Tuesday, April 12, 2011

April 9 tornadoes in NW Iowa: Contrasting daytime/nighttime environments?






Last Saturday's tornadoes in northwest Iowa with a persistent supercell complex over a period of several hours were quite interesting. The town of Mapleton took a direct hit before dark with a large dusty EF3 tornado developing from a rather high cloud base after 7 pm CDT (thankfully, there were no deaths or life-threatening injuries). Then, after a "break" of about an hour, the same supercell reorganized and began producing several large long track tornadoes during a 2-3 hour period after dark (see nighttime video shot above of wedge tornado near Odebolt, from Shawna's and my storm chase that night). It is very fortunate that none of these tornadoes after dark hit towns such as Sac City or Pocahontas directly (see tornado track maps above).

The first weak dust whirl tornado near Onawa around 6:50 pm CDT (see photo above, a "tornado" because it occurred within a mesocyclone both on radar and visually ahead of an RFD, under a condensation funnel at cloud base) was a precursor to the tornado from the same supercell that hit Mapleton about 30 minutes later. The environment at that point had relatively high MLLCL heights (around 1500 m), with an 83/61 F surface ob at Onawa, and a fairly steep low-level lapse rate (approaching dry-adiabatic, see the 00 UTC Omaha observed sounding above) with little MLCIN. The later Mapleton tornado was mainly visible due to thick swirling dust rising from the ground into a wedge shape under a similarly "high" cloud base.

The tornadoes after dark were associated with a somewhat different environment (see 02 UTC Denison IA RUC analysis profile above). Low-level lapse rates were no longer steep, and MLLCL heights had lowered to below 1000 m with diurnal cooling while MLCIN of 50-100 J/kg had built in. However, the low-level jet had intensified to around 60 kts just after dark, at least doubling the 0-1 km SRH present before dark; according to my colleague Andy Fischer, VAD wind profiles from the NWS Des Moines radar suggested that 0-1 SRH feeding the NW Iowa supercell complex may have actually been greater than 700 m2/s2! Additionally, MLCAPE increased (opposing the typical diurnal trend) as moist advection via the low-level jet caused surface dew points to rise 2-3 deg F. The resulting strong CAPE/SRH combinations (0-1 km EHI > 10) along with large deep layer shear (> 50 kts) probably helped updrafts and low-level mesocyclones overcome increased near-surface MLCIN to support significant tornado potential after dark. This environment matched those ingredients typically associated with tornadoes after dark in the Plains, as discussed in Davies and Fischer (2009).

Forecast-wise, this day stood out for 2-3 days ahead of time with consistent model forecasts of strong CAPE and SRH values (see NAM 0-1 km EHI forecast above) coinciding with an area of low-level CAPE (see NAM 0-3 km MLCAPE forecast above), implying a strongly unstable and sheared evening and early nighttime environment that would be relatively surface-based over eastern NE/western IA. The only big question was whether warm temperatures aloft (the "cap", not shown) would spread too far out across the Plains area in SSW to NNE mid-level flow ahead of the deep 500 mb trough in the western U.S. (see NAM 500 mb forecast above) to inhibit convection along and south of a northward moving warm front (not shown).

On the morning of the event, however, the NAM forecast of derived radar reflectivity (see above) strongly suggested storms would fire along and south of the warm front over northeast NE and northwest IA around and after 00 UTC 4/10/11. In addition, the NAM 12-hr forecast 850 mb forecast (also above) showed an early evening S to N low-level jet developing over eastern KS/western MO flowing north into the warm frontal area over northwest Iowa, providing moisture advection and convergence along with excellent low-level shear for tornadoes increasing into the early nighttime hours.

- Jon Davies 4/12/11

11 comments:

Northern Illinois Storm Chaser said...

Excellent write up Jon! Glad to see you verify our Onawa tornado. Makes me feel a little better. NWS OAX Bryon Miller and I have talked and he seemed intent on saying it was not a "true tornado" I theorized a high based funnel developed and the RFD kicked up all the dust you see blowing to the south of the funnel. I am not sure though. We were 1/2 mile away from it and it didn't look to have much in the way of surface rotation. Cloud base was rotating and the tornado had strong inflow. Mother Nature always finds a way to amaze me!

Jon Davies said...

Danny:

Regardless of whether one calls the dusty but weak Onawa IA vortex a "true tornado", it was definitely associated with cloud features (RFD and cloud rotation) suggestive of a small but rapid mesocyclone evolution (matching radar). This was a definite red flag signal for stronger things to come from the same storm. Those kind of _organized_ features are an important heads up for spotters.

Northern Illinois Storm Chaser said...

Absolutely Jon. We had sat in our spot near Whiting, IA for 20 minutes watching the storm evolve. We knew a circulation was developing because the wind had shifted from SE to NE and actually blew quarter sized hail storns from NE to SW. At this time we noticed a spiraling (almost cinnamon roll) feature in the relatively high based RFB. We finally noticed the classic occlusion which actually drew in smoke from the fire that was sparked by the train that passed earlier. I am just glad someone else verifies this as a tornado as those who are officially rating it are hesitant to do so. Excellent work as always.

tornadodude said...

yeah, I was with MIDSCAR when we called that report in. IMO, regardless of whether it was a legit tornado or not, calling that in and extending the tornado warning saved lives

mesoextreme said...

Jon...always great information contained in your severe weather analysis, pre and post event. Excellent information for those in the EM field and chasers alike...always a pleasure to read your posts and gain your insight.

Corey Sloan

Thomas said...

Excellent write up Jon. It really does a great job of explaining tornado formation on Saturday, and why the event was so extreme, at least in terms of nocturnal storms. It was quite something to watch the lightning-illuminated tornadoes on live stream feed from my friends at Convective Addiction. Thanks.

DM said...

Jon-Here's a look from the opposite end on your first tornado photo.

http://www.flickr.com/photos/stackedplates/5611865106/in/photostream

Great write-up as usual!

Dick

Jon Davies said...

Dick:

Thanks for posting the shot with the surface dirt plume and cloud structure above. Both the high based RFD and short condensation funnel are visible, suggesting that this was a weak tornado.

Northern Illinois Storm Chaser said...
This comment has been removed by the author.
Northern Illinois Storm Chaser said...

One more photo to boot... It nice to see this surrounding view. I can't see how OAX still is hesitant in calling this a "true" tornado. Anyway, seeing this from a different perspective has put my mind to ease.
http://external.ak.fbcdn.net/safe_image.php?d=6e94e6c776f39ac34f363f48618ab1e1&url=http%3A%2F%2F3.bp.blogspot.com%2F-Fjxb0gF_Zxo%2FTaNawSJvxDI%2FAAAAAAAAA4k%2FxGm8E81fUN4%2Fs400%2FIMG_6980edited.jpg

WINGER said...

I just wanted to drop a line and say "Thank You" for these detailed reports. I enjoy reading and learning from you and your blog. I check your site after every outbreak. Thanks, Brad Winger ( Minneapolis, MN)