Thursday, August 11, 2005

Tropical Storm Irene Update 120300Z

At  11 PM EDT, the center of was near 26.4 North 64.9 West, 410 miles south of Bermuda, 875 miles southeast of Cape Hatteras  and moving to the west-northwest at 15 mph.  Maximum sustained winds are 50 mph  and minimum central pressure is estimated to be 1000 millibars (29.53").

Satellite imagery suggests that Irene's organization has not changed much in the past few hours.  Intensity estimates from two of the three agencies are unchanged, so  the intensity estimate for the advisory package is the same as before.  The enviroment along Irene's path is favorable for development and the intensity forecast, which is a blend of model guidance, brings Irene to hurricane strength in 48 hours and maxxes out intensity at 90 mph.  Locating the center continues to be a challenge, which makes the estimate of motion difficult as well.  Best guess is a heading of 300 at a speed of 13 knots.   Forecast millibars show a mid-level center of high pressure developing over Bermuda and then moving to the west late in the period.  It is expected that Irene's forward speed will decrease as the storm approaches it.  Beyond 36 hours, the models diverge significantly.  Track forecast is similar to the previous one and shows slowing in the forward speed after day three,  It is "in reasonable agreement with the FSU Super-Ensemble".


Up through sunset today, it looked like Irene's organization was improving in a way that would justify higher intensity estimates.  Since then however, she hasn't looked quite so good.   The Air Force Weather Agency intensity was slightly higher than the other two agencies, but as tends to be the norm, if two of the three are agreeing, then NHC goes with the majority (assuming of course that there are no other observations to go by).

There are several unanswered questions that cloud the track forecast.  First and foremost is the position of the center.  There is nothing clear-cut about its location in the satellite imagery as the three agencies continue to produce estimates that are significantly different.  The second one is the position and strength of the Bermuda High.  Tomorrow afternoon will bring some answers as a reconnaisance flight is scheduled to enter Irene at about 1 PM tomorrow afternoon.  That will help answer the first question.  Some help in the second question will come from a flight of NOAA's Gulf-Stream IV jet, which will sample the upper-air enviroment in front of Irene.  Those observations will be fed into the evening runs of the forecast models.  In the past, the observations have helped the models significantly in generating an accurate forecast.

At the moment there is no change in my thinking for landfall potential that was detailed in my previous post.

Via comments, Brendan Loy asks what chances I think Irene has of affecting New England.  The scenario is what my Virginia-centric state of mind has thought of as the 'overshoot' scenario.  The storm makes an approach to the Outer Banks but turns parallel to the coast and runs north until it finds land.  The most recent example of it in pure form is Bob of 1991, (which affect my family since we were living in Brunswick, Maine at the time).  Gloria of 1985 is another. 

Due to the uncertainties of a five day forecast, it is a bit hard to have high confidence in rating the probability of such a scenario, but here goes anyway.  I'm a bit too young to be familiar with the atmospheric setup that caused those tracks, however, one can reckon the positioning of high pressure that would have caused such a track (The NHC report on Bob offers merely a sentence or two on the matter).  It seems like it would take the center to be positioned to be either a little bit further north than is normal (or elongated north-south), and somewhat off the coast.  One thing noticable about these types of storms is that they are very much under the influence of a well-defined steering current, as their forward progress increases tremendously as they pass the Outer Banks. 

Looking at what setup appears to be, I don't see such a scenario ocurring.  Right now the options plausible to me are a quick turn out to sea, or a stall/near-stall off the coast that leaves the storm spinning until the next front passes through (carrying the storm off to the east).   I don't think there are any examples of storms approaching the coast slowly (in the fashion that some models are creeping Irene forward in the mid-late term of their forecasts)and then  lurching to the north.

As always, the thoughts below the tear line are my own and do not reflect an official forecast by any organization.


