04/11/05: TALK: Mississipi Valley/Gulf Coast

[nickgrillo]

Tornado potential is increasing in Kansas along the surface low ... with a red box expected within the next hour...

Further south ... the first high risk of the year has finally arrived ... NE LA has been updated to a high risk by the SPC!

Although I should be paying attention to that, I guess I'm focusing more on NE Kansas/SE Nebraska/SW Iowa right now ... as tornadoes are a bet there and that's where the best chase oppurtunities are. I doubt anybody is chasing the gulf today, but who knows...

..Nick..

 

[Glen Romine]

Ya know, I look at some of the conditions there and short-range model forecasts, and I have a hard time seeing the high risk forecast based on current trends. Even the PDS box currently in effect seems premature if nothing else. Obviously, these guys have looked at the situation much closer than I have so I'm surely just not seeing it, but there are a few factors that don't seem that favorable right now. First, the widespread cloud cover. Yes, a few breaks here and there, but mostly well to the east, such that the current moist axis west thermal axis east situation is further reinforced. Quite a bit of cinh is in placed based on convective trends and mesoscale analyses, while 300 mb divergence is working to erode, 700 mb warm air advection is working against it. Certainly the shear profiles look fabulous right now, but low-level shear has taken a slight hit with increasingly deep mixing as surface heating continues.

I'm REALLY needing to see the 18Z soundings to figure out what is actually happening down there. Always problematic when upper air trajectories are from over the Gulf of Mexico - so a lot of model guesstimation in what the thermal profiles are right now. Also, RUC model forecast convective development appears misplaced - which could be messing up the analysis fields pretty badly. This makes using the RUC model output guidance for short-range forecasting rather suspicious. Things maybe will come together better later, or maybe this is just a case of early season jitters. We shall see.

Glen

 

[nickgrillo]

Convection has fired in the red box in NE KS and SE NE ... with a line stretching from Washington County, NE southwest to Republic County, KS. All storms are moving to the north-northwest. The strongest cell currently located in Jefferson County, NE has shown some indication of large hail the past few scans...

I expect warnings to start flying for the storms along this boundary within the hour...

..Nick..

 

[Morgan Palmer]

I don't have much of a problem with the high risk for LA and MS, as I think the sheer number of severe reports might warrant it... however, I too, don't see everything coming together for the PDS hose box. I haven't had a chance to look at too terribly much.

I would be very interested in the (sortof) tail end cell about to hit LCH.

 

[Skip Talbot]

One thing that caught my eye looking at the F5Data RUC plot is the 03z sig tor parameter up to 13 across MS :shock: . 1km SRH between 300-400, CAPE to 2000, and 60-70 knots speed sheer: perhaps this show won't be kicking off until this evening.

 

[nickgrillo]

Concerning the developing Lancaster County, NE supercell...

SVR has just been issued for Lancaster County ... however, I've been noticing somewhat of a appendage on radar, with some notable rotation ... I'd keep an eye on this, as this storm is quickly strengthening and could very well become tornadic.

..Nick..

 

[Scott Lincoln]

The storm near Lincoln, NE, appears to be mostly a pulse severe... It quickly had an increase in rotation, but that has been dropping off. I could see it cycling and increasing once again, but the storm is moving in an unfavorable direction. The threat for tornadoes from that storm at this time would probably be low. Storms moving to the N or NE ahead of the surface low probably have the best chances of producing tornadoes.
If you look at the volumetric data for that storm, the rain shaft has been collapsing... it may be in the dissipating phase if the storm is unable to get another updraft going.

 

[Scott Lincoln]

Another storm west of Lincoln just had a quick pulse of rotation, enough for a tornado warning, but it too is now decreasing in intensity. My thinking is that this trend will continue.
The lack of widespread sunny conditions in this area will limit the threat of long lived supercells, but some mini-supercells could quickly form and produce a tiny weak tornado and quickly dissipate. Unless some conditions in this area improve, there shouldn't be much increase in the tornadic potential.

