April 25, 2019

Another Intense Tropical Cyclone Makes Landfall in Mozambique

Tropical Cyclone Kenneth made landfall in storm-battered Mozambique late Thursday night with the equivalent strength of a category four hurricane. The remnants of the storm are expected to slow to a crawl over the next couple of days, bringing significant wind damage and a deep storm surge to coastal communities in the path of the eyewall, as well as potentially devastating flooding from heavy rain across northern Mozambique and southern Tanzania. Kenneth will make landfall a few hundred miles north of the region devastated by Cyclone Idai in March.

The center of the cyclone made landfall in a concave bay north of Pemba. The town of Pangane and a small coastal community east of Mucojo appear to have experienced the full force of Cyclone Kenneth’s eyewall. Satellite imagery shows hundreds of homes—many likely poorly constructed—right along the coast or on flat terrain within a mile of the water. The wind and surge will likely result in the destruction of these communities.

Source: Tropical Tidbits

The latest forecast from the Joint Typhoon Warning Center (JTWC) shows Kenneth slowly making its way inland and meandering over northern Mozambique for the next couple of days. This will lead to catastrophic flooding from more than a foot of rain in many areas.

Analyses right before landfall late Thursday night showed that Cyclone Kenneth was the equivalent of a category four on the Saffir-Simpson Hurricane Wind Scale, packing maximum sustained winds of about 140 MPH just before the eye crossed land. This makes Kenneth one of the strongest storms ever recorded on landfall in Mozambique.

This cyclone is an especially devastating blow as the country is still reeling from the effects of Cyclone Idai, which made landfall in central Mozambique on March 15. Idai reportedly destroyed 90 percent of the port town of Beira with a storm surge deep enough to fully submerge most one-story buildings. The storm killed more than 1,000 people in Mozambique and surrounding countries, leaving hundreds of thousands of residents in a humanitarian crisis that will continue for years—now made even worse by the added effects of Cyclone Kenneth.

This is now the most active cyclone season on record for the southwestern Indian Ocean. Unfortunately, as we’ve seen so many times in the United States, a hyperactive tropical cyclone season greatly increases the odds that a powerful storm will make a direct landfall.

It’s unusual for Mozambique to see one cyclone in a year, let alone two major storms in a month’s time. Mozambique is typically protected from major cyclones by the island nation of Madagascar, which sits a few hundred miles off the coast. Cyclones often weaken as they cross Madagascar—however, Idai was able to rapidly strengthen in the Mozambique Channel, and Kenneth skirted Madagascar to the north.

Tropical cyclones are called different names around the world. Mature tropical cyclones are known as hurricanes around North America, typhoons in the northwestern Pacific Ocean, and simply "cyclones" everywhere else in the world. While the name changes depending on the ocean basin, the basic structure and effects are the same no matter where they form.

[Top Image: UW/SSEC]

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April 19, 2019

Storms With 70+ MPH Winds Possible in Carolinas and Virginia on Friday

A moderate risk for severe weather exists across parts of the Carolinas and Virginia on Friday afternoon as squall lines capable of producing winds in excess of 70 MPH sweep across the region. It's been a long time since this part of the country has seen a moderate risk in an outlook from the Storm Prediction Center. Take today's severe weather threat seriously and make sure you've got a way to get warnings the moment they're issued.

Today is the final day of a three-day severe weather outbreak across the southern United States. And intense squall line is expected to develop along the cold front extending off the low that's caused all the trouble for the last couple of days.

Winds are ripping not too far above the surface, and it won't take much for a strong thunderstorm to mix those winds down to the surface. The Storm Prediction Center issued a moderate risk because of the chance for significant wind gusts in excess of 70 MPH, mainly across North Carolina and Virginia.

In addition to the risk for significant wind gusts, tornadoes and (to a lesser extent) large hail are also possible. The best chance for tornadoes is over central North Carolina.

Central parts of North Carolina and Virginia haven't seen a moderate risk for severe weather in an SPC outlook in a long, long time. Greensboro, N.C., hasn't been under a moderate risk since June 13, 2013. This is the first moderate risk in Raleigh, N.C., since February 24, 2016. You have to go back to September 2012 to find the last moderate risk in south-central Virginia.

