Hurricane Matthew dumped a swath of 8 to 18 inches of rain across inland portions of eastern North Carolina during the period of 07 October through 09 October 2016. Several locations reported incredible rain amounts including 18.38 inches in Elizabethtown NC, 17.00 inches in Hope Mills NC, 16.71 in White Oak NC, 16.28 in Godwin NC, 15.62 in Fayetteville NC, and 15.56 in Goldsboro NC. The precipitation pattern was notable for several reasons, including the fact that the area of heaviest rain in North Carolina was generally located well removed from the coast. The axis of heaviest rain in North Carolina fell near the Interstate 95 corridor as shown in the precipitation image shown above/to the right from the Southeast Regional Climate Center.
The animated regional radar reflectivity loop to the right is from 2358 UTC on 06 October through 0258 UTC on 09 October which shows the evolution of the precipitation across the Southeast during Matthew. Note how very little precipitation is occurring east of the storm track as the system moves near the South and North Carolina coast. During most of the time the tropical cyclone impacted North Carolina, the storm had a Left of Track (LOT) precipitation distribution that favored the heaviest rainfall across inland locations in NC. This was not a surprise as two related conceptual models supported this pattern, including the LOT model from Atallah et al. (2007) and the expectation that Matthew would be undergoing extratropical transition (ET) as it approached the Carolinas.
The Atallah et al. (2007) composite argues that storms that have most of their rainfall distributed on the west side of storm’s track interact with an upper level trough and potential vorticity maximum in a synergistic way. A LOT precipitation distribution is often characterized by a positively tilted mid-latitude trough approaching the tropical cyclone from the northwest. The precipitation stretches out well north of the system. The trough often transitions from a positive to a negative tilt during its interaction with tropical cyclone. Some notable tropical cyclones in the Carolinas with a left of track precipitation distribution include Alberto (2006), and Floyd (1999).
The objective analysis from the Storm Prediction Center Mesoscale Analysis Page showed a well defined upper-level trough and potential vorticity axis across the Ohio and Tennessee Valleys at 18 UTC on 08 October in the 400-250 mb layer. This pattern is consistent with the conceptual model presented by Atallah et al. (2007).
Similarly, Extratropical Transition (ET) has been shown to impact the distribution of precipitation of tropical cyclones where the precipitation area expands poleward of the center and the heaviest precipitation is shifted to the left of the tropical cyclone track. ET is a gradual process in which a warm-core tropical cyclone loses tropical characteristics and become more extratropical in nature (Jones et al. 2003).
The cyclone phase space diagram developed by Hart (2003) provides a mechanism to objectively determine whether a system is warm or cold core and whether the thermal structure will promote asymmetry. The diagram describes the evolution of a cyclone including tropical, extratropical, subtropical, and hybrid structures, and provides a way to visualize the location of the cyclone in the continuum between tropical and extratropical. In the cyclone phase space diagram, ET is identified when the system moves from symmetric/warm-core to asymmetric cold-core. For Hurricane Matthew, this occurred from 07 to 09 October as the storm moved from off the Florida and Georgina coasts to near and along the North Carolina coast.
There are two cyclone phase space diagrams produced by the Cyclone Phase Evolution: Analyses & Forecasts web page for Hurricane Matthew. The cyclone phase space diagram A, which shows thermal asymmetry versus lower-tropospheric thermal wind, produced from the GFS model, is shown above and to the right (click to enlarge). The diagram shows that Matthew evolved from a symmetric warm-core system to an asymmetric warm-core system on the 7th and 8th of October. The National Hurricane Center (NHC) Matthew Discussion from 11am EDT 08 October noted that “The cloud pattern associated with Matthew is beginning to acquire some extratropical characteristics. The wind field is expanding, and the area of heavy rains is now northwest of the center.”
In addition, the cyclone phase space diagram B, which shows upper vs. lower-tropospheric thermal wind, produced from the GFS model, is shown above and to the right (click to enlarge). This diagram shows that Matthew evolved from a deep warm-core system to a more shallow warm-core system on the 8th and 9th of October. The NHC Matthew Discussion from 11pm EDT 08 October stated that “Matthew is undergoing extratropical transition, and there is barely enough convection near the center to keep the system classified as a hurricane…”
Forecasters used these conceptual models during the days leading up to Hurricane Matthew to assess the flooding threat, weigh confidence in the NWP, and localize precipitation guidance from the WPC. The left of track precipitation distribution was noted in an Area Forecast Discussion from the National Weather Service Raleigh, NC which noted
“…the approach of an upper trough and cold front that will lead to a left of track precipitation distribution should lead to storm total rain amounts that range near a foot…”
Atallah, E., L. F. Bosart, and A. Aiyyer 2007: Precipitation Distribution Associated with Landfalling Tropical Cyclones over the Eastern United States. Mon. Wea. Rev., 135, 2185-2206.
Atallah, E and L. F. Bosart, 2003: Extratropical transition and precipitation distribution: A case study of Floyd (1999). , 131, 1063-1081. Brennan, M. and R. Knabb, 2008: Extratropical and Subtropical Cyclones: NHC Operational Challenges and Forecast Tools.
Hart, R.E., 2003: A cyclone phase space derived from thermal wind and thermal asymmetry. Mon. Wea. Rev., 131, 585-616.
Jones, S.C., P.A. Harr, J. Abraham, L.F. Bosart, P.J. Bowyer, J.L. Evans, D.E. Hanley, B.N. Hanstrum, R.E. Hart, F. Lalaurette, M.R. Sinclair, R.K. Smith, and C. Thorncroft, 2003: The Extratropical Transition of Tropical Cyclones: Forecast Challenges, Current Understanding, and Future Directions. Wea. Forecasting, 18, 1052–1092.