Mile-wide tornado smashes Windsor, Colorado; plus, hurricane season commentary

By: Dr. Jeff Masters , 1:23 PM GMT on May 23, 2008

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A mile-wide tornado swept through Colorado between 11am and noon yesterday, ripping the roofs off buildings, tossing cars into the air, and killing at least one person in Weld County, northeast of Denver. Hail up to 2.75" in diameter accompanied the storm, which took an unusual north-northwesterly track , parallel to the Rocky Mountains north of Denver. Hardest hit was the town of Windsor, between Fort Collins and Greely, where damage appeared to be at least EF3. A local TV station took some impressive live video of the tornado, and a wunderground web cam video in Windsor, Colorado (Figure 1) caught the funnel as it passed east of the camera.


Figure 1.Webcam view looking east at 11:45am MDT in Windsor, Colorado as the tornado passed by. Note the golf ball-sized hail covering the ground. Image credit: windsorweather.com.

The Weather Underground's tornado expert, Rob Carver, had this explanation of yesterday's tornadoes:

Two low pressure systems over the Western U.S. were the cause of the severe weather outbreak of May 22. A strong upper-level low over the Great Basin formed a surface low in the lee of the Rockies, and brought a strong southerly mid-level jet over the Plains of Colorado and Kansas. As the surface low formed, it brought warm moist air northwards, forming a warm front. This warm air moved westward, rising with the terrain, causing thunderstorms to form. Once these storms formed, the juxtaposition of easterly flow at the surface with southerly flow aloft (i.e., wind shear) produced significant spin in the horizontal direction, which was tilted by the storms to vertical spin, which triggered the formation of tornadic thunderstorms. This movement of the warm front up the slope of the Rockies to help trigger tornadic thunderstorms is a rare occurrence.



Figure 2. Radar reflectivity image (top) of the Windsor, Colorado tornado at 11:45am MDT May 22, 2008. Note the classic hook-shaped echo associated with the tornado. 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. For those interested, we've saved an animation of the reflectivity and Doppler velocity.

Severe weather forecast
Severe weather is expected today over Kansas, Nebraska, and surrounding states. NOAA's Storm Prediction Center has placed portions of this area 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, which SPC later upgraded to "High Risk" once the tornadoes started pounding Colorado. More severe weather is expected Saturday and Sunday over the Midwest as the upper-level low pressure system responsible moves slowly eastward. The Weather Underground Severe Weather page and Tornado page and WebCam 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, who was in Kansas yesterday, and has posted many spectacular photos of yesterday's storms. According to Mike's blog:

Today Cloud 9 Tours saw 4 tornadoes and ONE developing almost overhead. We experienced winds over 100mph from the circulation of the meso that passed overhead as a cone tornado developed. Stay Tuned....


Possible development in the Eastern Pacific late next week
The past four days, the ECMWF model has been predicting the formation of a tropical storm in the the Eastern Pacific, just off the coast of Guatemala, around May 29. The GFS model has also been predicting something might develop, but in the Western Caribbean near Mexico's Yucatan Peninsula. The GFS has been rather inconsistent with its handling of this potential storm, and I am inclined to discount its forecast--especially since last night's long range runs of the NOGAPS, Canadian, and UKMET models all show development in the Eastern Pacific, not the Caribbean. All five models predict a northward shift in the jet stream and substantial relaxation in wind shear over the Eastern Pacific and Western Caribbean next week. It is common to see May tropical storms in the Eastern Pacific, and it would not be a surprise to see something develop there. I'd put the odds of something popping up in the Western Caribbean or Gulf of Mexico at less than 10%, though.

NOAA's seasonal hurricane forecast
NOAA issued its annual seasonal hurricane forecast yesterday, which I discussed in detail in yesterday's blog entry. Yesterday's forecast came out with a little more uncertainty attached to it than previous forecasts, which is a good thing. The media attention and fanfare that accompanies these forecasts is rather excessive, given the low skill they have. In fact, we don't even know if the NOAA forecasts have ANY mathematical skill, because they've never released a verification study of their forecasts. I doubt that the skill is very high--as Eric Berger of the Houston Chronicle pointed out in a blog yesterday, NOAA has blown its forecast of Accumulated Cyclone Energy (ACE) each of the last six years. NOAA held its usual press conference to announce the forecast; this year, it was held at the home of the NOAA Hurricane Hunter aircraft, MacDill AFB in Tampa. The NOAA dignitaries present said all the right things, preaching the need for preparedness regardless of the forecast for the upcoming hurricane season. Still, I wonder if NOAA might be hurting themselves by making such a public spectacle over the release of a forecast that no one knows the accuracy of, and has performed poorly by some measures in recent years. I do like the fact they are issuing public hurricane forecasts, as I expect their accuracy and value will improve in coming years, but they're definitely not worth the attention they're getting at present.

