July 11, 2019

Life-Threatening Flooding Likely As Tropical Storm Barry Crawls Toward Louisiana

Extremely heavy rain associated with Tropical Storm Barry will lead to life-threatening flooding across southern Louisiana and the lower Mississippi River Valley over the next couple of days. The slow-moving storm could reach hurricane strength before it makes landfall on the Louisiana coast on Saturday. Latest forecasts show double-digit rainfall totals across the eastern part of the state, compounding ongoing flooding issues across the region.

NHC Forecast

The disturbance we've been watching for the last couple of days developed a closed surface circulation on Thursday morning, allowing the National Hurricane Center to upgrade the system to Tropical Storm Barry. The NHC's 7:00 PM CDT advisory shows the storm slowly organizing over the next two days, possibly strengthening into a hurricane before making landfall in southern Louisiana on Saturday morning.

Conditions across the northern Gulf of Mexico are conducive to strengthening if the storm can get its act together fast enough to take advantage of its environment. Barry's structure on Thursday afternoon was...lacking...with an exposed low-level circulation south of the mouth of the Mississippi River, and almost all clouds and convection fanned-out on the southern side of the system.

The window for strengthening will begin to close the longer the storm remains disorganized. That's great news in terms of wind and storm surge, but Barry is set to bring flooding rains to the northern Gulf Coast regardless of its strength at landfall.

Rainfall Forecast



While wind speeds and the phrase "hurricane warning" will get the most attention, the real story of this storm is the water. Barry threatens to wring out a tremendous amount of tropical moisture over the northern Gulf Coast through this weekend.

The latest rainfall prediction from NOAA's Weather Prediction Center shows the potential for 20" or more—yes, that's twenty inches or more—of rain across southern Louisiana. A wider swath of half a foot or more of rain spreads up the Mississippi River toward the Memphis area.

Not everyone will see all of the rain in the forecast. But the storm will move slow enough that rain bands and thunderstorms will be able to tap into a deep reserve of tropical moisture and produce copious amounts of rain in a short period of time.

It's important to note that these predicted rainfall amounts will change as the storm gets closer to land and forecasters get a better handle on the structure and future track of the storm. Small changes in intensity, organization, and track will shift the bullseye for heavy rain with time. The overall point is that everyone in Louisiana should prepare for a potentially significant flooding event.

Flooding Potential

NOAA/NWS
The combination of accumulated water from heavy rain, the Mississippi River rising from excess upstream runoff, and a potential storm surge could severely strain the ability of New Orleans and surrounding areas to stave off floodwaters.

The Mississippi River is already in flood from months of heavy rain across the central United States. Add that on to the flash flood emergency that played out across New Orleans on Wednesday—dropping more than half a foot of rain in a couple of hours—and it won't take much heavy rain to cause major flooding along the area's already-strained waterways.

River flooding forecasts from the National Weather Service show the Mississippi River in New Orleans cresting at 19 feet if current precipitation forecasts hold up, which would be the highest crest recorded there since February 1950. The levees along the Mississippi in New Orleans are only 20 feet tall, so the water would only be about a foot away from the top.

The entire city of New Orleans sits below sea level. Not only does it face a threat from the bodies of water that surround the city, but rainwater has to be pumped out of the city because it can't seep into the ground. The pumps can handle rainfall rates of 1.00" in the first hour and 0.50" in every subsequent hour after the rain stops. Rain that falls faster than that will cause flooding in spots around the city until the pumps can catch up with the excess water.

It's not just New Orleans, either. The excessive rainfall amounts forecast across Louisiana and throughout the lower Mississippi River Valley will cause extensive and life-threatening flash flooding across the region.

A scenario like this played out not too long ago. A disturbance over the northern Gulf of Mexico brought a widespread swath of 10"-20" of rain to central and eastern Louisiana back in the summer of 2016. The resulting flooding was some of the worst in Louisiana's modern history, killing more than a dozen people and causing more than $10 billion in damage. Many homes destroyed by the floods in 2016 didn't have flood insurance, as the owners thought they were safe from flooding.

