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

mrobinson

I'm starting this thread and left it for a broad range of states since I am heading out and wanted to be sure a topic was started today.
 
Hmm... Seems like an odd situation to have a PDS tornado watch... Areas south of the warm front (the only areas with surface-based instability) have markedly veered winds (southwest in some areas, resutling in only abismal 0-3k helicity) and 25-35 dewpoint depressions... That's make LCLs what, 1500-1700+m. That's very high for significant tornadoes. There is the potential for the mid-50 tds to mix out further. Hmm....

The winds will probably back east of the low in the next few hours, however. And with ample heating north of the warm front, there is a chance for some decent instability to develop along that front. I can't seem to find an 18z MPX sounding, which surprises me greatly.

I guess I'm looking at right along the warm front... There appears to be attempts at intiation out in west central MN as evidenced by towering cumulus per satellite imagery and weak radar returns...
 
Are you chasing jeff? Convection should develop soon huh...
 
Nope... Unfortunately, I'm down in Norman OK for school... LOL I don't think I've ever thought I'd say that in mid-April!

I think we have initiation now. Looks like the TCU near Wilmar and southwest of St. Cloud is beginning to take root and deepen. The last few radar scans have increasing returns, now into the 40s. The veered surface flow out here is very concernign as far as tornado potential is concerned, however. I'd be worried if I lived in St. Cloud, however...

EDIT: Right now, I'm far from impressed about the situation, at least for the next couple of hours. Helicity is weak in the warm sector, as there is very little directional shear. Instability is decent, but far from impressive. In addition, LCLs are still very high south of the warm front. This will likely change in a few hours, but for the time being, I'm unimpressed.
 
Perfect Forecast nail on the spot.

Math proves again very useful, in predicting Tornadoes 1 day in advance, instead of 1-3hrs. The Watch Box isued was issued exact a day before this event, and a Tornado Warning iwas issued by NWS smack in this watch area I forecasted...

http://www.powow.com/orbiteradds/severe.html
 
Re: Perfect Forecast nail on the spot.

Originally posted by Kevin Martin
Math proves again very useful, in predicting Tornadoes 1 day in advance, instead of 1-3hrs. The Watch Box isued was issued exact a day before this event, and a Tornado Warning iwas issued by NWS smack in this watch area I forecasted...

http://www.powow.com/orbiteradds/severe.html

I think most people for the past couple of days have been marking the warm front for good tornadic action today. It was just a question of where the front would set up by afternoon...
 
Atother one to be added to the list in Minnesota: Kanabec County.

* AT 415 PM CDT...RADAR INDICATED A TORNADO 4 MILES SOUTHWEST OF
GRASSTON...OR ABOUT 9 MILES SOUTHEAST OF MORA...MOVING EAST AT 20 MPH

LOCATIONS IMPACTED INCLUDE...
GRASSTON.

THERE WAS AN UNCOMFIRMED REPORT OF A TORNADO TOUCHDOWN WEST OF DALBO
IN NORTHWESTERN ISANTI COUNTY WITH THIS STORM.
 
Another one in Isanti County:

AT 427 PM CDT...RADAR INDICATED A DEVELOPING TORNADO 17 MILES WEST
OF CAMBRIDGE...OR ABOUT 4 MILES SOUTH OF PRINCETON...MOVING
NORTHEAST AT 20 MPH.

* LOCATIONS IMPACTED INCLUDE...
BRAHAM.
CAMBRIDGE.

GO TO THE BASEMENT OR A SMALL INTERIOR ROOM ON THE LOWEST FLOOR!
 
Re: Perfect Forecast nail on the spot.

Originally posted by Kevin Martin+--><div class='quotetop'>QUOTE(Kevin Martin)</div>
Math proves again very useful, in predicting Tornadoes 1 day in advance, instead of 1-3hrs. The Watch Box isued was issued exact a day before this event, and a Tornado Warning iwas issued by NWS smack in this watch area I forecasted...

http://www.powow.com/orbiteradds/severe.html[/b]

Forecasters usually make a general forecast 1-3 days in advance, and then as the event comes closer, they will adjust it as necessary. I think to issue a watch 1 day in advance would be setting up for a total forecast bust. I was just wondering, how exactly do you calculate your severe weather threat?

<!--QuoteBegin-Jeff Snyder

Nope... Unfortunately, I'm down in Norman OK for school... LOL I don't think I've ever thought I'd say that in mid-April!

I think we have initiation now. Looks like the TCU near Wilmar and southwest of St. Cloud is beginning to take root and deepen. The last few radar scans have increasing returns, now into the 40s. The veered surface flow out here is very concernign as far as tornado potential is concerned, however. I'd be worried if I lived in St. Cloud, however...

EDIT: Right now, I'm far from impressed about the situation, at least for the next couple of hours. Helicity is weak in the warm sector, as there is very little directional shear. Instability is decent, but far from impressive. In addition, LCLs are still very high south of the warm front. This will likely change in a few hours, but for the time being, I'm unimpressed.

Jeff...I agree with you very much on this one. Looking at RUC 1 and 2 hour forecast soundings...LCLs are 3-5K FT AGL! LFC are also very high...Close to 750MB...So it is very unlikely we will see significant tornadoes. The LCL/LFC may drop later this evening as the T falls toward the Td...But that may also weaken instability somewhat... Directional shear is decent...But I have yet to see helicity much over 250/300m2/s2 in the main threat region. Yesterday across IA actually had a better setup IMO - CAPE was around 2500J/KG with 0-3KM helicity of 550m2/s2!

NOTE: Here is my forecast graphics issued earlier (~12PM), I will wait until later tonight or tomorrow to see how well it verifies, if at all (the X is my "would be target"... http://www.yourimagehost.com/is.php?i=4119...img=outlook.gif


Robert
 
Re: Perfect Forecast nail on the spot.

Originally posted by Kevin Martin
Math proves again very useful, in predicting Tornadoes 1 day in advance, instead of 1-3hrs. The Watch Box isued was issued exact a day before this event, and a Tornado Warning iwas issued by NWS smack in this watch area I forecasted...

http://www.powow.com/orbiteradds/severe.html


Wow. You need to start letting us know the day before instead of waiting till it's underway. Might save a life someday.
 
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....
 
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 :eek:

Robert
 
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...
 
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.
 
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
 
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
 
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
 
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