This is all to say that today is a day where you have to take the weather extremely seriously. All of the severe weather we've seen in recent years occurred on days with a lower SPC outlook than the one we've got today. The dynamics are there to produce widespread and significant straight-line winds if storms are able to take full advantage of the environment available to them.

(I apologize for any errors or formatting glitches I missed. I had to write this post on my phone today. Technology!)

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April 18, 2019

Nighttime Severe Thunderstorms Are Possible in Mississippi and Alabama on Thursday

A risk for nighttime tornadoes—some potentially strong or long-lived—will unfold this evening and tonight across parts of southern Mississippi and Alabama. Damaging straight-line winds and large hail are also possible. Severe weather is dangerous no matter when it occurs, but it's especially threatening at night when people aren't paying attention or they're asleep.

Thursday's severe weather risk is part of a multi-day severe weather outbreak that began Wednesday on the southern Plains. The system responsible for the severe weather will lead to the development of one or more rounds of severe weather that will affect parts of Louisiana, Mississippi, and Alabama on Thursday evening and overnight into Friday.

A squall line was already underway at the time of this post's publication. The above radar image shows what the storms looked like at 3:30 PM CDT. The line of storms moving into south-central Mississippi had a nasty bite to them, with severe thunderstorm and tornado warnings in effect for various parts of the line. All of this action will continue lumbering east through the evening and overnight hours.

The greatest threat for severe weather covers much of the northern Gulf Coast, including New Orleans, Biloxi, Gulfport, Mobile, and Pensacola. The latest forecast from the Storm Prediction Center paints an enhanced risk for severe weather across these areas. While this is the "bullseye" for dangerous weather, the threat for damaging winds, large hail, and isolated tornadoes extends much farther to the north toward Tennessee and Kentucky.

The best dynamics for severe weather seem to be setting up over southern Mississippi and Alabama, as well as the western Florida Panhandle. The SPC placed the areas under a 10% risk for tornadoes—a number that seems small, but is relatively large when it comes to tornadoes. The black hatching indicates the area where the atmosphere could support strong, long-lived tornadoes.

Tornadoes are most likely in discrete thunderstorms that develop out ahead of the main squall line, though tornadoes are also possible embedded in the storms within the squall line. Embedded tornadoes can happen very quickly in a situation like this, cutting down the tornado warning lead time most of us are used to.

Damaging winds can produce just as much damage as a tornado over a much wider area. The SPC seems particularly concerned about the risk for damaging wind gusts from one or more squall lines along a stretch from western Mississippi—including Jackson and the surrounding areas—through western Alabama.

Nighttime tornadoes are terrifying because people stop paying attention to severe weather once the sun goes down and lots of people don't have a reliable way to hear warnings once they're asleep. If you're in the areas at risk for severe weather tonight, stay aware and get prepared for dangerous thunderstorms overnight.

You won't see the tornado coming when it's dark and the tornado is wrapped in rain. Tornado sirens are outdoor warning systems that are not meant to be heard indoors. Your weather app may not alert you to a tornado warning in time, either, especially when the storms are moving quickly and every second counts.

Check to see that the "emergency alerts" are activated on your mobile device. The Wireless Emergency Alert System—the system they tested on the national level last year that freaked everyone out—delivers a push notification to your device direct from the National Weather Service the moment they issue a warning. The system triangulates your location based on your cell signal to determine if you're in a tornado warning. It works. It's saved lives.

The risk for severe weather tonight is its own advertisement for the benefits of a NOAA Weather Radio. Modern weather radios have the ability to program your county into the device so you're alerted to watches and warnings the moment they're issued for your county. The radio sounds a loud alarm and automatically tunes to the feed so you can hear the warning read out loud. Weather radios may feel like an antiquated technology in the smartphone era, but they're like smoke detectors for the weather. (If you ever need help setting one up, email me!)

The threat for severe weather will shift east tomorrow and stretch from the Mid-Atlantic to the Florida Peninsula. I'll have a post on that later this evening or tomorrow morning.

