A recipe for possible historic flooding in the Carolinas

Discussion started by Mike Mogil 3 years ago

Flooding in the Carolinas is not an unusual weather outcome. When widespread flooding happens, it is often linked to tropical storms and/or hurricanes interacting with stalled weather fronts (e.g., Nicole in 2010; the twin impacts of Irene and Lee in 2011).

Well, widespread flooding and flash flooding is now occurring and is expected to continue to occur across a large part of North and South Carolina. This is because heavy rainfall has fallen across the area in the past week and additional excessive rainfall is expected the next three days (Fig. 1).  Some places may see an additional twelve to eighteen inches of rainfall, some of it occurring in a very short time frame.Fig-001-past-expected-rainfall-151003

To emphasize the potentially historic character of this event, NOAA’s National Center for Environmental Prediction (NCEP) included the following wording in its forecast discussion late Friday evening (Oct. 2, 2015), “… (this situation) represents at least a one in 200 year event…and more likely a one in 500 year event...”

Note that Joaquin is NOT part of the story today. That is because computer model run cycles for the past two days have continued to show consensus that the storm track will remain well offshore, even affecting locations like Bermuda (Fig. 2).Fig-002-Joaquin.track-151003-09Z

Still, abundant tropical and Atlantic moisture, a stalled frontal zone, a very favorable and slow moving upper level wind pattern, focused and repeating training of heavy showers and thunderstorms, and some terrain uplift will all be contributing to the evolution of this already major flooding event. “Life-threatening,” “destructive,” and “significant” have all been put forth by various National Weather Service (NWS) offices in the area affected. As of mid-day Friday, there was already one reported flood death and one flood rescue in South Carolina from this weather event.

Flood and flash flood watches and warnings cover the Carolinas and many other parts of the mid-Atlantic early this morning. Note how closely the warnings (red areas) align with the long precipitation band extending from the western Atlantic into west-central North Carolina (Fig. 3).Fig-003-radar-warnings-151003

In addition to the expected excessive rainfall, persistent northeast winds (generally in the 15 to 25 mile per hour range at the coast, but stronger just offshore and along some coastal areas) will be pushing water onto East Coast beaches. Storm and high wind warnings (gusts up to 60 miles per hours…and NOT associated with Joaquin) are expected to lash coastal areas and offshore waters from southern New Jersey into the Delmarva region. Coastal flood advisories and warnings have been posted from Long Island southward to northeast Florida. Areas from Long Island to southeast Florida remain at risk for rip currents.

Even without Joaquin’s influence, this weather situation is significant and potentially life-threatening. One of the best safety rules put forth by NWS is to “turn around, don’t drown.” When faced with flooded roadways, it is best not to trust visual clues as to the speed of moving water, the depth of water, and the integrity of the roadway beneath the water surface.

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John Manetta
John Manetta
As you mentioned above, the impacts of Irene and Lee caused major problems, that said, the consistent, training and extremely heavy rainfall that is falling is made that much worse by it being tropical in nature. As tropical rainfall combines with the low heights/atmospheric "lag"/stalled to barely moving front/training rainfall...this forcing is extreme and should not be taken lightly. Of, course you mentioned all of this above, but the prospect of feet of total rainfall in a relatively short period of time means that there will be severe to historic creek/small stream/river flooding and days after the rain stops the crests will occur, causing even more flooding. South Carolina is getting pounding today. Lastly as you know, the rainfall that occurred prior to the more extreme rain has saturated the ground and run-off will rapidly occur causing extremely fast rises in streams, creek and rivers. Nice and informative blog as always.
3 years ago
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