April 10, 2005

Storm Chase

Central Oklahoma

Editor:  Putnam E. Reiter

Forecast/Setup -

Risk Area:  Slight Risk
Initial Target Area:  Anadarko, Oklahoma

Models consistently progged a shortwave trof to enter the western U.S. and close off over New Mexico early on Saturday.  This year has been characterized by an active southern jet stream with closed lows developing in the central or eastern U.S. every four to five days, for the past three weeks.  Each of these systems has been far enough south latitude to scour out the Gulf, resulting in meager moisture return when the next system approaches.

This system had the same meager moisture to work with, however moisture was still progged to be slightly better than prior systems.  Also, this system was farther west when it was progged to close off.  Models have had difficultly with the pattern and progression of the systems.  Many of the models progged this system to remain progressive even though it was progged to close off.  As such, deep layer lift and cold air advection at the cap level were progged into western Oklahoma by 7pm Sunday.  The result was to be a squall line formation and this activity would then move quickly east across Oklahoma and parts of Texas.  SPC started out the Day 3 with a slight risk and a 10% hatched area over eastern Oklahoma.  The morning Day 2 continued the slight risk and general probabilities.  The afternoon Day 2 saw an upgrade to a moderate risk.  The early morning Day 1 continued the moderate risk with a 15% tornado threat over much of Oklahoma.  The mid-morning Day 1 downgraded the risk area to a slight and it stayed that way the remainder of the updates.  At first we disagreed with the downgrade.  When all was said and done, the downgrade was appropriate.  I am still not in favor of upgrades or downgrades on the 13:00z Day 1.

The actual atmospheric evolution was somewhat different than what the models progged.  The upper system didn't close off until Sunday morning, slowing significantly in the process.  This slow down resulted in timing changes for expected cold air advection at the cap level and deep layer lift.  Our forecasting did not catch this change, however we did anticipate a slightly slower solution.  This year the models have been decent with the dryline prog and on this day they nailed it.  I have to admit I'm usually the first to scream the models are over-driving the dryline.

During the overnight hours convection developed in the Texas panhandle and moved rapidly northeast into northwest Oklahoma.  As daylight occurred widespread drizzle/light rain developed across all of central Oklahoma.  This drizzle slowly migrated east during the day.  The 18z OUN sounding showed a saturated layer up to 650mb.  Not sure why this drizzle occurred, some believe it was the warm air advection pattern.  It did shift slowly east during the day, which would certainly indicate that was the case.  The dryline moved quickly into western Oklahoma and was steadily marching east by early afternoon.  It made the greatest eastward push by 3pm when it ended up near Highway 281.  The early morning showers seriously offset daytime heating across much of central Oklahoma.  In western areas, near the dryline, skies cleared by noon and strong surface heating began.  The showers moved out of central Oklahoma around 4pm and resulted in some warming.  Eastern sections did not clear out.

Before the showers offset heating, the main concern was unidirectional wind shear due to a vertically stacked upper system.  GFS and NAM didn't really agree on it with the GFS offering more helicity in strongly backed surface flow.  With the expected quick progress of the upper system and unidirectional wind shear, a large squall line was progged to develop, as noted above.  While we agreed that a squall line was likely, we felt it would be later in the evening, allowing a few hours for discrete storms.  Alas, none of this was correct as Mother Nature had a few tricks for us.

We expected convective initiation by 15:00 CDT in Oklahoma, however this did not occur.  Eventually storms developed around 18:00 in central Oklahoma.  A single supercell moved across Norman into southeast Oklahoma County and produced a tornado seven miles south of Choctaw.  This storm eventually dissipated as it moved into a hostile environment.  A squall line develop around 19:00 in south-central Oklahoma and quickly moved east into southeast Oklahoma.  However, it wasn't the expected squall line.  By far the best activity occurred in in west-central Kansas where several supercells lined up in this area.

SPC Products:  SWO Day 1 Graphics:  Risk Area - Tornado Prob - Damaging Wind Prob - Hail Prob - Storm Plot - LSRs
Surface Maps:  Temperature - Dewpoint - Wind Speed - CAPE - LI
Text Products:  Watches - Mesoscale Discussions - Tornado Warnings - Warning Decision Updates
Soundings:  OUN
Mesoeta 00z Initialization:  850mb Temperatures - 850mb Dewpoints - 850mb Windspeed - 850mb Vertical Velocity - 700mb Wind - 700mb Temperatures - 700mb Relative Humidity - 700mb Vertical Velocity - 500mb Vertical Velocity - 500mb Temperature - 500mb Vorticity - 500mb Wind - 250mb Wind - 250mb Relative Humidity

The Chase -

Team 1 - Don Clark, Rob Ferguson, Ray Hardy, and Putnam Reiter
Team 2 - Bryan Youngblood
Miles Driven -  180
Departure Time -  13:00
Return Time -  20:45

