About six days ago, our forecasting team got together to discuss the possibility of tropical development in the Gulf of Mexico. After some debate, we decided to push an update out to clients that detailed the potential for Harvey — then an open wave — to regenerate into a tropical storm again and pose a threat to the Gulf Coast, perhaps reaching minimal hurricane strength. However, we never could have imagined, at the time, that the situation would become this dire or significant, as Harvey rapidly intensified from a tropical depression into a category 2/3 borderline hurricane over the past 24-36 hours. It’s usually quite difficult for an open wave to rapidly intensify after days of weakening over the Yucatan Peninsula. But Harvey defied those odds, which will result in devastating impacts.
Hurricane Harvey continued to strengthen slowly this morning, with the pressure slowly dropping into the 940mbs. As of 11am maximum sustained winds were still at 110mph, just underneath Category 3 strength. The hurricane continues to move north-northwest with outer rain bands now reaching the Texas Coast, with some tropical storm force winds. Harvey is expected to continue to intensify today into a major hurricane, while it moves over very warm sea-surface temperatures and heat content over the Western Gulf of Mexico, with a continued very favorable mid-upper atmosphere for strengthening.
Harvey’s strengthening has slowed down from the overnight, as an eyewall replacement cycle has taken place. This cycle is a development of two concentric eyewalls, with two double wind maxima around the eye. The cycle completes when the new outer eyewall replaces the older, inner eyewall. After this happens, the eye can contract, allowing the central minimum pressure to fall rapidly and then winds in the eyewall to increase. This is expected to happen by this afternoon and evening. But there are some uncertainties on the exact duration of this cycle, as if it takes a bit longer, Harvey may not have time to re-strengthen before landfall. But if the cycle completes early enough, another period of rapid intensification into a strong category 3 hurricane would be possible.
Harvey is expected to make landfall somewhere near Corpus Christie, Texas. Just before making landfall, Harvey will be moving slowly north-northwest over the shallow waters off the Texas Coast. This is may result in the upwelling of cooler waters and cause Harvey to weaken slightly. However, Harvey is still likely to be a major hurricane as it makes landfall later tonight or early tomorrow morning. The highest and most destructive winds and storm surge will be in the northeast quadrant with Harvey.
But very heavy rainfall and significant flooding will be widespread along the Texas coast. Harvey is still expected to slow down and stall over South Texas through the weekend into early next week, as any steering influence is being canceled out by two adjacent ridges. This stalling of an extremely moist tropical system combined with a prolonged fetch of extreme Gulf moisture converging in the same locations for days creates a textbook situation for extreme rainfall.
Models still indicate the potential for anywhere between 20-40” of rainfall, and even locally higher totals over the next several days. This could lead to flooding at prolific or even catastrophic levels, especially over the more flood prone areas of Texas. It’s hard to even illustrate the threat to life and property from this storm, especially from flooding — just know that it’s life-threatening and potentially catastrophic.
Model and ensemble guidance also suggest that Harvey could back over the Western Gulf of Mexico next week, as a north to south gradient between a ridge in the West and a trough digging in the Midwest “pushes” Harvey southward. Then with atmospheric conditions remaining favorable, it restrengthens into a strong tropical storm or hurricane and makes landfall again along the Upper Texas Coast or the Western Louisiana Coast. We still have low confidence on this solution, as the storm’s inner core could be severely disrupted if there is more land interaction beforehand. Also some upwelling of cooler waters prior to Harvey’s first landfall may inhibit more rapid intensification. So this will continue to be closely monitored as well, through much of next week.
However, every single 06z GEFS ensemble member shows Harvey’s remnants backing into the Gulf and restrengthening before a second landfall, so it’s certainly something to strongly consider — though the track of any potential second landfall is still up in the air.
With all of this in mind, here is an overview of the expected hazards along the Central and Southern Texas coasts:
Flooding Rainfall: By far the most concerning threat with Harvey, flooding rainfalls of prolific nature appear increasingly likely this weekend across Southern Texas. Precipitable water values in the atmosphere are forecast by all ensemble guidance to increase to near record values. This immense tropical moisture coupled with strong lift and a slow, stalling tropical system could lead to extreme rainfall amounts capable of resulting in catastrophic flooding.
The best area for this flooding potential currently appears to be the larger Southern Texas coast. GFS and ECMWF model data is in good agreement on a very large swath of prolific rainfall totals over this region including a stretch from Corpus Christi to Houston. It’s possible that a relatively wide swath of 20” or more of rain could fall. These rainfall totals can produce life-threatening flash floods. Low lying areas and poor drainage regions will almost certainly face catastrophic damage. Even flood-protected zones should prepare immediately to protect life and property.
Storm Surge: As the storm system gains strength and breadth in the Gulf of Mexico, it will begin to accumulate water and begin to advect this water northwestward with its track. As the system makes landfall, a significant storm surge appears possible along parts of the Texas Coast and even inland along waterways. The NWS has issued a Storm Surge Watch for this regions — and this will be most impactful near or just to the east of the landfall region.
Strong Winds: Strongest winds associated with the tropical system will likely be confined to the northeast quadrant of the storm system as it makes landfall this weekend. Please see the official NHC track for the latest pinpoint landfall forecast.
Additionally, here are the preparedness steps that need to be taken. At this point, time is running out, so these should be the final preparations.
Preparation Steps: All interests along the Texas Coastline should be making final preparations for Hurricane impacts. A plan should be in place specifically for flood waters outside of normal climate and reaching toward extreme storm levels. Tremendous rainfall amounts are increasingly likely to pose a risk for life-threatening flooding in poor drainage, low lying, or coastal areas. Storm surge will pose an additional risk for coastal damage. A plan should additionally be in place to handle tropical storm and hurricane force winds along the area coastal plain.
We will continue to refine our forecast as the landfall location becomes more apparent.Have an emergency plan and kit handy including the necessities (flashlight, batteries, etc) if you, for some reason, cannot leave. Pay close attention to the latest forecasts as we continue to gather new information.
This post was written by Miguel Pierre and Doug Simonian