Keeping A Close Eye on Hurricane Irma

As of 5 pm ET, Hurricane Irma is a category 3 storm (115 mph sustained winds) located at 17.3° N and 34.8° W and is moving west northwest at 12 mph. Irma was only a tropical storm this morning but underwent rapid intensification. Irma has remained in a relatively low-shear environment; however, this kind of quick intensification was actually a bit surprising. While sea surface temperatures were above the threshold for tropical development (above 26°C/79°F) when Irma quickly intensified, these temperatures were not overly warm. While the National Hurricane Center has Irma strengthening further over the next day, I actually wouldn’t be surprised if there is some slight weakening or at the least, no additional strengthening for about a day. There is a bit of dry air out ahead of the system, and as I mentioned, sea surface temperatures are just above the threshold for development. Regardless, Irma will eventually move over much warmer sea surface temperatures (with higher ocean heat content), and given that vertical wind shear should remain light, there’s no reason that this system won’t rapidly intensify into a stronger hurricane, possibly reaching category 4 status over the next few days. To sum it up, after a brief break in strengthening, Irma should strengthen further. If it ends up wrecking homes near you, it might be a smart idea to talk to Ballwin Commercial Roofing Pros to better understand how to repair homes during the aftermath.

dry air hurricane irma

Figure 1: There is some dry air that Hurricane Irma could temporarily encounter.

We are going to be tracking Irma for a long time, which gives us a decent amount of time to figure out where Irma is going. Also, if Irma were to directly impact the contiguous U.S., it won’t be for at least 10 days, give or take a day. It could even be a bit longer. What I’m basically saying is NOT to cancel any plans just yet, but instead, follow the latest tropical updates very closely if you live near the coast or have vacation plans.

Hurricane Irma projected path

Figure 2: The National Hurricane Center’s 5-day projected path of Hurricane Irma

Hurricane Irma Discussion:

The big question over the next several days will be to determine if Irma could be a threat to the U.S. Along with a mid to upper-level low (northwest of Irma) to the south of the Bermuda ridge creating a weakness, numerous shortwave features will ride along the northern periphery of the Bermuda ridge, keeping this ridge on the weaker side over the coming days. In addition to this, the latest guidance indicates that a longwave trough could develop over the eastern U.S. next week. If at this point Irma were to be far enough west, then it would likely recurve and miss the East Coast. However, it appears that the eastern trough will move out in time, and Bermuda high pressure will build westward, possibly extending along the East Coast. This would increase the threat of Irma eventually impacting the East Coast, but also, some guidance builds the Bermuda ridge far enough west and southward that Irma slides between Florida and Cuba into the Gulf of Mexico. At this point, there are a plethora of scenarios, but two of several things to watch closely will be the strength of the Bermuda ridge and how long the longwave trough remains established over the eastern U.S.

If you’re going to follow the forecast models, don’t actually be too focused on where the models show Irma going at this point. The model guidance will continue to vary quite a bit on exact track, so as stated above, the focus should be on the following two features: the strength of the Bermuda ridge and the longwave trough that will be building over the eastern U.S. Also, keep a close eye on how far north Irma gets over the next several days, because a more northward track increases the odds of either impact to the East Coast OR a recurvature out to sea. If the trough fully moves out and Bermuda ridging builds westward quickly enough, then we potentially have a problem in the Gulf of Mexico.

From the looks of it, Typhoon Sanvu in the western Pacific may play a major role in influencing the downstream pattern over the U.S. in about a week. Sanvu is going to gain latitude with time and make an extra-tropical transition as it gets absorbed into the westerlies. Recurving typhoons in the western Pacific can sometimes cause the jet stream to buckle as excess amounts of latent heat gets transferred to the atmosphere, resulting in a wavy jet stream downstream over the U.S. I suspect this is the cause of the western ridge/eastern trough pattern that will evolve over time, and while this would be a great pattern for steering hurricanes away from the U.S. (assuming the hurricane remains high enough in latitude), the forecast model guidance just doesn’t have this pattern sticking around long enough. Some models lift the trough out quickly but leave a piece of energy behind over the southern U.S., and some fully lift out the trough. The problem is that as soon as this trough moves out, the Bermuda ridge will most likely build westward and become well-established. That’s what we don’t need to happen.

Jet stream hurricane irma

Figure 3: Model guidance shows a western ridge/eastern trough configuration over the U.S. next week.

