8/19/10 FCST: SD, NE, IA, MN

[Josh VandenTop]

Be gentle, this is the first fcst thread that I have started so here it goes. I've been watching this set up for a couple of days now and it looks like things could really light up Thursday afternoon. Looking at today's 12z NAM runs its showing decent instability and sheer all coming together over SE SD. I think that once the cap breaks there could be some pretty good storms going. I was kind of curious if anyone else had any thoughts on this day.

 

[Dean Baron]

Things do look good along the SD/NE border. The biggest concern to me is the H7 temps which are progged to be in the 12-14C range across the warm sector. It's gonna take some strong forcing to get past that cap.

EDIT: Looking at the WRF it shows a sizable bulge on the northern edge of that front (dryline?) on the SD/NE border. If this verifies, this is where I would want to be as it will give us some extra forcing. One other thing to note is that the models have not been very good at forecasting the cap up here in the northern plains this season. I have been on quite a few chases this year where models showed the cap holding through the evening only to actually have storms go up in mid-afternoon. Whether or not that will be the case on Thurs. I dont know, but it's worth noting. If things can go with the current progged parameters, it could be a very interesting day.

 

[Skip Talbot]

once the cap breaks

The NAM is trending towards a stronger cap with huge amounts of CINH. Its so strong on this morning's run that I don't think it will break at all:

http://www.twisterdata.com/data/mod.../17/12/NAM_221_2010081712_F60_CIN_SURFACE.png

Last night wasn't so bad though with things opening up in the evening, and yesterday morning's run looked more favorable still. If the strengthening cap trend continues I don't think will have much of a shot at a chase. On the other hand, the models are nowhere near as accurate pinning things down like inhibition as they are the upper level pattern. You don't want to turn your back on the kind of shear and instability combination the NAM is depicting. If the NAM is wrong about the inhibition, and surface based storms do go up, monster supercells and tornadoes are possible.

 

[jamesmseitz]

Rob Hurkes and I have been looking at this as well. The WARM(HOT)temps around the 850 MB have us more then worried about this one. The 0-1KM helicity/EHI are great and cape seems to be in place If the cap does give way at all this would be a great late season chase..... we will be watching very closely

 

[Jeff Duda]

Rob Hurkes and I have been looking at this as well. The WARM(HOT)

Yeah...wha...?

If by "WARM(HOT)" you mean the cap, then yes, ugh...I don't think I've seen CIN values as great in magnitude in any other setups this year as I do for this one. No way any surface based convection will fire with CIN < -100 J/kg, and that is a conservative value. Also, the GFS doesn't put much of anything into this setup. However, the SREF is much friendlier with the cap, putting the nose of the 12C isotherm at 700 mb at or just south of Sioux Falls at 00Z. It is a little less friendly on the shear, however, but it does put out precip all up and down the line in SD and NE.

 

[Jeff Duda]

The 18Z NAM offers little improvement in the capping situation. After checking a large number of forecast soundings in SE SD, NW IA, and much of N/E NE, convective temperatures are in the low to mid 100sF across the board, and surface temps struggle to exceed 30C (86F). Thus the very high CIN continues to show. Also, the lid strength index never goes below 1.0 through 00Z, and only in very spotty areas goes < 0 in SE SD after 00Z. It seems that CIN decreases with southern extent, but so does the shear. In fact, to get into areas of only modest CIN, one would have to go clear into C NE, but by that latitude, shear is pretty terrible. The 18Z GFS has changed little also, only shortening the amplitude of the shortwave trough slightly and making it more compact (now only one lobe of vorticity).

 

[Jeremy Den Hartog]

Call it a hunch but I *think* the cap will weaken and be breakable in time somewhere roughly from Brookings, SD to Montevideo, MN - maybe some 6PM (or slightly later) magic is in store? The shear in this area looks good as well (at least on the NAM...not so much on the GFS though). I know trying to forecast a cap this far out is next to impossible but it would be logical to think the further north you are the better the chances of the air cooling and the cap weakening/breaking - at least it seems its trying to do this in a relatively narrow area along or near the warm front. Like I said, this is all based on a hunch and I could be completely wrong but it seems to me the models are at least trying to hint at something like this happening? Hmm...something to keep an eye on anyway...

 

[James Hammett]

0Z NAM continues to show nice hodos in SE SD now with slightly cooler H7 temps. The big change is a weaker cap before 0Z with a larger area of <2 LSI and <50 CIN in SE SD as well. Still wishcasting on the cap 48 hours out though.

 

[Jeremy Den Hartog]

Still liking an area roughly between Brookings, SD and Montevideo, MN. The 12Z NAM continues to show an area of breakable CIN and good shear in this area. It also breaks out some precip between 0Z and 03Z friday. The latest WRF also breaks out some precip in the area around 0Z friday. Still wishcasting at this point but I'm not convinced this will be a bust. Will obviously know more come tomorrow morning though which is when I will decide if I'm going out or not...

