Colin ready to re-form; TSR keeps their Atlantic hurricane forecast numbers high

The remains of Tropical Storm Colin continue to generate heavy thunderstorm activity over the waters a few hundred miles northeast of Puerto Rico. Colin's remains are in a rather unfavorable environment for re-development, since the storm is passing beneath an upper-level low pressure system with dry air and high wind shear. Despite the high wind shear of 20 - 25 knots, Colin's remains have grown more organized during the past day, and a low-level circulation has formed. A pass of the Windsat satellite last night revealed that ex-Colin is already generating tropical storm force winds of 40 mph in isolated regions. Recent satellite imagery shows that heavy thunderstorm activity has increased in intensity and areal coverage this morning, and a low level circulation that may or may not persist has formed near 23.5N 65.5W (Figure 1.)

Forecast for Colin
The latest SHIPS model forecast predicts that wind shear will drop from 15 - 25 knots today to a moderate 10 - 20 knots on Friday. This relaxation of shear prompts most of the major models to predict re-development of Colin sometime in the next three days. NHC is giving Colin's remain a 50% chance of developing into a tropical depression by Saturday morning. A major trough of low pressure will move off the U.S. East Coast on Friday, and this trough will pull Colin to the north and cause it to slow down. All of the major forecast models are predicting that the trough of low pressure will be strong enough to fully recurve Colin out to sea early next week. Colin's remains may pass close to Bermuda on Saturday, with the latest 06Z (2am EDT) run of the GFDL model predicting that Bermuda will experience tropical storm force winds on Saturday as Colin passes to the west of the island. The Hurricane Hunters are scheduled to investigate Colin's remains at 2 pm EDT this afternoon. It currently appears that Colin will only be a threat to Bermuda and Newfoundland.


Figure 1. Morning satellite image of Colin's remains show a low-level circulation, exposed to view, has formed at the edge of a region of heavy thunderstorms.

92L
A tropical wave (Invest 92) a few hundred miles south of Jamaica is moving west at 15 - 20 mph. This wave is over warm water and is experiencing low wind shear of 5 - 10 knots, and could show some development over the next two days. However, the wave's rapid westward motion should bring it ashore over Nicaragua and Honduras on Friday and the Yucatan Peninsula on Saturday, and 92L probably does not have enough time over water to develop into a tropical depression. NHC is giving a 10% chance of this disturbance developing into a tropical depression by Friday morning.

Elsewhere in the tropics
The GFS model is predicting that a tropical wave that moved off the coast of Africa yesterday will develop into a tropical depression early next week. None of the other models is showing any obvious tropical cyclone development over the coming week. The current phase of the Madden-Julian Oscillation (MJO) over the Atlantic favors upward motion and enhanced probabilities of tropical storm formation, so it would not be a surprise to see a new tropical depression form next week between Africa and the Lesser Antilles Islands.

TSR keeps their forecast numbers for the coming hurricane season high
The British private forecasting firm Tropical Storm Risk, Inc. (TSR) joins CSU in calling for a very busy Atlantic hurricane season. The latest TSR forecast issued August 4 calls for 17.8 named storms, 9.7 hurricanes, 4.5 intense hurricanes, and an Accumulated Cyclone Energy (ACE) 183% of average (assuming an ACE of 101 is average.) These storm numbers are a slight drop from their July 6 forecast of 19.1 named storms, 10.4 hurricanes, and 4.8 intense hurricanes, but this is still a very aggressive forecast. The 50-year average is 10 named storms, 6 hurricanes, and 2 intense hurricanes. TSR predicts a 96% chance that this season will have an above-average ACE index, and only 4% chance it will be near normal. TSR rates their skill level for August forecasts as 51% above chance at forecasting the number of named storms, 64% skill for hurricanes, and 47% skill for intense hurricanes.

TSR projects that 5.7 named storms will hit the U.S., with 2.6 of these being hurricanes. The averages from the 1950-2008 climatology are 3.2 named storms and 1.5 hurricanes. TSR's skill in making these August forecasts for U.S. landfalls is 19% - 23% above chance. They give an 89% chance that the U.S. landfalling ACE index will be above average. In the Lesser Antilles Islands of the Caribbean, TSR projects 1.8 named storms, 0.8 of these being hurricanes. Climatology is 1.1 named storms and 0.5 hurricanes.

TSR cites two main factors for keeping their numbers high: below-average trade winds and near-record warm sea surface temperatures in the tropical Atlantic. TSR expects trade winds in the Main Development Region (MDR) for hurricanes over the Atlantic (the region between 10° - 20° N from Central America to Africa, including all of the Caribbean) to be 1.53 meters per second (about 3.4 mph) slower than average in this region, which would create more spin for developing storms, and allow the oceans to heat up, due to decreased mixing of cold water from the depths and reduced evaporational cooling.

Forecast numbers for the coming hurricane season
Here are the number of Atlantic named storms, hurricanes, and intense hurricanes predicted by the various forecast groups in their late May or early June forecasts:

23 named storms: PSU statistical model
20 named storms: UKMET GloSea dynamical model
18.5 named storms, 11 hurricanes, 5 major hurricanes: NOAA hybrid statistical/dynamical model technique
18 named storms, 10 hurricanes, 5 intense hurricanes: CSU statistical model (Phil Klotzbach/Bill Gray)
17.7 named storms, 9.5 hurricanes, 4.4 intense hurricanes: Tropical Storm Risk (TSR), hybrid statistical/dynamical model technique
17 named storms, 10 hurricanes: FSU COAPS dynamical model
10 named storms, 6 hurricanes, 2 intense hurricanes: climatology

And here are the new late July/early August forecast numbers so far:

15 named storms, 8 hurricanes: FSU COAPS dynamical model (July 15 forecast)
18 named storms, 10 hurricanes, 5 intense hurricanes: CSU statistical model (Phil Klotzbach/Bill Gray)
17.8 named storms, 9.7 hurricanes, 4.5 intense hurricanes: Tropical Storm Risk (TSR), hybrid statistical/dynamical model technique (Note: TSR had higher numbers of 19.1, 10.4, and 4.8 in their July 6 forecast)

This season has had three named storms so far (Alex, Bonnie, and Colin.) It will be difficult to have a season with 19 or more named storms, since the four seasons that had at least 19 named storms all had at least five named storms by this point (August 4.) These four seasons were 1887, 1933, 1995, and 2005.

Next update
I'll have an update on Friday morning.

Jeff Masters