Bullet Point Summary Of “Stuff” To Watch Over The Next 5 to 7 Days:

  • Watch the strength of the Bermuda ridge and see how the upper level low to the south and extra-tropical systems to the north influence its strength.
  • Watch to see if Irma gains some latitude, given that any amount of latitude gain could increase the chance of an East Coast impact OR a recurvature out to sea.
  • Watch the building trough over the eastern U.S. and see how quickly or slowly it will be moving out of the region.
  • Watch how Typhoon Sanvu influences the downstream pattern in about a week, given that this will probably be the culprit behind the western ridge/eastern trough configuration
  • Understand that models have a difficult time handling such meridional flows (wavy jet stream patterns). Throw a hurricane into the mix, and it gets very complicated.
  • Don’t really bother looking at where the models have Irma going beyond 4 to 5 days at this point in time. Focus on the above bullet points for now.

If you read this article and have come to the conclusion that we have no clue what’s going to occur, then, to an extent, you’d be correct. I could wait several days to write these articles until there is a bit more certainty, but I like to go ahead and introduce the Firsthand Weather audience to what I’m actually watching. I will be referring to features such as the Bermuda ridge, the eastern trough, etc. over the next week to two weeks, so I wanted to go ahead and give everyone a very early introduction to what’s going on. It will only make the forecast better over time and will help all of you with your planning for this potential hurricane.

More Heavy Rain To Houston?

First, I would like to send my good thoughts to Houston and all the other communities and towns that have been impacted by Harvey across Texas and Louisiana. If there is anything Firsthand Weather can do to assist you all, please do not hesitate to send us a message!

The rain has begun to shift eastward to far eastern Texas, and towards Louisiana and Mississippi. This will allow for better weather conditions as damage assessments and cleanup begins. Unfortunately, numerical guidance is indicating the potential for deep tropical moisture to move into coastal areas of Texas and Louisiana by mid next week. A moisture axis in the southern Caribbean will move over the Yucatan Peninsula late this week/early weekend into the southwestern Gulf of Mexico. This may lead to the development of a surface low off of the Texas coast by early to mid next week.

This would aid in deep tropical moisture potentially being pulled into Texas as an upper-level low is situated across the Panhandle of Texas–leading to precipitation chances. At this time, it appears a surface cold front will be situated somewhere across Texas, which could enhance precipitation, too. As the low moves eastward, it will aid in precipitation chances for Louisiana.

Current moisture axis in Caribbean (GFS)

Deep moisture in southwestern Gulf of Mexico by Monday morning (GFS)

Deep moisture in southwestern Gulf of Mexico Wednesday afternoon (GFS)

Precipitation forecast through 7 days from NWS (please note: this map will change over the next few days and it looks plausible that higher precipitation amounts will shift northward)

This is still several days out, so a lot will change; however, this scenario is plausible and needs to be closely monitored. It’s to early to determine the strength of this low, but it is possible a tropical cyclone (TD or TS) could evolve. Any additional rainfall next week will quickly cause flooding. Keep checking back for updates!

Tropical Storm Watch in the Carolinas

Tropical Storm Watches have been posted from the South Santee River in South Carolina to Duck North Carolina, including the Albemarle and Pamlico Sounds.


Tropical Update

Florida has been experiencing heavy rains for the past several days at this system spun in the area.  Some areas have seen several inches of rain over the past few days as this system slowly meandered up the peninsula.  This system has been watched for a while as it slowly moved up, but interactions with land and conditions that were neutral or negative for tropical development continued to prevent this system from developing further.   Now that the system is heading over the warm water of the gulf stream, convection is beginning to increase and the storm is organizing despite its sheared environment, which Hurricane Harvey has been helping to maintain.  These storms are starting  to get some separation and we expect to see Tropical Storm Irma form in the next couple of days.


Model Analysis

Steering currents across the South of the US have been weak as of late.  This is why both this system and Harvey are moving so slowly, as you can see in the early movement of this system.   This will begin to change as a trough moves into the Central United States and begins to make this system move.

This system moves slowly north towards the border of South and North Carolina.  It will come very close to making a landfall here but much of the circulation will remain over water, which will slow any weakening.  This system will flow along the coast up towards the North Carolina Virginia border.  This system has the potential to hit the coastline hard with storm surge and high waves ahead of the storm as it strengthens.

