Convective Addiction

Chase Report: IA – 04/03/11

Posted on 06. Apr, 2011 by Jesse in Uncategorized

I chased this day with Dr. Tom Williams and Rich Sample, both from Western Illinois University. We initially targeted Mt. Pleasant, IA, but later decided to drop south to near Keosauqua, IA after some of the short-range numerical models were showing convection erupting across parts of far N MO and far S IA between 21z and 00z.  The Storm Prediction Center had outlooked the chase target area in a high end slight risk, with a slightly enhanced 10% Tornado Probability area across the region, with an attendant risk for very large hail and even some damaging winds.

A trough was perambulating towards the Great Lakes region, with a 90-100 kt H3 jet core approaching the Upper Midwest by 18z Sunday. At the mid-levels, a 70-80 kt jet core associated with a shortwave perturbation was approaching the Missouri valley, placing parts of the Middle and Upper Mississippi Valley within the region of maximum lift as as associated region of low pressure gyrated eastwards towards the western Great Lakes.

H3 Upper-air profile at 18z (1 PM) Sunday

500 MB Mid-level profile - 18z SUN (1 PM)

By late afternoon the low pressure had moved into far NC Iowa, with a cold front extending SSW into central Kansas, along with an attendant warm front fixated ESE from N IA into the Ohio Valley region.  Though some clouds had worked in behind the front, a surge of warm air and some later clearing would rapidly destabilize the atmosphere across the region, with the one real caveat being a strong EML around 800 mb that would prevent convection from firing until better forcing approached the region by later afternoon.

Surface composit at 21z SUN (4 PM CDT)

Hopes for tornadogenesis were hampered across the region due to a somewhat veering wind profile and unfavorable temperature/dewpoint spreads, with surface temperatures warming well into the 80s, with a rather shallow axis of moisture behind the front, as dewpoints only approached the lower 60s in most spots.  Nonetheless, a stout LLJ fueled H85 winds of 50-60 kts into the region, providing some enhanced shear profiles.

Note the absence of really deep moisture indicated by the darker colors for higher dewpoints

850 mb profile at 18z (1 PM CDT) SUN

You may also note that the most optimal SRH profiles were shunted further to the east, though shear profiles were still favorable for supercells.

21z (4 PM CDT) storm-relative helicity profiles - extent of directional shear in the lowest 1km of the atmosphere

0-6 km shear profiles of 40-45 kts exists across the region at 21z (4 PM CDT)

As the area of low pressure moved east, you will notice that the winds became more veered as the day progressed, aligning more parallel to the surface cold front that was approaching from the I-29 region by very early afternoon.  This surface convergence would later ignite the first cells across the northern target in central IA after 22z (5 PM).

Note the change of surface wind direction and speed as the day progressed.

The 12z DVN sounding indicates that the environment across the region is favorable for supercells with ample shear, yet some dry air is evident, which, along with cooler temperatures aloft, will later serve to enhance lapse rates and contribute to a large hail threat.

12z (7AM CDT) DVN observed rawinsonde

To the west, a special sounding from Topeka, KS at 20z (3 PM CDT) shows a strong cap in place, though this would begin to erode east of I-35 by early evening.

Topeka, KS (TOP) special 20z rawinsonde (note the EML near 800 mb)

However, as the afternoon progressed and instability materialized as forecast, stronger forcing began to approach from the WSW.  By later afternoon the SBCAPE values across the region stood AOA 2,000 J/KG, with the convective inhibition aloft (CIN) having gradually eroded.  The MLCAPE values were approaching 3,000 J/KG just across the border into N MO.

21z (4 PM CDT) Surface-based CAPE (instability)

21z Mixed-layer (MLCAPE) CAPE (instability)

Despite the ample instability that had materialized across the region, LCL heights were not looking quite as favorable in regions further south, away from the warm front, which had earlier shown promise of perhaps a discrete cell or two forming along the moisture axis near the main low and, as had been hoped, perhaps riding parallel to the front and remaining rooted in the boundary layer.

21z (4 PM CDT) LCL Height Profile

Nonetheless, conditionally instability was present due to favorable cooling with height, as lapse rates across the region at the mid-levels were approach 8-9 C/1000 m; 0-3 km low-level lapse rates were approaching 7.5-8 C/km.

21z (4 PM CDT) 0-3 km lapse rates

21z mid-level lapse rates

As surface convergence approached from the west, we can watch the following visible satellite loop as storms literally explode across SC IA between 2215z (5:15 PM) and 2245z (5:45 PM).

We can also see how rapidly the storms exploded on a radar loop from the NWS DMX (Des Moines) between 2230z (5:30 PM) and 2300z (6:00 PM).

We chased the first cell that erupted near Sigourney, IA, and played the cell east to the Hwy 218 corridor, dropping south the near Ainsworth before giving up the chase due to darkness.  We saw some great structure, a few wall clouds with disorganized, weaker rotation, and some golfball sized hail.  No tornadoes were reported across the immediate region on this day.

JLR