04/09/11 Chase Report – Sac and Pocahontas County, Iowa Tornadoes
Posted on 11. Apr, 2011 by Jesse in Uncategorized
I chased this day with Brad Goddard and Jodi Irvin; we initially targeted Council Bluffs, IA as a preliminary starting spot, but decided to move north to the Missouri Valley truck stop exit when a cumulus field started to build northwest of Omaha between 4 PM and 5 PM. A healthy storm popped up west of Columbus, NE and trekked towards the I-29 corridor, so we decided to move north to the Sloan, IA exit in order to intercept the storm after it crossed the Missouri River around 6:30 PM. We observed the cell produce several grandiose gustnadoes (and possibly one brief tornado) between Whiting, IA and Mapleton, IA on County Hwy L12 before dropping south of Castana and re-intercepting the cell about 5 miles east of Mapleton. Though we missed the Mapleton tornado, we watched this cell produce more tornadoes after dark from near Arthur, IA all the way up to near Pocahontas, IA.
I can honestly say that is was my most magnificent nighttime chase ever, and I witnessed the most spectacular, substantial RFD gusts and gustnado show that I have ever seen.
Brad created a GPS log overlay with the NWS damage assessments, which shows our path in blue, along with the trek of the tornadoes.
View GPS Track and Suveyed Tornado Path 4/9/11 in a larger map
You might notice that we crossed paths with one of the earlier tornadoes. Towards the beginning of the nighttime tornado footage, you’ll notice that we had to turn around because of a tree that was down, which was likely caused by the first tornado (A); we likely came across the tree damage will filming tornado (B), which was off to the right and clearly visible through power flashes.
As you can see below, the Storm Prediction Center had the area outlined in a moderate risk for severe weather, with a 10% tornado probability area denoted across the region, though the major risk for the day was large hail, which prompted the upgrade to a moderate risk on the morning outlook.
SPC's 20z (3 PM CDT) SWODY1
SPC's 20z (3 PM CDT) SWODY1 - Tornado Probability
Synoptic and Mesoscale environment
A powerful upper-level trough currently situated over the eastern Great Basin region of the United States, digging eastward as it prepared to take aim on the Upper Midwest, is visible on this image loop from 18z (1 PM CDT) to 03z (10 PM CDT).
The associated 500 mb mid-level perturbation (see loop below from same time frame as above image) with associated height falls began to spread ENE across the far southwestern Plains, with a 90-100 kt jet core having rounded the basal side of the mid-level trough as it ejected ENE on the leeward side. By late Saturday afternoon, the heightened risk area lay downstream the exit region of the main mid-level jet core, which tends to be an area traditionally favored for enhanced PVA, divergence and WAA.
However, unlike major severe weather outbreaks, the absolute strongest mid and upper-level dynamics were still well south of the region, but would provide the focus for what was progged to be a more substantial risk of SVR weather on Sunday.
At the surface, a 990mb low pressure cyclone was stationed over NE CO, with an attendant WF just north of Omaha (OAX) extending SSE into SE IA, while a pacific front/dryling extended southward to the Red River Valley. Extremely ambient low-level shear was maximized along the front, which would provide the main locus for tornadogenesis as the cells that we intercepted rooted in that frontal boundary and rode parallel to this warm front, taking advantage of the magnanimous helical environment (turning of the winds with height).
Satellite/Surface Composite Map at 2230z (5:30 PM CDT)
On this afternoon and evening surface map loop, note the winds backing out of the SSE in the vicinity of the warm front as it nudged into far NW IA, ahead of pressure falls moving eastward ahead of the parent surface cyclone. Also, another area of low pressure moves into the far northeast NE/southeast SD border region per pressure readings and surface wind orientation on the METAR plots. This likely served as a focal point to enhance low-level convergence and strengthen the shear as t-storms initiated south and east of this meso-low.
A 30-35 kt 850 mb low-level jet was nudging northward behind the surface warm front into the region of maximum warm air advection, allowing copious amounts of gulf moisture to stream northward ahead of the maturing trough. You’ll note that the winds actually increased to 50kts after 02z (9 PM); this increase in wind speed with time will serve to enhance hodographs (clockwise turning of the winds with height) and would explain why the region experienced a spike in tornadic activity in the hour or so following the Mapleton tornado, which fellow Convective Addiction member Skip Talbot, along with riders Mike Boik and Jennifer Brindley, intercepted and can be viewed in this video.