 

[Jeff Lawson]

The storm near Lincoln, NE is behind the cold front. Despite that, OAX issued a TOR for the storm a few minutes ago. I could understand if the storm was just now crossing the boundary (or preparing to), but 10 miles behind?

I'm tied up all afternoon, unfortunately.

 

[Glen Romine]

Originally posted by Glen Romine

I'm REALLY needing to see the 18Z soundings to figure out what is actually happening down there.
Glen

Well, the 18Z soundings are in, a quick summary. Lake Charles showed only slightly greater than moist adiabatic lapes rates and deep moisture up to around 610 mb, but very little cinh. Further east, Slidell shows much better mid-level lapse rates, but above a sharp inversion at 800 mb that won't be penetrated anytime soon. Up in Jackson, one finds somewhat of a blend of the other two environments - with weaker mid-level lapse rates but still a substantial inversion - though not as high up (around 880 mb) yet still won't be broken anytime soon. As such, much of the lead convection could remain elevated for a quite a while longer. Persistent upper level divergence continues across the region per SPC mesoanalyses, so perhaps by late evening conditions could start to rapidly deteriorate. We shall see.

http://www.rap.ucar.edu/weather/upper/lch.gif
http://www.rap.ucar.edu/weather/upper/sil.gif
http://www.rap.ucar.edu/weather/upper/jan.gif

Glen

 

[nickgrillo]

Washington Co, KS SVR-warned cell has been showing rotation, along with the potential for golfball-sized hail...

..Nick..

EDIT: The cells just split and wouldn't be surprised if a tornado drops out of this...

 

[Glen Romine]

Originally posted by nickgrillo


EDIT: The cells just split and wouldn't be surprised if a tornado drops out of this...

Doesn't look like a split Nick, looks more like a new cell just developed on the western edge of the cold pool from the cell near Washington. Winds at the surface are fairly light in the area, with low 70 temps and dews in low to mid 50's. I'm starting to think the cells back further west may have a better chance in the short term, like those in Jewell county where better moisture north of the front could compensate for cooler sfc temps.

Glen

 

[Jeff Snyder]

I read the MCD regarding the categorical outlook upgrade this morning, looked at radar, and said "Huh?". Yes, shear profiles look(ed) very good, but instability was marginal at best (SBCAPE <1000 nearly everywhere, save southern part of LA). In addition, there was a semi-well developed squall line in place... I'm glad to see the HIGH downgraded to MDT, since I didn't really see instability nor storm mode favoring such a high risk. I think the lack of instability has resulted in relatively few reports, though there does continue to be a hail/wind threat. I think there is a tornado risk in southeastern LA and along the southern part of the LA/MS border. I do wonder if there will be some examination into the verification of PDS boxes thus far this year. Not meaning to be overly critical (who am I to say anything? These folks are experts in severe weather forecasting, so my comments are speculation of course), but I don't think there has been a single verified PDS box so far this year. In fact, it seems that the only tornadoes that were F3 or greater were NOT in PDS boxes... It seems that there have been 5-6 PDS boxes thus far this year, so this is somewhat disconcerting. There may be redevelopment in western/central LA later this evening, but low-level shear is progged to be low pretty low... With widespread convection, I wonder about the stabilizing effects, which could further lessen any threat.

 

[Scott Olson]

I must say after a glance at the 18z data im a little suprised with the PDS. Doesn't the SPC have a record of 4-5 PDS per year? Or im I thinking of the Public Severe Weather Outlook? Anyhow, I expect that last weeks 32 tornadoes (some long-track) probably influenced their decision. Wish I would have seen the potiental in Eastern Nebraska last night.

 

[nickgrillo]

Your probably thinking of the Public Severe Weather Outlook, which is issued on high risk days...

The SPC usually issues roughly a dozen PDS boxes every year ... with 2003 and 2004 having the most. I'd take a rough guess and say that there was most likely two or three dozen PDS watch boxes issued last year, heck, on May 30th there was about seven or eight issued alone.

A bunch of unorganized hailers continue to fire in NE Kansas. Quite a few storms showing 1"+ hail indications, but nothing really serious...

The tornado forecast for today hasn't really panned out too well.

..Nick..