Dr. Jeff Masters' WunderBlog

Odds of a Weekend Tropical Depression Rise; Dodge City Dodges Multiple Tornadoes

Posted: 3:36 PM GMT on May 25, 2016

Showers and thunderstorms continue over portions of the Bahamas and nearby waters in association with an upper-level trough interacting with a weakening cold front. On Wednesday morning, NHC designated this area of interest as Invest 91L. This activity is expected to coalesce into an area of low pressure on Friday a few hundred miles north of the Bahama Islands. This low has the potential to develop into a tropical depression as it moves northwest towards the Southeast U.S. coast over the weekend. Sea surface temperatures (SSTs) over the Bahamas are about 28 - 29°C (82 - 84°F), which is 1 - 2°C (1.8 - 3.6°F) above average. These waters are plenty warm enough to support formation of a tropical storm. Phase space diagrams from Florida State University have been consistently showing over the past few days that this storm will be a symmetric warm core system--technical lingo for a storm that is tropical in nature, rather than subtropical or extratropical.

Figure 1. The area of disturbed weather near the Bahamas that we're tracking, as seen by MODIS on Tuesday afternoon, May 24, 2016.

Figure 2. Predicted wind speeds for the Southeast U.S. at 2 pm EDT (18Z) Monday, May 30, 2016 from the 00Z Wednesday, May 25 run of the European model (left) and the 06Z Wednesday May 25, 2016 run of the GFS model (right). Both 5-day forecasts were predicting a possible tropical depression off the Georgia/South Carolina coast. Image constructed using our wundermap with the "Model Data" layer turned on.

Increasing model agreement on genesis
In my 2013 blog post, Genesis of New Atlantic Tropical Cyclones: Which Model Should You Trust?, I explained that we have three models that have proven to be fairly reliable for predicting the genesis of tropical depressions up to four days in advance: the American GFS model, the European ECMWF model, and the British UKMET model. About 50% of the time, at least one of these models will successfully predict tropical cyclone genesis up to four days in advance. When all three models agree on genesis, confidence increases in the forecast. On Tuesday, the UKMET model was not forecasting genesis, while the GFS and European models were. However, the Wednesday morning (00Z) runs of all three of these models showed the potential for Invest 91L to develop into a tropical depression a few hundred miles north of the Bahamas this weekend; this increases our confidence that genesis will occur. In a special Tropical Weather Outlook issued at 8:15 am EDT Wednesday, the National Hurricane Center bumped up their development odds in the 2-day and 5-day time ranges to 10% and 50%, 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. The 00Z Wednesday runs of the models indicated a possible threat to the coasts of northern Florida, Georgia, or South Carolina early next week--though the European model showed the storm staying just off the coast through next Wednesday. I'll keep you updated each day this week with the latest prognosis for this potential early-season storm. Should it become a tropical storm, it would be named Bonnie.

Close call for Dodge City: A marathon tornado sequence narrowly misses town
Tuesday produced one of the biggest tornado outbreaks of 2016 thus far, with 30-plus twister reports logged by the NOAA/NWS Storm Prediction Center as of early Wednesday. A long-lived supercell dropped several tornadoes across northeast Colorado, and a brief twister damaged several structures near Bristow, Oklahoma. More than 5 inches of rain fell from Tuesday to Wednesday morning across parts of eastern Oklahoma (see embedded tweet at bottom), and flash flood warnings were in effect Wednesday morning over parts of northeast Oklahoma and far northwest Arkansas. One person drowned early Wednesday morning after a car was swept off a roadway in high water near Davenport, OK.

Figure 3. One of the series of tornadoes that moved through Ford County, KS, along a north-south line just west of Dodge City. Image credit: Bob Henson.

The tornadic storm of the day on Tuesday was a monster supercell that ground its way northward across the Dodge City, Kansas, area. The storm formed near the intersection of the Southern Plains dryline and an outflow boundary from Monday night storms that had pushed through most of Kansas. Winds north of the outflow boundary were from the southeast, which provided plenty of low-level spin for the storm to ingest, and very warm, humid air (temperatures well above 80°F and dew points in the upper 60s) led to extreme levels of instability. With upper-level winds relatively weak, the Dodge City storm hung close to the slow-moving boundary intersection or “triple point”, spitting out more than a dozen tornadoes of various shapes and sizes (cone, wedge, elephant trunk, rope, etc.) in the space of 90 minutes along the storm’s track of about 30 miles. Observers reported at least three instances of two simultaneous tornadoes from this storm’s mammoth wall cloud--a la the twin tornadoes of Pilger, Nebraska, from June 2014--and there was reportedly a brief third tornado at one point. The sequence may end up ranking among the most prolific in the annals of cyclic tornado production from a single storm. At least a dozen structures were damaged or destroyed along the storm’s path, including several on the western outskirts of Dodge City, but no serious injuries had been reported as of early Wednesday--a very fortunate outcome for Dodge City’s closest tornado call in many years. Near sunset, an impressive light and cloud show unfolded on the storm’s west side.

Figure 3. A spectacular display of anticrepuscular rays opposite the setting sun near Dodge City on May 24, 2016. Image credit: Bob Henson.

Figure 4. The departing storm filled the eastern sky over Dodge City with mammatus at sunset on May 24, 2016. Image credit: Bob Henson.

Figure 5. Severe weather outlook for Thursday, May 26, 2016, from NOAA's Storm Prediction Center (SPC). SPC is calling for an enhanced risk of severe weather (orange) across portions of Nebraska, Kansas, and Oklahoma. This Day 2 outlook will be updated around 1:30 pm EDT Wednesday.

The severe weather outlook for Wednesday and beyond
The active pattern across the Southern Plains will take a partial breather on Wednesday, as very warm mid-level air will help suppress thunderstorms for much of the day. A few scattered supercells may pop up by evening across SPC’s broad slight risk area, which extends from Texas to Minnesota. Pockets of weakness in the upper-level wind pattern will be generally unfavorable for tornadic storms, leaving large hail as the main threat toward the south and strong winds toward the north along a cold front. Activity should refocus along the dry line from Texas to Kansas on Thursday, when a stronger upper-level wave may trigger another round of multiple tornadic supercells. SPC has placed the region just east of the dryline under an enhanced risk of severe weather for Thursday. The stubborn upper-level trough that’s driven four consecutive days of tornado activity will languish across the western U.S. through Memorial Day weekend into next week, but it will gradually weaken as it does so. As a result, the intensity of the Great Plains severe weather looks likely to ramp down as next week unfolds, with plenty of thunderstorms popping over the Southern Plains but less of an overall tornado threat.

Jeff Masters (tropical), Bob Henson (severe)

About This Author:
Jeff Masters co-founded the Weather Underground in 1995 while working on his Ph.D. at Michigan. He flew with the NOAA Hurricane Hunters from 1986-1990.


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