Winds

Flooding is the greatest threat, but we can't ignore the winds. Hurricane warnings are in effect for the central Louisiana coast for the potential that Barry could strengthen into a hurricane before reaching land. In practical terms, though, the difference between a 70 MPH tropical storm and a 75 MPH hurricane is negligible.

Wind gusts above 70 MPH and soggy ground will allow trees and power lines to fall with relative ease. Widespread power outages are likely where the strongest part of the storm makes landfall. Strong winds will easily snap tree limbs and blow around small debris—stuff like trash cans and yard decorations.

Tornadoes

Tornadoes are always a threat in the right-front quadrant of any landfalling tropical system. Folks along and to the east of Tropical Storm Barry's track will stand the greatest threat for tornadoes. Tropical tornadoes are different from tornadoes you'd see in a "regular" thunderstorm. They can happen so quickly that forecasters can miss them between radar sweeps. Eastern Louisiana, Mississippi, and Alabama will have to be on the lookout for tornadoes as the storm makes its way inland.

[Satellite Image: NOAA]


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July 9, 2019

Tropical System In Gulf Likely To Strengthen; Greatest Threat Is Flooding From Heavy Rain

A tropical disturbance entering the Gulf of Mexico has a high chance of growing into a named storm by the end of the week. Some models have the system intensifying before it makes landfall on the northern Gulf Coast this weekend. Flash flooding from heavy rain is far and away the greatest threat with this system, no matter what it's called at landfall.

Tuesday afternoon's update from the National Hurricane Center gave the disturbance, dubbed "Invest 92L" for now, an 80 percent chance of developing into a tropical depression by Thursday morning. We've been watching this trough of low pressure mosey southward across the southeastern United States for a couple of days. The disturbance emerged in the northeastern Gulf of Mexico on Tuesday morning, opening the window for potential development over the next day or two.


Conditions over the Gulf of Mexico are capable of supporting a tropical system. Sea surface temperatures in the northern Gulf are in the mid- to upper-80s, which is plenty warm enough to support tropical development. Low wind shear will allow thunderstorms to develop without getting ripped apart, and ample moisture should stave off intrusions of dry air from suffocating the storm.

The major complication here is the structure of the disturbance itself. It's pretty disorganized at the moment. Assuming the system develops, exactly where it takes root and how strong it gets will ultimately determine how far west it travels across the northern Gulf. A stronger storm may be able to tap into winds that could steer the storm farther west across the Gulf.

Water is the greatest threat with this system no matter what it does. The best case scenario right now is that this system remains disorganized and weak. The worst case scenario is that the system gets its act together in a hurry, posing a threat for wind damage and power outages in addition to lots of heavy rain.


Either scenario would bring heavy rain to the Gulf Coast. The latest forecast from the Weather Prediction Center shows more than five inches of rain falling across a wide swath of the southern United States, with a maximum near the coast where the system ultimately makes landfall. This swath of heavy rain will shift in location and intensity as forecasters get a better handle on exactly what will happen. Everyone along and inland of the Gulf Coast is at risk of seeing flooding rains through next weekend.

We've seen so many storms in recent years—from tropical storms to major hurricanes—leave behind horrendous flooding in their wake. Folks in Texas and North Carolina are intimately aware of the threat for flooding in a landfalling tropical system. Even so, it's still a major battle to get folks in harm's way to appreciate the threat of water over the threat of wind.

The threat of winds can't be completely ignored, of course. Strong winds could cause damage if this system reaches shore as a tropical storm or hurricane. Tree damage, power outages, and some structural damage would be possible in that scenario. Those are significant hazards, made even worse by the potential for flooding from heavy rains.

Forecasters (and weather models!) will have a better idea of what's going to happen over the next couple of days once—and if—the system develops and there's actually something there to analyze. It's a good idea to prepare for an extended period of heavy rain if you live in any of the southern states. Good questions to ask yourself right now include "do I have multiple routes to get around if roads are flooded out?" and "do I have food and supplies to get through a couple of days without power?"

