Tropical Storm Debby growing in size and strength

Tropical storm warnings are flying from Alabama to the Panhandle of Florida and along much of Southeast Louisiana coast, as Tropical Storm Debby inches to the north at 2 mph. The heaviest rains of Debby have moved ashore along much of the Gulf Coast of Florida, with up to two inches of radar-estimated rainfall in the Apalachicola, Florida area so far. Several buoys to the north and east of the center of Debby are receiving tropical storm-force winds, including SGOF1 (56 mph, gusting to 68 mph at 9 am EDT) and buoy 42022, 100 miles off the coast from Tampa (42 mph sustained winds at 7am EDT.) Our Wundermap for the surrounding ocean areas shows a large region of 25 - 45 mph winds off the Southeast Louisiana coast. An Air Force hurricane hunter aircraft flying through Debby at 5,000 feet found an area of 60 mph surface winds about 100 miles south-southwest of Apalachicola, Florida at 7am EDT. Visible satellite loops show the classic signature of a medium-strength tropical storm undergoing substantial wind shear. The heavy thunderstorms of Debby are all on the east and north sides. Upper-level winds out of the southwest creating a moderate 10 - 20 knots of wind shear that is driving dry air to the southwest of the storm into Debby's core. This dry air can be seen on Water vapor satellite loops. However, Debby is steadily overcoming this dry air and wind shear, and the storm has increased in organization, size, and in intensity this morning. Ocean temperatures are about 28.5°C (83°F) in the Central Gulf of Mexico, which is about 1°F above average, but these waters do not extend to great depth, which will limit how strong Debby can get.


Figure 1. Morning radar image of the rainfall from Tropical Storm Debby over coastal Florida.


Figure 2. True-color visible satellite image of Debby taken at 12:45 pm EDT Saturday June 23, 2012, before it became a tropical storm. Image credit: NASA.

Forecast for Debby
Debby's slow motion will make rainfall the primary threat from the storm, with up to 10 inches likely in some regions along the coast from Southeast Louisiana to Pensacola, Florida. Unfortunately, this part of the coast is not under drought, and does not need the rain. Farther to the east, along the rest of the Gulf Coast of Florida, moderate to severe drought prevails, and flooding from Debby will be less of an issue. The slow motion of Debby will also slow down intensification of the storm, since its winds are stirring up cooler waters from the depths to the surface that then cool down the storm. Debby's close proximity to land places a portion of its circulation over land, which will also tend to slow down intensification. Wind shear is expected to remain in the moderate range through Tuesday. The latest SHIPS model forecast gives Debby a 19% chance of undergoing rapid intensification--a 30 mph increase of winds in 24 hours. The 8 am EDT NHC wind probability forecast is giving Debby a 29% chance of becoming a hurricane by early Tuesday morning. Given the current increasing trends in Debby's organization and intensity, these odds should probably be closer to 50%.

Steering currents for Debby are very weak, resulting in an unusually large spread in the model forecasts for where the storm might go. Take your pick from the various model solutions: Debby could make landfall anywhere from South Texas to Tampa Bay. The official NHC track west towards Texas should definitely not be viewed as gospel.


Figure 3. Pick a model, any model: the model forecasts for Debby are all over the place, making the current official forecast a low-confidence one.

Debby's place in history
Remarkably, Debby's formation on June 23 comes a full two months ahead of the usual formation date of the season's fourth storm in the Atlantic, August 23. Debby's formation beats by twelve days the previous record for formation of the fourth named storm of the year in the Atlantic, set in 2005, when Hurricane Dennis was named on July 5. An early start to the Atlantic hurricane season has been increasingly common in recent years. In 2008, I blogged about the research of Dr. Jim Kossin of the University of Wisconsin, who published a 2008 paper in Geophysical Research Letters, titled "Is the North Atlantic hurricane season getting longer?" He concluded that yes, there is a "apparent tendency toward more common early- and late-season storms that correlates with warming Sea Surface Temperature but the uncertainty in these relationships is high". Three out of four of this year's early quartet of storms--Alberto, Beryl, and Debby--formed in ocean areas that were more than 1°F above average, which is an unusually high amount of warmth. We should expect to see more early-season Atlantic tropical storms as a consequence of global warming, since cool ocean temperatures are a key impediment to formation of such storms. However, this assumes that factors such as wind shear and atmospheric stability won't grow more hostile for tropical cyclone formation during the early part of hurricane season, and this is uncertain. If we do end up seeing a substantial increase in early-season tropical storms as a consequence of global warming, this is not necessarily a bad thing. Early-season tropical storms are often more boon than bane, bringing much-needed drought-busting rains, like Tropical Storm Beryl did for North Florida last month. With drought frequency and intensity predicted to increase for much of the Gulf Coastal states in coming decades, an increase in rainfall from early-season tropical storms may do more good than the damages inflicted by the high winds and flooding these storms may bring. There is typically a lot of wind shear around in May, June, and July, making it difficult for early season storms to reach major hurricane status. According to Wunderground's list of major early-season hurricanes, since record keeping began in 1851, there has been only one major hurricane in May, two in June, and nine in July. Three of these occurred in the past ten years, so there has not as yet been an observable large increase in early-season major hurricanes due to global warming.

References
Kossin, J., 2008, "Is the North Atlantic hurricane season getting longer?", Geophysical Research Letters, Vol. 35, L23705, doi:10.1029/2008GL036012, 2008.

Record heat in Colorado
On Saturday, for the second consecutive day, record heat scorched Colorado. According to wunderground's weather historian Christopher C. Burt, the 104° reading in Denver tied for the hottest June temperature on record in the city, last set on June 26, 1994. It was also just 1° short of the all-time record of 105° (set on July 20, 2005 and August 8, 1878.)

Colorado Springs tied its all-time record for warmest temperature ever measured on Saturday, with 100°. The city has hit 100° four other times, most recently on July 24, 2003.

Pueblo, Co reached 106°, a daily record; this was just 2° shy of the monthly record of 108°. The all-time record is 109° on 7/13/2003.

Lamar, Co hit 109°, just 2° short of the all-time record of 111° set on 7/13/1934.

The Colorado heat did no favors for firefighters, who are struggling to the contain the massive 81,000 acre High Park fire fifteen miles northwest of Fort Collins. The fire is the second largest and most destructive wildfire in Colorado's history, and is 45% contained. A new fire erupted Saturday in Estes Park, at the boundary of Rocky Mountain National Park, and destroyed 21 homes.

Jeff Masters