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Hurricane Earl will pose a threat to residents along much of the eastern coast of the United States later this week. If you recall, last week we had Hurricane Danielle that became a powerful hurricane but turned north from the tropical regions into the Central North Atlantic and really was nothing more than a concern for shipping, though it may eventually raise issues along the SE Greenland coast, Iceland, England or mainland Europe. Danielle turned north because there was a trof in between ridge in the North Atlantic and a ridge over the eastern third of the United States. Since that time, the trof lifted up and out and the ridge has filled in to a large degree.

Consequently, Earl has been traveling more westward around the expanding Atlantic ridge at the lower latitudes of teh North Atlantic than Danielle did. The idea has been that this storm would turn north by now but the models did a lousy job of anticipating the expansion of the Atlantic ridge. However, each model run from the last 48 hours have been taking the track of Earl farther and farther west. At this point, none of the hurricane models that make up the spaghetti model depiction take Earl into the United States but does bring it close enough to cause an increase in tides and also provide windy weather in squalls. However, midday Monday, Earl was passing North of the Virgin Islands with a vector of 285 degrees, or just 15 degrees north of due west, which means that Earl has not begun its turn.

Remember, coriolis parameters will want to turn Earl north but the ridge is preventing that. Any weakness in that ridge will allow Earl to turn. As it stands, the ridging along the US East Coast is not all that strong so some turning northwest is anticipated. But the major turn will come when a trof digs in from the Great Lakes and down the East Coast. If that happens, then Earl will turn north and increase in forward speed. There should also be some decrease in intensity as shearing with the trof will tend to mess up the upper support. However, Earl will remain a significant hurricane. At 5pm, winds were estimated at 135 mph with gusts to 155 mph. Several models take Earl to Category 5 status on the Saffir Simpson Scale over the next few days before backing off.

The issue is when does Earl turn? If the trof does not dig as deep as the models advertise, then the turn could be a more gentle, wide turn and that would increase the possibility of it running along the East Coast. If the trof does not come through fast enough, then the storm will be farther west when it makes its turn and then it affects the US. None of the hurricane models at this point show the eye of Earl on land in the US. A couple of conventional 12Z models run it along the VA coast but, in general, they too keep it off shore. Most models time the storm as off the VA coast on Thursday night. The majority of the 12Z hurricane models take it to 131 to 140 mph winds (cat 4) at its maxima but decrease intensity as it runs up the coast. What concerns me is that if you look at the spagetti models at 18Z, they initiate Earl moving in a heading something a little greater northwest than what I observe from the actual satellite loop. It could just be a wobble but its possible that Earl may have a bit more of an initial westward motion than the models think and that could greatly affect the ultimate track, depending on how the trof shakes out. The 5PM Discussion has the intial motion at 290 degrees, which is 5 degrees farther north than the 11AM discussion, but the NHC has once again shifted the track farther west for the first 72 hours. Again, this bears watching on the East Coast

The 18Z hurricane models also raise the stakes and initiate Earl as a Category 4 hurricane, yet, at 3pm EDT it was only 125 mph. Several raise its intensity to category 5 before backing it off somewhat. Regardless, tides will be running very high later this week all along the East Coast. Folks from North Carolina to Maine will feel some effect of Earl for the latter half of the work week…it just remains to be seen how much of an effect. The National Hurricane Center added this last remark to its 11AM Monday discussion, which should be enough for East Coast residents to pay attention:

THIS IS A GOOD TIME TO REMIND EVERYONE THAT NHC AVERAGE TRACK

FORECAST ERRORS ARE 200 TO 300 MILES AT DAYS 4 AND 5. GIVEN THIS

UNCERTAINTY…IT IS TOO SOON TO DETERMINE WHAT PORTION OF THE U.S.

EAST COAST MIGHT SEE DIRECT IMPACTS FROM EARL.

