Tropical Storm Colin Chugs Toward Florida, May Strengthen Off Southeast Coast

Despite its disorganized structure, ragtag Tropical Storm Colin gained a bit of strength Sunday night as it continued churning toward an expected landfall Monday night on the Big Bend of Florida’s northwest Gulf Coast. As of 11 AM EDT Monday, Colin was located at 27.0°N, 87.0°W, or about 285 miles west-southwest of Tampa, FL, moving north-northeast at 16 mph. Tropical storm warnings are in effect from Indian Pass to Englewood on the Florida Gulf Coast and from Sebastian Inlet, FL, to South Santee River, SC, on the Southeast U.S. coast. Colin’s top sustained winds increased to 50 mph overnight, leaving it in the weaker half of the tropical storm range. Little if any further intensification is expected before tonight’s landfall, as the storm’s structure is not conductive to strengthening. Vertical wind shear of 20 - 30 knots (25 - 35 mph) is helping to keep nearly all of Colin’s showers and thunderstorms (convection) displaced well to the east of the storm’s low-level center. The resulting comma shape starting to become evident on Monday morning is a classic marker of a tropical cyclone under the influence of increasing upper-level flow.


Figure 1. Infrared NOAA GOES image of Tropical Storm Colin as of 1445Z (10:45 am EDT) Monday, June 6, 2016. The center of Colin is faintly visible in the grey cloud mass just to the west of the bright red/orange area (which indicates very high, cold cloud tops). Image credit: NASA/MSFC Earth Science Office.


Figure 2. Unlike a more prototypical tropical cyclone, Colin features a broad area of southerly low-level winds ahead of its center, rather than winds across a large area rotating around its center. Shown here is data posted at 1502Z (11:02 AM EDT) Monday, June 6, 2016, from the Advanced Scatterometer (ASCAT) aboard the European Metop-A satellite. Image credit: NOAA/NESDIS and ASCAT/MetOp

Colin will sweep by Southeast U.S. coast over next 24 hours
Colin is being steered by a large area of southwesterly upper winds ahead of an upper-level trough extending from the northeast U.S. into the northwest Gulf of Mexico. This trough will keep the storm on a fairly well-defined track and will keep its structure asymmetric. A gradual curve to the right will take Colin across the Florida Peninsula overnight, with the storm emerging over the Atlantic just off the Southeast U.S. coast on Tuesday. Computer models agree that Colin will hug the coast while gaining forward speed on Tuesday, with the center likely just southeast of the Outer Banks by Tuesday afternoon and well east of the U.S. coast by Tuesday night. Tropical storm warnings may be extended northward along the Southeast coast, and Colin’s top sustained winds may actually increase slightly during the storm’s brief trek along the Southeast coast, as energy from upper levels feeds into Colin and the storm begins the process of transitioning into a post-tropical/extratropical cyclone. Because of the forward component of motion and the storm’s pre-existing asymmetry, the winds on Tuesday will be considerably stronger on the offshore side of Colin than on the coastal side. The well-defined steering currents will keep Colin from attempting any leftward curve that could hook it into the East Coast. Colin may reach its peak intensity as a strong tropical storm or post-tropical cyclone late Tuesday as it moves well east of the Outer Banks of North Carolina.


Figure 3. WU depiction of official forecast track of Tropical Storm Colin as of 11 AM EDT Monday, June 6, 2016.


Figure 4. Radar image for Tropical Storm Colin at 11:24 AM Monday, June 6, 2016. The image was produced using WU’s WunderMap with the “radar” feature turned on.

Torrential downpours will likely be the most widespread impact from Tropical Storm Colin. A large area of heavy rain on Monday morning extended from the Florida Panhandle into South Carolina, with intense rainbands streaming northward into Florida’s West Coast (see Figure 4 above). NOAA’s Storm Prediction Center has the northern half of Florida and far southeast Georgia in a slight risk for severe thunderstorms through Monday night, with a few tornadoes possible. As Colin sweeps to the northeast, heavy rain can be expected along the immediate Southeast coast late Monday night and Tuesday morning. Flash flood watches are in effect from the northern Georgia coast to the Outer Banks, which have already been hard hit by torrential rains associated with Tropical Storm Bonnie. Cape Hatteras, NC, has received 6.15” of rain in the first five days of June after a record 12.67” in May, most of that during Memorial Day weekend. This is already the second wettest June on record for Cape Hatteras (although the 14.34” racked up in June 1949 will be hard to top).

Storm surge risk along Florida’s Gulf Coast
Strong southwest flow on Colin’s east side may produce a storm surge along Florida’s Gulf Coast on Monday. The risk for high water will be greatest to the east of the eventual point of landfall, with more general enhancement along the coast several hours earlier, around the time of the mid-afternoon high tide. NHC is projecting a potential storm surge of 1 to 3 feet (perhaps higher in some areas) along the immediate coastline from Indian Pass south to Tampa Bay, with 1 to 2 feet possible south of Tampa Bay to Florida Bay. According to storm surge expert Hal Needham, the most vulnerable place for high water from Colin may be the Big Bend area from Apalachee Bay to Cedar Key, where the topography of the concave coast is especially conducive to producing surge. In a blog post published midday Monday, Needham projects a surge in the Cedar Key region of around 2.5 feet near high tide on Monday afternoon (around 3 PM EDT). Combined with tidal levels, this would produce a total water level (storm tide) at Cedar Key of around 4.7 feet above mean sea level (dubbed NAVD 88). This would be Cedar Key’s ninth highest storm tide in records going back to 1920, and it would be several inches higher than the water level produced in Cedar Key by Tropical Storm Debby in 2012.


Figure 5. If Tropical Storm Colin produces a peak storm tide on Monday afternoon of 4.72 feet, as projected by storm surge expert Dr. Hal Needham, it would be the ninth highest storm tide in records going back to 1920. This image was generated through Needham’s “U-Surge” dataset. Image credit: Hal Needham.

Colin is the earliest “C” storm in Atlantic history
Never before have we been tracking the Atlantic’s third named storm of a calendar year this early in the year. As noted in a weather.com article, there have been two other “C” storms as early as June since current naming practices began in the 1950s: Hurricane Chris (which began as a named subtropical storm on June 18, 2012) and Tropical Storm Candy (June 23, 1968). Going all the way back to 1851, the previous earliest appearance of the season’s third storm was June 12, 1887, although some early-season storms were undoubtedly missed during the pre-satellite era.

Bob Henson