Jeff Masters

Tornado1 (kd7tda)
One of several tornados. This one was near Grainfield, KS
Tornado1
Tornado Damage 3 (jlg)
Damage from today's Tornado in Windsor, Colorado
Tornado Damage 3
()
Mean Looking Meso with Cone Tornado (MikeTheiss)
Photo os a wallcloud with a developing tornado passing just to the north of our location on May 22nd, 2008. Photo copyright Mike Theiss
Mean Looking Meso with Cone Tornado

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957. nash28
3:48 PM GMT on May 24, 2008
12z is at 48hrs now...
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956. nash28
3:48 PM GMT on May 24, 2008
Yep JP. Already a STW until 9pm for just about everyone EXCEPT Tampa Bay.

Figures.
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955. hurricane23
11:48 AM EDT on May 24, 2008
Here is what the 06z was showing earlier for south florida.12z should be complete about about an hour or so.

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952. Weather456
11:41 AM AST on May 24, 2008
I think we have to look at it both sides...suppose the GFS is correct but instead of the WCARIB, its the EPAC. I think its goin to be a 50/50 chance. On one had we have 2 models, climatology and current conditions favor EPAC development, on the other hand, we have climatology and 2 models favor the WCARIB disturbance. Until the MJO shows its present we are stucked with a probability of 40/60 in favor of the EPAC. Then again, early development in the EPAC would benefit the WCARIB.
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950. nash28
3:42 PM GMT on May 24, 2008
A nice wet TD would be heavenly to Flordians.
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949. hurricane23
11:41 AM EDT on May 24, 2008
938. FLWeatherFreak91 11:36 AM EDT on May 24, 2008
But Adrian, the GFS has been consecutively predicting this storm for about a week now. Usually these 'fantasy storms' show up on one model run and none more. I don't expect the storm to be as strong as the GFS says now but I am willing to bet we will have a Barry-type system here in the next 8 days

Sure it is possibe in the next week or so as climo tracks would generaly bring what ever is down there towards florida.As of now iam expecting an increase in moisture into florida next week and will hold of on anything more then that until we get into mon-tues of next week and we see how things are shapeing up.





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948. nash28
3:40 PM GMT on May 24, 2008
So good in fact, Phil Collins invited it back to tour with them:-)

LOL!

Sorry Drak.... I initially when straight to the 06z 300hr mark. I see that now...
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947. nash28
3:39 PM GMT on May 24, 2008
The GFS was good with genesys last year...
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946. Drakoen
3:38 PM GMT on May 24, 2008
940. nash28 3:37 PM GMT on May 24, 2008
Ok... I see the GFS is back on the SW CB Low coming into or around Tampa area at 300 hrs...

I'll believe it when I see a Low form and have consistency.


What run are you looking at? The 06z showed the storm hitting SFL. Track isn't important right now.
Member Since: October 28, 2006 Posts: 57 Comments: 30490
945. TerraNova
11:39 AM EDT on May 24, 2008
12z GFS is running now.
Member Since: July 30, 2007 Posts: 76 Comments: 4063
944. atmoaggie
3:35 PM GMT on May 24, 2008
A geostrophic meteorological model is one in which the horizontal wind vector in the equations defining the model is replaced everywhere by its geostrophic approximation. In a quasigeostrophic model the horizontal wind vector in some instances is replaced by the geostrophic approximation but not in others. In the standard quasigeostrophic model it is also assumed that the Coriolis parameter f is constant except where its derivative %u2202f/%u2202y=%u03B2 is taken. (Justification 1) This is known as the beta-plane approximation.Link

910: Good one, Pat. A complete desciption. In the link, you guys can partly see there why Mets have to take more math than most of the Engineering students. Only the EEs take as much, at A&M anyway.

One class away from an accidental minor in it.
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943. StormJunkie
3:35 PM GMT on May 24, 2008
I hear ya 23

This is not 07 though, and I am much more interested in seeing how it does on the actual genesis as opposed to track and strength. I don't think anyone is giving it too much stock right now, more just feeling out how the verification of the GFS may or may not go this year.
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942. Drakoen
3:37 PM GMT on May 24, 2008
935. atmoaggie 3:34 PM GMT on May 24, 2008
Heck, if there is a system of any type in the general vicinity 300hrs from now, I would be very impressed with the GFS.

I believe it was Drak that correctly called it the GFS Kaleidescope.


