10/4/05 FCST: Mississippi Valley

[Dan Cook]

Probably just grasping at straws as I want storms badly(lol), but I took notice to the GFS around the 120 hour area. GFS is showing dewpoints finally raising to respectible levels, around 60-65, with a strong cold front coming in. Winds don't look to be too bad in the Iowa area, so maybe things'll be busy.

While both DVN and ILX mention storms for Wed, it's too early to see if they'll be severe.

Thoughts?

 

[nickgrillo]

It could get interesting on the 4th (you said the 5th, but the timeframe you are talking is the 4th) with this cold front activity.

Strong 30-40kt southwesterly 850mb-near surface flow will advect in deep moisture - with Td's in the 60-70F degree range making it as far north as MI/WI. There should be a lot of convection forming BEHIND the cold front (which wouldn't be much of a concern) although any activity which can develop in the warm sector could access this deep/rich moisture and instability, and could be capable of severe weather. Now, winds aloft are not particularly strong - although this can certainlty change since it's +100hrs out LOL. It's WAY to early to tell yet...

 

[Robert Dewey]

I sure hope it changes back to the 06Z GFS... I'm not liking the 12Z one bit. It's strongly positively tilted, wind fields are cut way down, and height falls are considerably less... I'm thinking we are gonna need a pretty dynamic system to get any significant severe weather...

 

[nickgrillo]

This could very well be a big event... Things are looking a bit more negatively-tilted (NW-SE) and a bit better then the 12z GFS run. Overall, the system could use some more improving - but it's still a whopping 4 days out. The latest 0z 84hr NAM advects in >500 SBCAPE at 12z all the way into northern WI/MI!

NCEP is already picking up on it (not sure why they wouldn't include points further north/east, though):

http://www.cpc.ncep.noaa.gov/products/pred...ats/threats.gif

EDIT: 0z NAM forecast sounding from EAU shows near 2000J/KG of CAPE and 200 SRH with temperatures in the low 60s... At 12z TUE, wow!

 

[Robert Dewey]

I'm really liking the 12Z NAM... The 12Z GFS is also trending further south with the wave, which is promising.

I would really like to see a closed off 500MB shortwave, like the NAM is showing. I just hope that if it is a closed system, it doesn't wind up and shear out to the west of the MS Valley/Great Lakes (which they tend to do). There is lots of energy, thus lots of potential for a big outbreak... This system looks like a season buster - in other words, it will likely be the last system we see around these parts for quite some time given the strength of the cold front and advection of dry air all the way down to the Gulf... Let's hope it's a good one!

 

[Scott Olson]

Keeping in mind that it's still 84hrs out, it seems like a pretty favorable synoptic scale for severe weather in MI/MN/WI. Cape seems reasonable (1500-1750 SBCAPE J/KG) and the winds are nice (30-50kts in NW WI @850hpa and 40-50kts @700hpa with a pretty strong southerly component to the winds and a deepening low pressure system. Im not going for the more closed system just yet as both the GFS & ECMWF still show the trough a little slower with not so much neg tilt. The GFS solution is pretty sore on instability but if it goes in the NAM's favor we can expect good differential advection.

 

[nickgrillo]

There is certainly a broad spectrum of good-looking factors with this system... Like Rob mentioned, this is probably the last event we'll see in here in quite some time (possibley the last one of the year).

A strong shortwave will move into the longwave trough - pushing the cold front east/southeast into MN and s/w in NE. Across the region, significiant SFC moisture will advect ahead of the frontal boundary (Td's in the 65-75F range). Strong speed/directional shear through the 0-6KM layer will be accompanied by widespread modest SBCAPE of 1000-2000 along/near the front. Given the relatively-forced nature of this system, I'd think we'd have a pretty good squall line developing. It's still +3 days out, so we'll see...

 

[Robert Dewey]

Still a ways out, so I'm not gonna get in depth since things will likely change...

Overall, I am liking the trend in the models though (over the past few days) - they have the SFC much further south than what was originally thought (it was supposed to be way up in Canada), the 850MB jet and thermal structure is much more organized, and the 500MB wave appears to take on a neutral to negative tilt near the Great Lakes. I am also really liking the 60-90KNT jet at 500MB the models are bringing in, leaving me to believe we will have quite a dynamic system. It also appears that there will be quite a bit of low level moisture around, and with very strong forcing likely along the front, thunderstorms should kick up quite easily...

Overall, I agree with Nick that storm type would be a squall line... Looks almost like a late season hybrid derecho setup.

 

[nickgrillo]

The latest NAM is weaker with wind fields... In addition, it has positioned the cold front more south/west then previously. It appears SPC has only gone with a slim 5% risk, and as they noted, they are waiting for the timing of the wave to become more clear. There is plentiful thermodynamics in place, it's just too bad the winds aloft weren't stronger. The best shear is still behind the front...

