9/19/05 FCST: GREAT LAKES

[Robert Dewey]

Looks like Monday could be an active day across the Great Lakes region.

A nice shortwave trough at 500mb swings into the region during the afternoon hours, pushing in a mid level jet of nearly 80KNTS. At the SFC, an area of deepening low pressure should be located around Lake Superior at 00Z TUE, with nicely backed SFC winds out ahead of the front. The main concern is instability - the NAM has a very narrow axis of AOA 2000J/KG, and a nice ridge of theta-e. Looking at 850-500MB RH fields between 12Z MON and 00Z TUE, it would seem as though cloud cover shouldn't be an instability-limiting factor - more than likely, the big factor will be how fast SFC td's can advect into the region before the front plows through.

With a decently dynamic system and plenty of shear (both speed and directional), if CAPE in excess of 2000J/KG is realized, there would probably exist a pretty good threat for severe thunderstorms.

 

[Kurt Hulst]

The timing seem to have been pushed ahead in the latest 12z run. Last nights run was slower. i havent looked at the GFS yet but it looked like it was pushing for a slower movement. By 00z iTues it looks like the PCP will be well to the west of western MI. I hope this thing slows down in the mean time. Though the axis of instibility is narrow it seems that it has given more insitbility of higher CAPE values in this run than the 00z run. The 18z time frame has a nice hodograph with backed to veering winds which i biasedly would like to see where they are during the 00z time period since i have to work all day monday.

 

[Kurt Hulst]

so far from what i have looked at through the UCAR 00z ETA runs the system as i suspected slower than this mornings runs. Now it depends on if the clouds will interfere with insolation in MI. But again this can all change back to a fast pace track in the 12z run sunday morning. but im begining to see that i might need to get out of work a little earlier if at all possible.

 

[Robert Dewey]

Yeah, it's looking pretty interesting...

My main concern is the SFC Td values on the NAM - are they too high? Well, only time will tell. Helicity is excellent, at nearly +400M2/S2 according to RAP/UCAR, and this is a fairly dynamic system... But mid and upper level winds (500MB and up) will be screaming at +65KNTS, so storms will be flying.

Storm mode is another question... Clearly, +400M2/S2 would support at least SOME threat for tornadoes. With such strong dynamics, I'd be inclined to say a squall line... But, I just have a funny feeling that supercells (most likely shallow/low topped) won't be too uncommon if the NAM plays out.

We haven't had much in the way of severe weather in MI this year, so maybe mother nature was bottling it all up for 9-19-2005 :lol:

 

[nickgrillo]

Originally posted by rdewey
Yeah, it's looking pretty interesting...

My main concern is the SFC Td values on the NAM - are they too high? Well, only time will tell. Helicity is excellent, at nearly +400M2/S2 according to RAP/UCAR, and this is a fairly dynamic system... But mid and upper level winds (500MB and up) will be screaming at +65KNTS, so storms will be flying.

Storm mode is another question... Clearly, +400M2/S2 would support at least SOME threat for tornadoes. With such strong dynamics, I'd be inclined to say a squall line... But, I just have a funny feeling that supercells (most likely shallow/low topped) won't be too uncommon if the NAM plays out.

We haven't had much in the way of severe weather in MI this year, so maybe mother nature was bottling it all up for 9-19-2005 :lol:

:lol: :lol: LOL... Wouldn't that be nice?

I'd think the threat for any type of supercellular convection would be confined to the southwest part of the state... Which is where the juxtaposition of extreme low-level shear (400-600 0-3km SRH) and marginal-moderate instability (sbCAPE 1000-1500j/kg) will be located (in the early afternoon). Assuming the NAM initializations are nearly close to correct, the boundary layer along and east of the cold front will be at least relatively unstable and highly-sheared.

Overall, as we've both have mentioned - the relatively strong wind speeds aloft will mostly be productive of fast moving storms. However, given the instability and shear - these storms should pack a pretty good punch...

 

[Robert Dewey]

Shear is looking stronger this go around... NAM shows helicity up and over 500M2/S2, a nice 50KNT LLJ, and 60-70KNTS at 500MB (westerly). At the SFC, winds will be backed to the SSE, which will contribute to over 90 degrees of directional shear between 0-6KM, and thus the high helicity values. There is definitely more than enough shear to support severe thunderstorms and tornadoes... But, the main problem will be instability.