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April 13, 2019

A Dangerous Severe Weather Outbreak Is Possible in the Deep South on Saturday

A significant outbreak of severe thunderstorms is likely across parts of the southern United States on Saturday. The Storm Prediction Center has issued a moderate risk for severe weather for parts of Louisiana, Arkansas, Mississippi, and Texas, which is the second-highest category on the agency's scale for measuring the severe weather threat.

Threat Areas

The environment in this part of the south on Saturday will be capable of supporting powerful supercells, or thunderstorms with rotating updrafts. Rotating updraft makes the storm stronger and more resilient than a "normal" thunderstorm. These storms could produce strong, long-lived tornadoes, destructive wind gusts, and large hail.

The greatest threat for severe weather includes (or is very close to) Shreveport, Alexandria, and Monroe in Louisiana, as well as Jackson, Vicksburg, and Greenville in Mississippi.

The outlook in this post was issued by the Storm Prediction Center at 1:00 AM CDT. It's a good idea to keep one eye on the radar all day and listen out for warnings if you're anywhere near this region on Saturday.

The above image shows the tornado risk for Saturday. These may seem like small percentages, but they're relatively large in the grand scheme of things. The black hatching indicates the risk for significant tornadoes that could stay on the ground for a long time.

It's important to remember that the severe weather risk isn't confined to just the high risk area. Much of the Mid-South and northern Gulf Coast are at risk of seeing severe thunderstorms today. A damaging wind gust or a tornado is dangerous no matter where it forms, so don't assume you're out of the woods if you're not in the predicted "bullseye" for this event. The highest-risk areas exist where the best severe weather dynamics are in place. There are plenty of cases where damaging tornadoes and particularly destructive squall lines formed well away from a moderate or high risk.

The Setup

Source: TwisterData.com

A low-pressure system will move across central Texas during the day on Saturday, setting the stage for a round of dangerous thunderstorms to develop in the warm sector on the eastern end of the storm. Strong instability behind the warm front will allow severe thunderstorms to quickly develop across the risk areas on Saturday afternoon.

The above image shows surface Theta-E, which is a great way to visualize different airmasses because it takes into account both temperature and moisture. You can easily see the warm sector that will fuel the storms on Saturday.
Source: TwisterData.com

Wind shear will drive the threat for tornadoes and damaging winds on Saturday. The combination of southeasterly surface winds blowing toward the developing low and upper-level winds from the southwest will create a region where extremely favorable wind shear will allow any storm that develops to quickly become severe.
Source: TwisterData.com

The above image shows the Energy Helicity Index (EHI) from the Friday evening run of the NAM. EHI takes into account both instability and wind shear to standardize the threat for supercells across an area. Values above about 3.00 are particularly concerning as they show an environment sufficient for supercells that could support stronger tornadoes.

Limiting Factors

All of the factors are there for an explosive day of severe weather on Saturday. Instability, lift, ample moisture, lots of wind shear...checkmarks on down the list. But there are limiting factors that could determine who sees severe weather and how severe the thunderstorms will be.

1)  Environments that are ripe for severe weather can falter because of ongoing clouds and precipitation that can limit instability. Several rounds of showers and thunderstorms developed over central and eastern Texas overnight into Saturday. These storms will move east and could linger in the risk areas through the morning hours. Any lingering thunderstorms will limit the ability of future storms to develop. The severe weather threat will depend on how much contamination there is from overnight storms.

2) Storm mode will determine how the day goes. The "storm mode" refers to the types of thunderstorms that develop. Squall lines carry a different risk than supercells. A classic tornado outbreak with textbook supercells on radar require clean, discrete thunderstorms that can fully engage with the favorable environment around them. If the storms form into clusters and squall lines, the predominant threat will be damaging straight-line winds with possible embedded tornadoes.

The weather rarely follows textbook examples. It only takes one storm to break through and thrive. That's what happened in eastern Alabama back on March 3—there were lots of sloppy clusters of storms that day, but one supercell managed to find an opening and take off, producing an EF-4 tornado that killed 23 people.