2005 Hurricane Season, Weather comment []11:28:44 PM   trackback [] 

Tropical Storm Irene Update 112100Z

At  5 PM EDT, the center of Tropical Storm Irene was at 25.7 North 63.9 West, 460 miles south of Bermuda, 950 miles southeast of Cape Hatteras North Carolina,   and moving to the west-northwest at 15 mph.  Maximum sustained winds are 50 mph and minimum central pressure is 1000 millibars (29.53 ").

Convective pattern has not changed much today.  Best estimate of the position of the center is that it is on the western edge of the clouds, not in the deep convection.  Intensity estimates from the three agencies varied, NHC took the average of the three for the initial intensity, but notes it could be a bit higher.  Initial motion estimate is 300 degrees at a speed of 13 knots.  Models have not changed their forecasts much today.  NOGAPS and UKMET models are to the right because they have Irene being a larger feature than the high pressure ridge, a scenario that the forecaster finds unlikely.  Official forecast is close to the GFDL and GFDN models, which bring Irene closer to the Outer Banks and feature a decrease in forward speed late in the period.  Official forecast is essentially unchanged and is on the left edge of the guidance envelope.  Convection is expected to increase in intensity tonight as the source of dry mid-level air that has been inhibiting it  is dissipating.  Irene is expected to become a hurricane by this time tomorrow.  The current intensity forecast maxxes Irene out at 85 mph.


Obiously, someone forgot to tell Irene that dry air was supposed to inhibit her today.  Looking at the infrared loop , we see a large amount of convection, with some intense areas sprouting up in the past hour or two.  However, the convection is not particularly organized at the moment as it doesn't seem to be right on top of the circulation center.  That holds down the satellite estimates.  Regardless, the expansion of it does indicate intensification, and I would not be surprised to the the 11 PM advisory featuring 55-60 mph winds.  As mentioned in the previous update, dry air was the only inhibiting factor.  Everything else is favorable.

As far landfall potential possibilities go, history shows the probability of a storm affecting the U.S. mainland from Irene's current position is low.  The closest is a storm that I mentioned in my 5 AM update: Emily of '93 , which brushed the Outer Banks, but still managed to cause significant damage to the southern parts of that area.   Carol  of '53 is another plausible analog.  While not extremely close to Irene's current position, the possibility of a collapse in steering currents is reminiscent of Felix of '95.

Here is the 18Z model guidance.  The track labeled NGPI is the NOGAPS referenced by the forecast; GFDI and GFNI are two of the other models that were referenced by the NHC discussion.  One other forecast track worth mentioning from that graphic is CONU, which is a consensus of several of the models. 

Irene as a weak tropical depression gave the model fits as they just could not handle her well.  They tended to either over-estimate her and take her on a more northerly track than warrented or they wanted to kill her off. With Irene intensifying, she is fitting into a mold that the models are more familiar with.  Their overall performance from here on out should be significantly better than that of the past five days or so. 

As has been the case for the past day or so, my only concern for land being affected by Irene is the Outer Banks and as long as Irene persists on a more northerly track, that concern is reduced.  Worst case scenario seems to be a situation like Emily where the southern Outer Banks were hit fairly hard and beach erosion was significant, but other nearby areas were much less affected as she turned quickly to sea.  Residents of those area and people planning to be on vacation there during the next week should continue to monitor Irene's progress.

All remarks below the tear line are my personal thoughts and by no means reflect an official forecast by any organization.

2005 Hurricane Season, Weather comment []5:31:58 PM   trackback [] 

Tropical Storm Irene Update 111500Z

At  11 AM EDT, the center of was at 25.1 North 62.6 West, 515 miles east-southeast of Cape Hatteras North Carolina and moving to the west-northwest at 17 mph.  Maximum sustained winds are 50 mph and minimum central pressure is estimated to be  1000 millibars (29.53").

