The Tribune

By DR JEFF MASTERS

Very dangerous Hurricane Matthew is maintaining Category 4 strength as it heads northwards at 6mph, and is already dumping potentially catastrophic rains on Haiti and the Dominican Republic as it moves towards the Bahamas.

An unusual area of extra spin and low pressure that has been embedded on the east side of Matthew’s circulation for days is generating intense rains in excess of one inch per hour.

A portion of this feature rotated ashore over southern Haiti and the Dominican Republic early Monday morning, and the mountainous terrain of this region undoubtedly caused additional uplift that resulted in rainfall rates much higher than one inch per hour. A personal weather station in Cabo Rojo, on the southern coast of the Dominican Republic near the border with Haiti, recorded 20.05 inches of rain in 11 hours between 3am and 2pm on Monday, including a remarkable 5.33 inches between 6am and 7am.

While Personal Weather Station data is often suspect, these are believable rainfall amounts based on the satellite presentation of Matthew. The outer spiral bands of Matthew are also affecting Jamaica.

An Air Force hurricane hunter aircraft made four passes through the eye of Matthew on Monday morning and early Monday afternoon and found surface winds as high as 140mph. The central pressure stayed in the 940 - 941 millibar range during all the passes, so it does not appear that Matthew is undergoing intensification. Satellite loops on Monday morning showed that Matthew’s eye was clearing out and becoming more distinct, and the hurricane’s cloud pattern was becoming more symmetric - signs the storm may be about to intensify, though. Light wind shear of five to 10 knots is affecting the storm, and Matthew is over warm ocean waters of 29°C (84°F) and has plenty of moisture to work with: 70 to 75 per cent relative humidity at mid-levels of the atmosphere, as analysed by the Statistical Hurricane Intensity Prediction Scheme (SHIPS) model.

Matthew passed over National Oceanic and Atmospheric Administration (NOAA) buoy 42058 early Monday morning, and top winds during passage of the weaker portion of the eyewall were 74mph, gusting to 92mph. Seas were 34 feet, and the buoy recorded a minimum pressure of 943 mb. The wind measurement height on the buoy was five metres, so an upwards correction of about 10mph is needed to adjust these numbers to the standard 10-metre observing height for winds.

Intensity forecast

The latest SHIPS model forecast predicts that wind shear will remain light to moderate, five to 15 knots, for the next five days. Ocean temperatures will be very warm, between 29° and 30° C (84° to 86°F) and the heat content of the ocean will be high to very high, which argues for intensification of Matthew. At any time, though, Matthew could undergo an eyewall replacement cycle (ERC), where the inner eyewall collapses and is replaced by a larger-diameter eye, with a new eyewall formed from an outer spiral band. This process usually causes a weakening to the storm’s top winds for a day or so.

The down side of an ERC is that is spreads out the storm’s hurricane-force winds over a wider area, resulting in severe impacts over a wider area. Our top three intensity models - the HWRF, LGEM, and SHIPS models - were predicting on Monday morning that Matthew would be at Category 3 or 4 strength for the next four days. The SHIPS model gave Matthew an 11 per cent chance of rapid intensification of 30mph or more by Tuesday morning. Landfall in eastern Cuba/northwest Haiti on Tuesday could act to disrupt the hurricane and destroy its inner core, which might knock Matthew down to Category 2 strength for several days. However, Matthew will probably re-intensify by at least 20mph in the two days after its landfall in Cuba/Haiti.

Three-day track forecast

Matthew will make landfall or pass very close to the southwest tip of Haiti early Tuesday morning, then make a second landfall in eastern Cuba on Tuesday afternoon. Matthew will then traverse the Bahamas from southeast to northwest Tuesday evening through Thursday morning. In their 11am EDT Monday Wind Probability Forecast, the National Hurricane Centre (NHC) gave highest odds of hurricane-force winds to Les Cayes in southwest Haiti (35 per cent) and to Guantanamo Bay, Cuba (41 per cent). Lower odds were given to Kingston, Jamaica (0) and Port-Au-Prince, Haiti (2). In the Bahamas, hurricane-force wind odds of 39 per cent were given to Great Exuma, 24 per cent to New Providence and 35 per cent to San Salvador.

Beyond the

Bahamas

A significant westward shift in computer model guidance on Hurricane Matthew has occurred, and this could have big implications for the hurricane’s potential impact on the US East Coast.

The main reason appears to be stronger ridging south of an area of low pressure associated with a tropical wave (designated Invest 98L) north of Matthew than earlier predicted, which may help to nudge Matthew far enough west for major impacts along the south east US coast. Last night’s 50 ensemble runs from the 00Z Monday European model included a number of tracks making landfall along the US East Coast.

Most concerning is that, for the first time in Matthew’s life, all four members of the Euro “high-probability” cluster - the members that most closely match the operational run - depict Matthew making landfall on Florida’s east coast.

Dr Jeff Masters co-founded Weather Underground in 1995 and flew with the National Oceanic and Atmospheric Administration ‘Hurricane Hunters’ from 1986 to 1990. This is his blog post from www.wunderground.com yesterday