HURRICANE EARL DISCUSSION NUMBER 22

NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL AL072010

500 PM AST MON AUG 30 2010

THE RECONNAISSANCE AIRCRAFT MEASURED 116 KT FLIGHT-LEVEL WINDS AND

108 KT SFMR SURFACE WINDS SHORTLY AFTER 1500 UTC. THIS WAS THE

BASIS FOR THE EARLIER INCREASE IN INTENSITY. SINCE THAT TIME…THE

PRESSURE HAS CONTINUED TO DROP AND WAS DOWN TO 955 MB ON THE LAST

DROPSONDE RELEASED IN THE EYE AROUND 1700 UTC. SINCE THE AIRCRAFT

DEPARTED EARL…THE EYE HAS CONTINUED TO CLEAR AND IS SURROUNDED BY

CLOUD TOPS TO -70 DEGREES CELSIUS OR COLDER. OBJECTIVE SATELLITE

INTENSITY ESTIMATES HAVE INCREASED TO JUST ABOVE 6.0 ON THE DVORAK

SCALE AND SUPPORT AN INITIAL INTENSITY OF 115 KT…CATEGORY FOUR ON

THE SAFFIR-SIMPSON HURRICANE WIND SCALE.

EARL IS EXPECTED TO REMAIN IN A LOW SHEAR ENVIRONMENT AND OVER WARM

WATER DURING THE NEXT FEW DAYS AND SOME ADDITIONAL STRENGTHENING IS

LIKELY IN THE SHORT TERM. THEREAFTER…EYEWALL CYCLES WILL LIKELY

INDUCE SOME FLUCTUATIONS IN INTENSITY…WHICH ARE DIFFICULT TO

PREDICT. THE NHC FORECAST IS AGAIN NEAR THE HIGH END OF THE

INTENSITY GUIDANCE. AT DAYS 4 AND 5…THE HURRICANE IS FORECAST TO

MOVE OVER COOLER WATER AND INTO AN AREA OF INCREASING SHEAR WHICH

SHOULD RESULT IN WEAKENING.

THE INITIAL MOTION IS 290/13. THE FORECAST REASONING REMAINS

UNCHANGED AS EARL IS EXPECTED TO TURN NORTHWESTWARD IN ABOUT 24

HOURS AROUND THE WESTERN PERIPHERY OF THE SUBTROPICAL RIDGE. THE

TRACK GUIDANCE HAS SHIFTED NOTICEABLY WESTWARD DURING THE FIRST

48 HOURS…WITH THE GFS AND ECMWF ON THE EASTERN EDGE OF THE

ENVELOPE. THE OFFICIAL FORECAST HAS BEEN ADJUSTED WESTWARD…

PRIMARILY THROUGH 72 HOURS…AND LIES BETWEEN THE GFS/ECMWF AND THE

MULTI-MODEL CONSENSUS.

THIS IS A GOOD TIME TO REMIND EVERYONE THAT NHC AVERAGE TRACK

FORECAST ERRORS ARE 200 TO 300 MILES AT DAYS 4 AND 5. GIVEN THIS

UNCERTAINTY…IT IS TOO SOON TO DETERMINE WHAT PORTION OF THE U.S.

EAST COAST MIGHT SEE DIRECT IMPACTS FROM EARL.

FORECAST POSITIONS AND MAX WINDS

INITIAL 30/2100Z 19.3N 64.7W 115 KT

12HR VT 31/0600Z 20.2N 66.3W 125 KT

24HR VT 31/1800Z 21.7N 68.1W 130 KT

36HR VT 01/0600Z 23.4N 70.1W 130 KT

48HR VT 01/1800Z 25.8N 72.1W 125 KT

72HR VT 02/1800Z 31.6N 74.7W 115 KT

96HR VT 03/1800Z 38.0N 71.0W 95 KT

120HR VT 04/1800Z 45.5N 63.0W 70 KT…INLAND

$$

FORECASTER BROWN