Harvey Monday evening forecast: It’s not a great one

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.

Harvey’s satellite appearance at 6pm CT. (NOAA)

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.

Rain accumulation forecast for 7pm Monday to 7pm Tuesday. (National Weather Service)

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

Early Monday afternoon Harvey forecast update

Monday, 2:15pm CT— Good afternoon. After our break last night, rains have returned to the picture this morning and afternoon. Any rain adds insult to injury right now, but at least the rainfall rates have been, for the most part, under 1.50 inches per hour. This is small comfort obviously, but versus the rates we saw on Saturday night and Sunday, the word “manageable” keeps coming up. As manageable as something can be in this situation.

Now

As of 2 PM, radar is active, with steady moderate to locally heavy rainfall across the southern half of the Houston metro area.

Weather radar as of 2 PM is not optimal. (GR Level 3)

North of I-10 is seeing steady light to moderate rain persist as well. Rainfall rates as of 2 PM are about 0.25″ to 0.50″ per hour north of I-10 and 0.50″ to 1.50″ south of I-10. Again, insulting, infuriating, frustrating, but mostly manageable. It just really slows down drainage. Steady northeast winds inland and southeast winds at the coast are not helping matters either. There have been gusts to 40 or 45 mph, and that looks to continue, although significant increases beyond this are unlikely.

Rest of Today

What you see is what you get. Expect this rain to stay in place much of the afternoon. Rain may fall heavily at times, especially south of I-10. Winds will stay occasionally gusty. Wish we had better news on this front, but it’s going to be a pretty miserable Monday, and hopefully the rates stay at this manageable level. We feel they ought to.

Tonight and Tuesday

I don’t expect a lot of change tonight. Rain will likely fall steadily through the night, and into Tuesday morning. Harvey’s general east or southeast movement will pretty much lock things in. As Harvey begins to shift northward on Tuesday, we will start to see the rain become a little more sporadic I think. Steady moderate rain much of the time Tuesday morning into the afternoon. Harvey should make another landfall Wednesday near High Island, TX as a weak to moderate tropical storm (no significant strengthening is likely).

Official NHC forecast is not changed much, with our horrible storm finally exiting on Wednesday. (NHC)

This remains first and foremost a rain issue, and you should continue to monitor the water. Steady rain should end Wednesday and daily storm chances trend to close to zero by Thursday. We should see sunshine and much better weather to end the week, and we can begin our road to recovery. Stay safe all.

Posted at 2:15 PM CT Monday by Matt

Harvey almost certainly the biggest US flood-producing storm

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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Harvey late Monday morning: Rains remain, but storm finally moving

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).

Tropical Storm Harvey at 10am CT on Monday. (NOAA)

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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