Killer tornadoes rip Iowa, and Minnesota; tropical depression possible late this week

The 2008 Memorial Day Weekend tornado outbreak will continue to hammer the U.S. today, even as residents from Iowa and Minnesota clean up from the devastating tornadoes that killed eight people Sunday afternoon. A mile-wide tornado plowed through Parkersburg, Iowa between 5pm and 6pm CDT yesterday, killing five people in that city, and two in nearby New Hartford. It was the deadliest tornado in Iowa in more than 40 years. The tornado passed just north of the airport in Waterloo, Iowa, which recorded sustained winds of 64mph, gusting to 94 mph at 5:37pm CDT. Damage appeared to be at least EF-4 in photos I saw, and possibly EF-5 (over 200 mph). In Minnesota, another powerful twister killed at least one person and injured 9 in the Minneapolis suburb of Hugo.


Figure 1. Satellite image of the supercell thunderstorms that spawned the Parkerburg, Iowa and Hugo, Minnesota tornadoes on May 25, 2008.

The slow-moving low pressure system responsible for all the mayhem began in Colorado on Thursday, when the Storm Prediction Center (SPC) recorded 48 reports of tornadoes, including the EF-3 mile-wide twister that killed one person in Windsor, Colorado. On Friday, an additional 63 tornado reports occurred, mostly in Kansas. The tornado that hit Quinter, Kansas on Friday was the eighth violent EF-4 tornado of the year. Also on Friday, two people were killed in Cairo, Kansas when a tornado smashed a car trying to flee the storm. If you want to see why one should not try to escape a tornado in a car, take a look at what the tornado did to the car. Saturday was relatively quiet, with only 13 tornado reports, but Sunday's tally of 43 brought the 4-day total from the 2008 Memorial Day weekend outbreak to a remarkable 157 tornado reports. Some of these tornado reports are undoubtedly of the same tornado, so the actual number of tornadoes for the 4-day outbreak may be less than 150. BBC has some awesome aerial footage of the weekend tornadoes.



Figure 2. Radar reflectivity image (top) of the May 25, 2008 Parkersburg, Iowa tornado. The position of Parkersburg is marked by a circle with a cross in the middle. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex.



Figure 3. Radar reflectivity image (top) of the May 25, 2008 Hugo, Minnesota tornado. The position of Hugo is marked by a circle with a cross in the middle. Bottom: Doppler velocity image of the tornado, showing a small core of red and blue colors right next to each other, denoting strong winds towards and away from the radar, the classic signature of a tornado vortex. We've also saved a 12-frame radar animation of the Hugo cell, thanks to wunderground member Todd S.

Tallying up the numbers
The death toll from Sunday pushes this year's tornado deaths to 111, the most since 1998, when 130 were recorded. Assuming that the Parkersburg, Iowa tornado was an EF-4 or EF-5, there have been nine violent EF-4 or EF-5 tornadoes this year. This is the most since 1999, when 13 such twisters were recorded. The total number of tornadoes this year is approaching 1100, and we may challenge the all time record for tornadoes in a year of 1817, set in 2004. Could this be a sign of climate change? No, I don't think so, and I'll explain why in a blog later this week.

Severe weather forecast
Severe weather will pound the U.S. again this Memorial Day, with the main action expected to stretch more than halfway across the country--from Texas to New York. NOAA's Storm Prediction Center has placed much of Kansas, Oklahoma, and the Texas Panhandle under its "Moderate Risk" category for severe weather, one step below its highest level of concern, "High Risk". Yesterday was also a "Moderate Risk" day in Iowa and Minnesota. The Weather Underground Severe Weather page and Tornado page are good places to go to follow the severe weather. Also, tune in to the chase accounts and awesome storm photos from Wunderblogger Mike Theiss. After today, it appears the severe weather outbreak will finally diminish, with only a slight risk of severe weather expected Tuesday, and no severe weather expected Wednesday.

Possible development in the Western Caribbean or Eastern Pacific late this week
For the past 3-6 days, our most reliable global computer weather forecast models have been predicting the development of a low pressure system near or over Central America by Friday of this week. Given the persistence in the models in developing this low, we need to be alert to the possibility of a tropical depression forming in either the Western Caribbean or Eastern Pacific, on either side of Costa Rica and Nicaragua. It is uncertain which ocean basin such a storm might form in, and whether or not there will be a tropical wave around to help kick off development. It may be that the low pressure region will stay anchored over land south of the Yucatan Peninsula, preventing any development. This is the solution preferred by the ECMWF model in its last few runs. However, the GFS, NOGAPS, and Canadian models all predict a tropical depression might form in the Western Caribbean near Mexico's Yucatan Peninsula. In contrast, the UKMET shows development in the Eastern Pacific, on the Pacific side of Central America. Climatologically, May tropical storms are much more common in the Eastern Pacific than the Western Caribbean, so we should not discount the UKMET solution, even though it is an outlier. All five models predict that the Central American low pressure area will move northward towards the Gulf of Mexico, and wind shear may fall enough to allow a tropical depression to form should the low's center emerge over water. I'll be posting daily updates on the situation this week.

Jeff Masters