As seen in the track above, as on this model image, the system is expected to make landfall along the North Carolina coast and impact the Outer Banks.  Beyond that, the forecast gets more difficult.  Some models have brought the system further out to sea, but the GFS is bringing the system closer to the coast of New England on some runs and this possibility will need to be monitored in future articles.



Voluntary Emergency Evacuation Order (Houston)

The Army Corps of Engineers will likely release water from the Addicks and Barker reservoirs beginning Monday morning at 2:00AM CDT (Addicks) and 11:00AM CDT (Barker).

What are the Addicks and Barker reservoirs?
The Addicks and Barker reservoirs are 26,000 acres of reservoirs close to 17 miles west of downtown Houston. These large reservoirs are the considered the largest municipal park reservoirs in the United States, and were engineered to help protect downtown Houston and the Houston Ship Channel (includes Galveston) from flooding.

What does the water release mean?
The release of water from the reservoirs will cause flooding to some neighborhoods downstream, and the Buffalo Bayou will rise about five inches per hour. According to city officials, voluntary evacuations are expected to be issued for residents near these reservoirs. Please listen to the County Officials of Harris and Fort Bend Counties if you’re in this area. They are currently in coordination to evacuate certain areas. Here are the neighborhoods that have a voluntary emergency evacuation order in place:

For more official information please click HERE!

Harvey May Re-intensify Dumping Feet Of Additional Rainfall

More heavy rainfall and tornadoes are expected late tonight through at least Wednesday for Houston as well as the surrounding areas. This is bad news for these areas because 30″ of rain has fallen in some areas. Currently, Tropical Storm Harvey is situated over Texas but is inching closer to the coast–likely reemerging over the northern Gulf by Monday morning. Harvey has maximum sustained winds of 40 mph at this hour with a slow movement of 2 mph to the SE.

NHC Cone

The position of Harvey and slow movement place Houston and much of southern Texas and central Texas in a favorable area for very heavy rainfall for the next few days. As aforementioned, once Harvey moves back over the Gulf of Mexico by Monday morning, it is possible that Harvey will re-intensify into a strong tropical storm (possible maximum sustained winds of 60 to 65 mph). Harvey will then move northeastward, slowly, making landfall just south of Houston by Wednesday morning. This scenario would aid in heavier rainfall rates and an increased tornado threat for much of the mid-Texas coast, upper-Texas coast, southern parts of north Texas, and western Louisiana.

European (Wednesday morning)

3km NAM Rainfall Totals (through Tuesday evening)

A big concern is tonight (Sunday night) for Houston and its suburbs. Short range guidance indicates a feeder band from Harvey may situate itself over this area. These feeder bands can produce rainfall rates of 4-8″ per hour and tornadoes. This will only exacerbate the flooding issues across Houston and do so during the most dangerous time for flooding (at night). The HRRR is indicating 10-24″ may fall over the next 18 hours in the Houston area.

HRRR Future Radar (tonight)

HRRR Rainfall Totals (through 18 hours)

Once Harvey moves inland, again, into Texas on Wednesday, the path becomes uncertain. The uncertain path, and the slow meandering of Harvey over the past few days, is because there is a lack of upper-level features to act as a magnet and steer Harvey out of Texas. The steering currents are too light, and Harvey is ‘stuck’ between a mid-level high to its west and the subtropical ridge across the southeast. This may change by late week into the weekend however, however. An upper-level trough appears to dig into the central plains, which may draw Harvey northeastward. This would place parts of northeastern Texas, southern Arkansas, northern Louisiana, and western Mississippi in a favorable area to see rainfall from the remnants of Harvey. The remnants would produce flooding for these areas. By Saturday, it is possible isolated areas in southeastern Texas may see 50-60″ of rainfall.

Spaghetti Plot

European (Friday morning)

WPC Precipitation Forecast (through 7 days)

Firsthand Weather will have updates as needed!

Tropical Update: Harvey and Hurricane Safety

Hurricane Harvey, now 115 miles southeast of Corpus Christi Texas, has seen the maximum sustained winds increase to 110 this morning, just shy of major hurricane status. The minimum central pressure has dropped 947 Millibars. Harvey is expected to become a major hurricane today with winds increasing further to 120 mph. As of this 11 AM EDT, the following watches and warnings are in effect.

A Hurricane Warning is in effect from Port Mansfield to Sargent Texas. Hurricane conditions will be occurring in these areas within the next 12-24 hours.