We left the house about on time, after waiting for Ray to get out of bed.  Conditions in central Oklahoma looked pretty bad, but Mesonet showed warming just to our west.  We headed south on I-44 towards Chickasha.  Once there we took Highway 62 west to Anadarko.  We stopped and filled up, then headed over to a good viewing spot to see what was around.  At this time we had some cumulus, but little in the way of vertical growth.  We looked at data and enjoyed the mild temperatures.  About 15:00 we elected to move east as the dryline was getting close.  We headed back to Highway 62 and went east to Highway 81 north.  We went north about eight miles and stopped to watch the sky a little more.  We noticed vertical growth in the cumulus, but not quite towering cumulus.  After hanging around for a while we drove around Highway 81 for a little bit getting a change of scenery.  About 17:30 we noticed towering cumulus and these observations where confirmed by a severe thunderstorm watch.

Noting that the best activity was to our south we headed back south and turned east towards Amber.  Once in Amber we went south on Highway 92 to Highway 9.  We watched an area of towering cumulus to our north which didn't do much.  We decided to get over to Highway 39 east and head to Blanchard to intercept a better area of convection.  We headed north on Highway 76 and were thinking about making a pit stop and maybe some dinner.  About this time Norman NWS was asking for spotter reports near Dibble.  Seeing that area was just south of us, we went east out of Blanchard and decided to wait for the storm.  To our southwest we had a clear view of the storm's updraft region.  RADAR showed the storm to be unimpressive and visuals indicated the same thing.  However, around 18:30 we started to see signs of persistent area of updraft and some slight rotation.  We watched it for a long time as there wasn't anything great, but periodically it looked interesting.

The storm was moving rather slow at this point and gave us plenty of time to sit there and watch it.  Eventually we did migrate east a mile at a time.  The storm developed supercell characteristics and became cyclic with RFD cycles every 10-12 minutes.  We had to adjust for these cycles as it is easy to focus on the wrong area.  We got caught once south of Goldsby when the NWS asked us for a visual on the wall cloud just south of Norman.  Seeing the error of our ways we got east to I-35 and went north to Highway 9.  We headed east and had a good visual on the updraft area.  The storm continued to show persistent rotation with the area reforming after each RFD cycle.  We continued east on Highway 9 to Northeast 24th, where we went north.  We went to Robinson and then a little east.  Here we were able to watch the wall cloud pass over Norman.  At first it was real broad rotation and then in the span of a few minutes tightened up.  The RFD came around and cut if off.  We saw it reform to the east and drove to northeast 60th and went north.  At this point we got into some serious tree coverage and had trouble getting good visuals.  However, we were still able to monitor the cloud base features.  One time we thought we saw a cone shaped funnel but it had dissipated by the time we got a good view.

At Indian Hills Road we had to turn east and decided to jump over to northeast 72nd.  Here we turned north and continued with the same cloud base observations.  We got to southwest 149th and had to turn east again.  We did so and turned back north at Anderson.  Here is where I made a navigation error.  To this point we had stayed in the inflow area just to the east of the storm.  When I turned us north we got to far north and got overtaken by the storm.  However, night time was upon us and the storm wasn't a huge deal to us.  We got off the main road and allowed the storm to pass.  We got nickel size hail and some 40-50 mph wind gusts.  After this we headed back to I-240 and the west to home.

The storm went on to produce a tornado about 5-6 miles from our location.  It was a brief tornado and the NWS rated it as an F0.

We are happy with how we changed with the atmospheric evolution and how the chase went.  The four of us did a good job of working together and it made the chase a lot of fun.  While the atmosphere didn't completely cooperate we did get the supercell we forecasted and it did produce a tornado.  We did have some trouble getting the gear to work, but a little persistence got things going.

Thanks to Jay and David for data support.

Lessons Learned -

This was a really good chase for us.  The only two things I can come up with dealt with 1) forecasting as the atmosphere did not do what we forecasted and 2) I made a minor road choice error towards the end.  The error didn't cause a problem, as it was getting dark and time to go home.

Multimedia -

No video with this one.

Stills:

Anadarko, pre-convection boredom:  1, 2, 3, 4, 5, 6, 7, 8, 9, 10
East of Blanchard, small rotation area:  1, 2, 3
East of Norman, wall cloud, sporadic funnels:  1, 2, 3, 4, 5, 6, 7
Our Chase Track

Encounters -

Engaged Storm:  East of Blanchard
Tornado:  No - did produce one after dark, but we had disengaged.
Funnel:  Yes
Hail (larger than 0.75 inches):  Yes - Nickel
Wall Cloud:  Yes
Wind (above 57.4 mph):  Got 40-50mph with the RFD near Lake Stanley Draper