A beautiful funnel aloft in Iowa, and the April 13-15, 2018 tornadoes

Last Friday through Sunday (April 13-15, 2018) were active severe weather days across the central, southern, and mid-Atlantic states, and sadly, there were two deaths related to tornadoes.  I'll touch on the setting for these severe weather days later in this blog.  But first, I'll briefly discuss an interesting storm chase to southwest Iowa on Friday, April 13 (see image above).

Near the Kansas City area where I live, Friday's storm environment didn't look very supportive of tornadoes. There wasn't much low-level wind shear (storm-relative helicity or SRH), and cloud bases looked to be too high with large temperature-dew point spreads around 20 degrees F or more.  The best tornado potential on 4/13/18 was farther south in Arkansas, northern Louisiana, and northeast Texas. But in southwest Iowa, east of a surface low near a warm front/stationary front, things looked at least a little interesting. 

So, my wife Shawna and I decided to take a quick trip north to the Iowa front to see if anything of note would happen there.  We were rewarded with a tornado-warned storm, mesocyclone, and lowering that briefly tried to produce a tornado just north of the front, north-northeast of Hastings, Iowa:

A second storm then produced a fascinating high-based horizontal funnel right over our heads (!) that lasted around 10 minutes near Emerson, Iowa as this newer storm crossed the same stationary front:

The surface map below shows the east-west front at about 3:00 pm CDT (2000 UTC), and also lowest elevation radar reflectivity (inset with storms labeled) about 45 minutes later (at 2044 UTC) in the Red Oak (RDK)-Atlantic (AIO), Iowa area:

We found that the temperature contrast when driving back and forth across the frontal boundary just north of Emerson was quite sharp, a nearly 15 degree F drop across only 2 miles!  That probably explains why neither storm produced a tornado with all that near-surface cool air, even though low-level shear and SRH increased significantly north of the front.  Notice on the SPC mesoanalysis panels below that the large values of SRH were north of the front, while surface-based CAPE (from surface lifted parcels) was restricted to the area south of the front:

With little or no overlap between these two environments, there was little chance for tornadoes as storms crossed the boundary, moving rapidly into the inhibiting cool surface air.  However, the storms north of the front still had enough "elevated" CAPE (from lifted parcels above the cool surface air) to become supercells and produce sizable hail and even some wind damage near Atlantic, Iowa.

And, of course, there was that cool funnel cloud as the second storm crossed the boundary :-).

On a much broader scale, the graphics below show the upper wind flow pattern and forecast environmental CAPE-SRH combinations (via the energy-helicity index, or EHI) for Friday evening April 13 (0000 UTC 4/14/18), and Sunday afternoon April 15 (2100 UTC 4/15/18).  SPC storm reports for those two days are also shown (for brevity, Saturday April 14 is omitted):

Notice how the large upper trough (sharp "dip" in the jet stream) moved eastward, and how tornadoes, in a broad sense, were most numerous between the jet stream branches (large white arrows) spreading out east of the upper trough.  These areas also had sizable EHI values (CAPE-SRH combinations) that were more supportive of rotating storms.

Here's a couple photos of tornadoes from this 3-day tornado-producing storm system; one in southwest Arkansas near Umpire on Friday evening 4/13/18 (very large and ominous), and one near Greensboro, North Carolina on Sunday afternoon 4/15/18:

Unfortunately, a toddler was killed on Friday night in an EF1 tornado near Shreveport, Louisiana, and a man was killed near Greensboro, North Carolina from winds located just west of the EF2 tornado shown in the image above. Both deaths were from falling trees, a significant problem regarding both wind and tornadoes in the eastern half of the U.S., where trees are more numerous.

-  Jon Davies  4/17/18