A Tropical Storm Warning is in effect from south of Port Mansfield to the Mouth of the Rio Grande River and from North of Sargent to High Island Texas

A Tropical Storm Watch is in effect for South of the mouth of the Rio Grande to Boca de Catan Mexico

A Storm Surge Warning is in effect from Port Mansfield to High Island Texas. A Storm Surge Warning means there is a danger of life-threatening inundation from rising water moving inland from the coastline in the indicated locations. This is a life-threatening situation. Persons located within these areas should take all necessary actions to protect life and property from rising water and the potential for other dangerous conditions. Promptly follow evacuation and other instructions from local officials.

A Storm Surge Watch is in effect from south of Port Mansfield to the Mouth of the Rio Grande. A Storm Surge watch means there is a possibility of life-threatening inundation from rising water moving inland. Persons located within these areas should take all necessary actions to protect life and property and be on the look out for rising seas.

Forecast Information


Hurricane Harvey continues to increase in strength this morning and is now approaching major hurricane status. Harvey is expected to be a category 3 storm when he moves ashore in Texas. Sustained winds are expected to be between 120 and 130 miles per hour with higher gusts. Harvey will be bringing life-threatening storm surge, rainfall, and wind hazards to portions of the Texas coast. Preparations to protect life and property should be completed this morning, as tropical storm force winds will first arrive in the hurricane and storm surge warning areas later today.

Life-threatening storm surge flooding could reach heights of 6 to 12 feet above ground level at the coast between the north entrance of the Padre Island National Seashore and Sargent. Devastating and life-threatening flooding is expected across the middle and upper Texas coast from heavy rainfall of 15 to 25 inches, with isolated amounts as high as 35 inches, from today through next

Harvey will hug the coast after he moves inland, which could help prevent the quick weakening associated with land falling hurricanes. While Harvey will still weaken, it may be a slower process and he could maintain Tropical Storm strength longer than usual. Harvey is forecast to impact this area for several days.

Hurricane Harvey Hazards

Storm Surge and Storm Tide

Storm Surge and large waves are the greatest threats to life and property along the coast. A storm surge is an abnormal rise of water generated by a storm’s winds. Many will remember the surge from Hurrican Ike is 2008. Storm Surge with Harvey is expected to be as high as 12 feet in some locations (was 20 feet in Ike). Please check your local media sources for the exact surge totals expected for your area. Surge related flooding will be dependent on the exact timing and the tide and can vary greatly over short distances. Large waves will also be a factor at the coast. Based on the tide charts, Harvey should hit as tides are moving out and close to low tide, but onshore winds are possible during several tidal cycles.


Hurricanes frequently produce tornadoes, usually in the embedded thunderstorms in the rain bands now beginning to hit the Texas coast line. They can also be associated with the eye wall. Tornadoes produced by these systems are usually weak and short lived, but they can be a threat to where they hit. A Tornado watch is expected to be posted for coastal areas of Texas and Louisiana later today.


It goes without saying that winds are a major issue with Hurricanes. Sustained winds with Harvey are expected to reach up to 120 mph with gusts reaching as high as 150 mph with some locally higher gusts. Harvey is forecast to be a major Hurricane at landfall.


Forecasts for Harvey have indicated significant rainfall is possible, with some models showing over 30 inches of rain. This will be a very long duration event and flooding and flash flooding will bea major issue for many areas.

Hurricane Safety


Many areas in Texas have had evacuation orders given, for those who are evacuating, you will need to determine a safe evacuation route inland. While evacuating, you’ll want to monitor the latest information to ensure that you will be evacuating to a location that is not also under a risk. Public services in these areas will already be strained and adding many evacuees will only cause more strain for everyone. Learning the location of official shelters will be helpful both in the event that you evacuate as well as for those who do not evacuate. Areas that are not under evacuation orders should also have shelters for the residents who end up in trouble due to hurricane hazards. Tornadoes, lightning, power outages, and property damage due to falling trees can force residents from their homes, even if a mass evacuation isn’t needed. The good news is that a quick check of traffic in this region doesn’t show any, so many heeded these orders in advance.

