4-18-13 Illinois forecast

[chrisbray]

Hi everyone, I am a novice when it comes to using forecasting systems, but have been amateur storm chasing for 8 or so years now in Illinois. I'm trying to look at the models for Tomorrow in North Central / North Eastern Illinois in terms of when exactly the front / squall line would be passing through. I guess I am just needing a little bit of help in determining what is likely to happen. So If I am running the NAM or GFS and it shows the line of cold surface temps passing through at 00 UTC 19 April, meaning in the evening, is that a good indicator? It Based on what I have read they are forecasting the squall line/storms late morning and early afternoon. Is there a different model I should use, or a way to show the precipitation instead? I could only find it for accumulated precipitation.

Sorry for the noob question. Just trying to figure out if getting of work at noon tomorrow would be early enough to head out and maybe get some pictures! I was using twisterdata.com to access the models, if that matters

 

[chrisbray]

Well I guess I have my answer looking at the radar this morning and seeing the line of storms preparing to move in already!

 

[N Parker]

I am watching with you. Nothing much is ever said on here once a weather system gets past the Plains. Waiting to see if anything can get going out ahead of the squall line. Atmosphere seems to be quite ready to spin one up with shear well over 250 m2/s2. Wouldn't be surprised to see another PDS watch box south of the one already issued. Be safe out there to all who chase in the land of the hills!

Forgot to add...Is very overcast here. And from the looks of the visable loop, isn't going to clear up anytime soon.

 

[chrisbray]

I am watching with you. Nothing much is ever said on here once a weather system gets past the Plains. Waiting to see if anything can get going out ahead of the squall line. Atmosphere seems to be quite ready to spin one up with shear well over 250 m2/s2. Wouldn't be surprised to see another PDS watch box south of the one already issued. Be safe out there to all who chase in the land of the hills!

Forgot to add...Is very overcast here. And from the looks of the visable loop, isn't going to clear up anytime soon.

Wish I would be in Southern instead of Northeastern Illinois. I've found that many of these systems are forecast to be impressive the day before, but once the overnight convection persists it pushes along the squall line a lot faster than anticipated, and the next day the target areas have shifted dramtically east into Indiana/ far southern and eastern Illinois.

Regardless, I'm doubting there will be much in the way of discrete cells at least in the northern part of this system.

 

[chrisbray]

Gotta say I am a bit surprised they issued a PDS tornado watch with this. Maybe the line will intensify later this afternoon, but it doesn't seem like much at the moment

 

[chrisbray]

This is interesting to me, a lot of parameters seem favorable for severe or tornadic storms in the PDS watch, but the yare already discussing clearing large portions of it. Is the main problem that the squall line is so far away from the actual fronts, meaning there isn't a forcing mechanism? Is it something else?

 

[Matt Hunt]

Chris, that has a little to do with it, but also that line moved through northern IN before daytime heating could destabilize the area. I'm not sure why they issued a PDS watch. At this point, IMO, northern IN isn't going to see anything. From Indy south could get some tornadoes later this afternoon/evening, as CAPE is expected to reach 2000 J/kg in some areas based on the SPC mesoanalysis page, which I believe uses the RAP. I think that's a bit overstated, though, especially with all of the cloud cover. My target area is basically Terre Haute south, when the line hits it around 4-5pm. Hodo's are definitely favorable in this area, the only question is will updrafts be strong enough, and will it be a broken line, a line with embedded supercells, or completely linear? If it's completely linear, the tornado threat should be reduced.

 

[chrisbray]

Thanks, I was just really confused on why they issued a PDS. Were they thinking the line would hold off its progression until later? I agree with you, it seems like everything with the northern part of the moderate risk is pretty much washed out at this point, but the southern half seems to have some hope. The squall line kind of broke apart earlier, with the north half continuing but the southern stalling.

 

[Matt Hunt]

Aaaand nothing! I've seen tor watch's before that never had a tornado, but never a PDS watch! I'm not sure what happened, but I suspect that they really expected instability to increase much more than it did. There was never a good updraft today, really anywhere north of Memphis!

 

[chrisbray]

Yeah, if you look at the PDS watch's initial areas, there are just a couple wind reports from this morning, basically right when the watch was issued! I don't think I have seen a PDS not only have no tornadoes, but practically no modes of severe weather reported whatsoever! Anyone recall something similar???

 

[Matt Hunt]

I tried to look it up, and I found this study: http://www.spc.noaa.gov/publications/dean/pdswatch.pdf

It's for a limited time period, but it looks like during that time period 82% of PDS watches had at least 1 tornado. So I guess 18% had no tornadoes.

He states at the beginning of the report, "PDS tornado (TOR) watches are issued when the forecaster has high confidence that multiple strong (F2-F3 on the Fujita Scale) or violent tornadoes (F4-F5 on the Fujita Scale) will occur in the watch area..."

 

[chrisbray]

What I really wish the SPC would do, would be to have an article explaining what went wrong with the forecast today. Or maybe someone else will make a writeup about it.