3/29/98 Chase Summary by Stephen Jascourt


From: Stephen Jascourt (jascourt@MAILBAG.COM)
Subject:      Re: The reason for no tornadoes in Iowa
I too was among the chasers who ended up in northwest Iowa and not in
Minnesota. I have not seen the 0z Omaha sounding from Sunday evening, the
usual places I look did not have any data for Omaha then. The 18z special
sounding does show some cap, and a mid-afternoon satellite picture showed
wave clouds in southwest Iowa, indicating a stable layer. However, I think
the problem may be the cap was at too high an altitude. I saw lots of
moderate cumulus all across west-central Iowa, in short lines and clusters.
They looked fresh but they weren't growing-- I think they were evaporating
at the top from cloud top entrainment and were being fed afresh from below,
almost a steady state in the mean. The result is the boundary layer was
getting the moisture sucked out of it. Or maybe the cap didn't hold at all
as cooler air aloft came in late in the day, but with the southwesterly
surge aloft came the dry air, so the moist layer was shallow, and the
cumulus were ingesting some dry air above the moist layer. The flanking line
of rainshowers (and a little bit of lightning, yes I saw some!) east of
Sioux City at sunset, along the cold front, looked anemic, hardly an anvil,
low-topped; obviously the updrafts were not healthy. The surface CAPE was
still fine despite some surface drying, so something else, such as dry air
above the shallow moist layer feeding into the updraft, was choking off the
buoyancy.
If anyone has the 0z OAX sounding, please post it on the list so we can see
the depth of the layer with decent CAPE and the strength and height of any
cap.

One function of the organized sustained convergence associated with the warm
front and triple point is to deepen the moist layer. This may have been the
critical factor in this case, given that surface based convection
in the warm sector in western Iowa was insufficient for generating strong
cells. Convergence along the cold front and dry line may have been too weak
to sufficiently deepen the moist layer after the southwesterly surge stripped
away part of it.

Note also the tornadoes occurred approximately where/when the nose of the
mid-level jet crossed the warm front. The nose of a mid-level jet crossing
some critical boundary seems to be a recipe for strong tornadoes in other
cases I've noticed, but I have not systematically investigated this. What
went wrong for me in this case, in addition to the flanking dryline/cold front
not lighting up, was I expected rain along the warm front to leave the
surface boundary south of where it was forecast, so it wouldn't advance well
into Minnesota, and for the surface winds a little south of the warm front
to be backed to southeast (as forecast by several runs of the RUC), creating
a convergence zone south of the warm front which would intercept the feed of
unstable air. This always seems to happen when the warm front is to my
south but never when it's to my north.

Stephen Jascourt   jascourt@mailbag.com


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