moderate risk from SPC for upper Miss Valley and midwest

[WXBUFFJIM]

Folks living in eastern Iowa, Illinois, parts of Missouri, western Kentucky and Indiana need to be on the lookout for a severe weather outbreak potential on Wednesday. Cities and communities impacted potentially include Davenport, Chicago, Rockford, Joliet, St Louis, Springfield, Illinois, Indianapolis, Indiana, and Evansville, Kentucky. Very large hail up to baseball sized and a few strong long tracked deadly tornadoes appear likely on Wednesday as a potent disturbance approaches from the southwest.

A strong low pressure system will track from western Kansas tonight into eastern Iowa by later Wednesday and into southern Wisconsin by Wednesday evening. In the mid levels a negatively tilted northwest to southeast axis trough will track from the mid plains into the upper Mississippi Valley. The air aloft should spread out in a diffluent pattern, which will allow the air to rapidly rise up from below. Strong deep layer shear combined will allow storms that develop to rapidly become severe and even tornadic.

Some of the most dangerous tornadoes are likely to develop from southern Wisconsin into Illinois and western Kentucky by late afternoon as winds at low levels back slightly from the southeast.

While moisture supply will not likely be totally out of control, just enough moisture at low levels warrants a moderate risk severe weather forecast in the above mentioned area talked about at the beginning of the discussion.

This has the potential to be one of the biggest severe weather outbreaks of the 2005 season in the upper midwest with very large hail and POTENTIALLY strong and deadly tornadoes a possibility later Wednesday. Very large hail is my biggest severe weather concern with baseball hail or large likely on Wednesday. The slower progression of this system has likely resulted in this situation occurring during peak heating as opposed to earlier in the day as originally anticipated. Storms could move into the Ohio Valley and lower Michigan Wednesday night with a severe wind threat continuing.

Jim

[PurdueWx80]

Nice discussion, Jim - looks like we saw the new day1 outlook and got excited at exactly the same time!!

[WXBUFFJIM]

that makes two of us. Will keep an eye on Wednesday, looks pretty bad potentially. We remember Utica last year. We hope no one is killed in a similar setup later Wednesday.

Hail baseball to softball sized and tornadoes appear to be the biggest risk.

Jim

[jkt21787]

1300z update now has moderate into Western TN and Northern MS, where the higher probabilities of tornadoes now exist. Ann F-2 or greater is not out of the question. Here is a discussion I wrote on another board just minutes ago that relates to this area.

OK, I am going to write this as kind of an overview of the situation coming together this afternoon. This is all from the 6z NAM.

Off the Skew-Ts, at 18z this afternoon for KMEM (Memphis) things are relatively unstable. Li is 1.6, cape 782, and lapse rate 4.7C. Most importantly, the cap is 2.1C. The morning satellite shows low level clouds across the area, but these should quickly mix out this morning. The resultant sunshine should lead to rapid destablization as temperatures rise to the upper 70s if not near 80 in some areas. At 21z, things begin to change quickly. Li drops to -5, cape 2240, lapse rate 7.0C, and cap index now mostly broken, less than 1C. EHI is 2.22, 0-3 km Helicity 248.9 m/s2. Then, at 00z, instability peaks, Lis -6.9, cape 2630, EHI 3.9, lapse rate 8.4C. The cap strength is now negative. By this time, thunderstorms should have already developed, but where the front is will determine where they start.

At 00z, the significant supercell composite should be 1 or greater. The significant tornado parameter is 3 or greater, with 5 or greater from across West Tennessee. The regular supecell parameter is 10 or greater, with areas just east of Memphis 15 or greater. The significant severe paramter is 50 or greater, and Memphis is 70+! All of that points to a likelyhood of supercells with tornadoes. The Skew-T showed a supercell potential peaking between 60-70% initilally.

As far as moisture, the dewpoint in Memphis is up to 52 this morning, and should easily clmb above 60 as the afternoon progressess and southerly winds continue. The jet dynamics are also favorable. We are in the left exit region, which is favorable. the 850 mb jet is 45kts With the jet 80kts plus at the mid levels, and 100 kts plus at the upper levels.

So my forecast for now is that somewhere between 3 pm and 6 pm explosive supercell development will occur near or just west of the Mississippi river, with movement east or northeast. Initially, supercells will carry a high potential of very large hail and tornadoes. An F-2 or greater is not out of the question. As the storms progress east, they will merge into a squall line pattern as winds become more unidirectional, carrying a higher threat of damaging winds, and perhaps a couple of tornadoes still possible.

The greatest threat is over Western Tn and North Ms. However, some threat of severe weather, mainly a squall line form, should reach into Middle TN and Northern AL. Depending on initiation when the cap breaks, Eastern AR and the MO bootheel have a slight threat, but again the erosion of the cap is key.

One thing I am concerned about is that although the NWS is playing it up (as they should be), the TV mets are still sounding more concerned about tomorrow, when the threat is now much further south. They are only mentioning a slight chance of showers, but it is very likely that at least there eastern counties will be dealing with severe weather tonight. I hope they soon realize that, and the NWS teleconference this afternoon which I am sure they will have will clear that up. All persons in this area should closely monitor developments.

Again, one more time, the erosion of the cap is the most critical factor as to where and when thunderstorms form. All indications are is when that breaks, the development should be rapid and should be severe. Unlike the past events, things here are more favorable for mainly one reason, and that is moisture return, which I am confident will happen as it already is this morning. The instability will be a lot better too.

Any comments are welcome. Am I spot on or am I dead wrong?

[wlfpack81]

jkt21787

I have family in Memphis (one sister in city limits another in Cordova w/2 kids). This concerns me as they don't pay much attention to weather. I had to call them a weak ago or so to warn them about that storm that headed in Shelby Co. and produced some hell and prompted a tornado warning. I'll definitely be keeping an eye on radar, obs and the ruc model during the day today.