Matthew Rolls Through Lesser Antilles; Huge Rains for D.C. This Week

Not even classified as a tropical depression early Wednesday morning, Tropical Storm Matthew rolled through the Lesser Antilles as a mid-strength tropical storm on Wednesday afternoon. As of 5 pm EDT, the National Hurricane Center placed Matthew about 65 miles west of St. Lucia, moving west at 18 mph with top sustained winds of 60 mph. Some observers may have done double takes when they saw Matthew debut as a 60-mph tropical storm in its very first advisory (11 am EDT Tuesday). NHC typically relies on Hurricane Hunter data before upgrading a system like Matthew, and we had a gap between flights on Tuesday afternoon and Wednesday morning. Hurricane hunter flights are now scheduled every 12 hours for the next couple of days. Moreover, in their first pass through Matthew on Wednesday morning, the Hurricane Hunters did not find a closed circulation, so the storm had 60-mph winds without being a tropical storm with a closed circulation. The last “instant tropical storm” of this magnitude was Tropical Storm Karen in 2013, whose first advisory was also as a 60-mph tropical storm. Update: According to Weather Channel tropical expert Michael Lowry, Karen's initial top winds were reduced to 45 knots (50 mph] in NHC's best-track reanalysis, so the only system aside from Matthew that has debuted as a 60-mph tropical storm was the very unorthodox Hurricane Debbie (1961).

Satellite imagery of Matthew on Wednesday afternoon revealed a very healthy tropical storm, with excellent outflow ventilating the upper reaches of the storm and an expanding area of intense thunderstorms at its core.


Figure 1. Satellite image for Tropical Storm Matthew as of 5:36 pm EDT Wednesday, September 26, 2016.


Figure 2. Radar imagery from weather.com for the Lesser Antilles as of 4:49 pm EDT Wednesday, September 26, 2016.

Heavy rains and gusty squalls were sweeping across the Lesser Antilles as Matthew moved through on Wednesday afternoon, with some of the strongest activity east of Matthew’s center still to reach the islands. Winds at Dominica’s Melville Hall Airport gusted to 53 mph at 10 am EDT Wednesday, and Martinique’s Le Lamentin Airport reported a sustained wind of 40 mph with gusts to 60 mph at 5:00 pm EDT.

Wunderground member java162 posted this report at 1834Z (2:34 pm EDT): “It has been pretty blustery here in Dominica. So far we have had just over 2 in. of rain. The wind is the real issue. Constant gusts between 60 and 90 kph [37 - 56 mph]. What I have been hearing is of a number of trees down throughout the island and quite a few areas are without electricity.”

Outlook for Matthew through the weekend
Long before it poses any possible threat to the United States, Matthew could mean big trouble for parts of the Greater Antilles. Computer models agree that Matthew will continue on a general westerly track for the next couple of days. Because Matthew’s center is a bit further north than earlier expected, the storm may be able to avoid too much interaction with the land mass of South America. By this weekend, a large upper-level low over the Mid-Atlantic (see below) should provide a pathway for Matthew to take a sharp right turn. While quite unusual, such a sharp turn is hardly unprecedented, as we noted yesterday. Tropical expert Brian McNoldy (University of Miami/RSMAS) delves into one powerful analog--Hurricane Hazel (1954)--in a very timely post today at Capital Weather Gang.

Should Matthew take its expected turn to the north, it would have a very hard time avoiding landfall somewhere between Cuba and the Dominican Republic. It’s too soon to pin down the longitude of that northward turn, but models have been gradually converging on a path that could bring Matthew somewhere near the Windward Passage between eastern Cuba and Haiti, perhaps as soon as Sunday. Residents of eastern Cuba, Jamaica, Haiti, and the Dominican Republic should keep especially close tabs on Matthew over the next several days.


Figure 3. NHC track and intensity forecast for Matthew as of 5:00 pm EDT Wednesday, September 26, 2016.

How quickly will Matthew strengthen?
Adding to the concern for the Caribbean is the potential for Matthew to strengthen quickly. Through Friday, the relative humidity in the lowest few miles of the atmosphere will be only modestly supportive of development (50-60 percent), but wind shear will be very low (5 - 10 knots) and sea-surface temperatures quite high (around 29°C or 84°F). In addition, Matthew will be entering a region with high oceanic heat content, between 50 and 100 kilojoules per square centimeter (see Figure 4 below). Even higher values of oceanic heat content lie further downstream, south of Cuba and Haiti. CSU/CIRA/RAMMB notes: “For tropical cyclones in favorable environmental conditions for intensification (i.e., vertical wind shear less than 15 kt, mid-level relative humidity > 50 %, and warm SSTs [i.e., > 28.5°C]) and with intensities less than 80 knots, values of ocean heat content greater than 50 kJ/cm^2 have been shown to promote greater rates of intensity change.”