When evacuating, put together a go-bag. Include a disaster supply kit, including a flashlight, batteries, cash, first aid supplies, medications, and copies of your critical information if you need to evacuate. Snacks and stuff to drink will also be good for longer evacuations, especially with children. While you may stop for gas, many others are also evacuating with you so its possible that many supplies will be sold out along your route. If you haven’t already, fill up your gas tank in your local area, gas supplies may also be strained along your route. Cell phones should be charged before you leave. Have one member of your family turn their phone off while travelling together. This will ensure that if one phone does run out of battery, you will have another phone to turn on and use. All family members should have all numbers for your phones and know to call multiple numbers if they can’t get a hold of you.

You should also inform someone of your plans. If you fail to arrive at your destination due to a car accident, your contact can alert the authorities. Having them know the route you planned to take is essential in locating you.

Stay tuned to local news outlets as you travel. Conditions may change and areas you were heading to that were going to be safe may not be anymore. Harvey is brining a large area of rain to Texas and the location you were heading for may end up under a flood warning when it wasn’t before. Always ensure that the safety of your location is the top priority.

Staying at home

For those of you who are not leaving home, I would first implore you to heed local evacuations if you are under them. Evacuation orders are given for a reason. If you are under those orders it is absolutely not safe where you are. While I do realize that some properties remain even in massively damaged areas, you only put yourself and those who would need to rescue you in the event of an emergency in danger by taking that gamble. It is never just your survival on the line.

If you have not been ordered to evacuate, there are several things you should plan for. First, as I mentioned above, have a plan to be able to evacuate to a local shelter if your house if one of the unfortunate locations that falls victim to one of the more local hurricane hazards. Lighting occurs frequently within a hurricane and can strike even in areas where hurricane conditions are not occurring. You should also have a plan for any pets you may have. Not all shelters accept pets so ensure the one you may go to does.

What to bring to the shelter

You will need to make sure that you have everything you need at the shelter. While the shelter will have supplies, they don’t usually carry specific medications or your specific brand of baby food. Ensure that you have a first aid kit with all the medications taken by your family. If you are running low, see if your local pharmacy can give you more. Baby food and diapers will be needed. Your baby should be prepared for a multiple day stay at the shelter. Bring things to do as well. Books, games for children, headphones and a source of music (we all know some of you still have a Walkman) will all be desired. The shelter is not exactly a fun place to be. Bring your toiletries and blankets. The shelter will have some but we all prefer our own. Flashlights and batteries are good to have in case the power goes out at the shelter as well. Also ensure you bring identification, cash and credit cards as well as copies of your essential documentation like proof of insurance.

Protecting your home

Be aware that Hurricane winds can cause trees and branches to fall, so before a hurricane trim or remove damaged trees and limbs to keep you and your property safe.
Secure loose rain gutters and downspouts and clear any clogged areas or debris to prevent water damage to your property. When it comes to repairing your rain gutters, it’s as easy as checking out sites such as, finding the right materials and replacing your damaged/old gutters. This will help protect your property, which is what you need to consider, especially after weather conditions like thunderstorms and heavy rain. This can also help keep them lighter and not fall under the weight of all the rain. Reduce property damage by retrofitting to secure and reinforce the roof, windows and doors, including the garage doors. Plywood over windows or close storm shutters if your house has them, this can protect them from wind and debris. If you are construction inclined, hurricane clips can be installed in your attic to help secure your roof to the house. You may also want to get in touch with a home improvement service similar to Mastershield Atl that may be able to provide support for homeowners who are looking for a way to protect their roof from water damage. While it is certainly too late to hire a contractor to do this for Harvey, other storms will come along and this can be done for future storms. You can also brace your garage door and doors that lead outside with planks to keep it from blowing in. You should also close all interior doors to compartmentalize the house. This way, if a window does break in one area, the remaining areas will have protection against the elements coming in. Purchasing a portable generator or installing a generator for use during power outages is also a good idea. Remember to keep generators and other alternate power/heat sources outside, at least 20 feet away from windows and doors and protected from moisture. You should never try to power the house wiring by plugging a generator into a wall outlet. If power does go out, use a flashlight. Candles are a fire hazard.

