4/18/04 NOW NE/IA/SD/MN/KS

[Jeff Snyder]

There are a couple of very nice supercells between the Twin Cities and Duluth... I have captured the radar image from the storm in Pine County, which has a massive hook. VILs with this thing are >70! I'm hearing reports of a rotating wall cloud, but no confirmed tornado yet.

That storm and the one to it's southwest are riding along (or very close to) the warm front. SPC mesoanalysis shows that instabilities in WI are considerably weaker than in MN, but this is changing as warm advection continues on south-southwest surface winds. Low-level shear via helicity is maximized invof that front, so both of these things may be able to do something....

 

[Robert Dewey]

Nice hook echoes, but I'm still having doubts about strong tornadoes (>F3) as LCLs and LFCs are still high...

http://www.yourimagehost.com/is.php?i=4125...F1_MPX_2153.GIF

Best looking storms (DLH): N0R: http://www.yourimagehost.com/is.php?i=4129...F1_DLH_2201.GIF
N0S: http://www.yourimagehost.com/is.php?i=4128...0S_DLH_2201.GIF

FSD:
http://www.yourimagehost.com/is.php?i=4126...F1_FSD_2154.GIF

Robert

 

[Robert Dewey]

Just saved a RUC II 21Z Initialization sounding for the supercells south of DLH...LCL and LFC in that area are good for tornadoes...Generally want <1000m (~3000FT AGL)...And they are at ~2500FT AGL...LFC is also low, and CAPE values are near 2500J/KG, 0-3KM helicity are at 346M2/S2...So given that, I would expect strong rotation to continue...With the potential for a decent sized tornado, for at least the next hour or so.

Sounding 25M South of DLH... http://www.yourimagehost.com/is.php?i=4130...g=sounding.jpeg

EDIT: Actually, after looking at the rest of the 21Z forecast data from RUC...Thinking the supercells may weaken somewhat as they run into weaker instability (as Jeff noted)...Instability is advecting into the region ahead of the cells, but I do not think it is advecting in fast enough...We shall see

Robert

 

[Robert Dewey]

Yet another update... Saved a Storm Relative N0S image - http://www.yourimagehost.com/is.php?i=4133...0S_DLH_2231.GIF ... Check out the rotation on the northern most supercell! Nearly 100knts of gate to gate shear in the lowest scan! I would be willing to bet this thing is producing a pretty good tornado attm...

Robert

 

[Jeff Snyder]

Beat me to it! I just saved that image and was going to post it... Very intense shear with that supercell... I can't seem to get anyhing from the MPX radar, though... I can't get a new image since 2213z, over half an hour ago... Looks like the line has really filled in from west of MSP all the way down to Nebraska.... Cells in that line are currently in very poor low-level shear per SPC mesoanalysis... only about 50m2/s2 0-3k currently, and 2000m LCLs. Meanwhile, that big northwestern WI cell is supposedly in a high-shear, very low instability (surface-based, though decent elevated), and high CIN environment (CIN up over 120 j/kg).

LOL... Just looked at surface obs... upper 40 tds near ahead of that line in southern MN. Nothing beats 40 degree dewpoint depressions for tornado potential...

 

[Matthew Hatton]

here in the UK we are involved in the usual armchair chase this evening. Have to say those two supercells do look mightly impressive on radar now. That said, north of Rice Lake they look like they could be merging somewhat.

 

[Robert Dewey]

Originally posted by Jeff Snyder
Beat me to it! I just saved that image and was going to post it... Very intense shear with that supercell... I can't seem to get anyhing from the MPX radar, though... I can't get a new image since 2213z, over half an hour ago... Looks like the line has really filled in from west of MSP all the way down to Nebraska....

Yeah, I just checked out the latest (2241Z) from DLH...And it looks like shear is weakening a bit with the supercell...Outbound velocity has weakened to ~10-20knts, while inbound velocity is still around 40knts (I believe I still see a pixel of 50knts). This is probably because the air is more stable, and the cell is losing some of its organization as a result...Its still very intense though.

I also tried MPX...The last data I got was also 2213z...What a time to go down...

Just checked the latest 2251z from DLH, and it looks like rotation is very disorganized now...Guess the weak SFC based instability is really taking its toll...

EDIT... Added a 22Z SFC plot/satellite image with CAPE values from the 21Z RUC... http://www.yourimagehost.com/is.php?i=4137...fc_vis_garp.gif

Aslo, still no MPX data...I

Robert

 

[Jeff Snyder]

Solid line starting to fill in indeed. Had to resort to local TV station radars to get a closer view. Just saw a tornado warning for Brown and Redwood county (southwestern and southcentral MN), saying "A large tornado is on the ground!"...

Still can't believe a signficant tornado threat with such veered surface flow and very high dewpoint depressions and associated LCLs...

The associated Severe weather statement for the large tornado: http://kamala.cod.edu/mn/latest.wwus53.KMPX.html

 

[Dan Cook]

New PDS watch for parts of MN, WI, and UP of MI.

 

[Robert Dewey]

Well, after an hour away from any weather data...I have come back to take another look. I see that SPC has issued another PDS tornado watch...But it appears as those MOST of the storms have formed into a squall line, but it would be possible for a spin-up or two.

Looking at the 23Z RUC...The biggest area of concern right now is west of MQT with the northern most cell (usually its the southern most cell that produces supercell characteristics)...Current hours RUC forecast soundings indicate between 1000-1500J/KG of CAPE and 0-3KM helicity of close to 700m2/s2 is available right along the this storm. LCL is around 1000m...and LFC isn't too bad...And after looking at MQT reflectivity and storm relative velocity (as of 0057Z)...There appears to be decent low level rotation with a storm on the MI/WI border (though not nearly as good as the supercell earlier). This cell will likely weaken, as it encounters the stable air in MI.

RUC also shows decreasing instability the next 3-4 hours over IA and WI, likely due to loss of diurnal heating...So expect the squall line activity to weaken over the next several hours from north to south...

EDIT: Speak of the devil...Now that I posted this, I took another look at the MQT storm relative image, and most of the low level rotation has weakened...

Robert

 

[Kevin Statler2]

Spotters reported a tornado on the ground with that supercell in NW IA.