This scale was updated last year as we’ve learned more about wind speed and the damage it causes and is now called the Enhanced Fujita scale, with tornadoes given a rating of EF0-EF5. The old scale was simply the Fujita scale with ratings of F0-F5. Its named after Dr. Ted Fujita who was a leading pioneer in many aspects of tornado research including some current theory of how they form and in terms of doing damage surveys. Damage surveys are done by the local National Weather Service office after a tornado is reported and is important because the Enhanced Fujita scale is a scale of damage, not a scale of wind speed. Through research however, we know that it takes a certain wind speed to do a certain type of damage so we can estimate the wind speed of a tornado fairly accurately.

The two images above lead me to believe at this time that the tornado will be rated an EF3 as the train was taken off the tracks and a significant portion of several well built homes were destroyed. However, the rating of a tornado is more complicated than finding the “worst” part and calling it an EF3 or whatever the case may be.

If indeed the tornado was on the ground the entire time from near Platteville around 11:27am to outside of Wellington an hour later (and some radar suggests it may have lifted at least once and a second tornado may have formed), I’m positive in saying that it wasn’t an EF3 the entire time. Tornadoes, a lot like the parent storm itself, will strengthen and weaken throughout its lifetime. So we may find that for portions of its journey northward it was EF0, EF1 and EF2 at different points. It will also be interesting to see if the NWS observers find any EF4 damage. We’ll also get confirmation through the NWS folks if it was one single tornado or several that did the much talked about damage that has dominated national news today.

At 11:54am a large tornado approached the town of Windsor, which is located in extreme western Weld County about 9 miles southeast of Fort Collins and 50 miles NNE of downtown Denver. At this time on the reflectivity image above, the storm displays a classic hook echo just south of the “Windsor” label. This is about the time the video was being shot by KUSA-TV in Denver that is circulating on all national news outlets this afternoon. One important note about this storm is that the core, or downdraft and significant rain and hail is located near I25, to the west of the updraft. Storms are not typically oriented in this fashion in Colorado, but the steering wind aloft was from SSE to NNW ahead of large area of low pressure approaching from the west.

In the image above, I’ve gone “inside” the storm to look at the velocity or wind flow field (enclosed by the black circle. Radar can only measure velocity along the radar beam, in effect only measuring if wind is moving toward or away from the radar and to what magnitude it is doing so. The data is from the Denver NEXRAD, located east of Denver at Front Range airport, on a line from the storm to just left of the bottom right corner of the screen and extended from there to the airport. This line is important because thats the axis we are able to measure windspeed on for this particular case. Note the green colors just to the southwest of the “Windsor” label, these colors indicate windflow TOWARD the radar site (this by the way is the rain cooled Rear Flank Downdraft), while just to the right of there, the red/magenta colors indicate flow AWAY from the radar (this is the area of inflow into the storm). These flows sitting right next to each other within the hook echo indicates via radar that a tornado may be present and obviously in this case we know it was.

Zooming in closer to the velocity data, I’m here measuring what we call gate-to-gate shear, or the velocity of adjacent data pixels. Notice that the green color is showing -30mph (or 30mph toward the radar) and the adjacent magenta pixel is measuring +74mph (or 74mph away from the radar). Add them up for a total shear of 105mph. At  this point the storm was approximately 55mph to the northeast of the radar and given the limitations to measure speed at this distance with a widening radar beam it is safe to assume that the windspeed within this tornado was greater than 105mph at this time.

It will be interesting to see what the Boulder NWS office determines this tornado to be on the Fujita scale and if there were multiple tornadoes that were produced by this storm, or if one tornado was on the ground from about Platteville through Milliken to Windsor. I’ll talk more about the Fujita (now the Enhanced Fujita scale) scale and how its used to determine tornado strength later tonight if I have time…or maybe tomorrow. Please feel free to post your comments or well wishes to the folks affected by this storm.

