Eric Berger

Unsettled weather remains for the first half of the week

As expected, Houston saw periodic heavy rainfall this weekend from the outer bands of Hurricane Hanna, but totals were more than manageable. The heaviest rains came near the center of the storm, which moved inland near Port Mansfield and proceeded to drop as much as 15 inches of rainfall over South Texas, leading to widespread flooding. Hanna has now become a depression in northern Mexico and should dissipate today. Despite our region’s dynamic weather over the last week, we have more to come.

Monday

For the next couple of days our weather will be determined, to some extent, by “cutoff low” in the upper atmosphere. Presently this atmospheric disturbance is east of Houston, producing widespread showers over southwestern Louisiana and over the Gulf of Mexico. This upper low should slowly sag toward Texas, bringing a healthy—50 percent chance of rain—later today. Some of these storms could briefly produce heavy rainfall and lightning. The potential for showers and thunderstorms this afternoon should help limit high temperatures to about 90 degrees, with partly sunny skies. Winds will be light for most of the day and rain chances should slacken overnight—but not entirely go away.

NOAA rainfall accumulation forecast for now through Wednesday. (Pivotal Weather)

Tuesday

This should be another day like Monday, with the atmospheric disturbance again influencing our weather. Storms likely will again develop over parts of the metro area during the late morning hours, and into the afternoon. These storms will have the potential to produce locally heavy rainfall but should be progressive enough to not lead to flooding. An unsettled atmosphere should again limit high temperatures to about 90 degrees.

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Hurricane Hanna reaches Padre Island, brings heavy rains to South Texas

At 5 pm CT on Saturday Hurricane Hanna made landfall along Padre Island, with maximum sustained winds of 90 mph. For Houston, our forecast remains unchanged. We will see periods of at-times heavy rainfall, but it should be intermittent enough that the worst impact we anticipate is some brief street flooding. Heavy rains should be more scattered on Sunday and backing off further on Monday. The rest of this post will focus on South Texas, where Hanna’s rainfall potential is much more serious.

Radar image of Hanna shortly after landfall. (RadarScope)

The rapid intensification of Hanna over the last 24 hours has helped drive a storm surge of 4 to 6 feet into locations near Port Aransas and Corpus Christi. The storm’s strongest winds have come in the largely unpopulated area between Corpus Christi and South Padre Island; and as Hanna moves inland its winds should continue to die down. Looking ahead, inland rainfall and the resulting flooding will almost certainly be the biggest concern.

Torrential rainfall continues to fall across South Texas, and this is likely to continue from now through Sunday. Hanna is moving, but it is only sliding slowly to the southwest at about 8 mph. For the next 24 hours to 36 hours, before Hanna moves into Mexico, away from the Gulf of Mexico, and begins to dissipate, the storm will be capable of tapping the warm Gulf waters to produce heavy rainfall.

NOAA rainfall accumulation forecast between Saturday evening and Monday evening. (Pivotal Weather)

Some high resolution modeling suggests that an additional 6 to 12 inches of rainfall may fall in locations such as Hidalgo and Cameron counties during this time period. And with these kinds of widespread totals we can probably expect some locations to receive as much as 15 inches of additional rainfall. This will lead to significant flooding problems in a region of the state already strained by the COVID-19 pandemic.

Hurricane Hanna will make for a wet weekend in Houston

Good morning. As of 7am CT, the National Hurricane Center has just upgraded Hanna to hurricane status, with sustained winds of 75 mph. The storm should make landfall near or just north of Port Mansfield later today, and may strengthen further before doing so.

Storm surge projections for Hurricane Hanna. (National Hurricane Center)

For the greater Houston area, this ultimately will probably yield little change to our overall forecast for a rainy and breezy weekend. We are going to see elevated tides along the coast of 1 to 3 feet from Freeport to Galveston, and along other coastal areas. In addition, these regions near the water will see some stronger winds, with gusts likely above 30 mph, while inland areas see gusts of 25 mph or above.

Houston radar at 7:24am CT on Saturday. (RadarScope)

In terms of rainfall, we’re still expecting a wet weekend, with 2 to 4 inches of rainfall coming down across most of the area as Hanna’s outer rain bands traverse the area. Generally, these bands should be spread out, and keep moving—helping to limit accumulations. Some streets may briefly flood due to high rainfall rates, however. We can’t rule out some isolated accumulations of 6 or more inches over the southern part of the Houston region.

Healthy rain chances will continue through Monday or Tuesday before we finally return to somewhat more normal weather for summer, which is to say highs in the low 90s and scattered afternoon showers.

Hanna, Gonzalo, and a third tropical system. (National Hurricane Center)

After Hanna moves off, the tropics should remain active. While Gonzalo is expected to die in the southern Caribbean Sea there is another system behind it that will be spinning westward across the Atlantic Ocean toward the Caribbean. Most of the global guidance is bullish on this system eventually becoming a hurricane (it would be named Isaias), and it will probably be something for the islands, and eventually the United States to track. But it will be several days before we have any sense of where the storm may ultimately go.

In Houston, we are lowering our expectations for the Gulf depression

Houston has about two more months to go, in terms of prime time, for the 2020 Atlantic Hurricane Season. As long time readers of this site will know, the odds of Texas being hit by a hurricane after September 24th, are quite low. By all appearances, the next two months in the tropics will be quite busy, with the Atlantic’s Main Development Region already starting to heat up. So we are going to have plenty of systems to monitor. To survive this period of anxiety, therefore, we should prioritize which tropical systems are worth worrying about.

And we’re here to tell you, Houston, Tropical Depression Eight does not have the look of something we think the greater Houston area should worry too much about. The usual disclaimers apply, of course. This is the tropics. Things change. But at this point we’re trending toward moderate to minimal effects.

(Space City Weather)

To that end we’re lowering the entire metro area to a Stage 1 flood alert on our flood scale, bringing the coastal areas (previously Stage 2) into alignment with the rest of the area. Generally, we think most of the Houston area will see 1 to 4 inches of rainfall this weekend. The coast may see some blips above 5 inches, but over the course of a couple of days this should be manageable.

The biggest area of concern, in terms of rainfall, now appears to be an area from Matagorda Bay south to Corpus Christi, or Port Mansfield. We think there may be some bullseyes of 10+ inches in these areas. The depression, which may be on the verge of becoming Tropical Storm Hanna, is simply moving westward fast enough that it will hopefully limit long periods of heavy rainfall. We anticipate a landfall on Saturday morning, somewhere between Corpus Christi and Brownsville. Most of the strongest winds will be to the north of the landfalling location.

The 12z European model shows a landfalling storm at around sunrise on Saturday. (Weather Bell)

We will of course continue to track this depression, and if our forecast thinking changes we will update post haste. For now, however, you can expect a gray and wet weekend, but likely one without widespread street flooding, or worse.