Friday, August 12, 2005



Tropical Storm Irene Update 130300Z



At  11 PM EDT, the center of Tropical Storm Irene was at 29.2 North 68.3 West, 300 miles southwest of  Bermuda, 590 miles southeast of Cape Hatteras and moving to the northwest at 10 mph.  Maximum sustained winds are  70 mph and minimum central pressure is 991 millibars (29.26").


Irene is not as well organized as it was when the recon plane was in Irene.  Because of that, intensity remains at 60 knots.  However, due to a drop in pressure observed by a buoy near the center, the minimum central pressure was adjusted downward a few millibars.  Best guess at current motion is nine knots on a heading of 310.   Data from the Gulfstream-IV jet shows a ridge slightly stronger and more west than that depicted by any of the forecast models.  This may cause more westward motion than was previously forecast.  Models were in agreement with a turn to the north and then northeast.  However, due to the observations from the Gulfstream, there may be a westward shift in the 00Z models.  For now, however, the track forecast is essentially unchanged.  Brief periods of intensification are expected to occur, however dry air will impede it.  Current forecast makes Irene a hurricane in 12 hours, but keeps her as a category one throughout the period.


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I said this morning (121500Z update) that if the models were misrepresenting the high pressure ridge then their forecasts could all be wrong.  Well, as the discussion states, they've been been depicting it weaker and further east then it actually is.  It would not be surprising to see rather significantly different forecasts from the models this evening, possibly bringing Irene closer to land than currently forecast. 
  
Elsewhere in the tropics, the disturbed area east of the Lesser Antilles that the National Hurricane Center has been mentioning in their Tropical Weather Outlook  has not changed much in the past six hours (the Quickscat pass did not show a center any more closed than the previous pass).  However, development into a tropical depression is still expected during the next day or so.

Next update probably won't be until 5 PM EDT tomorrow.  I have drill with the Naval Reserves this weekend, so my availability to post will be limited.  If there is no update from me on Saturday, then next will be on Sunday evening.  As always, check with the National Hurricane Center for the latest and greatest on Irene and other tropical weather systems (Tropical Depression 10, perhaps).

2005 Hurricane Season, Weather comment []11:09:30 PM   trackback [] 


Tropics Watch 0812 continued



Latest Tropical Weather Outlook :

SATELLITE DATA AND SURFACE OBSERVATIONS INDICATE THE WELL-DEFINED
LOW PRESSURE AREA CENTERED ABOUT 1150 MILES EAST OF THE WINDWARD
ISLANDS HAS CONTINUED TO BECOME BETTER ORGANIZED. THUNDERSTORM
ACTIVITY HAS BEEN GRADUALLY INCREASING AND ENVIRONMENTAL CONDITIONS
APPEAR FAVORABLE FOR A TROPICAL DEPRESSION TO FORM DURING THE NEXT
DAY OR SO AS THE SYSTEM MOVES WEST-NORTHWESTWARD AT ABOUT 10 MPH.

Further comments in the Tropical Weather Discussion

1012 MB LOW HAS DEVELOPED FROM THE TROPICAL WAVE PREVIOUSLY 
ALONG 39W. THE BROAD CENTER OF THE LOW IS NEAR 11N41W AND IS
MOVING WNW 10-15. THIS IS A COMPLEX DISTURBANCE WITH A LOW/MID-
LEVEL CENTER SEEN ON VISIBLE SATELLITE NEAR 12N42W WITH ANOTHER
LOBE OF ENERGY ROTATING AROUND THE E SIDE NEAR 12N39W. SURFACE
OBSERVATIONS INDICATE A LARGE CLOSED CIRCULATION WITH BUOYS
REPORTING WINDS OF 20-25 KT. THE LOW HAS FAVORABLE CONDITIONS
FOR FURTHER DEVELOPMENT WITH WARM WATER NEAR 83F... LIGHT
SHEAR... AND IT SEEMS TO BE FAR ENOUGH SOUTH TO AVOID
SIGNIFICANT DRY AIR IN THE MID-LEVELS. THERE IS SOME EVIDENCE
OF BANDING THOUGH CONVECTION HAS FADED A BIT THIS AFTERNOON.
ISOLATED MODERATE FROM 10N-13N BETWEEN 40W-45W.

The Quikscat pass just ocurred, but the data has not been put out yet.  If it indicates a closed circulation, then we could see a Tropical Depression Advisory package at 11 PM.  Otherwise, provided that the organizing trend continues, we'll see the first advisory tomorrow.  Something people with interests in the Leeward Islands will want to pay attention to.


2005 Hurricane Season, Weather comment []5:42:25 PM   trackback [] 


Tropical Storm Irene Update 122100Z

At  5 PM EDT, the center of Tropical Storm Irene was at 28.7 North 67.5 West, 295 miles southwest of Bermuda, 650 miles southeast of Cape Hatteras and moving to the northwest at 10 mph.  Maximum sustained winds are 70 mph and minimum central pressure is 997 millibars (29.44"). 


Shear and Sea Surface Temperatures are favorable for strengthening, but since guidance does not bring Irene above category one strength, not much intensification is forecast.  Movement is to the northwest (310) at 9 knots.  Irene has been steered by a strong mid-level ridge.  As the ridge shifts eastward, Irene is expected to turn to the north-northwest and then north into an area of light steering currents.  She could meander for awhile before westerly flow takes Irene away from the U.S.  "The official track forecast has been gradually shifting eastward in agrement with guidance.  This lessens the potential threat to the United States East Coast".

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Not much to say here as things are playing out as expected so far. 



