For a more recent update on Hurricane Bill, CLICK HERE

Hurricane Bill is providing a pretty good lesson on how hurricanes work and forecasting. If you look at the National Hurricane Center Hurricane Bill Discussion, they note that the winds have decreased and that the pressure was up a bit to 954 mb. Also, the satellite imagery makes it hard to find the eye. The Hurricane Hunter Aircraft reports an eye of 14 miles. If you recall, earlier it had been running about 35 miles and was clearly visible on the satellite images. One thing that the discussion does not say is that the vortex message indicates that the eye is open in the Southwest quadrant. On the surface, this all appears to mean that Bill is dying. In a sense, it is because it is headed toward colder water and a more hostile environment….but not just yet. Again, in the discussion it talks about a second wind maxima. What appears to be going on is that Bill is going through another eyewall replacement cycle. Most likely, the open part of the eye

is indicative of a deteriorating inner eye all and the second maxima is indicating that a new eye is forming. The smaller eye is probably a result of the old eye collapsing in. The pressure rise was not all that dramatic and fairly typical of a storm going through a replacement cycle. The idea here is that as the storm bends on Saturday from its Friday evening 340 degree vector to a 360 degree heading, it will still be in a relatively weak shearing environment and be over warm water. It will have completed the eyewall replacement cycle and so it should ramp up a bit in intensity. This eyewall replacement process is why it is really tough for a mature hurricane to maintain a maximum intensity for a long period. For that reason, it is rare for a category 5 hurricane to make landfall in the United States. I think its only happened 3 times..Camille in 1969, Andrew in 1992 and a Florida hurricane in the 1920s or 1930s…maybe the Keys Hurricane. The timing just has to be right..or wrong as the case may be.

Anyway, Bill’s uptick will probably not be back up to category 4 status and whatever increase in intensity will not last long. The big long wave trof has made its way across the Ohio Valley and weekend temperatures will be in the 70’s over much of the region. It’s a fall like pattern and Bill is behaving like a fall-like storm. As Bill moves north….probably about the time its even with Washington DC, it will start to encounter strong southwesterly winds ahead of the trof and that will start to rip it up. It will also be running into cold water and as it approaches Nova Scotia, Bill will start to take on a northeastward component and its forward speed will increase rapidly and markedly. It will lose its tropical characteristics and probably be in Scotland by the middle of next week. But, even though it may parallel the coast of Nova Scotia and be losing its tropical characteristics, the pressure gradient between it and the big ridge in the Atlantic will still be large and therefore strong winds, perhaps still running about 100 mph will be pushing water on the right hand side of the storm up in the form of a storm surge. Harbors, bays and inlets in Nova Scotia and other land masses will have a pretty rapid and significant sea rise. What’s interesting is that with the storm moving so fast, if it does parallel the coast, then they may see a rapid retreat of extreme tidal surges as the wind shifts around from the north on the back side. If I’m in Halifax or anywhere in Nova Scotia, I’m still heeding the local warnings and advisories.

WTNT43 KNHC 212040

TCDAT3

HURRICANE BILL DISCUSSION NUMBER 26

NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL032009

500 PM AST FRI AUG 21 2009

AN AIR FORCE HURRICANE HUNTER AIRCRAFT REACHED BILL THIS

AFTERNOON AND FOUND THAT THE CENTRAL PRESSURE HAS REMAINED AROUND

954 MB…BUT THE WINDS HAVE DECREASED AND INITIAL INTENSITY IS NOW

90 KT. THE PLANE REPORTED A SMALL EYE OF 14 N MI DIAMETER.

GIVEN THE CONCENTRIC RING STRUCTURE OBSERVED ON MICROWAVE AND THE

DOUBLE WIND MAXIMA PROVIDED BY THE PLANE…IT IS LIKELY THAT THE

HURRICANE IS GOING THROUGH AN EYEWALL REPLACEMENT AND THE REASON

FOR THE CURRENT WEAKENING. THERE IS AN OPPORTUNITY FOR BILL TO

REGAIN SOME STRENGTH DURING THE NEXT 12 TO 24 HOURS WHILE THE SHEAR

IS LOW AND BEFORE THE HURRICANE REACHES COOLER WATERS.

THEREAFTER…A STEADY WEAKENING IS INDICATED AND BY 72 HOURS…BILL

SHOULD ALREADY BE AN EXTRATROPICAL CYCLONE INTERACTING WITH THE

HIGH LATITUDE WESTERLIES.

BILL IS NOW MOVING TOWARD THE NORTH-NORTHWEST OR 340 DEGREES AT 17

KNOTS. THE HURRICANE SHOULD GRADUALLY TURN MORE TO THE NORTH AROUND

THE BERMUDA-AZORES HIGH TONIGHT AND SATURDAY AND SHOULD BE FORCED TO

RECURVE AS A LARGE MID-LATITUDE TROUGH MOVES EASTWARD ACROSS THE

EASTERN UNITED STATES. THERE HAS NOT BEEN ANY CHANGE IN TRACK

GUIDANCE WHICH IN FACT CONTINUES TO BE TIGHTLY PACKED…MAINTAINING

HIGH CONFIDENCE IN THE TRACK FORECAST.

LARGE SWELLS GENERATED BY THIS HURRICANE WILL AFFECT A LARGE PORTION

OF THE WESTERN NORTH ATLANTIC COASTAL AREA DURING THE NEXT FEW

DAYS. THESE SWELLS WILL LIKELY CAUSE EXTREMELY DANGEROUS SURF AND

LIFE-THREATENING RIP CURRENTS. PLEASE CONSULT STATEMENTS ISSUED BY

YOUR LOCAL WEATHER OFFICE AND METEOROLOGICAL SERVICES FOR MORE

DETAILS.

AT THIS TIME…TROPICAL-STORM-FORCE WINDS ARE NOT FORECAST TO REACH

NEW ENGLAND. HOWEVER…A SLIGHT SHIFT OF THE FORECAST TRACK TO THE

LEFT WOULD REQUIRE A TROPICAL STORM WATCH OR WARNING TONIGHT. THE

WIND SPEED PROBABILITY PRODUCT INDICATES THAT THERE IS ROUGHLY A 25

PERCENT CHANCE OF THESE WINDS REACHING EXTREME SOUTHEASTERN NEW

ENGLAND.

FORECAST POSITIONS AND MAX WINDS

INITIAL 21/2100Z 29.4N 66.9W 90 KT

12HR VT 22/0600Z 31.7N 68.0W 100 KT

24HR VT 22/1800Z 35.1N 68.4W 100 KT

36HR VT 23/0600Z 39.0N 67.2W 85 KT

48HR VT 23/1800Z 43.0N 64.0W 75 KT

72HR VT 24/1800Z 49.5N 47.0W 50 KT…EXTRATROPICAL

96HR VT 25/1800Z 54.5N 20.0W 45 KT…EXTRATROPICAL

120HR VT 26/1800Z 60.0N 6.0W 35 KT…EXTRATROPICAL

$$

FORECASTER AVILA