Tuesday, August 09, 2005



Tropical Depression Irene Update 100300Z



At  11 PM EDT, the center of Tropical Depression Irene  was at 22.4 North 55.5 West, 890 miles southeast of  Bermuda and moving to the west at 10 mph.  Maximum sustained winds are 30 mph and minimum central pressure is 1008 millibars (29.77").

While Irene appeared to be on a re-intensification trend earlier today, it has not materialized.  Irene's organization is very poor at the moment.  Because the enviroment is expected to become more favorable, Irene is expected to intensify; if she survives.  "This is a low confidence forecast".  Placing the center of Irene was even more difficult than previously.  The initial motion is an uncertain 280 (west-northwest) at 9 knots. The forecast track is on the left side of the envelope, but is not as far west as the ECMWF model.  The latter part of the forecast features a slow forward motion partially because of the uncertainty that exists in how Irene will move around the high pressure center that is expected to form off-shore.


It sounds highly contradictory, but it is the truth more or less.  Either Irene will die shortly, or she will be a hurricane in five days. 

My skepticism of the HPC's asessment of Irene's organization has proven to be well-founded.  Irene looks terrible tonight.  Convection is almost non-existent.  And while that was the case a couple of nights ago as well, there was at least a clear low level circulation at that time.  For there has been no evident center of circulation.  That is why the forecasters are having such a hard time placing a center and determing the initial motion.  The last two Quikscat passes over the storm show no hint of one. 

In the past there were models that were calling for her demise, but Irene's persistence seemed to overcome their dire predictions.  At this time, however, they may be coming true.

But, for the sake of continuity, if nothing else, the NHC assumes that she's alive and more or less well.  The cloud mass has continued to move more or less due west, which increases the probability of affecting the mainland (even though it is still low).  The ECMWF forecast brings Irene fairly close to northeast Florida before running her due north into South Carolina.  The track of the GFDN model (also mentioned in the discussion) also takes Irene further west and less north than most other models.  Since the models in general have been too eager to turn Irene to the northwest, the NHC forecast favors these models.  A reasonable line of thinking.

The intensity forecast is very problematic.  Irene has persisted in underachieving against the intensity guidance, something that is rare to see for days on end.  The 120 hr guidance five days ago called for 44 knots.  14 knots too high.  96 hr guidance four days ago was 46 knots.  16 knots over.  48 hr guidance two nights ago called for 47 knots.  17 knot error!  There was no 0000Z (8 PM EDT) guidance last night, but the guidance from six hours before and six hours both called for a little bit of strengthening in 24 hours.  That has not happened. 

It is reasonable to think that strengthening will occur if Irene survives, but it is one of those deals where I'll believe it when I see it.  Right now Irene's organization is so poor, that I doubt much intensification would occur in 24 hours even if conditions became perfect right at this moment.  After all, since there she may not even be a tropical depression at this time since there is no evident low level circulation. 

So, Irene... tropical wave or future hurricane?

We'll see in the morning. Maybe.

2005 Hurricane Season, Weather comment []11:30:19 PM   trackback [] 


Tropical Depression Irene Update 092100Z

At  5 PM EDT, the center of Irene was at 22.6 North 54.4 West, 925 miles of  southeast of Bermuda and moving to the west at 9 mph.  Maximum sustained winds are 35 mph and minimum central pressure is 1008 millibars (29.77")

Irene's appearance on visual satellite has improved significantly today.  Placement of the intial position and setting the intial heading proved to be a problem, but the best guess for heading was 280 at a speed of 8 knots.   Track forecast is very problematic as there is a lot of divergence in the models.  The forecaster turned to the long-range/large-scale pattern for guidance.  On the basis of model guidance of that form, he expects a high to form over the western Atlantic.  Forecast track is shifted to the south and west.  The atmospheric enviroment around Irene appears to be more favorable than previously.  On that basis, modest intensification is expected over the next five days (bringing Irene just short of hurricane strength on Sunday afternoon). 

Like earlier this morning, I disagree somewhat with the HPC forecaster's description of Irene.  There are some aspects of her appearance that were better this time 24 hours ago.  Convection was more intense and the low level circulation was more evident.  While the visible satellite gives the appearance of more 'stuff' going on, it is (1) unorganized and (2) infrared satellite shows it to be rather unimpressive. 

With the low level center being impossible to locate with any degree of confidence, it is very difficult discuss the track forecast.  After all, if you are unsure of what movement of the past six hours was, then you are really unsure of what movement is going to be in the future.  As I've stated in previous posts, history shows that it is rare for storms this far north and east to affect the U.S. mainland.  However, with Irene slipping to the west over the past 24-30 hours, the chances of Irene affecting land have increased, slightly.  Right now I would rate an outside chance of Irene affecting the Carolina's on a classic 'run up the coast and turn to sea' trajectory.  However, as I've said with the current motion being so doubtful, that is a low confidence forecast.  In any event, Irene is still far enough out to sea that everyone has the luxury of checking up on Irene only once or twice a day for the next couple of days rather than needing to maintain a steady watch.





  

2005 Hurricane Season, Weather comment []5:25:45 PM   trackback [] 


Tropical Depression Irene Update 091500Z



At 11AM  EDT, the center of Tropical Depression Irene was at 22.3 North 53.9 West, 960 miles southeast of  Bermuda and moving to the west-northwest at 10 mph.  Maximum sustained winds are 30 mph and minimum central pressure is 1009 millibars (29.80").

Irene continues to have deep convection near her center, but it is not organized enough to call her a tropical storm once again.  Irene should come under the influence of an upper level high currently to her northwest that would give Irene an enviroment with lower shear and enable her to become a tropical storm in 36 hours.  The ECMWF, NOGAPS, and UKMET are in general agreement on a west-northwest track for Irene while the GFS offered a different situation featuring Irene being sheared apart and her remnants taken north.  The official forecast is favoring the first scenario and is slightly to the left (west) of the previous forecast


Irene has a very messy appearance this morning.  In some ways she looked better without the convection.  I must say I disagree with the forecaster's asessment that Irene is better organized than she was at 5 AM (note that the forecaster is not a National Hurricane Center forecaster but instead belongs to the Hydrometeorological Prediction Center). 

For the past day or so Irene has defied the track forecast and moved more or less due west.  However, her position is still far north enough and far east enough to make the possibility of affecting the U.S. mainland a low one.


  

2005 Hurricane Season, Weather comment []12:02:53 PM   trackback []