As the aforementioned low-level jet strengthened, we can see a little THETA-E ridging develop in NW IA in the region with warmer low-level temperatures and dewpoints in the vicinity of the front. This gives us a measure of instability and strength of the low-level jet, too.
In the wake of ample surface heating behind the warm front, a large portion of the warm sector had destabilize throughout the afternoon. In fact, rather capacious instability values >/=4,000 J/KG (SBCAPE) and 4,000 J/KG (MLCAPE) were present across the region which, when combined with the shear, made for an extremely volatile combination once the convective inhibition eroded. This was slightly higher than what had been progged by some of the models the night before. The two CAPE images below depict this widespread unstable airmass that was present across the region in the hour or so before dark.
Mixed-layer CAPE at 00z (7 PM CDT)
Surface-based CAPE at 00z (7 PM CDT)
This extreme instability was juxtaposed within a region of 6 km bulk shear values of 60 kts and 0-1000m SRH values AOA 350 m2**s2 across NW IA.
0-6 km Shear at 00z (7 PM CDT)
0-1000m storm relative helicity at 00z (7 PM CDT)
These combined helicity and instability parameters, sometimes referred to as the energy-helicity index (EHI), were quite large across the region (as high as 7), indicating a favorability of significant supercells once convection initiated. The SPC’s significant tornado parameters were also showing a bullseye over this region.
0-1 km EHI at 00z (7 PM CDT)
00z (7 PM CDT) Significant Tornado Parameters
The stout southwesterly flow in the lower-levels proffered the existence of an elevated mixed layer (EML) throughout most of the day, which prevented initiation from occurring too early and allowed instability to maximize itself in the wake of good daytime heating.
The EML is a layer of stable air in the lower levels of the atmosphere that prevents thunderstorm updrafts from penetrating this “lid.” Since this is essentially a layer of warm air aloft over cooler temperatures at the surface, it stabilizes the region and prevents what would oftentimes be explosive and vigorous updraft development. It’s denoted by the red line on the sounding, which is atmospheric temperature, moving to the right at around 850 mb, denoting warm temperatures in an otherwise inverse proportion between falling temperatures and increasing height. More on how to read a SKEW-T sounding can be found here on our education page.
A special penultimate afternoon sounding (4PM – 21z) at Omaha denotes a strong cap, which had weakened significantly by the time the last evening sounding (7 PM – 0z) is launched. It’s obvious on these soundings that lapse rates are also very strong, and this plume of steep lapse rates in the favored hail growth zone of the mid-levels helped contributed to a large hail threat. Surface-based lifted indices (LI) of -11 indicated that explosive parcel accelerations were likely with vigorous updraft development as convection initiated.
21z (4 PM) special sounding at Omaha (OAX) denotes a strong cap still in place
00z (7 PM CDT) sounding reveals that the CINH has weakend significantly
By 9 PM CDT (02z), the LCLs (level of parcel saturation for condensation) across northwest IA had dipped to around 750m; LCL heights from approximately 500m to 800m above ground level are oftentimes associated with stronger tornadoes. The lower LCL heights and low surface dewpoint depressions, indicative of higher levels of low-level relatively humidity, suggest a thermodynamically bouyant RFD, which is believed to be a significant ingredient in tornadogenesis.
The satellite loop below shows an agitated cumulus field developing between 2145z and 23z (4:45pm to 6 pm), and then initiation occurs shortly thereafter as the atmosphere destabilizes completely in the wake of the eroded EML, allowing explosive thunderstorm development. Once the storms reach the warm front and ride parallel to the boundary, they capitalize on the ambient helical environment, as we previously discussed, and metamorphose into tornadic stallions.
We can also watch this explosive growth on a radar loop from Omaha during the same time frame, as the storms take a good hour or so to mature, and then blast east into the evening, riding the boundary and producing a spectacular nighttime tornado festival.
Following the Mapleton tornado, which we missed, explosive tornadic development occurred as the LLJ ramped up after dark. Preliminary damage surveys by the National Weather Service in Omaha (click here) and Des Moines (click here) have indicated that no less than seven tornadoes touched down on Saturday night, and further reports are likely to be forthcoming.
This was an extremely intense chase for us, as we witnessed a number of tornadoes, some large, as well as plenty of damage. However, there were no serious injuries or deaths, due in large part to the efforts of a diligent NWS warning process and their dedicated personnel, along with chasers and spotters who relayed timely details that provided the NWS with ground truth verification of what they were seeing on radar. Meteorologist Mike Smith wrote a blog on this today, and it is worth a read (click here).
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