Footage captured by residents along the North Carolina coast shows flooded streets and houses in the wake of Hurricane Matthew. (Erin Patrick O'Connor/The Washington Post)

The rainfall from Hurricane Matthew was extreme, even for hurricanes that make landfall, inundating a large arc from Georgia through the Carolinas and into Tidewater, Va. As shown below, amounts in the 12-inch range were common, with areas of 12 to 18 inches concentrated over parts of the coastal Carolinas.

Although winds and the storm surge were the most feared hazards initially, it soon became clear that this hurricane would deliver rainwater by the gigaton as it hugged the coastline. Here we examine the inside story behind these incredible rains.

An incredible amount of rain fell during Hurricane Matthew — even by hurricane standards. (Jordan Tessler)

Record-breaking moisture levels

Total precipitable water (TPW) is a measure of the amount of water vapor in an entire atmospheric column. Envision a one-meter-square tower stretching from the surface to the top of the atmosphere’s weather-bearing layer. Condense all this vapor instantly and you get a precipitated water depth. Data collected by weather balloons are routinely used to compute TPW. In at least two locations — Jacksonville, Fla., and Charleston, S.C. — air feeding into Hurricane Matthew contained record-high levels of TPW: 2.93 inches and 2.85 inches, respectively.

Rarely does the East Coast see values this extreme, even during the height of summer. But the waters over the western Atlantic, including the Gulf Stream, have been exceptionally warm, nearly record-breaking in their own right. This drove evaporation into high gear, rendering the overlying air sopping wet. The figure below shows a bull’s eye of 2.9 inches TPW in the very core of Matthew at 8 p.m. Friday. Most of that moisture was being converted into rain.

Black contour lines show precipitable water levels early Saturday as Matthew tracked up the coast. Extensive rain shield is seen extending to the north. (Adapted from NOAA)

Moisture channel feeding the storm

High TPW values within the storm can explain some of the rain, but the storm also gathered vapor-laden air over a large region surrounding it. At times, the influx became concentrated into narrow channels. Like a straw dipping into the Atlantic, a deep channel of moist Atlantic air developed along the northeastern side of Matthew. As shown below, a swath of air with high dew-point temperatures (63 to 64 degrees, shown by the green shades) was drawn into Matthew as it crossed the Carolinas. The higher the dew point temperature, the more moisture in the air. These are high values considering that this analysis is at 5,000 feet. At the surface, dew-point temperatures in the onshore moisture feed were in the low-to-mid 70s.

Dew-point temperatures (shaded green regions), wind speeds (solid red lines) and wind direction (orange wind barbs) at 5,000 feet Friday evening. (Adapted from NOAA)

Also shown in the figure are peak winds at 5,000 feet. Winds from the southeast in the 70 to 80 mph range were delivering vast amounts of moisture-laden air directly into Matthew’s inner core. The influx of moisture, which is the product of wind speed, air density and water-vapor concentration, was truly astronomical. And the moisture channel was also an unstable plume, meaning it had a tendency to erupt into thunderstorm clouds, many of which stayed concentrated close to the storm’s center as it raked the coastline. Satellite views revealed that these convective clouds were soaring to great heights, manufacturing rain at rates of two to four inches per hour.

Focusing the rains: Coastal front

As Matthew moved up the coast, its converging airstreams drew in air masses from different regions. As shown in the figure below, a channel of cool, inland air flowed down from the northeast (a la Appalachian cool-air damming). Meanwhile a huge influx of warm, humid Atlantic air surged in from the east-southeast. The battleground between these two air masses, setting up just inland of the Carolina coastline, is called a coastal front.

A striking convergence of dissimilar air masses. The boundary between them serves as a focus for heavy rainfall, as the humid ocean air lifts above the cooler, denser air along the coast, condensing into thick clouds and rain. Note the coincidence of this front with pockets of rain exceeding one foot (see Fig. 4). (Lance Bosart at SUNY; annotated by author)

The imprint of the coastal front extended deeper into the overlying atmosphere. In the lowest areas, at 3,000 to 5,000 feet, converging airstreams were increasing the temperature contrast in a process called frontogenesis (translated literally, “a front being created”). Frontogenesis can be described mathematically, with the results graphed as shown below. The purple frontogenesis contours, which look like a bull’s eye, extend northeast from Matthew’s center, align perfectly with the surface coastal front.

Detailed surface analysis showing the location of the coastal front and converging air masses over the Southeast on Saturday morning. (Adapted from NOAA)

The upshot of all of this was the creation of a vigorous, deep frontal boundary that intercepted and lifted all of that moisture streaming inland off the Atlantic, within the aforementioned moisture channel. Additionally, high-speed air streaming off the ocean encountered greater friction over land, decelerating, thus increasing the convergence (or “piling up” of air) along the front. Comparing the graphics in the previous figures, the heavy rain, lifting and moisture-enhancing mechanisms all line up.

Wind shear plays a role

Wind shear refers to the increase in wind speed or change in wind direction with altitude. As Matthew’s tropical circulation moved north-northeast, it began to encounter increasing winds at high altitudes, as it entered the belt of westerly flow that straddles the mid-latitudes. Wind shear disrupts a hurricane’s inner vortex, causing it to lean or tilt downwind. This typically weakens the storm, and this was the case with Matthew.

But wind shear also tends to rearrange the storm’s dynamics, concentrating the uplift of air in certain areas of the storm. With strengthening wind shear directed parallel to the Southeast coast, from southwest to northeast, an asymmetry developed. Rising air may have become concentrated along Matthew’s northwest side — which happened to overlie the coastal Carolinas. (The author has published research showing this to be the case in another example of a weakening Atlantic hurricane).

Police filmed a rescue where they saved a mother and toddler from a car trapped in flood waters in Fayetteville, N.C. on Saturday, Oct. 8. (Fayetteville Police Department)

Moving at a crawl: The rain piles up

This factor is often so obvious that it is sometimes overlooked and barely mentioned. From Friday through Saturday, Matthew was moving at a crawl, at about 11 to 12 mph. This increased the “dwell time” of the storm in any one location — allowing rainfall at rates of two to four inches per hour, in a pileup. It’s simple arithmetic: The longer a heavy-raining weather system remains overhead, the more rain will accumulate in an area.

Morphing storm: Extra-tropical transition

Many of the heavy rain processes described here, including the role of wind shear and formation of a coastal front, are part of a bigger transformation called extra-tropical transition (ET). This occurs when a hurricane moves into higher latitudes, feeling the effects of stronger winds aloft, cooler and drier air masses, increased Coriolis deflection (the effect of Earth’s rotation) and other factors. During ET, heavy rains frequently shift to a storm’s northwest side, and the wind vortex weakens but expands over a larger area. All of these effects began to play out Saturday.

In terms of catastrophic rainfall, Hurricane Floyd serves a historical analog to Matthew. In Sept. 1999, this storm moved up the Carolina coast and underwent ET. Its heavy-rain area, shown below, generated North Carolina’s flood of record. Floyd might edge Matthew in terms of absolute heaviest rainfall at a singe location (24 inches), but the jury is still out, as careful post-storm analysis of Matthew’s rain will take several days. Floyd’s heavy-rain area was also directed slightly more inland, and its metamorphosis was driven by much stronger dynamical processes in the upper atmosphere involving the jet stream.

Rain swath generated by Hurricane Floyd. (NOAA)