The activity that develops today over the central Plains today shouldn't reach us until late MON, so I am not too concerned about that, though there could be a bit of debris cloudiness. As for moisture, Td's are currently in the upper 50's to low 60's... With upper 60's to near 70F located in KS. If we can hit at least 65F, that combined with the NAM's projected SFC temp of 80-83F, would probably yield a moderately unstable airmass. Provided the shear isn't TOO strong, severe thunderstorms would be a very good bet. Right now, it appears to me that convection will be located just to the west of the instability gradient as it cross through MI. This area also coincides with the highest speed and directional shear, with helicity in excess of 500M2/S2. Even with a main storm type of small lines and bows, any discrete storm that can manage to form would likely be supercellular, with a decent chance at dropping a tornado. The best time frame for severe weather would probably be between 21Z and 03Z (from west to east, obviously).

This event really does remind me of the WI setups that produce quite a few tornadoes. It seems to be the conditional to moderate instability days that catch everyone off guard up here in the Lakes.

 

[Alex Lamers]

Originally posted by rdewey
This event really does remind me of the WI setups that produce quite a few tornadoes. It seems to be the conditional to moderate instability days that catch everyone off guard up here in the Lakes.

Thank you for pointing that out. June 23 2004 MLCAPE was only around 1000 j/kg. It's when we have strong shear and surprisingly enhanced SRH values and convective rolls left over from previous convection when we get our tornado outbreaks.

I'm leaning away from the NAM and I guess thus the NGM because their surface low tracks are near the very bottom of all model tracks (which is easy to see on a spaghetti plot). I'm expecting this to be pretty dynamic so I think the low will be closer to online with an ensemble track from SE MN into the Upper Peninsula of Michigan. This means a more delayed front and possibly some early afternoon convection over eastern wisconsin and illinois. Anywhere that gets some clearing should be in for a ride although the further away you get from the surface low...but still in the strong jet flow...you should see more damaging winds.

I think sups with tornadoes are possible but look out for more quasi-linear structures with peak gusts 60-80 mph.

Great and some people just got their power back here...

...Alex Lamers...

 

[nickgrillo]

The latest 12z runs continue to make me more confident in tomorrow's situation... If the NAM solution plays out, then we could likely have the best severe weather in MI of the year.

At 0z TUE NAM shows 1500-2500j/kg of sbCAPE with widespread 250-600 0-3km SRH across southern MI. Second, without any real cap, the strong convergance along the frontal boundary will ensure expolosive development shortly after 18z across western MI. Given the relatively strong deeplayer shear (40-50kts) and near/extreme low-level shear (300-600M2/S2 SRH) some of these storms will likely take on supercell charioristics - and will be capable of producing damaging winds and isolated tornadoes. Given the strong low-level shear, which should yield a productive and strong supercell. isolated tornadoes are a pretty sure bet if this solution plays out.

Upscale growth from low-topped supercells to squall line should occur slowly as the storms push east. The thread for embedded supercells and tornadoes should be pervasive until the late night hours... With instability increasing by nightfall (with NAM soundings showing >2000J/KG of CAPEs) and low-level shear remaining quite favorable.

I should be able to chase by tomorrow afternoon when I get off work, so I'm hoping this will play out.

 

[Jeremy Lemanski]

A place to look for enhanced severe threat will be near the lake michigan shoreline. With the wind out of the south, off lake michigan, locally higher dewpoints will likely be found along and slightly inland from the coast. If the wind is more SSW ahead of the front, it spreads that add moisture farther inland.

I'm looking for a large squall line with some poss. LEWPs spinups. there also maybe one or two sups. ahead of the main line, but if they do form, they will likely be quickly overtaken by the large line.

 

[nickgrillo]

Originally posted by Jeremy Lemanski
A place to look for enhanced severe threat will be near the lake michigan shoreline. With the wind out of the south, off lake michigan, locally higher dewpoints will likely be found along and slightly inland from the coast. If the wind is more SSW ahead of the front, it spreads that add moisture farther inland.