The National Hurricane Center will begin issuing advisories on this system every 3-6 hours if/once it develops into a tropical depression. In the meantime, they issue tropical weather outlooks every six hours at 2:00 and 8:00 AM/PM Eastern Time.


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July 7, 2019

A Trough Over Georgia Could Become A Tropical System In The Gulf—And It's Not As Weird As It Sounds!

The National Hurricane Center is monitoring a trough of low pressure over the southeastern U.S. for signs of tropical development once it reaches the Gulf of Mexico in a couple of days. A disturbance turning into a tropical cyclone once it moves from land to sea isn't as weird as folks are making it sound on Facebook and Twitter—after all, where do you think Cape Verde hurricanes come from? (It ain't the stork!)

Sunday's 8:00 PM EDT update from the NHC gives the disturbance over northern Georgia a 60 percent chance of developing into a tropical system once it reaches the northern Gulf of Mexico later this week. Conditions are favorable for the system to slowly organize by late next week as it meanders in the northern Gulf.



Even if the storm doesn't develop, it looks like a good bit of rain will fall along the northern Gulf Coast and points inland once this whatever-it-is starts moving inland next weekend. The latest rainfall forecast from the Weather Prediction Center shows a boatload of rain (technical term!) falling across parts of Florida, Alabama, and Georgia, through next Sunday, and these totals could easily tick higher if the system organizes and grows stronger once it reaches the Gulf. Flash flooding is likely in areas that see too much rain too quickly.

Everything I've seen about this system on social media so far makes a hullabaloo about the National Hurricane Center mentioning it in their outlooks while the trough was still over Tennessee. If that seems weird, it shouldn't. Lots of tropical systems begin as clusters of storms that form over land and move over the ocean.

You know how we talk about Cape Verde hurricanes and "tropical waves" moving off of Africa in August and September? Those are disturbances that begin over western Africa and move over Atlantic Ocean. Most of the big hurricanes we remember—Katrina! Andrew! Rita! Charley!—started as troughs or thunderstorms over the African continent.

Most folks just aren't used to hearing about this process occurring over the United States. Well, that, and the fact that it's a little unsettling to see a giant X over Tennessee on a map produced by the National Hurricane Center. But this is certainly one method of tropical development this early in the season, and the Gulf of Mexico is a prime location for storms to develop in July.

Oh, and if you're wondering..."Barry" is the next name on this year's list of tropical cyclone names for the Atlantic Ocean. (We used the name Andrea back in May.)


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July 3, 2019

Anchorage Could Break Its All-Time High As An Intense Heat Wave Builds Over Alaska

High temperatures will soar into the 90s near Anchorage over the next week as a historic heat wave builds across Alaska. The actual air temperature is astounding—90s in southern Alaska!—but the duration of this heat wave will take an especially hard toll on residents not used to such warm temperatures. The heat isn't forecast to break until the middle of next week.
The upper-level ridge over Alaska this Saturday, per Wednesday afternoon's run of the GFS model. | Source: Tropical Tidbits


This heat wave is the result of a strong ridge of high pressure building over Alaska this week. The ridge will peak this weekend before waning early next week. Calm, sinking air beneath the high will allow temperatures to climb into record territory for an extended period of time.
An animated map showing the NWS's forecast highs across Alaska between July 4, 2019, and July 9, 2019.
The latest forecast from the National Weather Service shows highs easily climbing into the 80s across most of central and southern Alaska. The Anchorage area will see the worst of the heat. The official high temperature in Anchorage*, recorded at Ted Stevens International Airport on the west side of the city, could meet or exceed the all-time high temperature of 85°F set in June 14, 1969. Temperatures will climb into the 90s for the duration of the heat wave in the valley north of Anchorage, which is home to communities like Wasilla. The average high in Anchorage at the beginning of July is about 65°F.

