whether the weather's blog

hurricane hanna part one

 abuse

The 5 am forecasts are out from the National Hurricane Center and they do not look good for us. And yes, I used the term Hurricane rather than the technically accurate Tropical Storm since hurricane is what we're gonna be dealing with.

Currently, TS Hanna is located just off the coast of Haiti and has begun its turn to the north. The central pressure is 993 mb and winds are estimates at 60 mph. Hanna is expected to strengthen over the next three days and make landfall potentially in South Carolina as a strong cat 1 or weak cat 2 storm. Projected landfall is currently in the middle of the night on Saturday morning with the eye passing over or close to Raleigh during the day on Saturday, 6 September.

This is going to be a fast moving storm, accellerating in its forward motion starting early Friday morning. That is good with respect to rainfall; the faster to forward motion of the storm, the less rain that falls on a given location. The downside is that the same forward motion adds to the wind speed already located in the eastern eyewall.

Presently, the projected path center takes the eye just to the east of Raleigh, although keep two things in mind.

1. The cone of probability is currently 400 miles wide at our latitude.

2. The probability of the storm's center location is exactly the same across that entire cone.

Hanna has an equal chance of passing through eastern Tennessee as it does staying completely out in the Atlantic. But realistically, that center track provides legitimate guidence in terms of making preparations.

Now, the scary part.

AccuWeather is predicting sustained winds in Raleigh of 80 mph with gusts to 109 at 4 pm EDT on Friday, 5 September. Winds are expected to start picking up at about sunrise that morning and finally drop below 40 mph (with gusts to 60 mph) around 5 am EDT on Saturday. They are also predicting three inches of rain out of the storm. It is important to note that AccuWeather is bringing the storm in 16 hours in advance of the National Hurricane Center projections.

Remember, that is a prediction based on computer models and the skill of forecasters to interpret those models. Hanna may miss us completely or they may be dead on accurate. The important part here is to anticipate the possibilities and prepare properly for them because if you wait until the path and intentity is sure, it is too late to prepare.

The timing of the accelleration to the north will make a huge difference when the storm actually hits and where it makes landfall. We will know a lot more about that factor by Thursday afternoon, but always keep in mind that starting to make preparations at that time may be too late.

Here is the official National Hurricane Center forecast discussion for 5 am. Further information may be obtained at http://www.nhc.noaa.gov/index.shtml on not only Hanna, but Ike and Josephine. I have bolded some of the terms and defined them below the discussion.

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TROPICAL STORM HANNA DISCUSSION NUMBER 26
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL082008
500 AM EDT WED SEP 03 2008

FIXES FROM AN AIR FORCE RESERVE UNIT RECONNAISSANCE AIRCRAFT
INDICATE HANNA HAS ALMOST COMPLETED A TIGHT CYCLONIC LOOP AND IS
NOW POSSIBLY TURNING SLOWLY NORTHWARD. THE INITIAL INTENSITY WAS
DECREASED TO COME MORE IN LINE WITH THE CENTRAL PRESSURE...WHICH IS
ESTIMATED TO BE 993 MB BASED ON A DROPSONDE REPORT THAT ALSO
MEASURED 25-KT SURFACE WINDS.

THE INITIAL MOTION ESTIMATE IS 360/02. THERE IS NO SIGNIFICANT
CHANGE TO THE PREVIOUS TRACK OR INTENSITY FORECAST REASONINGS.
HANNA IS EXPECTED TO GRADUALLY ACCELERATE NORTHWESTWARD OVER THE
NEXT 72 HOURS AND MAKE LANDFALL OVER THE SOUTHEASTERN UNITED STATES
...POSSIBLY IN THE SOUTH CAROLINA AREA. IT IS WORTH MENTIONING THAT
THE 00Z MODEL RUNS CONTAINED UPPER-AIR DATA FROM DROPSONDES
RELEASED AROUND HANNAFROM THE NOAA GULFSTREAM-IV JET AIRCRAFT...
WHICH USUALLY PRODUCES BETTER TRACK FORECASTS. THE OFFICIAL
FORECAST IS SIMILAR TO THE PREVIOUS TRACK AND IS CLOSE TO THE NHC
MODEL CONSENSUS.

THE INTENSITY FORECAST BASICALLY REMAINS UNCHANGED AND FOLLOWS THE
SHIPS INTENSITY MODEL...WITH HANNA EXPECTED TO REGAIN HURRICANE
STATUS BY 36 HOURS AND CONTINUE TO INTENSIFY UNTIL LANDFALL OCCURS.

FORECAST POSITIONS AND MAX WINDS

INITIAL 03/0900Z 20.2N 72.2W 50 KT
12HR VT 03/1800Z 21.5N 72.8W 55 KT
24HR VT 04/0600Z 23.2N 74.1W 60 KT
36HR VT 04/1800Z 24.8N 76.0W 65 KT
48HR VT 05/0600Z 27.2N 78.1W 75 KT
72HR VT 06/0600Z 33.7N 80.0W 70 KT...INLAND
96HR VT 07/0600Z 42.5N 71.5W 35 KT...EXTRATROPICAL INLAND
120HR VT 08/0600Z 51.0N 55.0W 35 KT...EXTRATROPICAL INLAND

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Some definitions:

• KT or one knot is equal to 1.15 miles per hour. So 75 knots wind speed equals 86 mph, or a middling category one storm.
• a dropsonde is a little tube-like thingie that the Hurricane Hunters drop out of their airplane as they fly through the storm. It contains instrumentation that measures things like barometric pressure, temperature, humidity, and horizontal wind speed at different levels of the storm as it drops to the ground or ocean.
• SHIPS, GFS, LBR and other strange looking acronyms are the names of various computer models that are used in storm prediction. Generally, multiple models are run which give different results depending on the parameters of the model. Then the results are viewed in the aggregate and best shot forecasts are developed.
• The letter "Z" stands for Zulu, or essentially Greenwich Mean Time, or the current time at the Royal Observatory in London England. That is the standard by which time zones are measured against. Raleigh is four hours behind Zulu during daylight savings time and five hours behind during eastern standard time. 1800Z in this case is 6 pm in Greenwich or  2 pm in the eastern US. Time is in 24 hour military notation.
• When you see something like 360/02 as above, it represents the direction and forward speed of the storm. The first number is the direction of motion measured as a 360 degree circle with 360 being due north, 90 is east, 180 south, 270 west and so forth. The second number, in this case 02, is the speed in knots.

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