Keeping food and water safe

The most important thing when it comes to food is to buy nonperishable goods and to store water. You do not need to buy multiple gallons of water for activities like brushing your teeth. Simply refill an old milk or water bottle with tap water and use it. You can do the same thing for bath water, pre-filling your tub is also recommended. Tap water storage can be done with any container from large pots used for cooking to old bottles and even any large plastic container. Always remember to keep using the tap water until it goes out, you don’t want to needlessly use up your supply and not be able to replenish it. For any cold food you do have, turn your refrigerator and freezer to their maximum cold settings and open them as little as possible. This will help keep things cold. Try to use up any chilled foods first so that keeping things cold becomes unnecessary and have a supply of ice on hand to keep things cold longer. While you can buy ice at the store, you can also just use ice cubes. Simply dump your tray into a plastic bag and refill the trays. Dump new ice cubes as they form and keep refilling the trays. You will be able to continue this process for as long as you have power or tap water. This also creates an emergency water supply should you run out of water. Simply melt the ice. You can also use rain water if you have a safe way to get a bucket outside to collect some. You should always try to keep a lid on your water supply. This will keep dust and bugs out of it.

After the Storm

It will be tempting to go outside after the storm has passed or during the calm period in the eye. This should be avoided. The eye is only temporary and hurricane conditions can start very quickly. After the storm, there will be a lot of damage in the region. Floods could still be occurring and the water could be contaminated. You also can’t see what’s in the water. Animals and hazards in the water could be potentially dangerous to your health. The water could even be electrically charged from downed power lines.

We here at Firsthand will do our utmost to keep you up to date with the latest information. Our thoughts go out to those in the areas affected by Harvey. Stay safe everyone.

Robert Millette

Incident Meteorologist

Braintree Emergency Management Agency

Harvey Likely To Drop Excessive Rainfall Along Texas/Louisiana Coasts

Quick note: This is a forecast that I wrote specifically for a tropical meteorology class that I’m taking, which is why the format is a bit different than what you’re used to seeing from me. However, I figured all of you would find this information beneficial.

Discussion on Harvey (August 24, 8 am CT update):

As expected, Harvey re-strengthened back into a tropical storm last night and now has sustained winds of 50 knots. Harvey has picked up some forward speed and is currently moving north- northwest at 9 knots. Convection is now much better focused around Harvey’s center, and he does not have the elongated look that he did just last evening. At this time, no modifications/updates need to be made to last night’s forecast (below).

Hurricane warnings have now been issued for parts of the Texas coastline. Excessive rainfall is still expected, and at this point, it is not unreasonable to say that parts of Texas could exceed 20 inches of rainfall over the next 5 to 7 days.

Harvey Projected Path

Figure 1: Tropical Storm Harvey’s latest projected path from the National Hurricane Center

Discussion on Harvey (August 23, 7:00 pm CT update):

As of 7:00 pm CT, Tropical Depression Harvey is currently to the west of the Yucatan Peninsula and is moving very slowly to the northwest at 2 knots. After weakening and losing its closed circulation earlier in the week, Harvey finally made its transition back into a depression and will likely strength into a tropical storm by tonight or tomorrow morning. At this point, hurricane status may eventually be reached before landfall.

An upper-level low has been positioned just to the south of the Texas/Louisiana border over the Gulf of Mexico, which has resulted in an elongated region of convection firing from Harvey’s center of circulation extending northward away from this center in an area of heightened upper- level divergence. Harvey’s circulation has been quite elongated thus far, but convection should eventually become more confined to Harvey’s center as the upper-level low dissipates.

With time, vertical wind shear should decrease ahead of Harvey, as the upper-level low dissipates. There is currently moderate to strong vertical wind shear that extends south of the Louisiana coast to the northeast Gulf of Mexico. The southern extent of this stronger shear has likely played a role in Harvey’s disorganized and asymmetrical look so far this week, but over time, this should no longer be a significant hindrance to Harvey’s intensification, especially as this tropical system moves northwest.

atlantic vertical wind shear

Figure 2: Locations of vertical wind shear across the Gulf of Mexico and western/central Atlantic from CIMSS

atlantic upper-level winds

Figure 3: Upper-levels wind across the Gulf of Mexico and western/central Atlantic from CIMSS

Sea surface temperatures are anomalously warm over much of the western Gulf with temperatures running around 30 to 31°C on average. Even more noteworthy, Harvey will be moving over a region of higher ocean heat content, so while some upwelling could occur due to Harvey’s slow forward motion, it appears that this system has plenty of warm, Gulf of Mexico water to enhance strengthening. In addition to warmer sea surface temperatures and a low-shear environment, there will be very little dry air to hinder further development.

