Friday, September 24, 2010

Hurricane Lisa and Tropical Storm Matthew: September 24, Update A

I'm attaching (and on blog site) the latest Infrared satellite image of the Atlantic, taken by the GOES-East satellite. There are two blobs. One is over Central America (Nicaragua/Honduras) and has moisture and clouds associated with it that stretch across a large region of the Caribbean. The other is way over in the eastern Atlantic, at around 20N, 27W. Can you identify which storm is officially the weaker of the two (and is a weak Tropical Storm) and which is the one that has just strengthened to a Hurricane? Remember an Infrared image shows cloud top temperatures. The coldest cloud tops are marked by the red going into dark grey colours and one can expect pretty strong thunderstorms, torrential rain, and possible tornadoes in those areas, the orange areas are thunderstorms with heavy rain, the yellows (as cloud tops get warmer) indicate areas of rain, and the blues going into whites (warmest cloud tops) are just cloudy areas. (Answers in the rest of this entry - and er, in the subject header ;-)).

Hurricane Lisa:
She has some clouds, but not a lot of strong convection or rainfall. She also has vorticity (circulation) in the lowest half of the troposphere, but nothing at higher altitudes. Her center is officially at 20.0N, 27.8W and she's finally decided she's tired of hanging out in the same place so she's heading north at 8mph. Her central pressure is estimated to be 989mb and her wind speed is estimated to be 75mph, making her a cat 1 hurricane (range: 73-95mph). Hurricane force winds extend out a whopping 10 miles from the center. I agree, there is a little itsy bitsy eye trying to form, but yes, of course I disagree! ;-) Well... I agree with the name. The location is fractionally off - I think she's slightly to the east of that - and she's heading NNE. But the estimated winds I am really not sure about. I think she's weaker than that. I looked at the satellite wind field (also attached/on blog site) from a couple of hours ago. It's easy to see the circular pattern in winds that is Lisa. Even generously it seems to me that the wind speeds are about 40-45mph (35-40 knots; 1 knot = 1.15 mph), but if the hurricane force winds are only 10 miles from the center (which they are estimating from satellites as well) then we might not be able to detect it from this image.   

Tropical Storm Matthew:
I am really not sure about their assessment on this 'little' guy. He was our Tropical Depression 15 from yesterday and I still think that, like Karl, they underestimated his intensity. The vorticity is *really* strong (a lot stronger than Lisa's) in the lowest half of the troposphere and he has vorticity in the upper levels of the troposphere which indicates a strong Tropical Storm, possibly close to a weak hurricane. He has a LOT of convection! Very strong thunderstorms, torrential rain, possible tornadoes. He's made landfall in Nicaragua, but this very strong convection is across Honduras too. It looks like his center at the moment is at 15N, 83.7W  - at the Nicaragua/Honduras border. Officially he is moving West at 15mph - I think it's more WNW. His estimated central pressure is 998mb and winds are 45 mph, making him a weak (!?!) Tropical Storm (range: 39-73mph). A plane went into the system when he was still over water, about 7 or so hours ago and found winds of 45 knots... hmm... 45 x 1.15 = 51.75 mph winds. Not 45mph winds. He has made landfall since then, so I can see their reasoning that he'll have weakened. Sure doesn't look like it to me at the moment. I'm sure it doesn't feel like it to the people underneath either.
Earlier today (5am advisory - over 12 hours ago), the NHC wrote: "THE CURRENT SHEAR IS FORECAST TO SUBSIDE IN 12-24 HR...SO THE MAIN
FACTOR CONTROLLING THE INTENSITY WILL BE LAND INTERACTION."  I don't think the impact of the ocean is fully being taken into account in these intensities - even though it's been shown time and time again that the deeper the warm water is, the stronger the storm generally becomes (barring strong atmospheric inhibition). The most famous examples were Katrina and Rita in 2005 - they became cat 5 as they went over the Loop Current. 
Now he is over land there's certainly the possibility that the convection will decrease, especially as there is also a bit of wind shear at the moment, but he has got some lusciously warm waters to sustain him for a while. I think he's going to cause a bit of a mess in Central America. I wouldn't be overly surprised if his intensity increases a bit before decreasing. As for the track, the forecast has him staying on land (mostly), crossing Nicaragua, Honduras, and Guatemala, and then dissipating over Mexico by Monday afternoon. I can see this as a viable track, but if he is a little stronger then it might take him a little longer. Speaking of dissipation, that's what seems to have happened to the forecast track making a right turn and heading northeast. I can see the low pressure front that would have steered him in that direction - it's in the northern Gulf area at the moment. In fairness to the NHC, they did say yesterday that both the track and intensity forecasts for this storm had a high level of uncertainty and could change drastically.
More tomorrow. Time for some sleepings over here. Have fun over there!
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These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I was there and was going to "run away, run away" (Monty Python), I'll let you know.

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