Safety Tips

Most tornado safety tips are common sense. Seek shelter in the lowest level of a sturdy building and as close to the interior as possible. This is a swampy part of the country and not many homes have actual basements. The best place to seek shelter is an interior room like a closet or a bathroom. The goal is to put as many walls between you and flying debris as possible.

"Stay home" is easier said than done. Make sure you mentally scout out a place to take cover at home, work, school, the grocery store, the library, wherever. Try to avoid big box stores on a day like this—those buildings are not built to withstand major severe weather and they will collapse like a house of cards when put under significant stress.

Go somewhere else if you're in a mobile/prefab home. Many of those homes are not designed to withstand winds greater than about 80 MPH, some even less than that. The environment on Saturday is capable of supporting tornadoes that could level even a well-built home. Some mobile homes that existed this morning won't be recognizable by nightfall. It's best to spend the day somewhere safe rather than waiting out the storms in a mobile/prefab home and then leaving at the last second.

You probably won't be able to see these tornadoes, so there's no use standing in the window waiting to see one or, even worse, hopping in the car and heading on a makeshift storm chase. Many of these tornadoes will likely be wrapped in rain, making it nearly impossible to see them until they're on top of you. As it is, experienced storm chasers will have a hard enough time navigating the swampy, forested terrain looking for tornadoes that are probably socked in buckets of driving rain.

Put on a bicycle or motorcycle helmet if you're taking cover during a tornado warning. Most tornado-related injuries and fatalities are caused by a blow to the head. It may look and feel silly, but looking like a goofball is worth it. On that note, it's a good idea to wear closed-toe shoes to protect your feet if you have to walk across any type of debris—broken branches, shattered glass, and twisted pieces of metal will tear up your feet and shred a pair of flip-flops.

Make sure you always have a reliable way to receive severe weather warnings the moment they're issued. Tornado sirens are an unreliable source for tornado warnings—they're not designed to be heard indoors and they're exceptionally prone to failure. NOAA Weather Radios and emergency alerts on your cell phone are great ways to stay in the loop.

A watch means that conditions are favorable over the next couple of hours. A warning means that threatening weather is imminent and will occur within the next couple of minutes. The Storm Prediction Center issues severe weather forecasts throughout the day. Local National Weather Service offices are responsible for issuing warnings that require immediate action.

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April 11, 2019

The Snow in Minnesota Is Dirty From Blowing Dust in the Southwest

The powerful winter-but-not-really-winter storm cranking across the Plains and Midwest this week is still producing heavy, thunderous bands of snow as the storm starts to wind down and finally lift toward Canada. Many folks noted that the blanket of white was a little more off-white than they're used to. The sprawling storm resulted in such windy conditions in the southwestern United States that the subsequent dust storms traveled across the country and tinted the snow as far north as Minnesota and Wisconsin.

A record-breaking blizzard continued to produce heavy snow on Wednesday night across the northern Plains and Upper Midwest. A significant portion of South Dakota has seen more than two feet of snow from the storm with more to come. That would be an impressive snowfall total at any point in the winter, let alone near the middle of April.

The storm—often called a "bomb cyclone" in the news because it underwent bombogenesis, or rapid strengthening—isn't all about the snow. Folks on the southern end of the storm dealt with raging winds as the storm ramped-up across flat terrain. These strong winds, gusting at times to 60-70 MPH, picked up a ton of dust as they blew over the Chihuahuan Desert.

This lofted dust raced northward through the day on Wednesday and got ingested into the storm system overnight. By Thursday morning, pictures started emerging on social media of tan-tinted "dirty" snow showing up in places like Minnesota and Wisconsin.

Here's some dusty snow near Minneapolis, MN...

...and in Maple Grove, MN...

...and some more in La Crosse, WI...

...and even as far north as Green Bay, WI...

It turns out that the dust wound up accumulating on the falling snowflakes—and even likely serving as the nucleus on which the snowflake could form—once the layer of southwestern air reached the moisture and cold air on the northern side of the system. Airborne particulates like dust get ingested into weather systems all the time, though it usually involves liquid precipitation. It's common to see a dirty rain develop after a major dust storm, volcanic eruption, or wildfires.

