Usagi is currently affecting Taiwan. The radar image below clearly shows the outer bands of Usagi pushing inland with the eye clearly visible.
The Joint Typhoon Warning Center (JTWC) estimated winds on September 20th at 8:00 am EDT to be sustained at 130 knots (~240 km/h) with gusts of 160 knots (~300 km/h) over open waters.
Current forecast indications are that winds along the southeast coast of Taiwan could range from 75 km/h near the Chenggong region with 150 + km/h likely for the extreme southern tip.
On top of that, rainfall totals for the southwestern part of Taiwan could push the 20 inch mark (that’s about 510 mm) as indicated by the GFS rainfall forecast below.
That’s nearly the total average annual rainfall that Toronto receives -- this sits at about 685 mm, according to the 1971-2000 climate normals.
As Usagi interacts with Taiwan it is forecast to dissipate and continue to do so as it enters into the South China Sea. This will occur for a couple of reasons -- the first one being the frictional convergence induced by Taiwan.
The second is because sea surface temperatures (SST) within the South China sea are a little cooler than the surrounding Pacific.
The first graphic below shows SST and the second shows the JTWC’s forecast track of super typhoon Usagi. Comparing the two we can see that Usagi will be exiting waters that range from 28°C to 30°C into waters of 26°C-30°C.
That's not a drastic change, but it is enough to prevent Usagi from strengthening once it is west of Taiwan and free from its frictional influence.
The strongest part of any hurricane or typhoon is the northeast quadrant.
This section of the storm has the strongest winds and largest storm surge, because you have the storm-induced wind speeds as well as the storm's translational speed interacting together.
So, if a hurricane or typhoon’s winds are 120 km/h and it is moving north at 10 km/h, then the resulting winds in the northeast quadrant will equal 130 km/h with the opposing effect on the western flank.
I bring this up because this is the main concern for Hong Kong.
Model guidance has been back and forth as to whether or not the northeast part of the eye will hit Hong Kong: Some runs have indicated yes, and some have indicated no.
The latest model guidance from Friday morning (EDT) does show the eye of Usagi tracking just to the east of Hong Kong as well as the JTWC forecast shown above, sparing Hong Kong from the worst but not completely out of the clear.
Peak wind speeds are still forecast to range from 120-150 km/h and rainfall amounts could push the 150-180 mm range.
But a shift in the track to the to south by 2-3 degrees would but Hong Kong right in the path of the northeast quadrant of the storm, exposing the major city to the worst Usagi has to offer.