Yea anything past 168 hours or so is part of the kaleidescope.
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941. nash28
3:38 PM GMT on May 24, 2008
Yeah Adrian. I think the Hornets gave them all they could handle and they are just gassed now.
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940. nash28
3:36 PM GMT on May 24, 2008
Ok... I see the GFS is back on the SW CB Low coming into or around Tampa area at 300 hrs...

I'll believe it when I see a Low form and have consistency.
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938. FLWeatherFreak91
11:34 AM EDT on May 24, 2008
But Adrian, the GFS has been consecutively predicting this storm for about a week now. Usually these 'fantasy storms' show up on one model run and none more. I don't expect the storm to be as strong as the GFS says now but I am willing to bet we will have a Barry-type system here in the next 8 days
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937. nash28
3:34 PM GMT on May 24, 2008
Morning Adrian! Haven't taken the time yet to look at the latest GFS or ECMWF runs. Will do that now.
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936. StormJunkie
3:34 PM GMT on May 24, 2008
Man nash, now I wanna go fry up some fish for some reason :~)
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935. atmoaggie
3:33 PM GMT on May 24, 2008
Heck, if there is a system of any type in the general vicinity 300hrs from now, I would be very impressed with the GFS.

I believe it was Drak that correctly called it the GFS Kaleidescope.
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934. hurricane23
11:33 AM EDT on May 24, 2008
If have of what the mighty GFS came true lol florida would cease to exist.It does this every season predicting its fantasy storms all over the place.In 2007 it had if i remember correctly what 3 cat 5's were suppose to make lanfall in south florida.

When i actually see something pop down there then i'll pay a little closer attention.With the various models showing development it should actually remind folks the hurricane season is just around the corner and if you live in a hurricane prone area you should use this time wisely and be ready to go come june1.

Adrian
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933. Ivansrvivr
3:22 PM GMT on May 24, 2008
The only reason I believe development in the Caribbean is remotely possible is because frontal passage over Florida today (apparently there will be a low along the front to push it southward) which should end up well to our south by tomorrow afternoon. That could be the mechanism that focuses development from the E-Pac into the N.W. Caribbean and any development would tend to move along the front as it eventually lifts back northward as a warm front next week. This is more like a scenario that would play out in October as will be our temps Mon and Tues. I really don't see this happening but I dont see a high of 80 here in S.Fl monday either so anything is possible.
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932. StormJunkie
3:31 PM GMT on May 24, 2008
925.

The irony would just be uncanny pat! I am not even sure how all would take that.

Y'all have fun !~)
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931. nash28
3:30 PM GMT on May 24, 2008
Morning everyone! Hot as a cast iron skillet here in Apollo Beach.
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930. StormJunkie
3:24 PM GMT on May 24, 2008
921.

That sounds like a Barry type storm, and although that is what may happen, that is not what the GFS has been showing the past few runs. I just don't think anyone can make a real assessment of what this system will look like 300hrs out. Heck, if there is a system of any type in the general vicinity 300hrs from now, I would be very impressed with the GFS.
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929. FLWeatherFreak91
11:30 AM EDT on May 24, 2008
Thanks Storm!
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927. atmoaggie
3:25 PM GMT on May 24, 2008
Iam still not buying the GFS caribbean solution as if right now.Until something starts to actually develope down there then its worth paying attention.Development in the eastern pacific seems a little more likely right now.As of right now the entire caribbean and gulf are covered in dry air

I hear ya. Way too much dry air, presently.

Looks like GFS wants to fix that, too. Look at the 850 mb RH...better than %70 for most of the Caribbean by Wednesday night.

Nowcast:


114 Hours (0 UTC Thursday)
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926. TerraNova
11:21 AM EDT on May 24, 2008
Heres another shot of the GFS 06z

Whatever it is; the GFS seems to think that it will have an eye-like feature or at least a asymetric wind field (possibly due to shear).

Also Drak, can you post a link to where you found that image? Thanks.
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925. Patrap
10:25 AM CDT on May 24, 2008
Im gonna be In Fla from Thurs till the 2nd.
Bad Karma maybe?
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924. Drakoen
3:24 PM GMT on May 24, 2008
921. hurricane23 3:24 PM GMT on May 24, 2008
In general increaseing moisture is expected to increase into next week over south florida.If anything were to develope next week what i see is a lopsided sheared system most likely ending up being a good rain maker for parts of florida which all in all would be great to see.


Whether or not its a sheared system will be dependent on if an upper level high is able to build over the system.
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923. Patrap
10:24 AM CDT on May 24, 2008
What will da nose of the Big H do with that one?

