Bob McDavitt's ideas for sailing weather around the South pacific

09 April 2017

TC COOK not following TC BOLA

In my last blog I made some early comparisons between TC COOK and TC BOLA.

TC COOK has NOT looped around Vanuatu like BOLA - and now is heading for New Caledonia, a place BOLA did not visit---
so it seems to me now that TC COOK is moving faster and differently than TC BOLA did
-so it doesn't compare well with BOLA after all.

See my illustrated edition at for a Track of BOLA and
see for a track of COOK.

As a Tropical cyclone with an eye-wall comes south, usually it will encounter increasing westerly winds aloft, often a jetstream. These winds blow the upper parts of the eye-wall downstream and "blow the top off" the feature, ending its tropical status and hastening its weakening.

TC Cook is expected to encounter that Jetstream on Wed--- that's also when it departs the tropics and may lose its eye-wall and its tropical status.

Just one more factor for possible comparison between COOK and BOLA may present itself:

When TC BOLA left the tropics and stalled north of NZ, it was between two jetstreams, and in a region of converging moisture. These combined to form a two-cell vertical circulation, maximising upward motion and thus rainfall. BOLA lingered for three days with this heavy rainmaker within it.

The Jetstreams around remains of BOLA (when it was just NE of NZ) are at from

"A Diagnostic study of the extratropical precipitation resulting from tropical cyclone Bola."

by Mark Sinclair (NIwa) in Monthly Weather Review V 121 p 2690-2707

It is interesting to see that the jetstreams on early Friday local time briefly offer a similar setup to the remains of TC COOK. To see the jetstreams go to

HOWEVER, it seems that TC COOK is moving steadily south now and not likely to linger, , so even if it does carry within it a two-cell vertical circulation / heavy rain maker, this feature is unlikely to stay over any one place too long. Fingers crossed.

Today, 10 April, is the 49th anniversary of the sinking of WAHINE in 1968.

The LOW involved in that event started out as Cyclone GISELLE . It had left the tropics and was weakening as it approached New Zealand, but then encountered a cold front /cold air. The Jetstream above it brought about a new period of deepening--- Taking air away from the top of the feature faster than it can be replaced and thus lowering the central pressure in the low. This sort of double-life for a low doesn't occur often.

Bob McDavitt

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