Eric Berger

Monday, 6:15pm CT—It’s been a wet day across an already waterlogged Houston, and unfortunately we can probably expect similar conditions to continue through the night. During the day we have seen a lot of dry enter into Harvey’s circulation along its eastern side, but unfortunately this has yet to substantially affect the convection on Harvey’s northwest side—which just happens to be continuously dumping rainfall on Houston.

This pattern has allowed Harvey to drop 3 to 8 inches of rain across most of the Houston metro area during daylight hours today, with the heaviest rains in northern Brazoria County and Southern Harris County.

Because Harvey is only moving to the east-southeast very slowly, about at the pace of a slow walk, we can probably expect this pattern to continue for tonight, and into Tuesday morning. For the most part these aren’t excessively heavy rains, but even rainfall rates of 1 inch an hour will cause bayous and floodwaters to rise if they persist long enough. Therefore, while we don’t expect floodwaters to rise rapidly tonight, steady, moderate rains could cause slow rises in bayous. This is about exactly what the Houston region doesn’t need right now.

As far as accumulations, the National Weather Service predicts that the following areas of Houston could get as much rainfall as the amounts shown in this graphic during the next 24 hours, through 7pm Tuesday. I’m hoping these totals are little high, but we can’t rule them out.

Looking for some good news? We have a little bit of that. The favorable area for tornado formation is moving to the east, so we should have fewer tornado warnings tonight, and then fewer to none on Tuesday. Also, we feel fairly confident that the potential for prolonged, heavy rainfall will end sometime on Wednesday, during the afternoon or evening hours.

Matt will have another update later tonight.

Posted by Eric at 6:15pm CT on Monday

Monday, 1pm CT—Houston remains in the midst of Hurricane Harvey’s devastating aftermath, and we are weeks if not months away from a full accounting of its toll on the fourth largest city in the United States. And while a definitive account of its epic proportions will have to wait, we can at least begin to make some preliminary assessments.

In this post we will discuss how Harvey compares to other major flood storms in US history, and also how it stacks up to the region’s previous flooding event of record, Tropical Storm Allison. (Spoiler alert: Allison had a good run, but Harvey has forever dethroned her).

Mississippi discharges

The Texas state climatologist, John Nielsen-Gammon, has been closely tracking Harvey and its attendant flooding. To gauge the overall impact of a rain event, he uses a metric that defines a target zone of 20,000 square miles (about the size of West Virginia, or about 30 times the size of the city of Houston), and then measures the rainfall rate over a 120-hour period in comparison to the discharge rate of the Mississippi River. Here are the top 10 Gulf Coast flood events, as measured in “Mississippi River discharges” over that time:

• 3.6 Hurricane Harvey 2017 (estimate)
• 3.4 Hurricane Beulah, 1967
• 3.1 Brazos River flooding, 1899
• 2.8 Hurricane Georges, 1998
• 2.6 Southeast Texas flooding, 1994
• 2.5 Louisiana floods, June 1940
• 2.4 Tropical Storm Alberto, 1994
• 2.3 November Texas floods, 1940
• 2.3 Louisiana floods, 1953
• 2.3 Tropical Storm Allison, 2001

Note that the initial estimate for Harvey may be too low. As of Monday morning, Neilsen-Gammon’s hand estimate for Harvey to date was 3.1, but more heavy rains were falling in the target area and a full five days not reached. Already, for 72 hours, Nielsen-Gammon said Harvey is the greatest event over this shorter time scale.

Under this metric of rainfall over a large area, then, Harvey will very likely produce the greatest total amount of rainfall in the United States from any single storm, at least during the last 120 years. (the Gulf coast is by far the most susceptible US regions to such events due to its proximity to the warm, humid body of tropical water).

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Monday, 10:30am CT—There is no way to escape the reality of the situation on the ground this morning: Houston bayous are flooding, and even after the region has received 30 inches of rain during the last two days, more is now falling over the central Houston area. It is a catastrophic situation. Finally, however, the end is in sight. Another day or two of this …

Harvey

Harvey remains barely a tropical storm, but its center moved into the Gulf of Mexico near Matagorda Bay this morning. Presently moving to the southeast at about 5 mph, some slight strengthening is possible over the next day or two as Harvey turns east, then northeast, but as discussed yesterday we are not too concerned about this possibility due to the lack of an organized core, and additionally some drier air is working into the storm. (See official track forecast).

Overall, Harvey’s movement into the Gulf may be a positive development, because the storm is getting closer to being picked up by lower pressure in the Midwestern United States, which should lift it north. By later Tuesday, or Wednesday, we should hopefully be on the back side of the storm, with northerly winds helping to push water from inland bayous out to the Gulf of Mexico. Heavy rains should end, hopefully, by Wednesday or Wednesday night.

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