Harvey’s Mammoth Deluge Potential: Some Models Are Showing Storm Could Produce Five Feet of Rain

Media, Texas, and Gulf Coast residents take note: the thing to be most concerned about with regards to Harvey is not its admittedly life-threatening storm surge and strong winds, but what is shaping up to be a potentially historic rainfall event.

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The latest update from the National Hurricane Center shows that Harvey continues to strengthen and is now a category 4 storm. Packing 130 mph winds and a 941 mb minimum central pressure, the storm is certainly now very powerful. This new intensity is above the NHC’s previously expected peak strength for the storm just prior to landfall late tonight. The situation is thus becoming ever more dangerous.

But with Harvey, the serious concern we are facing is not just the usual and notably very dangerous high winds and storm surge flooding that go along with a category 4 storm. We are also looking at very severe and long lasting flooding rains that will have the potential to cause damage and disruption for not just months but for years to come.

A Devastating Rainfall Potential

(Southeast Texas has never seen 60 inches of rainfall from a tropical system. But that potential exists with Harvey.)

Consensus models now predict that peak rainfall totals will be around 35 inches in association with Harvey. This is due to the dual facts that Harvey is currently a very moisture-rich storm and that the storm is expected to stall for between 5 and 10 days following landfall. The storm is predicted to hover along the coastline, drawing in an unusually intense flow of moisture from a much warmer than normal Gulf, and to generate severe thunderstorms hour after hour, day after day. And this kind of rain event, if it emerges, could produce a disaster of historic proportions for Texas.

It’s worth noting that rainfall totals could also exceed the consensus forecast. Some models are now predicting upwards of 50 or 60 inches of rainfall by the time Harvey leaves the Texas area later next week (see top image above). The highest rainfall amounts ever produced by a tropical cyclone, in our records, for Texas is 48 inches. But there’s at least some possibility, with the perfect rainstorm that appears to be shaping up in Harvey, that these ultimate rainfall totals will be exceeded and a disaster of unprecedented proportions could emerge. But even if this worst-case doesn’t emerge, a 35 inch rainfall event would wreck untold destruction upon Texas’s southeastern cities.

Harvey strengthening as it moves toward shore

( Harvey rapidly strengthening to CAT 4 as it nears landfall. Image source: National Hurricane Center.)

Normally soft-spoken forecasters like Bob Henson and Eric Holthaus are not mincing words over the potential severity of the present situation. Last night, Bob Henson on twitter asked people: “Please don’t fixate on whether Harvey arrives as a Cat 2, 3, or 4. It’s the mammoth rainfall amounts (up to 35″) that will affect millions.” Meanwhile, Eric Holthaus warned: “This is scary. I have never seen a rainfall forecast like this in my entire career. Texas will be recovering from for years.”

Of course, we could dodge a bullet and rainfall totals could be lower for Harvey. It’s just that this event is currently trending toward a near worst case or worst case deluge-type storm that produces very heavy rains over the same region over nearly a week-long period.

Conditions in Context — This is Not Your Father’s Atmosphere

(The number of record rainfall events has increased dramatically during recent years. An observation attributed to human-forced climate change. Image source: Increased Record-Breaking Precipitation Events Under Global Warming.)

During recent years, a warmer than normal atmosphere has been producing more and more intense rain storms. The number of record daily rainfall instances around the world has been rising precipitously (see image above). This increasing severity is, in large part, due to the fact that human-forced warming amps up the hydrological cycle — producing more intense rain storms and more intense droughts. In other words, the climate dice are loaded for extreme rainfall and droughts in the present atmosphere. And it is in this atmosphere that Harvey has emerged. So we shouldn’t at all discount the fact that Harvey’s potential worst impacts from rainfall are now higher than they would have been even just a few decades ago. And this is one of the major reasons why we are seeing such a historic potential out of Harvey.

UPDATED (4)

UPDATE (5): As of 800 PM CDT the eye of Harvey crossed the coastline somewhere between Corpus Christi and Port O’Connor. Unfortunately, this storm is just getting started as a very severe one week long rain event is about to follow.

UPDATE (6): As of 1000 PM CDT and just following landfall, Harvey’s minimum central pressure had fallen to 938 mb. An indication that the storm was still strengthening as it began to cross the coastline. According to these reports (here and here), this is the 4th lowest barometric pressure recorded for a hurricane at landfall on the Texas coast since 1900. Maximum sustained winds remained at 130 mph with gusts to 160 mph.

Links:

National Hurricane Center

National Weather Service

Increased Record-Breaking Precipitation Events Under Global Warming

Greg Carbin

Eric Holthaus

Bob Henson

Hat tip to Greg

Hat tip to Titania

Hat tip to Wili

Hat tip to Andy in San Diego

Hat tip to Wharf Rat

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