Convection has continued to increase and become better organized .  Difficulties in locating a low low level center continue, the latest position estimate essentially placed the center on a well defined mid-level circulation.  Two of the three agencies agreed on  an intensity of 45 knots (based on Irene's appearance on satellite).  Estimate of motion is 15 knots on a heading of 300.  Forecast models are in excellent agreement through 48 hours and the agreement ends there.  The UKMET and NOGAPS models turn Irene to sea at around 70 West, while GFDL and GFDN bring Irene very close to the North Carolina coast and the GFS persists in wanting to kill off Irene.  Official forecast track favors the GFDL/N solution.  Warm sea surface temperatures and low shear favor intensification of Irene, however dry air does lie in front of her.  Intensity forecast brings Irene to hurricane strength in 36 hours and holds her maximum intensity at 85 mph.


References for the intensification factors mentioned in the discussion: This Saharan Air Layer graphic shows the dry air that Irene will have to contend with in the near term.  Sea Surface Temperatures are warm along Irene's path though if she deviates too far north.   there is a drop off. The shear analysis shows a favorable upper-air enviroment.  SHIPS guidance has not done well in these dry air situations, so it wouldn't surprise me if Irene doesn't make hurricane strength when forecast. 

As mentioned in the center placing the center continues to be an issue.  It may not be clarified until the first reconnaisance plane investigates Irene tomorrow afternoon.

My track forecast thoughts are unchanged, see previous post for details.  As such residents of the Outer Banks and those planning to visit next week should monitor the progress of Irene.


2005 Hurricane Season, Weather comment []11:58:15 AM   trackback [] 

Tropical Storm Irene Update 110800L

At  5 AM EDT, the center of Tropical Storm Irene was at 24.2 North 61.0 West, 605 miles south-southeast of Bermuda and moving to the west-northwest at 14 mph.  Maximum sustained winds are 40 mph and minimum central pressure is estimated at  1007 millibars (29.74").

Organization of Irene continues to improve.  However, placing a center remains a problem as the position estimates from the three agencies had a spread of 90 nautical miles.  Two of the three agencies estimated 35 knot intensity (other estimated 40), so intensity is held at 35 knots. As previously, a gradual increase to hurricane strength is forecast.  Due to the uncertainty of the center, setting the initial motion is also a problem.  It appears to be 12 knots with a heading of 295.  Irene is expected to bend more to the west in the near term, but several models suggest that Irene could be curved away from the coast in the later periods.  There is too much uncertainty in the four to five day forecast to tell what kind of threat Irene could pose to the U.S.   Official forecast track is shifted slightly to the right, but is still to the left of most forecast guidance.


The big caveat to what follows is the true position of the center of the storm.  As I've said in the past, if you can't place the center accurately in the beginning of the forecast, then the forecast that follows is fairly certain to have low accuracy.  Right now I think the center is a little bit further north than the position that NHC is going with.  (And is probably more in line with the estimate that the Air Force Weather Agency offered earlier this morning).

I believe that if there is anyone that should be paying particular attention to Irene at this time, it is residents of the Outer Banks and those who plan to be in that area during next week.  If Irene affects anyone, that will be the place, I think.

However, I do not necessarily think there is much of a chance of landfall at this time.  A more plausible scenario to me is Emily of 1993, which merely grazed the Outer Banks (but still managed to cause a good amount of damage in the southern parts of that area, Buxton, in particular). 

It is very plausible that Irene could remain at sea as suggested by many of the models referenced in the National Hurricane Center's discussion. 

Under the rule of 'prepare for the worst' residents of the Outer Banks should be ready to complete preparations for a category one- category two hurricane by Monday.  Residents further south should maintain awareness of Irene's progress to ensure that it does not take a more westerly path than I am expecting.  After all, as the NHC discussion states, there is much uncertainty in the extended forecast. 

All remarks below the tear line are my personal thoughts and by no means reflect an official forecast by any organization.

2005 Hurricane Season, Weather comment []5:23:02 AM   trackback []