If you recall a few days ago I had talked about the challenges for Hurricane Earl maintaining its ferocity. One of the main concerns that I had was a huge amount of dry air that Earl was getting ready to move in to. I was a bit surprised that the NHC didn’t mention it more often and it appeared that Earl was able to fend off the dry air. However, it seems apparent now that dry air did indeed work its way into the storm and Earl rapidly decreased in intensity on Thursday evening. The dry air intrusion seems to have coincided with what so often seems to be the case which is that Hurricane Earl began an eyewall replacement cycle during the late morning on Thursday, prior to a potential Outer Banks of North Carolina landfall. Most likely, Earl yesterday reached its maxima when it topped out at 145 mph with gusts to 165 and a central pressure somewhere in the neighborhood of 930 mb. If you look at the satellite loop, you will note that the eye seems to fill in. That was not an indication that the storm is dying, but instead was indicative of a very healthy hurricane. What typically happens with an eyewall replacement cycle is that the maximum winds near the eye decrease somewhat, but the overall energy gets dispersed throughout the storm. Hence, a broader field of strong winds results. At 3:30 PM EDT Earl essentially had two eyes. The old eye was collapsing and a new, much larger, eyewall was developing around it. So, winds decreased and the pressure rose to 943 mb but its a much larger storm. By 5 pm Earl’s maximum winds had dropped to 115 mph and the pressure was up to 947 mb. We saw that with Hurricane Katrina. As it made landfall, the winds dropped off to 125 mph in far western Mississippi and near the mouth of the Mississippi River yet, all the way east over to Mobile, AL winds were in excess of 100 mph. So, regardless of exactly where the eye of the storm runs in relation to North Carolina, the winds on the Outer Banks and even for some distance inland will be hazardous.

On Thursday afternoon, another potential problem was that, if the replacement cycle was complete before it got to North Carolina, then it could have been in a re-intensification stage and perhaps deepening back toward a category 4 hurricane. At this point, that is not only unlikely but also rather irrelevant. As of 3:30 pm EDT on Thursday, Earl was moving at about 355 degrees, or just slightly west of due North. It had crossed just west of 75 degrees West Longitude. Cape Hatteras is at 75 degrees 15 minutes W. Long and 35 degrees 30 minutes North Latitude. I believe that there is a fair chance that Cape Hatteras will take a direct hit though if Earl is just off shore, then it could be in the western eyewall. If you look closely at the 2PM forecast discussion, the NHC forecast position at 2AM is 75 degrees West Long and 33.5 degrees North Lat, or right over Cape Hatteras. At two AM on Saturday, the storm is forecast to have maximum winds of nearly 100 mph and is located at 40.o N and 75.8 W or almost exactly over Marthas Vineyard. Keep in mind a wobble to the right by 25 miles would spare the eye passing over these locations but it would still be close enough to be potentially destructive. By 8PM EDT Earl’s maximum winds had decreased to 110 mph and the liklihood of the trend of winding down is rapidly diminishing. It would appear from the satellite depiction that upper shear is starting to work on Earl as well as dry air from the Eastern United States.

Once it passes Cape Hatteras, three things will start to work in conjunction to turn it North Northeast and then Northeast. First, there is the Coriolis Force, which is the force that wants to turn a cyclone to the right of its path in the Northern Hemisphere. That force increases as the storm moves poleward. Then there are the mid lattitude prevaling westerlys. That is the general persistent wind flow in the mid lattitudes of the Northern Hemisphere. Finally, there is a frontal boundary that is progressing Eastward. As of 3:30 PM EDT it was just west of the Ohio Valley. That is some distance from the coast and not moving as fast as we might like. But, as it approaches, winds ahead of the front or trof will increase from the Southwest. That will increase Earl’s forward speeed and also serve to knock up the upper air support of the hurricane. As mentioned earlier, by 8PM EDT the satellite imagery appears to indicate that the southwesterly shear was already taking a toll on Earl.

Essentially, a hurricane is a warm-core area of low pressure that is a giant heat engine, drawing its energy from warm ocean water. The ocean has to be at least 80 F. Not every surface low over the ocean becomes a hurricane though. It is essential that a hurricane get what I call a “hat” or an anticyclone aloft. That would be a big area of high pressure. That is why when you look at a satellite loop, you see clouds going clockwise, yet at the surface, the hurricane winds go counter clockwise. The upper level high serves to vent the storm. The structure of a hurricane is such that, at the surface, the flow is toward the center in a counter clockwise fashion. When the air reaches the center it has nowhere to go but up. Aloft, the high pressure features air that sinks and goes outward in a clock wise fashion. The high aloft helps push out the rising air below and as it does, it allows for a more even flow of air rising from below. When these winds from the southwest kick in, it will disrupt the ventiing flow aloft which will limit the rising air below. All of that should start to occur in earnest once Earl gets past the Outer Banks. At that same time, it will start to cross over water that is either near or below 80 F and eventually will get over water that is definitely too cold to support a tropical system.