I'm looking for a large squall line with some poss. LEWPs spinups. there also maybe one or two sups. ahead of the main line, but if they do form, they will likely be quickly overtaken by the large line.

I'd have to think that supercells will be the main convective mode with first initiation... Then, it'd transition and propagate eastward rapidly as a squall line.

If we could only see stronger instability (even just 500-1000 joules stronger), then we'd probably be looking at a MDT risk tomorrow...

 

[David Draun]

I am really excited about the setup for NE IL tomorrow, maybe because I have to head back to Iraq on Tuesday and this will be my last chance to see some good storms for a while. It looks like there will be a strong to severe MCS ongoing through the mid morning hours. Then I expect some clearing into the afternoon. Good insolation should get our temps into the low to mid 80's, and dewpoints should be pretty high from that LLJ from the SW. I think storms will initiate west of I-39 during the late afternoon as the front approaches. I will definitely be out and about tomorrow, because my father and I have planned to do some fishing.

I'm banking on a MDT risk in the morning.

 

[Andy Wehrle]

Setup in eastern Wisconsin does not appear as good as points further east, but a few severe storms are possible. Wind fields will be excellent as the deepening low tracks right across the state. If we can get some insolation ahead of the upper MS valley MCS it could intensify and produce severe gusts/hail.

 

[rdale]

LFC's seem a bit too high for my tornado sensor to be activated... And overall it seems the 00Z continues showing the best upper-air setup in northern LP but the best thermo fields are from southern LP into N IN/IL. I'm afraid that disconnect will spell trouble.

Other than that - if this was April we'd be on fire with th exact same setup. I'm excited but holding back (a bit) on my enthusiasm on-air tonight.

- Rob

 

[Alex Lamers]

Modifying the MKE 18Z NAM output sounding for tomorrow reveals several telling things. The first is that storms will need to be right movers in order to realize any substantial SRH as 0-3 KM AGL layer SRH will peak at around 90 m**2/s**2 for storms moving in the mean direction.

Shear should be just fine in the deep layer on the order of about 60 knots. Something just doesn't add up right for me here in SE WI. A lot of SREF members do show development from 18-22Z across SE WI and if this occurred we'd catch initial/discrete convection with a better chance of rotating sups.

I just don't like how the new NAM has shied away from giving us QPF in the afternoon and splitting best qg forcing north and south of the area.

...Alex Lamers...

 

[Robert Dewey]

NAM still showing plenty of shear, with some decent instability developing. NAM shows Td's in the mid 60's, which doesn't look TOO unreasonable given the values out to the southwest. Basically, nothing has really changed except an increase in the instability axis, which is now slightly further west than the previous run. The GFS is even stronger, with 60KNTS at 850MB, cranking up to +80KNTS at 500MB. It also brings in LI indicies of -4C or so by 00Z TUE. I do believe the GFS is overdone in regards to the depth of the wave and the wind fields... But, one thing is certain - there will be plenty of low level and deep layer speed and directional shear, even if the GFS is a bit overdone.

I also agree that the best forcing is located further north, but this may actually be a benefit if you want discrete cells. I would hate to see such high vertical velocity and vorticiy smacking into an uncapped airmass. But then again, with such exceptionally high shear, it might take quite a bit of forcing to get things/keep things going if instability doesn't hit at least 2000J/KG. On the other hand, wind convergence does appear decent, and there is a rather tight theta-e ridge located over the region - But as with past events, that usually proves worthless in developing convection alone.

In addition... The soundings (the last I checked, which was 12Z) showed a classic inverted "v" - This would indicate high LCL's and a lower tornado threat, but in contrast; an enhanced threat for some significant wind damage within any storm that manages to develop.

To sum it up, this appears to be rather complicated. Definitely not clear cut, as there are several questions:

1) Will low level moisture be sufficient?
2) Will the shear be TOO strong for instability?
3) Will there be enough forcing along the front to get things going, or will we just watch a big blob of rain pass by to the north with the best upper level support?

Now, if we could attain temps near 80F with Td's getting up around 70F (lower LCL's), and a bit more of a "punch" in the upper levels (or an MCV rolling by) we would probably be looking at a rather significant severe thunderstorm and tornado outbreak. But, that's just a "what if" situation, and I'll keep dreaming...