 

[Skip Talbot]

Both the GFS and NAM are still showing a stout cap. There is a thin little hole at 0z on the NAM, but I don't think that window will be open long enough to get a mature, surface based supercell if that's the case. SPC must be seeing something good on the ECMWF or SREF because they are pretty bullish on the wording and probs. If the NAM and GFS are accurate, however, I don't think we'll have much of a shot at tornadoes. The NAM is showing a lot of precip firing by early afternoon, but that's got to be elevated given the 100-300 CINH its showing. Great shear and instability combo, however, so this is definitely one to keep watching.

 

[Jeremy Den Hartog]

Both the GFS and NAM are still showing a stout cap. There is a thin little hole at 0z on the NAM, but I don't think that window will be open long enough to get a mature, surface based supercell if that's the case. SPC must be seeing something good on the ECMWF or SREF because they are pretty bullish on the wording and probs. If the NAM and GFS are accurate, however, I don't think we'll have much of a shot at tornadoes. The NAM is showing a lot of precip firing by early afternoon, but that's got to be elevated given the 100-300 CINH its showing. Great shear and instability combo, however, so this is definitely one to keep watching.

Your probably right Skip ..glad to see the latest NAM is looking more promising though

 

[Jeff Hawkins]

Please be "nice" as this is my first comment in a forecast thread.
From my observations of the Yankton to Vermillion SD area...
The 18Z+27 (~4pm 8/19/10) NAM gives nice 3000+ J/kg,
0-1km EHI >3.0,
0-3 km EHI >7.0,
MLLCL height <= 1000
Surface-LCL 250-750
Surface-1km Helicity >=125
Surface-3km Helicity >=350

BUT!!!!!

7H temps around 14 C
CIN (Mean 0-90mb AGL) around -300

Looking at the 18Z+30 (~7pm CDT)
the CAP looks much weaker around -30
7H temps around 12-14 C
0-1km helicity is around 125
0-3km helicity is around 225
850 LLJ is a bit weak around 30knots a bit further south in extreme NE Nebraska

Hodographs at 4pm looked slightly backed in this region, but 7pm are not.

These later dynamics suggest to me that initiation will be later PM perhaps after 5pm, and if there's any TOR's they'll be early on.

 

[James Hammett]

It seems SPC is picking up on the NAM consistently showing a SW-NE corridor of weakened CIN and except for a couple of earlier runs shows it somewhat juxtaposed with the area of interest (last two runs have swung back to "less CIN" again...a good sign).

However H7 temps are looking awful (awful-er) with the 0Z run - areas in E SD showing 15+ at 21Z and still hotter than previous runs at 0Z. 21Z SREF shows even fewer members breaking out precip than prior runs. Keeping my expectations low at this point.

 

[Brandon Sullivan]

I had pondered making the trip up there, but I don't think the 00z run is good enough to get me out the door..

First thing I dont like.. Hodographs arent as large, owing to the meager surface winds. Definitely would like more support @ H5... But the area will be on the southern periphery of the stronger flow aloft.

Looks like we are also dealing with some higher LCL heights too, ran through a quick Dprog/Dt and definitely looks like out of a few runs, the LCL's were higher.

Next, everyone is talking about H7 temps.. and they should be.. they are warm.. There is no HUGE forcing mechanism, but I do think that should storms fire, they will likely fire in proximity of the low before dark.... I wouldn't be surprised at a few supercells and a tornado report or 2 at first, but it should turn into MCS city quickly..

If I were going to go north, I would likely find myself around the Huron, SD area by 22z.

 

[Jeff Duda]

I agree with James and Brandon. Although the 12Z NAM today reduced the capping and CIN, it brought it back with tonight's 00Z run. Again I'm seeing consistent convective temperatures in the low to mid 100s F in the area where decent shear exists. Also I noticed that the 00Z NAM moved the better 500 mb flow farther west, thus reducing deep-layer shear. Although some of the 0-1 and 0-3 km SRH values look good in SE SD, the 0-6 km look pretty crappy, although effective shear looks a little better. However, I think the degree of stability in the 850 - 700 mb layer will probably be enough to preclude much of any daytime surface based convection. It's just too warm and surface temps won't make it high enough.

The hi-res WRFs (ARW and NMM) support this analysis. The 12Z hi-res WRF-NMM has no development in SD until 02Z-03Z, and it is linear/MCSish. It does have development in NW KS/SW NE, but that is in a region of very poor shear. The 12Z hi-res WRF-ARW has absolutely no development of any significance in either SD or NE through 00Z and only has weak showers in NC NE/SC SD by 01Z.