So, look for rapid weakening of the storm beginning off the North Carolina coast and thereafter. It will also lose its tropical characteristics probably before it gets to Cape Cod. Nevertheless, winds of at least hurricane force will continue as the storm moves north. The forecast track takes it very close to Long Island and even over Cape Cod. While it may lose its hurricane classification, it will also provide a very healthy storm surge and a high tide will only aggravate the situation. Click here to find the storm surge forecast for your location. The storm will also be moving very very quickly so, on the one hand, the effects will not last as long but the conditions will also deteriorate very rapidly. No one can tell with exact certitude where the center of Earl will travel. That should not surprise anyone when one considers that the earth is 25,000 miles around; a margin of error of say 50 miles represents just a fraction of a percent but, it can be the difference between terrible conditions and horrific conditions. Most of the time, people who try to ride out a storm vow they will never do so again. Sometimes, they lose their lives and by that time it is obvious they made a mistake. With hurricanes you do not get a second chance. Getting out of the way is the better part of valor. Also, Earl’s interaction with the front may enhance rain fall inland greatly. Flooding will be a concern from not only the surge, but also rainfall.

Behind Earl is Fiona which should not be an issue for the United States and perhaps no one. What is of greater concern is Tropical Storm Gaston. It is much farther south than Danielle was, or Earl or Fiona. Fiona will get picked up by the same trof that is picking up Earl and Danielle also got picked up by a trof. But, Gaston is so far south that it will most likely sneak under any trof and westerly flow around an Atlantic high should drive Gaston in a westerly fashion. Most models put Gaston either near or South of the Dominican Republic in about a week and most develop it into a hurricane. It will not be traveling over any water that has been worked over by Danielle, Earl or Fiona so it should be plenty warm enough and conditions should be conducive for development, perhaps even enough to support a major hurricane. Its location will offer scenarios for it to enter the Gulf of Mexico as well as potentially affecting the East Coast.

HURRICANE EARL DISCUSSION NUMBER 34

NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL072010

500 PM EDT THU SEP 02 2010

DATA FROM A RECONNAISSANCE PLANE INDICATE THAT EARL HAS WEAKENED A

LITTLE BIT THIS AFTERNOON. THE CENTRAL PRESSURE ROSE TO 947

MB…AND A BLEND OF THE SFMR MEASUREMENTS AND FLIGHT-LEVEL WINDS

SUGGEST THAT THE INITIAL INTENSITY HAS DECREASED TO 100 KNOTS. EARL

SHOULD CONTINUE TO WEAKEN GRADUALLY AS THE SHEAR INCREASES AND THE

HURRICANE SHOULD WEAKEN EVEN FASTER AFTER 24 HOURS AS IT MOVES OVER

A COLDER OCEAN. AFTER CROSSING CANADA IN 48 HOURS…EARL SHOULD

BECOME POST-TROPICAL/EXTRATROPICAL AND THEN DISSIPATE AS IT IS

ABSORBED BY A MUCH LARGER EXTRATROPICAL CYCLONE.

SATELLITE AND RECONNAISSANCE FIXES SHOW…AS ANTICIPATED…THAT EARL

HAS TURNED NORTHWARD AND IS MOVING 360 DEGREES AT 16 KNOTS. THE

STEERING PATTERN CONTROLLING THE MOTION OF EARL…AND THE

CONSISTENCY IN TRACK GUIDANCE…HAVE BEEN ADEQUATELY DESCRIBED IN

PREVIOUS DISCUSSIONS. EARL IS ABOUT TO BECOME EMBEDDED IN THE

MID-LATITUDE FLOW AND IT IS READY TO GRADUALLY TURN TO THE

NORTH-NORTHEAST AND NORTHEAST WITH AN INCREASE IN FORWARD SPEED.

ALTHOUGH THE MAXIMUM WINDS IN THE CORE HAVE DECREASED…EARL

CONTINUES TO BE A LARGE AND POWERFUL HURRICANE.

FORECAST POSITIONS AND MAX WINDS

INITIAL 02/2100Z 32.5N 75.2W 100 KT

12HR VT 03/0600Z 34.8N 74.8W 95 KT

24HR VT 03/1800Z 38.0N 72.7W 85 KT

36HR VT 04/0600Z 41.7N 69.0W 75 KT

48HR VT 04/1800Z 46.5N 64.5W 50 KT…POST-TROP/EXTRATROP

72HR VT 05/1800Z…DISSIPATED

$$

FORECASTER AVILA