Figure 4. Tropical cyclone heat potential, an index of the amount of heat in the upper ocean, for the Caribbean as of September 27, 2016. Image credit: NOAA/AOML.

The 12Z and 18Z Wednesday runs of the SHIPS statistical model give unusually high odds for rapid intensification of Matthew over the next 24 to 48 hours. Based on the starting-point conditions (50 knots, or 60 mph, at 2:00 pm EDT Wednesday), the 18Z SHIPS run projects the following likelihoods of a rapid increase in Matthew’s peak winds:

70% odds of a 25-knot increase in 24 hours
50% odds of a 35-knot increase in 24 hours
42% odds of a 45-knot increase in 36 hours
44% odds of a 55-knot increase in 48 hours

It should be stressed that these numbers are based on a set of statistical predictors that compare the environmental conditions now present with Matthew to those in play during past tropical storms and hurricanes. (See this PowerPoint for more details on the index.) This index has shown some modest skill in predicting rapid strengthening. Other tools used by SHIPS give Matthew considerably lower odds of rapid intensification. Forecasters at NHC also take into account satellite imagery and the results of dynamical models (such as the GFS, Euro, and UKMET) before issuing official predictions. The NHC forecast issued at 5:00 pm EDT Wednesday makes Matthew a hurricane by Thursday night and a Category 2 hurricane by Sunday. We cannot say with any confidence that Matthew will undergo rapid intensification beyond the official forecast, but the possibility is there.

Long-term outlook for Matthew
There remains huge uncertainty in Matthew’s fate beyond the weekend. A large minority of the members of the European ensemble model run from 12Z Wednesday take Matthew back westward toward the Gulf of Mexico as it is approaching Cuba and Haiti, while members of the 12Z Wednesday GFS ensemble are in unanimous agreement that Matthew will continue northward. We cannot yet discount the possibilities in the Euro ensemble, but assuming that Matthew moves into The Bahamas by early next week--as indicated by the 12Z Wednesday operational runs of the GFS, European, and UKMET models--Matthew’s subsequent path will hinge on the state of the upper-level low parking over the Mid-Atlantic into the weekend, as well as another upper-level trough that will be plowing eastward across the United States next week. The upper-level flow across North America and the North Atlantic will include several blocking features late this week into early next week, and these are notoriously difficult to predict. The most we can say at this point is that Matthew has the potential to make landfall somewhere along the Gulf or Atlantic U.S. coasts by later next week.


Figure 5. Forecasts from the 12Z Wednesday European (ECMWF) model ensemble (left) show a wide variety of potential tracks for Tropical Storm Matthew after it reaches the western Caribbean, while the members of the 12Z GFS model ensemble (right) are much more tightly clustered.


Figure 5. Predicted 3-day rainfall totals from 8:00 pm EDT Wednesday, September 28, 2016, through 8:00 pm EDT Saturday, October 1. Image credit: NOAA/NWS/WPC.

Epic multi-day rains for Mid-Atlantic
Even without the help of a tropical storm, the Mid-Atlantic low will bring a period of extremely heavy rain over the next several days, especially in the Washington, D.C., area. The 3-day precipitation outlook from NOAA’s Weather Prediction Center calls for widespread 3” - 6” rains from northern Virginia to southern Pennsylvania, with an 8” - 10” maximum possible close to the District of Columbia (see Figure 5 above). “The D.C. area should prepare for the possibility of the heaviest rain event in at least five years and possibly longer,” noted Jason Samenow (Capital Weather Gang). Given the drought conditions that emerged during the parched summer of 2016, much of this rain will be welcome and beneficial, but at least some areas will get too much, leading to the potential for “severe flooding,” according to the NWS/DC office.

Such heavy rain is very rare for D.C. outside of tropical cyclones. The city’s top 3-day rainfall of 10.34” (June 25-27, 2006, part of a billion-dollar flood event) was associated with slow-moving mid-latitude weather features together with an unclassified tropical low off the North Carolina coast. All but one of the runner-up totals--in 1955, 1928, 1933, 1972, and 2005--were directly associated with hurricanes or tropical storms. Deep moisture will be flowing from the Caribbean into the mid-Atlantic as the rains unfold. This flow will pass over a large section of the Northwest Atlantic that experienced record-warm sea-surface temperatures (SSTs) in August. SSTs continue to run 1-2°C above average throughout the Northwest Atlantic, and these record or near-record values will enhance the flow of water vapor heading into the Mid-Atlantic deluge. Should Matthew move north into this region, it could also benefit from the unusually warm waters, and any possible rain from Matthew in the D.C. area would fall on saturated ground.

We’ll be back with our next update on Matthew by late Thursday morning.

Bob Henson