*Note: The official temperature in Anchorage is recorded at the international airport, which is right on the water. The rest of Anchorage gets warmer as it's farther inland. The temperature label for Anchorage on the maps in this post are for the forecast point very close to downtown Anchorage, which more accurately reflects the temperature that residents will experience.

It can get quite warm in low-lying parts of Alaska that are on relatively flat land. Fairbanks, for instance, climbs into the upper 80s or low 90s at least a few times each summer. The state's all-time high temperature was 100°F set in Fort Yukon, north of Fairbanks, during the historic heat wave of 1915.

A long-duration heat wave of this caliber will pose a risk for heat-related illnesses in vulnerable populations. Most homes and businesses here don't have air conditioning. It's Alaska, after all. They rarely need it.

The lack of cooling systems, combined with the intense high-latitude summer sunshine and homes built "to hold onto every molecule of warmth," will lead to a long period of abject misery for most folks affected by this heat wave. It'll be hard or impossible to stay cool at home, leading to a heightened risk of heat exhaustion or heat stroke in the elderly and those in poor health.

Annual average temperature trends in Alaska since 1925. | Source: NOAA/NCDC
This event is an unsettling reminder that Alaska is steeped in the effects of our changing climate. Climate Central sums it up well:
Alaska is the fastest-warming state by far, as the Arctic is warming twice as quickly as the rest of the world. And what happens in the Arctic, doesn’t stay in the Arctic. Melting glaciers are fueling sea level rise and permafrost thaws are releasing more heat-trapping gases. According to a recent UN report, winter temperatures over the Arctic Ocean could rise 5-9°F by midcentury unless the world’s emissions reduction commitments are strengthened. 
One heat wave isn't a Day After Tomorrow-esque manifestation of climate change, but the warming trend in Alaska over the last couple of decades is undeniable. 2014, 2015, 2016, 2017, and 2018 were five of the ten warmest years ever recorded in Alaska. Climate change accentuates the extremes, making heat waves like this more likely in the future.


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July 2, 2019

Hurricane Barbara Grew From A Tropical Storm To A Cat. 4 Hurricane In 21 Hours

Hurricane Barbara rapidly strengthened into a category four hurricane on Tuesday as it spun over the eastern Pacific Ocean. "Rapid" isn't an exaggeration in this case—the storm grew from a tropical storm to a category four hurricane in just 21 hours. The hurricane is likely near its peak strength and will weaken over open waters later this week. 

The latest advisory from the National Hurricane Center shows the storm at or very near its peak strength. The hurricane will soon start losing steam as it moves over cooler waters to its north. While Hawaii is in the frame on the forecast map above, take note of the scale on the bottom-right—even by Sunday, the storm will be hundreds of miles from Hawaii, and the storm or its remnants should be weak (or even non-existent) by the time it reaches Hawaii.


Barbara was quite the healthy hurricane this afternoon. The storm has a well-defined eye surrounded by a thick eyewall. The classic "buzzsaw" appearance of the storm—those cirrus clouds radiating clockwise from the center of the storm—are indicative of good upper-level outflow, which is an essential to maintaining a storm's strength.

The storm took advantage of calm winds, warm waters, and decent organization to get going in a hurry. Barbara was a tropical storm with 70 MPH winds at 11:00 AM PDT on Monday. The storm nearly doubled in strength by the following morning. This is another point on a long list of hurricanes that underwent rapid intensification. Thankfully, this event occurred well away from land, but rapid intensification is a prime reason folks along coast need to pay extremely close attention to the latest forecasts when future storms threaten land.

The eastern Pacific hurricane season got off to an unusually late start this year. I wrote over at Forbes last week that the season's first tropical depression—which eventually strengthened into Alvin—was tied for the latest first tropical system since reliable records began in the mid-1900s. As we've learned so many seasons before, a late start doesn't necessarily mean a quiet season. There's likely another storm forming on Barbara's heels, and, like Barbara, it should also head out to sea without affecting land.


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