gulf of mexico ocean heat content

Figure 4: Gulf of Mexico and western Caribbean ocean heat content map from CIMSS

GOES-16 Harvey water vapor

Figure 5: GOES-16 water vapor image from early morning, August 24, 2017

Aside from the fact that Harvey could quickly intensify as he moves towards the Texas coast, one major concern at the moment is the significant flooding situation that could unfold across Texas and Louisiana. A region of high pressure is expected to build over the eastern Gulf of Mexico by this weekend, while ridging is expected to amplify over the western United States around the same time. The mean flow will generally be from the north on the east side of the western ridge and from the south on the west side of the eastern Gulf high pressure system, putting Harvey is a region of weak steering flow. Unless one of these high pressure regions becomes more dominant than the other, Harvey will likely meander along or just inland over Texas for quite some time. Broad troughing over the eastern U.S. could eventually pull Harvey northeastward next week, but at this point, that remains uncertain. Due to the complexity of this pattern, model guidance has had a difficult time determining where Harvey will go once he moves closer to the Texas coast. Given the moisture-rich environment that will be present along the Texas and Louisiana coasts and the frontal boundary that will remain stalled across that region, rainfall totals over the next 5 to 7 days could reach 10 to 20+ inches across those locations. Reaching such totals will be highly dependent on the overall track of Harvey, which will be dependent on the strength and exact placement of the two high pressure features.

harvey rainfall totals

Figure 6: 7-day rainfall forecast from the WPC

Discussion on the rest of the Atlantic (August 23, 7:00 pm CT update):

Aside from Harvey, the rest of the Atlantic is relatively quiet. There is a trough of low pressure that is currently located near the Florida peninsula, which is bringing unsettled conditions south of a frontal boundary that will continue to makes its way southward.
In addition to sea surface temperatures being anomalously warm across the Gulf of Mexico, temperatures are also warm across the main development region. This will be extra fuel for any African easterly wave that moves across that region. It is worth noting that there is quite a bit of dust that is now moving over the eastern Atlantic from Africa, which for the time being, will likely hinder the development of any wave in that region.

global sea surface temperature anomalies

Figure 7: Weekly global sea surface temperature anomalies from NOAA

saharan air layer map

Figure 8: Saharan air layer map from NOAA

Discussion on the eastern and central Pacific (August 23, 7:00 pm CT update):

For now, the tropics in the eastern Pacific are quiet. Kenneth recently dissipated after moving northward into a region of lower sea surface temperatures. While sea surface temperatures are anomalously cool over the central and eastern equatorial Pacific, sea surface temperatures are above average north of that region, including near Hawaii. As the hurricane season continues, that region will bear watching.

central pacific sea surface temperature anomalies

Figure 9: Sea surface temperature anomalies across the central Pacific from Tropical Tidbits

Tropical Storm Harvey Rapidly Strengthening

Harvey intensified overnight and is now a Tropical Storm with maximum sustained winds of 60 mph. Harvey is moving towards the Texas coast, to the NNW, at 10 mph. The latest analyses shows Harvey is in a favorable area for continued strengthening, and the latest microwave imagery shows Harvey has a closed eye wall. This means Hurricane status will likely be met later today as the winds ‘catch up’ with the closed eye wall and decrease in pressure, and Harvey will likely make landfall as a Hurricane near Corpus Christi late Friday night/early Saturday morning.

Microwave Satellite

IR Imagery

NHC Cone

Spaghetti Plot

Hurricane Warnings are in effect for Port Mansfield to Matagorda, Texas; Tropical Storm Warnings are now in effect north of Matagorda to High Island, Texas and south of Port Mansfield, Texas to the mouth of the Rio Grande. Once Harvey moves inland in Texas, it will likely meander in southern Texas for a day or two. This will cause extensive, deadly flooding for much of southern Texas and southern parts of central Texas. Widespread 10-25″ of rain are possible with isolated amounts in excessive of 30″. San Antonio, Austin, and Houston are included in this flood threat.

GFS Rainfall Totals

More details on the impacts across other Gulf States later today.

Harvey: Dangerous Storm For Entire Gulf

The National Hurricane Center (NHC) has reclassified the low in the southwestern Gulf of Mexico as Tropical Depression Harvey. Reconnaissance data indicates that Harvey has maximum sustained winds of 35 mph and is moving towards the NW at 9 mph. This motion is expected to continue through the week as Harvey develops into a hurricane near the Texas coast.

