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. Several all-time one day rainfall records were set as noted in the graphic below. The precipitation pattern was notable for several reasons including the fact that an ideal setup for enhanced rainfall associated with a tropical cyclone was in place and would contribute to the record breaking rainfall and subsequent flooding across eastern NC.
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 that much of central and eastern North Carolina had more than 12 hours of moderate to heavy rainfall as noted by the 35 dBz reflectivity values (yellow to orange or red shading). In fact, Fayetteville NC reported a consecutive 14 hours and 5 minutes of moderate or heavy rain between 449 am and 653 pm on 08 October 2016.
The issue of enhanced precipitation associated with tropical cyclones (TCs) and the distribution of the heaviest rainfall associated with TCs has been examined in numerous studies including Croke (2006) which looked at 28 tropical cyclones that made landfall or tracked along coastal North Carolina from 1953 to 2004; Atallah, et. al. (2007) which investigated the precipitation distribution associated with landfalling tropical cyclones over the eastern United States; and DeLuca, Bosart, and Keyser (2004) which examined the distribution of precipitation over the Northeast during landfalling and transitioning tropical cyclones.
The Croke (2006) study was conducted as a part of collaborative research between NC State and several Mid-Atlantic National Weather Service offices following the devastating impact of Hurricane Floyd (1999) which was responsible for killing 52 North Carolinians, including 36 from drowning. The goal of that research was to develop a conceptual model to determine the potential of enhanced precipitation due to the interaction of the TC with other meteorological features prior to landfall. This paradigm would give forecasters an indication of the potential for an enhanced precipitation event by identifying features that may exist at different temporal and spatial scales outside of the TC. From this work, a conceptual model for enhanced rain associated with tropical cyclones was developed. A cartoon of the primary features is shown in the image to the right, largely adapted from the Croke (2006) work.
The presence of the following features can create an environment favorable for enhanced precipitation across North Carolina:
1) An upper level trough over the Great Lakes and Ohio Valley
2) Strong upper-level divergence inland and poleward of the TC associated with a northern stream jet streak
3) Strong inland moisture flux prior to landfall
4) A cold air damming wedge of cooler/more stable air with a surface high pressure system centered over the northeast
5) Development of a coastal front
6) Slow to moderate TC translation speed and proximity to the Carolinas
Not surprisingly, the environment around Hurricane Matthew included all of these features to some extent which enhanced the precipitation across the region and lead to widespread record-breaking flooding. We’ll examine a few of these features relating to Matthew in the paragraphs below.
1) An upper level trough over the Great Lakes and Ohio Valley – the objective analysis from the Storm Prediction Center Mesoscale Analysis Page showed a well-defined mid and upper-level trough and potential vorticity axis in the 400-250 mb layer across the Ohio and Tennessee Valleys at 18 UTC on 08 October. This larger scale feature is often identifiable 48 hours or more prior to landfall. The strength and southward extension of the PV can often be correlated to the heavier rain fall events.
2) Strong upper-level divergence inland and poleward of the TC associated with the northern stream jet stream – the objective analysis from the Storm Prediction Center Mesoscale Analysis Page at 18 UTC on 08 October showed a well-defined upper-level trough across the Great Lakes with a jet streak at 300 mb across the eastern Great Lakes. A well-defined region of enhanced upper-level divergence was analyzed over North Carolina, likely increasing vertical ascent and enhancing rainfall.
3) Strong inland moisture flux prior to landfall – the objective analysis from the Storm Prediction Center Mesoscale Analysis Page showed a well-defined region of 850 mb moisture transport early in the morning on 08 October not only near the tropical cyclone center but also extending northward into the eastern and southern portion of North Carolina. This moisture transport analysis is from 06 UTC on 08 October as the heavy rain was poised to build and move into North Carolina.
4) A cold air damming wedge with a surface high pressure system centered over the northeast – while not as pronounced as in other TC events, a cooler and more stable air mass became established across the Piedmont of North Carolina on 08 October. The METAR plot from 18 UTC on 08 October shows surface dew points in the mid and upper 60s across the Piedmont while dew points were in the lower to mid 70s across the Coastal Plain. The parent high pressure center was located off the New England coast which resulted in an in-situ cold air damming wedge across North Carolina. The wedge likely enhanced the rain in North Carolinas as warm moist air on the forward side of the storm was lifted up and over the surface based stable layer.
5) Development of a coastal front – The establishment of the in-situ cold air damming (CAD) wedge across the Piedmont can lead to the development of a low-level boundary along the eastern perimeter of the CAD region. The objective surface frontogenesis analysis from the Storm Prediction Center Mesoscale Analysis Page from 06 UTC on 08 October showed a region of surface frontogenesis shown in red contours extending northeast of the center of Matthew across eastern North Carolina. The coastal front marked the boundary of the strong easterly to southeasterly surface flow with a cooler north to northeast flow further inland. Several studies have shown that a surface boundary can focus heavy precipitation in a mesoscale band driven by low-level frontogenesis that often develops left and poleward of the storm track with enhanced precipitation falling along or in cold sector of the boundary.
6) Slow to moderate TC translation speed and proximity to the Carolinas – while these features are rather intuitive, the Croke (2006) research suggests that at times they are not dominant. Other factors, such as synoptic or mesoscale features have proven to compensate for TC’s with less favorable tracks or translation speed. Still, absent of other features, a TC that is moving slower and closer to the North Carolina will have a greater potential for heavier rain across North Carolina than one that is moving faster and is removed from the coast.
The synoptic and mesoscale pattern across the eastern United States prior to and during the time in which Hurricane Matthew impacted North Carolina fit the paradigm for enhanced tropical cyclone precipitation. This event was a classic and efficient heavy rain producer with all of the features for enhanced heavy rain including: a strong upper level jet poleward of the tropical cyclone, an approaching upper trough, strong low-level moisture flux, a cold air damming region across the interior Piedmont, and a strong coastal front. Forecasters recognized the developing pattern during the days leading up to the storms arrival and it was highlighted in an Area Forecast Discussion from the National Weather Service Raleigh, NC which noted:
“.SHORT TERM /6 AM SATURDAY MORNING THROUGH SUNDAY/…
As of 345 PM Friday…
…Threat of life threatening flooding increasing across the Sandhills and Coastal Plain of NC as ideal setup for enhanced tropical rain becomes established…
The biggest threat and impact arises from the potential for extremely heavy rain with amounts forecast to exceed 10 inches across the southeastern and eastern portions of the CWA. The setup for tremendous rainfall associated with a non-land falling tropical cyclone are nearly ideal.
The combination of an approaching tropical cyclone with deep tropical moisture, a coastal front and a cold air damming air mass in the Piedmont providing an enhance region of ascent northward of the tropical cyclone and 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.
Croke, M. S., M. L. Kaplan, L. Xie, and K. Keeter, 2005: Examining planetary, synoptic, and mesoscale features that enhance precipitation associated with Tropical Cyclones making landfall over North Carolina. Preprints, 21st Conference on Weather Analysis and Forecasting/17th Conference on Numerical Weather Prediction, Washington, DC, Amer. Meteor. Soc.
Croke, M.S. (2006) Examining Planetary, Synoptic and Mesoscale Features that Enhance
Precipitation Associated with Landfalling Tropical Cyclones in North Carolina, Thesis
(M.S.), North Carolina State University.
DeLuca, D. P., L. F. Bosart, and D. Keyser, 2004: The distribution of precipitation over the Northeast accompanying landfalling and transitioning tropical cyclones. 20th Conference on Weather Analysis and Forecasting, Seattle, WA., Amer. Meteor. Soc.