Stay Tuned.
Member Since: July 3, 2005 Posts: 424 Comments: 128273
922. Patrap
10:22 AM CDT on May 24, 2008
UNYSIS GFSx Day 10 3JUNE 0Z

500mb hght pres (mb)Link

Surface pres Link
Member Since: July 3, 2005 Posts: 424 Comments: 128273
921. hurricane23
11:20 AM EDT on May 24, 2008
In general increaseing moisture is expected to increase into next week over south florida.If anything were to develope next week what i see is a lopsided sheared system most likely ending up being a good rain maker for parts of florida which all in all would be great to see.
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920. StormJunkie
3:19 PM GMT on May 24, 2008
Love all those folks pat! Just wish we could have thrown some more dollars and resources at them over the past 20 years.
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919. BahaHurican
11:19 AM EDT on May 24, 2008
917.

LOL, Pat.
Member Since: October 25, 2005 Posts: 19 Comments: 22090
918. StormJunkie
3:17 PM GMT on May 24, 2008
Morning Adrian

Genesis is predicted to occur around the 100+hr time frame. First that gets us through the next couple of days. Next, as someone pointed out last night, a tropical wave is predicted to enter the area and interact with the Panama low. Thus increasing moisture and the potential for something to develop
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917. Patrap
10:18 AM CDT on May 24, 2008
Dem guys write those paragraphs Like a religion SJ,.one can hear the Cathedral walls..
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916. Drakoen
3:18 PM GMT on May 24, 2008
914. StormJunkie 3:17 PM GMT on May 24, 2008
Whoa, big words, SCARY :~)

Morning Drak, thanks for the help last night!



No problem.
Member Since: October 28, 2006 Posts: 57 Comments: 30490
915. BahaHurican
11:15 AM EDT on May 24, 2008
908. atmoaggie 11:12 AM EDT on May 24, 2008
i remember last year we were using a site that had the expected wind speed based on millibars

Actually, you just described just how wind speed is predicted by all of the models.


I think there was a link to some page that gave a conversion chart or something; I think this is what he was asking about.
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914. StormJunkie
3:14 PM GMT on May 24, 2008
Whoa, big words, SCARY :~)

Morning Drak, thanks for the help last night!

Pat T-36hrs and some change!
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913. Drakoen
3:16 PM GMT on May 24, 2008
Heres another shot of the GFS 06z
Photobucket
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912. hurricane23
11:15 AM EDT on May 24, 2008
With all that dry air in place the gulf/caribbean are closed for business atleast over the holiday weekend.
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911. Weather456
11:08 AM AST on May 24, 2008
Incase anytone missed it....Awesome shot of the tropical wave at peak organization around 1200 UTC May 23.



Also,

What the GFS has been predicting is the development of a low pressure area in the monsoon trough just south of Panama. This normally causes development in the EPAC, but occasionally the monsoon trough shifts northward into the SW Caribbean normally in the early part of the season (May/June) and the late part of the season (October/November) which causes development to occur there rather than in the EPAC. Any passing tropical waves normally enhances development. Now if one was to look at an infrared image...one would see the monsoon trough lies in the EPAC indicated by the amount of shower activity, but any northward shift could make a difference between the whether the CMC/UKMET or GFS/ECMWF is correct.

Past examples were Beta in 2005 and Mitch in 1998.
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910. Patrap
10:13 AM CDT on May 24, 2008
A geostrophic meteorological model is one in which the horizontal wind vector in the equations defining the model is replaced everywhere by its geostrophic approximation. In a quasigeostrophic model the horizontal wind vector in some instances is replaced by the geostrophic approximation but not in others. In the standard quasigeostrophic model it is also assumed that the Coriolis parameter f is constant except where its derivative ∂f/∂y=β is taken. (Justification 1) This is known as the beta-plane approximation.Link
Member Since: July 3, 2005 Posts: 424 Comments: 128273
909. Drakoen
3:11 PM GMT on May 24, 2008
I see that the ECMWF dropped the EPAC system and is developing a weak closed low in the Caribbean. With the current difference in the reliable computer forecast models neither area is more favored than the other.
Member Since: October 28, 2006 Posts: 57 Comments: 30490
908. atmoaggie
3:08 PM GMT on May 24, 2008
i remember last year we were using a site that had the expected wind speed based on millibars

Actually, you just described just how wind speed is predicted by all of the models. Wind is nothing more than a product of pressure gradient, coriolis, and little contribution by thermal gradient. Predict the rest and calculate the wind. Called quasigeostrophic.
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907. hurricane23
11:12 AM EDT on May 24, 2008
Iam still not buying the GFS caribbean solution as if right now.Until something starts to actually develope down there then its worth paying attention.Development in the eastern pacific seems a little more likely right now.As of right now the entire caribbean and gulf are covered in dry air.SEE HERE
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.