NHC Forecast Cone

By Friday, it looks likely that Harvey will reside close to the mid-Texas coast. With low shear and warm sea-surface temperatures, it is likely that Harvey will be a hurricane at this time. This would create strong winds and potentially a damaging storm surge for parts of the Texas/Louisiana coast (depending on the strength and position of Harvey). Very heavy rainfall is another major threat with Harvey, and this rain threat is not only for coastal areas.

Since Harvey will likely slow down and meander around the coast late into the week/early weekend, this will draw in copious amounts of moisture into southern Texas and Louisiana–leading to an increased flooding threat. This meandering scenario looks likely due to the steering pattern associated with the TUTT low over the northwestern Gulf. There is uncertainty as to how long Harvey will meander near the Texas coast; the synoptic evolution is important in determining the eventual eastward or northeastward motion of Harvey–this is highly dependent on where Harvey resides on Friday. If Harvey is closer towards the southern Texas coast, more of an eastward motion may resume by late in the weekend. But, if Harvey is further towards the mid-to-upper Texas coast, then phasing will likely occur which would shift Harvey east-northeastward/northeastward shifting the heavy rainfall threat further north and eastward along the Gulf states through early next week.

Regardless, it appears widespread 10-20″ of rainfall are across southern Texas with several inches possible further inland towards southern parts of central Texas; depending on how long Harvey meanders along the coast, a few areas could see well in excess of 20″. Louisiana, Mississippi, and Alabama will likely receive heavy rainfall by late in the weekend into early next wee. Southern Louisiana may pick up 10-15″ of rainfall wile the rest of Louisiana and central/southern Mississippi may see 3-7″. Isolated much higher amounts are possible across the Gulf states.

Spaghetti Plot

WPC Precipitation Forecast Through 7 Days

GFS Rainfall Forecast

A Storm Surge Watch has been issued for the coast of Texas from Port Mansfield to High Island, aHurricane Watch has been issued for the coast of Texas from north of Port Mansfield to San Luis Pass, and a Tropical Storm Watch has been issued for the coast of Texas from the Mouth of the Rio Grande to Port Mansfield and from north of San Luis Pass to High Island. We will have updates as needed.

Latest Spaghetti Plot: Entire Gulf Needs To Remain Alert

Firsthand Weather is keeping a close eye on the Gulf of Mexico this week as the remnants of Tropical Storm Harvey move into the southwestern Gulf. Impacts to the United States are looking more likely–especially for Texas then there’s the potential the remnants may impact additional Gulf States (Louisiana, southern Arkansas, Mississippi, Alabama, and Tennessee), so I wanted to provide a brief post on the latest spaghetti plot with you all.

The National Hurricane Center (NHC) is monitoring the remnants of Harvey (currently crossing the Yucatan Peninsula), and is giving the low a 70% of tropical cyclone development over the next 48 hours and an 90% chance of tropical cyclone development over the next five days. The current analyses show decent spherical vorticity, light environmental shear ahead of the system in the Bay of Campeche, and above average sea-surface temperatures. This should allow the remnants to develop into a tropical cyclone by Wednesday afternoon. It is possible the NHC may label this as a Potential Tropical Cyclone by the afternoon hours today, which would allow local National Weather Service offices to begin issuing weather products (watches) on this system.

Numerical guidance has begun trending further northward over the past few runs. This has shifted the potential track from northern Mexico, more towards the Texas coast. This means the entire Texas coast needs to keep a close eye on future forecasts as confidence continues to increase.

On to the spaghetti:

So what exactly are spaghetti plots? Spaghetti plots are what meteorologists refer to when multiple numerical weather models are shown together. This creates a spaghetti like appearance because the individual model tracks are all plotted on the same image. These plots are important because they can give some insight into where a tropical system may track.

Here is the latest spaghetti plot for the remnants of Tropical Storm Harvey that’s heading towards the Gulf of Mexico. The 12Z guidance is clustering a landfall along the mid-Texas coast. This is a drastic shift northeastward over the past 24 hours. Guidance will continue to need to be assessed to determine if future shifts (northward or southward) are possible.

If you live along the Texas coast, you need to prepare now for potential tropical cyclone impacts. This is a precautionary measure in case impacts are felt late in the week. The other Gulf States should keep a close eye on this system, too, because it is possible it may phase and get pulled into Louisiana and Mississippi.

See yesterday’s article about the remnants of Harvey!