April 18 2003 Supercell & Tornadoes in Central Nebraska                 Photos and text by Jon Davies            

It's been several years since I chased a small fast-moving upper cold core low lifting out to the northeast... more often than not they seem to produce "surprise" tornadoes.  In the past I've usually positioned myself too far  south and east of the surface low along the dryline where dew points are larger.  But from studying similar events, it seems that the tornadic storms tend to occur closer to the surface low near the intersection of the warm front and dryline/pacific front (the "triple point") where the nearby cold air aloft generates good instability in spite of deceptively low moisture and dew points.  Clear skies behind the dryline/pacific also help by generating surface heating and increasing the instability near the triple point.   A composite diagram I've put together shows some typical features that come together for these cold core tornado events.

I left Wichita at 8 a.m CDT., determined to play this Nebraska system differently.  I arrived in southwest Nebraska around 1 p.m., with towers trying to develop near Cambridge along the dryline just east and south of the surface low.  Skies were clear in the dry air to the southwest, boosting heating for instability near the low, in spite of dewpoints only in the low 50s F.  Towers at times looked better to the southeast of me, but I stayed with an area of cumulus congestus that appeared to be just east of the surface low at the "triple" point ( intersection of  the NW-SE dryline with a WNW-ESE warm front, marked visually by low clouds and haze to it's north, see satellite photo and surface map further on down).

Things began to shape up around 3:15 pm near Eustis, with some low-based cumlus towers taking hold.  By the time this cluster of clouds approached Cozad on I-80, rain was falling and the easternmost cell base had some supercell-like characteristics, with cloud motions suggesting a rapid updraft.  Northeast of Cozad at 4 p.m., I saw a funnel or tornado develop out of a lowering on the back side of this cell, lasting 1-2 minutes (see photo).  I relayed information by cell phone to Hastings through North Platte NWS.   As I struggled to get east and north behind this fast-moving cell, about 4:15 p.m another feature that looked like a cone tornado was barely visible under the broadening wall cloud to my northeast.  It turns out that a Highway Patrol officer videotaped this tornado between Lexington and Oconto (see these video captures).  Unfortunately, the storm was moving rapidly into the sandhills, which cut off my view.  A slick hail-covered road soon brought my pursuit to an end.   In all, this cell and another that formed to its east produced 8 tornadoes (listed in Storm Data), mainly in open country.

Going back south then east on I-80, I watched the supercell in the distance to the north under beautiful blue skies behind the front/dryline.  Talking later to Mike Moritz at Hastings NWS, he informed me that damage to a farm had been reported southwest of Ansley where timely tornado warnings had been issued.  I finished a successful and educational storm chase with some photos of distant low towers lit by the setting sun behind the Hastings radar dome at Blue Hill, enjoying the crisp dusk air.  Photos and images follow (click on yellow-bordered images to get a larger view):      

Developing towers near Cambridge, Nebraska, view W	View to NNW around 19 UTC	Another developing tower on dryline, looking ESE	Base of developing cell, view SW between Eustis & Cozad
Cell approaches Cozad, view S around 2030 UTC	Rapid upward motion and turning at cloud base	Rapid upward motion passes overhead near I-80, view S	Updraft base and lowering cross I-80, view WSW
"Needle" funnel or tornado NE of Cozad, view NE at 21 UTC	Zoomed in view...hard to confirm as a tornado with the wet ground	Condensation funnel descends... photo enhanced for contrast	Wide view of wall cloud with funnel in poor contrast
View to E showing towers/cells on dryline	Another view of clear skies behind dryline, view to SE	View N, cone tornado S of Oconto (video thru windshield)	Hwy Patrol saw same tornado (video courtesy Hastings NWS)
Nebraska sandhills obscure my view at 2140 UTC (video)	Pea/marble size hail covers road S of Oconto, brings chase ro end	More hail...	One last view of back side of storm, view to E
Tornado-warned supercell, view N from I-80 about 23 UTC	Wider view of low-topped convection & blue sky, view N	Beautiful blue skies behind front and dryline, view to SE	Distant towers in setting sun... Hastings WSR88D at Blue Hill

Above, panoramic view from 3 photos, looking NE through SE from just east of Cozad, Nebraska.  This is the clear area south of the supercell near the "triple" point in the satellite photo below.  The Eta forecast for the tight upper system showed a vorticity max ("X") lifting into Nebraska.. The tornadic supercell appeared to occur right ahead of this upper system at the triple point where low-level instability (see Eta low-level CAPE forecast) was maximized by heating from the clear area just behind the dry line and the nearby cold system aloft.   <surface map

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