2005 Hurricane Season, Weather comment []5:05:00 PM   trackback [] 


Irene Recon Update 3

Just after I sent out the last update, recon recorded flight level winds of 75 knots... reduces to approximately 70 mph at the surface.  Pressure unchanged at 997 millibars.

2005 Hurricane Season, Weather comment []4:48:53 PM   trackback [] 


Irene Recon Update 2

Well, it's decision time for the NHC forecaster.  Flight level winds of 60 knots suggest surface winds of 55-60 mph.  However, the flight meteorologist seems to be observing sea state that suggests 65 knot winds , (which would make Irene a hurricane).   Given that the two intensity estimates that are public (from AFWA and SSD) are under hurricane strength, it is doubtful that NHC would go with  75 mph winds this advisory.  70 is a possibility... 65 more likely. We'll find out soon.

2005 Hurricane Season, Weather comment []4:24:50 PM   trackback [] 


Irene Recon Update 1



First VORTEX message was just sent out by the hurricane hunter.  It indicates a minimum central pressure of 997 millibars and places the center at 28.4 North 67.1 West.  It hasn't sampled the northeast quadrant yet, but (presumably on the basis of sea-state observation) they are estimting surface winds of 60 knots (70 mph).

2005 Hurricane Season, Weather comment []2:49:05 PM   trackback [] 


Tropics Watch 0812



From the morning Tropical Weather Outlook:

SURFACE OBSERVATIONS INDICATE THAT A LOW PRESSURE AREA ASSOCIATED
WITH A TROPICAL WAVE HAS FORMED ABOUT 1200 MILES EAST OF THE
WINDWARD ISLANDS...AND SATELLITE IMAGES SUGGEST THAT THE SHOWER
ACTIVITY IS GRADUALLY BECOMING BETTER ORGANIZED. THIS SYSTEM HAS
THE POTENTIAL TO BECOME A TROPICAL DEPRESSION DURING THE NEXT DAY
OR TWO AS IT MOVES TOWARD THE WEST OR WEST-NORTHWEST.
System is about 3-5 days west of the Islands depending on how its forward motion varies. It had been moving at a 15-20 knots clip, but it appears to be slowing now. Latest Quikscat indicates that there is a rather broad closed circulation (Later: Not quite completely closed... looking at it again and a high-resolution graophic, I don't see a northerly wind.)



From the morning Tropical Weather Outlook:

SURFACE OBSERVATIONS INDICATE THAT A LOW PRESSURE AREA ASSOCIATED
WITH A TROPICAL WAVE HAS FORMED ABOUT 1200 MILES EAST OF THE
WINDWARD ISLANDS...AND SATELLITE IMAGES SUGGEST THAT THE SHOWER
ACTIVITY IS GRADUALLY BECOMING BETTER ORGANIZED. THIS SYSTEM HAS
THE POTENTIAL TO BECOME A TROPICAL DEPRESSION DURING THE NEXT DAY
OR TWO AS IT MOVES TOWARD THE WEST OR WEST-NORTHWEST.
System is about 3-5 days west of the Islands depending on how its forward motion varies. It had been moving at a 15-20 knots clip, but it appears to be slowing now. Latest Quikscat indicates that there is a rather broad closed circulation (Later: Not quite completely closed... looking at it again and a high-resolution graphic, I don't see a northerly wind.)


2005 Hurricane Season, Weather comment []12:35:21 PM   trackback [] 


Tropical Storm Irene Update 121500Z


At  11 AM EDT, the center of Tropical Storm Ireen was at 28.3 North 66.8 West, 300 miles of  south-southwest of Bermuda and moving to the northwest at 13 mph.  Maximum sustained winds are 65 mph and minimum central pressure is estimated to be 994 millibars (29.35").


Cloud pattern is better organized then yesterday.  There is a low level center in the deep convection .  All three agencies (Air Force Weather Agency, Satellite Services Division , and Tropical Analysis and Forecasting Branch) agreed on an intensity of 55 knots.   However, a recon plane is scheduled to enter the storm this afternoon so that NHC is no longer relying solely upon satellite-based estimates.  Upper-air enviroment is favorable for development, however, with Irene passing over the waters recently occupied by Tropical Storms Franklin and Harvey, intensification past a category one hurricane is not expected. Irene is currently moving around the edge of a mid-level ridge.  As the ridge shifts east-ward Irene is expected to turn to the north-northwest  and a decrease in forward speed is anticipated.  Eventually, a westerly steering flow would become established that would send Irene to sea.  "That is too far in the future to be certain".


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As mentioned yesterday, today's big event is the recon flight and the upper-air flight.  One will let us know what Irene's true intensity is and the other will help the computer models with their forecast.  If the computer models do not shift much in their runs this evening (after taking in the observations from the upper-air flight) then confidence in the current forecast will be significantly higher.  As it is now there is almost unaminouts support for the 'turn to sea' forecast.  However, if they are all mis-representing the size and placement of the high pressure ridge, then their forecasts are all wrong and Irene could come in closer to land than currently forecast.  That is why the upper-air flight is so important: it will take the observations that will give the models accurate data.  In the case of Hurricane Isabel, the accuracy of the GFS model increased by as much as 40% after it started receiving observations from the Gulfstream-IV.  

Observations from the hurricane hunter will start coming in after 1 PM EDT.  The upper-air observations will be taken late this afternoon and evening and will be fed into the models when they start their 'runs' at 11 PM or so this evening.
 

2005 Hurricane Season, Weather comment []11:50:51 AM   trackback []