My attention is shifting towards lower MI on WED, as the front pushes east... Winds aren't super strong in the upper-levels, but there is enough shear/instability to produce some severe weather...

 

[nickgrillo]

It's too bad tomorrow didn't have better mid-upper flow ahead of the front. Otherwise, it could have a been a significant event (surging fall cold front with rich boundary layer dewpoints.) There is, however, a nice little setup in se Nebraska... Which is where low-level winds have a southeasterly component along the boundary... Which is right invof of a surface-low at 18z. Not much of a cap in place... So, deep/moist convection should develop off the relatively unstable/sheared airmass - and will be capable of isolated tornadoes/damaging winds.

 

[Mike Hollingshead]

The GFS brings in some hope for tomorrow. I think there is ample shear for supercells according to the 12z GFS run. What it does with the sfc is really interesting. It'll be interesting to see if the 18z eta picks up on this. I've seen this many times where the gfs has the better handle and on the following meso-eta it is picked up. I just really like the looks of that sub 1000mb low out there, lol. I didn't know they still made those around here. Prelim target of Bartlett NE. It would be fun to be part of another Oct 9th, but be able to chase it earlier than a 5:30 departure.

 

[nickgrillo]

Originally posted by Mike Hollingshead
The GFS brings in some hope for tomorrow. I think there is ample shear for supercells according to the 12z GFS run. What it does with the sfc is really interesting. It'll be interesting to see if the 18z eta picks up on this. I've seen this many times where the gfs has the better handle and on the following meso-eta it is picked up. I just really like the looks of that sub 1000mb low out there, lol. I didn't know they still made those around here. Prelim target of Bartlett NE. It would be fun to be part of another Oct 9th, but be able to chase it earlier than a 5:30 departure.

Yeah, it most certainly does...

Tomorrow can certainly be a player, with soundings from the area showing 2000-3000 CAPE and strong >300 SRH in the 18z-21z timeframe. Much of the area near the surface low will be expierencing backed low-level flow, and southwesterly at 40kts at 850mb, which will increase deep-layer/low-level shear sufficiant for supercells. Overall, the synoptic setup of tomorrow could have really set off a biggie, we just needed stronger mid-upper level flow and stronger instability/more intense height falls to set off a powerful squall line (aka October 24, 2001, but a bit more to the west). There is still plenty of deep moisture, instability, theta-e and height falls in the warm sector (IA/MN/WI) to setoff some squalls, but nothing too terribly organized.

Bartlett-Columbus-Norfolk (near the SFC low) area would be my choice for tomorrow, at the moment...

 

[nickgrillo]

A very buoyant trophosphere in the warm sector, ahead of a decelerating cold front, along with strong low-level wind fields/height falls should initiate deep convection across WI/NE/MN tomorrow. I'd have to put my focus on the area near the SFC low in eastern Nebraska...

The latest 0z NAM forecast sounding for OLU at 21z Tues. shows a parcel of 83/67F, which yields strong instability of 2543 j/kg (along with steep 7 C/KM lapse rates), Strong surface heating with temperatures in the mid-80s should help break the realtively weak cap across eastern Nebraska, with deep moist convection likely initiating ahead of the frontal boundary shortly after 21z Tues. Hodographs appear favorable for supercells... 0-6km shear is 40kts on the OLU sounding, with 200 0-3km SRH - given the relatively-backed flow evident at the surface layer (and veering to the southwest at 40kts at 850mb).

If I was chasing tomorrow, I'd probably go to the OLU area for start (along the boundary),

 

[Brett Adair]

Originally posted by nickgrillo
A very buoyant trophosphere in the warm sector, ahead of a decelerating cold front, along with strong low-level wind fields/height falls should initiate deep convection across WI/NE/MN tomorrow. I'd have to put my focus on the area near the SFC low in eastern Nebraska...

The latest 0z NAM forecast sounding for OLU at 21z Tues. shows a parcel of 83/67F, which yields strong instability of 2543 j/kg (along with steep 7 C/KM lapse rates), Strong surface heating with temperatures in the mid-80s should help break the realtively weak cap across eastern Nebraska, with deep moist convection likely initiating ahead of the frontal boundary shortly after 21z Tues. Hodographs appear favorable for supercells... 0-6km shear is 40kts on the OLU sounding, with 200 0-3km SRH - given the relatively-backed flow evident at the surface layer (and veering to the southwest at 40kts at 850mb).

If I was chasing tomorrow, I'd probably go to the OLU area for start (along the boundary),

I agree completely. The SFC frontal axis will be the feature of the event I have a feeling. Nice dry slot in the mid levels. I say convection fires post 22-23z in this region given that the cap may hold for a little longer. Plenty of positive bouyancy in the air.