Tropical Update: Guam Typhoon Threat, Next Atlantic Depression Likely

Stu Ostro
Published: July 29, 2014

- Atlantic system still trying to develop; as of this hour still having only limited success

- Headed toward eastern Caribbean islands by end of week

- Western Pacific active with a tropical storm and a large disturbance to its west


Invest 93L's development process had been following a steadily increasing trend for days, with convection (rain/tstorms) persisting & growing and becoming better organized, culminating last evening in a burst near the center along with the circulation tightening up.

Immediately thereafter, however, the system has taken a step backward, in fact it's looking more than a little less healthy on infrared satellite imagery. 

That's not the whole story, as visible satellite loops still show a solid spin, and it wouldn't take much of an increase in tstorms and tightening of the circulation for NHC to designate it as a tropical depression, but at least as of this hour the steady development trend has been interrupted, for which there are a number of plausible meteorological reasons.

Also, satellite estimates indicate quite modest winds with the system so far, so for the moment it's not like one of those tropical waves which already has tropical storm force winds and is just waiting for enough organization to be a tropical storm.

Many, though not all, models ultimately have the system becoming one; in any event all have a track in the realm of this "spaghetti" swath of possible tracks, heading to a position over/near the Leeward Islands by Saturday-ish. (Per the note last week about the pattern, it currently isn't like last year when many systems swerved far to the east of Bermuda, thus this system's ability to reach the Caribbean and at least get a bit closer to North America.) 

The ultimate track thereafter will depend in part on what the system is.  A stronger, deeper system would likely get pulled more toward the north-northwest and a weaker, shallower one would probably head more toward the west-northwest.  

A couple of our main longer-range models, the European (ECMWF) and the American (GFS), handle an upper-level trough of low pressure near the central-eastern Caribbean islands and Bahamas differently, the former holding in more hostile winds aloft, hence a weaker tropical system, and the latter predicting the tropical system to more successfully fight them off and be stronger. That model has a history of often reducing those "shearing" winds too much, though that's not always the case. 

We'll see what happens this time -- there's plenty of time to assess the prospects in the longer range; in the meantime, status quo from yesterday, which is for residents of and visitors to the islands to keep apprised of the status of this tropical system. 

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Not as much going on in the eastern Pacific now, as the final advisory has been issued on Hernan and there's just one other system NHC is identifying in its tropical weather outlook, with a near-zero chance of development in the short term and eventually a medium chance, and it is far out at sea.

The Central Pacific Hurricane Center is noting a bunch of systems.


In the western Pacific, the tropical depression near Guam has become Tropical Storm Halong and is forecast by the Joint Typhoon Warning Center to soon be a typhoon, and an enormous gyre known as a "monsoon depression" (not a well-defined enough, tight enough center of circulation to be designated a tropical depression or storm) is in the big circle west of Halong. 

Both are moving slowly WNW.


Our live wall below has the latest updates from our hurricane experts and from coastal National Weather Service offices. No need to refresh, the latest updates will appear at the top of the wall. Time stamps on each post are in Eastern U.S. time.


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