[Top Image: RAMMB/CIRA]

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April 10, 2019

This Week's Major Storm Will Plaster The Northern Plains With Up To Two Feet Of Snow

A wet, wind-driven snow will plaster the Plains and Upper Midwest this week as another memorable springtime winter storm rapidly spins-up over the center of the country. The storm—not too dissimilar from the major storm we saw this time last month—will produce everything from a historic -for-April blizzard up north to extreme fire conditions in the Southwest.

The storm will kick into gear on Wednesday as the lift from an upper-level trough crossing the Rockies coincides with the lift from strong winds in the jet stream over the southern Plains. Just like we saw last month, this storm should ramp up in a hurry, undergoing bombogenesis during the day on Wednesday as it crosses from eastern Colorado through central Kansas.

We could see another round of record-low air pressure readings for the month of April if the low's minimum central pressure deepens as much as current forecasts suggest. The current all-time low air pressure readings for the month of April stand between 977-980 mb across the region where the low will grow its strongest, which is right around the predicted minimum pressure for this storm.


A map of storm total snowfall predicted by local NWS offices, issued Tuesday evening.

This will be a significant and disruptive snowstorm for a large swath of land between Colorado and Ontario. Precipitation will fall in the form of snow across most of the northern expanse of the storm, likely producing more than a foot of snow from central Nebraska through to the western shores of Lake Superior. Communities looking at 12"+ of snow include Rapid City, Pierre, St. Cloud, and Duluth.

Timing: Snow is already falling across parts of the central Rockies tonight. Wintry precip will quickly pick up in coverage and intensity on Wednesday morning as the storm develops and strengthens. A blizzard warning is in effect for the Denver metro area from noon Wednesday until noon Thursday. The heavy snow will overspread the northern Plains and Upper Midwest on Wednesday night and continue through Friday morning, gradually ending from west to east.

Amounts: A sharp temperature gradient along the track of the storm, both at the surface and aloft, will mean there's a sharp cutoff between significant snowfall totals and not much of anything at all. This is a particularly unnerving scenario because Sioux Falls, Minneapolis, and St. Paul are all straddling that line between a nuisance snowfall and a crippling blizzard. Any southward jog in the track of the storm will force the heavy, sustained snowfall farther to the south and into the heart of these cities.

Some communities will see monthly snowfall records from this system. The most snow ever recorded during the month of April in Pierre, South Dakota, was 13.4" during the April of 1986. This storm could top that. The most snow recorded for the month of April in Duluth, Minnesota, was 12.7" set back in...well, last year. The latest NWS forecast shows this storm easily topping that with close to a foot-and-a-half of snow.


The big story with last month's storm wound up being the immense flooding that engulfed entire towns in the days and weeks after the storm. Major flooding is likely after this storm, as well, though probably not on the scale we saw last time around.

The snow that falls this week will be heavy and wet. The latest precipitation forecast from the Weather Prediction Center shows 3-4" of liquid-equivalent precipitation falling across the hardest-hit flood areas through the end of the week.

River flooding forecast from the NWS through the next week. (NWS)
Once the snow starts to melt, many rivers will again begin to overflow their banks and lead to major flooding in the surrounding areas. Several rivers in the region are already at moderate or major flood stage.

The National Weather Service expects most gauges along the Red River in North Dakota, James River and Big Sioux River in South Dakota, the Mississippi River from Minnesota on down, and many other smaller waterways in the region to experience moderate or major flooding in the next week because of the impending surge of rain and snow.

The situation shouldn't be quite as severe as we saw last month—only minor and moderate flooding is in the forecast in Nebraska—as the bulk of the precipitation will be farther north than it was in March and the ground isn't frozen anymore. The frozen, impermeable ground, along with forced the vast majority of the water to run off into waterways, exacerbating the flooding.

Severe Weather

Not only will the impending satellite images tomorrow grab the attention of weather nerds, but it's likely that a temperature map will also look pretty impressive during the day's peak heating. Only a short drive will separate seeing your breath from sweating to death. (Okay, well maybe not sweating that much, but "kinda sweating" doesn't rhyme.)

As you'd expect to see with any strong storm system and that kind of temperature gradient, there's a chance for severe thunderstorms along the storm's track on Wednesday. The dynamics aren't the greatest due to the relative lack of moisture accompanying the warmth, but conditions will be favorable for severe thunderstorms with the risk of large hail and some damaging winds. An isolated tornado or two is possible in a situation like this, the best chance for which would be right around the warm front.

The chance for severe weather will shift to the other side of the Mississippi River on Thursday afternoon, where severe thunderstorms that develop along the advancing cold front could bring damaging wind gusts to parts of eastern Illinois and western Indiana.

High Winds

A strong storm wrapping up on flat land is a recipe for high winds. (I think I typed that exact sentence with the last one, come to think of it.) High wind warnings and wind advisories are up for much of the Southwest and southern Plains in anticipation for wind gusts of 60-70 MPH when the storm cranks up during the day on Wednesday. The combination of high winds, dry air, and warm temperatures will lead to a high fire risk across parts of New Mexico and far western Texas during the day on Wednesday.

[Top Image: Tropical Tidbits]

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April 3, 2019

An Intense April Nor'easter Produced Atlantic Supercells And Snow In The Carolinas

A powerful nor'easter developed off the eastern coast of the Carolinas on Tuesday, bringing a burst of winter weather to a part of the country that hasn't seen much of it this year. The formation and track of the nor'easter has been in the forecast for a few days now, but this storm turned out to be particularly impressive due to its structure and the supercells it produced over the open ocean.

The unusual snow in the Carolinas was the storm's major headline for most of the day. Subfreezing temperatures just above the ground allowed frozen precipitation to reach the surface for a couple of hours from north-central South Carolina to north-central North Carolina. Folks around Charlotte saw the most, with some communities pleasantly surprised to measure about two inches of snow by the time the precipitation came to an end. Snow and sleet accumulated on cars as far north as Greensboro, while it was just pretty to look at closer to the Virginia border where I live.

The snow was mostly conversational and didn't cause too many problems thanks to warm ground temperatures and air temperatures that mostly stayed above freezing during the event. Charlotte only dropped to 36°F during the heaviest precipitation.

It's not common to see accumulating snow this far south this late in the year. This is just the second time in the past century that Charlotte has seen measurable snow during the month of April. Winter weather usually comes to an end across the Carolinas by the middle of March, though there are some exceptions. Parts of North Carolina saw accumulating snow during the second week of April last year, followed a few days later by a severe weather outbreak.

Source: Gibson Ridge

Things got even more interesting once the storm fully exited the North Carolina coast. The low-pressure system that would go on to become a full-fledged nor'easter quickly got its act together this afternoon thanks to a sharply tilted upper-level trough and favorable winds in the jet stream. Meteorologists started buzzing on social media this evening while watching the radar out of Morehead City, NC, as a squall line with multiple embedded supercells played out a few dozen miles off the Outer Banks.

I mean, helloooo:

Those thunderstorms actually wound up helping the low-pressure system deepen even faster than it would have otherwise. For a couple of hours this afternoon, this storm strengthened in part through the same processes that fuel tropical and subtropical cyclones. Strong updrafts in the thunderstorms near the center of the low pulled lots of air away from the surface, causing the minimum central pressure to deepen more rapidly than it would have otherwise. NOAA's Weather Prediction Center put it succinctly this evening: "[the storm is] a good reminder that cyclones exist along a continuum and that determining their type (extratropical, subtropical, or tropical) isn't always clear."

New England will see some gusty winds and a cold rain on Wednesday morning as the bulk of the storm passes the region to the east. Atlantic Canada will feel the full brunt of the storm as it makes landfall in Nova Scotia on Wednesday evening. Environment Canada has warnings out for wind gusts as high as 60 MPH along Nova Scotia's southern coast and 2-3 inches of rain across the province through Thursday. The storm will be somewhat kinder to New Brunswick and Prince Edward Island, bringing an inch or two of cold rain with some heavy snow possible on the western fringe of the storm.

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