Looks like we’re in for a classic, high plains tornado outbreak today so I wanted to dive into a little storm chasing discussion.  Figuring out where to go is complicated, both scientifically and logistically.  I’ll start with the Meteorology which is pretty textbook on a day like today.  The big storm to our west is dropping pressures along the Front Range which increases the upslope flow region wide.  This easterly flow drags low level moisture courtesy of the Gulf of Mexico northwestward.  The same system provides the wind shear and instability and supercell thunderstorms will be the result.  Storm chasers tend to use derived parameters when identifying a chase “target area”.  Fing the highest CAPEs (measure of instability) or the best HELICITY (wind shear) and go to those areas.  Problem is that the model grids aren’t really that accurate with placement of these things.  I think people often make this more difficult than it needs to be so I like to forecast based on storm “features” instead.  The maps below show the overall Meteorology of the day with the frontal features identified.  The blue spikes show a cold front, the red “scallops” the warm front and the green “scallops” the dryline.  A stationary front is shown where the blue “spikes” and red “scallops” alternate over northern Colorado…

Low level moisture will work westward behind the stationary front over northeastern Colorado, and while the overall moisture and heat combination at the surface will be less than for areas farther southeast, this area will also be underneath some colder air overhead which will ensure that there will be plenty of instability to work with.  Because this area is closer to the upper level system it also is the area where the first severe thunderstorms of the day will occur, maybe as early as lunchtime…

Surface moisture levels and warmth will be significantly better over western Kansas, and this is necessary as this area is farther from the upper level storm and so the air overhead is warmer too.  Instability ends up very strong over this area but the warm air overhead (CAP) will supress the development of thunderstorms until about mid-afternoon.  Storms will develop along the warm front and the dryline…

Hot humid air will extend southward into extreme southern Kansas, western Oklahoma and the Texas Panhandle where the warmest air overhead will keep thunderstorms at bay until late afternoon or early evening.  Surface instability may end up greatest here where thunderstorms may get a late start but will explode when they do finally manage to break the CAP…

So the question becomes where to go if you’re interested in seeing tornadoes?  Here is a map highlighting the 3 areas where I think tornadoes wil occur this afternoon and evening, keep in mind the storms and tornadoes will be of different character in each area…

So what to do?  Well, if logistics weren’t an option I think the best storm and tornado of the day will probably end up over the southern most area where the right combination of surface moisture and wind shear look to be.  Storms in this area will be very isolated, which isn’t bad mind you, but there is a risk that a storm here will struggle until light is waning.  Storms may be very widely spaced and it’s hard to know exactly where to set up shop, as fast as they may develop when they do get going it may be tough to recover if you’re not in the right spot when they start.  You’d want to set up where a localized area of a more easterly wind/and or enhanced surface moisture develops.  Another nice thing about this area is that storms will move a little slower than those farther northwest, makes them easier to chase.  At $4 a gallon I wouldn’t be driving that far when I could stay closer to home but isolated, long track tornadoes may occur here…

Western Kansas seems to be a slam dunk.  Storms that develop on the warm front will move northward quickly and become “elevated” over the cool air at the surface, these aren’t likely going to be the tornado makers but will produce a lot of hail.  Storms that develop south of the warm front, within say 100 miles along the dryline, will move northward until they encounter it.  Any mature storm that encounters the warm front will likely change direction and start moving more easterly, these storms will produce tornadoes today that may be significant, but maybe shorter lived then any farther south.  There will be many storms in this area which makes for tough chasing too as storms interact, there is some luck to this you know.  All things considered, this looks like the best and safest place to be today, expect a mob of other chasers in this area today.  I’d likely set up near the “triple point” where the surface low develops over the intersection of the warm front/dryline/stationary front in west/central Kansas, probably south of I-70…

Northeastern Colorado is closer to home and isolated tornadoes are a decent possibility, although tornadoes in this area would likely be short lived and weaker than those in the other areas.  Drawbacks here are storms developing early over the mountains possibly producing pools of cold air that may slide under the stronger storms farther east and interrupt surface based circulations, and cold air to our north may also undercut our storms which is not favorable for tornadoes.  On the plus side these storms will fire early and could give you a nice show by early to mid afternoon allowing for an early return and a restful night’s sleep, it’d also be easier on the pocket book and you’ll have a lot less human company with storms here.  I’d position myself along the stationary front away from the junk that will develop over the mountains and on the leading edge of where the deeper surface moisture looks to be rolling in from the east.  Storms here will move northerly in a hurry and be tough to follow initially, but as they mature you’d expect them to slow a little bit…

With gas where it is I think I’d forego the long drive for the beastly storms down south, avoid the mob of people in Kansas and stay closer to home.  If I were chasing I’d probably head out I-70 somewhere and wait to see how far west that better moisture gets.  Tomorrow I’ll post storm reports to see how my areas worked out.  Thanks for reading the book and try not to blow away today!

Marty V