Slab Ascent

[Dave Gallaher]

The SPC discussion of our severe potential for Thursday mentions "slab ascent" along the front. I can possible guesstimate what this is, but would much prefer an educated definition.

*Edit* The mention was on our local NWS page, not SPC.

Thanks

 

[Dave Gallaher]

Since there seems to be no response to this term, I am postulating a guess based on field observations. In the smaller scale of a gust front, moist air can be observed rising over a shelf or roll cloud in a variety of forms, from round blobs to sharp-pointed 'teeth' or 'claws'--or occasionally a visibly rising mass. Would the latter, expanded to the scale of a full-sized cold front, possibly be an example of slab ascent? I have emailed SPC (who should be way too busy to answer right now) and have heard from a friend in the Huntsville NWS office who said he has only encountered the term in SPC outlooks in recent months and is unaware of the official definition.

Opinions?

 

[David Drummond]

Have you tried emailing the local NWS office where you saw that in discussion and ask them how they were using the term? I have always found NWS offices to be very responsive to questions.

 

[Dave Gallaher]

Have you tried emailing the local NWS office where you saw that in discussion and ask them how they were using the term? I have always found NWS offices to be very responsive to questions.

Yes, thanks, David--Actually, the mentioned met at NWS Huntsville is an ST reader and saw this thread and emailed me, as indicated above. They are always more than helpful.

 

[Rich Thompson]

Dave,

I believe the forecaster was referring to rather uniform lift along the cold front as "slab ascent" - i.e., the immediate pre-frontal air mass ("slab") in the low levels is being lifted over the frontal surface.

The reason for using this terminology would be to support development of a sharp band of precip, which would take the form of a squall line if instability is present and the lift/front is deep enough.

Rich T.

 

[Dave Gallaher]

Excellent. That seems to be exactly what happened here, a narrow band of hard rain.

Thanks, Rich.

 

[Dave Gallaher]

Additional definition graciously supplied by Corey Mead via Andy Kula, both NWS:

"Probably the best description for 'slab ascent' is an unbroken, two-dimensional swath of ascent created by a frontal boundary and/or accompanying mcs cold pool as described by James et al. 2005."

The paper itself ("Enviromental Distinction between Cellular and Slabular Convective Lines" by P. James, J. Michael Fritsch, Paul M. Markowski) is online but viewable by subscription or purchase only. The abstract at AMS Journals Online gives three paragraphs. I have tried twice to place a link here, but it won't connect.

The "two-dimensional" parameter is interesting.

Thanks to all of you.