Saturday, October 14, 2017

Hurricane Ophelia Hits Category 3; Destructive Winds On Tap for Ireland

Following on this morning's hurricanes-in-Britain-and-Ireland-backgrounder we revisit Wunderground's Category 6:
The 2017 Atlantic hurricane season continued to astound on Saturday morning with the unexpected ascent of Hurricane Ophelia to major-hurricane status. Based on a very impressive satellite signature, the NOAA/NWS National Hurricane Center brought Ophelia’s peak winds up to 115 mph at 11 am EDT, making it a low-end Category 3 storm. The wind estimate may be conservative, said NHC forecaster Lixion Avila in the NHC forecast discussion. Ophelia was located about 220 miles south of the Azores, moving northeast at 25 mph. Ophelia is expected to pass within 100 miles of the Azores’ southeasternmost island, Santa Maria. The island will be on the hurricane’s weaker left-hand side, but winds could reach tropical-storm force, and squally weather is likely.
Much bigger impacts from Ophelia are expected in Ireland (see below).
Figure 1. A remarkably well-organized Ophelia in enhanced infrared satellite imagery from 1:45 pm EDT Saturday, October 14, 2017. Image credit: NOAA/NESDIS.
To call Ophelia unusual would be an understatement. For one thing, it became a major hurricane at longitude 26.6°W, further east than any other formation of a Category 3 in the Atlantic. The former record-holder was Frances (1980), which became a Category 3 at 12.8°N, 29.8°W. Ophelia’s achievement is even more impressive when you consider its latitude: 34.8°N. In data going back to 1851, no other major hurricane is known to have formed anywhere close to as far northeast as Julia. The runner-up at Julia’s latitude range, Michael (2012), developed some 900 miles further west (see Figure 2 below).

Ophelia also extends this year’s count of major Atlantic hurricanes to six, a tally last achieved in 2004. Only two years have notched seven major Atlantic hurricanes: 1961 and 2005. 

What’s a major hurricane doing in a place like this?
By conventional standards, one wouldn’t even expect Ophelia to be a hurricane, much less a major one. Sea surface temperatures beneath Ophelia are around 25°C (77°F), which is roughly 1°C below the traditional benchmark of SST levels warm enough to support tropical development. However, these waters are about 2°C (3.6°F) above average for the location and the time of year, and upper-level temperatures near the top of Ophelia are several degrees C below average. The result is enough instability to support well-organized showers and thunderstorms (convection), even though the convection is less intense than it would be in a warmer environment. A 2015 study led by Ron McTaggart-Cowan (Environment Canada) showed that a better threshold for systems like Ophelia that are transitioning away from the tropics would be based on potential instability between lower and upper levels of the hurricane, rather than on SSTs alone. Ophelia meets this threshold, according to Philippe Papin (University at Albany, SUNY).

Other things are also working in Ophelia’s favor. A strong outflow jet at upper levels on Ophelia’s west sides is helping to ventilate the hurricane, and the 12Z Saturday run of the SHIPS model showed that wind shear on Saturday was in the light to moderate range (about 10 – 15 knots). The shear will begin to increase rapidly by Saturday night, heralding a change to come in Ophelia’s structure....

Still looking to be a category 1 at landfall but the bump up to category 3 windspeeds should give pause to anyone in the path. The slight shift of the track to the west gives some hope but isn't yet enough; the front-right quadrant of a hurricane is the area of maximum wind strength which means Limerick gets walloped: