By:
Jeff Masters and Bob Henson
, 3:51PM,GMT on May 26,2016
Showers and thunderstorms have increased and grown more organized in
association with an area of low pressure that has formed between the
Bahamas and Bermuda (Invest 91L.)
This low appears increasingly likely to develop into a tropical or
subtropical depression as it moves west-northwest or northwest towards
the Southeast U.S. coast over the next few days. Should it become a
named storm, it would be called Bonnie.
Satellite loops show that 91L has developed a pronounced spin near the surface, though no well-defined low-level circulation center was in evidence yet on Thursday morning. Sea surface temperatures (SSTs) near 91L's center were about 26 - 27°C (79 - 81°F), which is 1 - 2°C (1.8 - 3.6°F) above average. These waters are only marginally warm enough to support formation of a tropical storm, and 91L is likely to take on the characteristics of subtropical system instead of a tropical system. As I explain in my Subtropical Storm Tutorial, a subtropical storm typically has a large, cloud free center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate, but tropical storms generate more rain.
Wind shear on Thursday morning was high, 20 - 30 knots, making significant development unlikely until the shear drops on Friday. A large area of dry continental air lies to the west of 91L, and this dry air will interfere with development. A hurricane hunter aircraft is scheduled to investigate 91L on Friday afternoon.
Figure 1. Lastest satellite image of 91L.
Figure 2. Predicted wind speeds for the Southeast U.S. at 2 pm EDT (18Z) Monday, May 30, 2016 from the 00Z Thursday, May 26 run of the European model (left) and GFS model (right). The GFS model predicted 91L would be inland over the South Carolina coast, while the European model had the storm just off the southern coast of North Carolina. Image constructed using our wundermap with the "Model Data" layer turned on.
A heavy rain threat for the Southeast U.S. coast
The 12Z (8 am EDT) Thursday run of the SHIPS model predicted that wind shear would fall to the moderate range, 10 - 20 knots, on Friday, then fall to the low range, below 10 knots, Saturday through Sunday. During this period, 91L will be traversing waters of 25 - 26°C (77 - 79°F), which should be just warm enough to allow the storm to spin up into a tropical or subtropical depression. The system will have to contend with a large area of dry air to its west, but if wind shear falls to the low range, 91L should be able to moisten its inner core enough to wall off the dry air and spin up. In their Thursday morning (00Z) runs, our three top models for forecasting tropical cyclone genesis--the American GFS model, the European ECMWF model, and the British UKMET model--all showed the potential for Invest 91L to develop into a tropical or subtropical depression on Friday or Saturday. In a special Tropical Weather Outlook issued at 8:25 am EDT Thursday, the National Hurricane Center bumped up their development odds in the 2-day and 5-day time ranges to 50% and 70%, respectively.
Invest 91L likely will not have enough time over water to become a strong tropical storm or hurricane, so heavy rain is the main concern from this system. Heavy rains should reach the coasts of South Carolina and southeastern North Carolina on Saturday night or Sunday morning, though the I expect the heaviest rains from 91L will stay out sea to the storm's east, in a large band of heavy rain typical for a subtropical system. The Thursday morning runs of our two top models for hurricane tracking, the GFS and European models, showed the center of 91L reaching the central coast of South Carolina near Charleston on Sunday afternoon. Steering currents will then shift, as 91L gets caught in the circulation associated with a trough of low pressure passing to the north early next week, forcing 91L to turn to the northeast by Sunday night. The center of 91L will likely track just offshore or just inland along the coasts of South Carolina and North Carolina on Monday and Tuesday, spreading heavy rains of 2 - 4" along its path. Ocean temperatures will cool as 91L pushes into the coastal waters of North Carolina, which should induce weakening on Monday and Tuesday.
Figure 3. One moment from the journey of a long-lived tornado that churned across central Kansas on Wednesday evening, May 25, 2016. A low-level inflow band is visible wrapping into the tornado from the right-hand side. See related video embedded at bottom. Image credit: Sam Ng, Metropolitan State University of Denver, used with permission.
Another day, another prolific tornado-producer in Kansas
After Tuesday’s sequence of highly visible tornadoes from a single supercell thunderstorm near Dodge City, KS, the atmosphere dropped countless jaws again on Wednesday as an isolated supercell marched along Interstate 70 in central Kansas. As was the case on Tuesday, this storm formed beneath relatively weak upper-level flow near the intersection of a dry line and an outflow boundary from earlier storms. Towering to 70,000 feet at one point, this all-alone storm generated 12 of the 14 preliminary tornado reports on SPC’s daily storm log by early Thursday. As it paralleled I-70 for much of its life, eventually angling across the highway, this storm produced what veteran storm chaser and damage expert Tim Marshall, who has filmed more than 200 twisters, called “the longest-lasting tornado of my chase career.” Update: This twister, roughly a half a mile wide, was rated EF4 and lasted for 90 minutes, according to the damage survey released on Friday afternoon by the NWS office in Topeka, KS. It is very rare when an individual tornado is confirmed to last more than an hour.
Almost miraculously, the tornado avoided any direct hits on large communities. It barely missed the tiny town of Talmage and passed just south of Abilene and the smaller town of Chapman; the latter was largely destroyed by a direct hit from a half-mile-wide EF3 tornado on June 11, 2008. About 20 homes were damaged or destroyed, but no serious injuries had been reported as of early Thursday.
Figure 4. A cyclic supercell spins just north of Enterprise, KS, as a long-lived tornado approaches the end of its hour-plus life cycle. Image credit: Victor Gensini, College of DuPage, used with permission.
Figure 5. The difference three hours can make: a clump of towering cumulus over central Kansas at 2215Z (5:15 pm CDT) Wednesday, May 25, 2016, had become a long-lived, tornado-spewing behemoth by 0115Z (8:15 pm CDT), as shown in this sequence of GOES visible satellite images. Preliminary tornado reports from this storm spanned the interval from 6:08 pm to 10:31 pm CDT, though tornadoes were not on the ground during that entire time. Image credit: NOAA and College of DuPage/NexLab.
The intensity of this tornado-generating machine may have surprised storm watchers, given that the NOAA Storm Prediction Center had indicated only a slight risk of severe weather for central Kansas as of Wednesday morning (and an even lower marginal risk at one earlier point). This is a good time to reiterate a key aspect of SPC convective outlooks: the risk categories are designed to emphasize probability, not intensity. The late morning outlook (issued at 11:30 am CDT) featured a 2% chance of tornadoes occurring within 25 miles of any given point (or about 1964 square miles) over a broad swath from North Dakota to Texas. This swath covered about 187,000 square miles, so the 2% odds across the swath were certainly of the right order of magnitude next to the modest amount of land area actually hit by tornadoes on Wednesday--even though nearly all of those twisters ended up occurring from a single storm.
Figure 5. Severe-weather risk areas for Thursday, May 26, 2016, issued at 8:00 am CDT Thursday by the NOAA/NWS Storm Prediction Center, include an enhanced risk for much of northern Kansas, southern Nebraska, and far southwest Texas, and a slight risk encompassing most of the southern and central Great Plains.
A serious severe threat for Thursday, with more Kansas tornadoes possible
A potentially volatile situation is lining up for Thursday, as upper-level winds strengthen and surface low pressure intensifies across western Kansas. Outside of thunderstorm-cooled areas, the air mass flowing north from the Gulf of Mexico remains unusually sultry, with dew points in the low- to mid-70s close to record levels for late May across Oklahoma and south-central Kansas. Wind shear should increase notably on Thursday, and instability will remain at extreme levels. As of Thursday morning, NOAA/SPC had placed much of Kansas and Nebraska, plus parts of southwest Texas, in the “enhanced” category of storm risk for Thursday, with a much larger slight-risk area (see Figure 5). Assuming the air mass recovers as expected, the greatest risk for tornadoes may once again be somewhere in western or central Kansas, near the intensifying dry line and surface low and a warm front expected to stretch eastward from the low. With more substantial upper energy flowing across the area today as opposed to earlier this week, storms will develop sooner and be a bit faster-moving, and severe weather should be much more widespread overall. I would not be surprised to see the enhanced-risk area upgraded and/or expanded as the day unfolds. Update: At 11:30 am CDT, NOAA/SPC enlarged the enhanced-risk area along the dry line from central Kansas to far southwest Texas. A moderate-risk area (fourth highest out of the five SPC categories] was introduced in central KS, reflecting a heightened chance of significant tornadoes in that area.
A large chunk of the stubborn upper-level low parked across the western U.S. all week will swing into the central U.S. on Friday, shunting the greatest severe threat into Texas. At least modest odds of severe weather could emerge during the holiday weekend across the southern Great Plains, as the soon-to-be-diminished western trough regains a bit of its strength.
Jeff Masters (tropical), Bob Henson (severe)
Satellite loops show that 91L has developed a pronounced spin near the surface, though no well-defined low-level circulation center was in evidence yet on Thursday morning. Sea surface temperatures (SSTs) near 91L's center were about 26 - 27°C (79 - 81°F), which is 1 - 2°C (1.8 - 3.6°F) above average. These waters are only marginally warm enough to support formation of a tropical storm, and 91L is likely to take on the characteristics of subtropical system instead of a tropical system. As I explain in my Subtropical Storm Tutorial, a subtropical storm typically has a large, cloud free center of circulation, with very heavy thunderstorm activity in a band removed at least 100 miles from the center. The difference between a subtropical storm and a tropical storm is not that important as far as the winds they can generate, but tropical storms generate more rain.
Wind shear on Thursday morning was high, 20 - 30 knots, making significant development unlikely until the shear drops on Friday. A large area of dry continental air lies to the west of 91L, and this dry air will interfere with development. A hurricane hunter aircraft is scheduled to investigate 91L on Friday afternoon.
Figure 1. Lastest satellite image of 91L.
Figure 2. Predicted wind speeds for the Southeast U.S. at 2 pm EDT (18Z) Monday, May 30, 2016 from the 00Z Thursday, May 26 run of the European model (left) and GFS model (right). The GFS model predicted 91L would be inland over the South Carolina coast, while the European model had the storm just off the southern coast of North Carolina. Image constructed using our wundermap with the "Model Data" layer turned on.
A heavy rain threat for the Southeast U.S. coast
The 12Z (8 am EDT) Thursday run of the SHIPS model predicted that wind shear would fall to the moderate range, 10 - 20 knots, on Friday, then fall to the low range, below 10 knots, Saturday through Sunday. During this period, 91L will be traversing waters of 25 - 26°C (77 - 79°F), which should be just warm enough to allow the storm to spin up into a tropical or subtropical depression. The system will have to contend with a large area of dry air to its west, but if wind shear falls to the low range, 91L should be able to moisten its inner core enough to wall off the dry air and spin up. In their Thursday morning (00Z) runs, our three top models for forecasting tropical cyclone genesis--the American GFS model, the European ECMWF model, and the British UKMET model--all showed the potential for Invest 91L to develop into a tropical or subtropical depression on Friday or Saturday. In a special Tropical Weather Outlook issued at 8:25 am EDT Thursday, the National Hurricane Center bumped up their development odds in the 2-day and 5-day time ranges to 50% and 70%, respectively.
Invest 91L likely will not have enough time over water to become a strong tropical storm or hurricane, so heavy rain is the main concern from this system. Heavy rains should reach the coasts of South Carolina and southeastern North Carolina on Saturday night or Sunday morning, though the I expect the heaviest rains from 91L will stay out sea to the storm's east, in a large band of heavy rain typical for a subtropical system. The Thursday morning runs of our two top models for hurricane tracking, the GFS and European models, showed the center of 91L reaching the central coast of South Carolina near Charleston on Sunday afternoon. Steering currents will then shift, as 91L gets caught in the circulation associated with a trough of low pressure passing to the north early next week, forcing 91L to turn to the northeast by Sunday night. The center of 91L will likely track just offshore or just inland along the coasts of South Carolina and North Carolina on Monday and Tuesday, spreading heavy rains of 2 - 4" along its path. Ocean temperatures will cool as 91L pushes into the coastal waters of North Carolina, which should induce weakening on Monday and Tuesday.
Figure 3. One moment from the journey of a long-lived tornado that churned across central Kansas on Wednesday evening, May 25, 2016. A low-level inflow band is visible wrapping into the tornado from the right-hand side. See related video embedded at bottom. Image credit: Sam Ng, Metropolitan State University of Denver, used with permission.
Another day, another prolific tornado-producer in Kansas
After Tuesday’s sequence of highly visible tornadoes from a single supercell thunderstorm near Dodge City, KS, the atmosphere dropped countless jaws again on Wednesday as an isolated supercell marched along Interstate 70 in central Kansas. As was the case on Tuesday, this storm formed beneath relatively weak upper-level flow near the intersection of a dry line and an outflow boundary from earlier storms. Towering to 70,000 feet at one point, this all-alone storm generated 12 of the 14 preliminary tornado reports on SPC’s daily storm log by early Thursday. As it paralleled I-70 for much of its life, eventually angling across the highway, this storm produced what veteran storm chaser and damage expert Tim Marshall, who has filmed more than 200 twisters, called “the longest-lasting tornado of my chase career.” Update: This twister, roughly a half a mile wide, was rated EF4 and lasted for 90 minutes, according to the damage survey released on Friday afternoon by the NWS office in Topeka, KS. It is very rare when an individual tornado is confirmed to last more than an hour.
Almost miraculously, the tornado avoided any direct hits on large communities. It barely missed the tiny town of Talmage and passed just south of Abilene and the smaller town of Chapman; the latter was largely destroyed by a direct hit from a half-mile-wide EF3 tornado on June 11, 2008. About 20 homes were damaged or destroyed, but no serious injuries had been reported as of early Thursday.
Figure 4. A cyclic supercell spins just north of Enterprise, KS, as a long-lived tornado approaches the end of its hour-plus life cycle. Image credit: Victor Gensini, College of DuPage, used with permission.
Figure 5. The difference three hours can make: a clump of towering cumulus over central Kansas at 2215Z (5:15 pm CDT) Wednesday, May 25, 2016, had become a long-lived, tornado-spewing behemoth by 0115Z (8:15 pm CDT), as shown in this sequence of GOES visible satellite images. Preliminary tornado reports from this storm spanned the interval from 6:08 pm to 10:31 pm CDT, though tornadoes were not on the ground during that entire time. Image credit: NOAA and College of DuPage/NexLab.
The intensity of this tornado-generating machine may have surprised storm watchers, given that the NOAA Storm Prediction Center had indicated only a slight risk of severe weather for central Kansas as of Wednesday morning (and an even lower marginal risk at one earlier point). This is a good time to reiterate a key aspect of SPC convective outlooks: the risk categories are designed to emphasize probability, not intensity. The late morning outlook (issued at 11:30 am CDT) featured a 2% chance of tornadoes occurring within 25 miles of any given point (or about 1964 square miles) over a broad swath from North Dakota to Texas. This swath covered about 187,000 square miles, so the 2% odds across the swath were certainly of the right order of magnitude next to the modest amount of land area actually hit by tornadoes on Wednesday--even though nearly all of those twisters ended up occurring from a single storm.
Figure 5. Severe-weather risk areas for Thursday, May 26, 2016, issued at 8:00 am CDT Thursday by the NOAA/NWS Storm Prediction Center, include an enhanced risk for much of northern Kansas, southern Nebraska, and far southwest Texas, and a slight risk encompassing most of the southern and central Great Plains.
A serious severe threat for Thursday, with more Kansas tornadoes possible
A potentially volatile situation is lining up for Thursday, as upper-level winds strengthen and surface low pressure intensifies across western Kansas. Outside of thunderstorm-cooled areas, the air mass flowing north from the Gulf of Mexico remains unusually sultry, with dew points in the low- to mid-70s close to record levels for late May across Oklahoma and south-central Kansas. Wind shear should increase notably on Thursday, and instability will remain at extreme levels. As of Thursday morning, NOAA/SPC had placed much of Kansas and Nebraska, plus parts of southwest Texas, in the “enhanced” category of storm risk for Thursday, with a much larger slight-risk area (see Figure 5). Assuming the air mass recovers as expected, the greatest risk for tornadoes may once again be somewhere in western or central Kansas, near the intensifying dry line and surface low and a warm front expected to stretch eastward from the low. With more substantial upper energy flowing across the area today as opposed to earlier this week, storms will develop sooner and be a bit faster-moving, and severe weather should be much more widespread overall. I would not be surprised to see the enhanced-risk area upgraded and/or expanded as the day unfolds. Update: At 11:30 am CDT, NOAA/SPC enlarged the enhanced-risk area along the dry line from central Kansas to far southwest Texas. A moderate-risk area (fourth highest out of the five SPC categories] was introduced in central KS, reflecting a heightened chance of significant tornadoes in that area.
A large chunk of the stubborn upper-level low parked across the western U.S. all week will swing into the central U.S. on Friday, shunting the greatest severe threat into Texas. At least modest odds of severe weather could emerge during the holiday weekend across the southern Great Plains, as the soon-to-be-diminished western trough regains a bit of its strength.
Jeff Masters (tropical), Bob Henson (severe)
Video 1. A view of the long-lived tornado with a low-level inflow band that started out in Bennington, KS. Video credit: Sam Ng (@DocWX, Metropolitan State University of Denver, used with permission.
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