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1. 8:00 pm CDT on August 13th. A second straight day of scattered marginal severe weather has just ended. Some impressive rainfall totals (see map below) caused minor flash-flooding in parts of western Oklahoma. Examine the Flash Flood Guidance from the Arkansas-Red River Basin Forecast Center.
- About how much rainfall in one hour would it take to cause flooding along I-35 south of OKC? In three hours? In six hours?
- What about Beckham and Greer Counties in far western Oklahoma?
- Why do you think these values are different?
Rainfall Totals
One-hour FFG
Three-hour FFG
Six-hour FFG
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2. It is now 8:30 pm on August 13th. There are still heavy rainshowers in parts of Oklahoma. The severe threat has diminished, according to the National Weather Service. Examine the latest forecast discussion, released earlier in the afternoon by the Norman WFO.
Norman, OK Radar
- What are the major threats mentioned in the forecast discussion?
- What are some issues of uncertainty, according to the forecaster?
- Why is the position of the surface front important to track?
- What datasets can you use to track it?
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3. Still 8:30 pm. Examine the following surface map from the Oklahoma Mesonet.
- Where would you place the surface front, based on Mesonet winds?
- Where would you place the surface front, based on Mesonet temperatures?
- Where would you place the surface front, based on Mesonet dew points?
- In what direction is it oriented (north-south, etc)?
- Are dewpoints high enough to support convection, provided other ingredients are in place?
Surface Map
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4. Still 8:30 pm. Examine some upper-air maps (925mb, 850 mb, 500 mb) from 0000 UTC (7pm).
- The 925 mb level is very near the surface. From what direction are the winds at 925 mb? What appears to be their ultimate source region?
- The 850 mb level about 5,000 ft above sea level. From what direction are the winds at 850 mb? What appears to be their ultimate source region?
- Would you characterize the moisture available to Oklahoma and North Texas as "abundant"?
- From what direction are the 500 mb winds across Oklahoma?
- Are the 500 mb winds strong or weak? (At 500 mb, 50 knots is considered "weak")
- Compare the 500 mb winds with the orientation of the surface front. If the forecasted convection indeed occurs, what direction will it go? How quickly will it propagate?
- Why are these ingredients important on this day?
925mb
850mb
500mb
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5. Examine the following 8-hour loop, valid from 10:00 pm on the 13th to 6:00 am on the 14th. BREF1 from Norman is shown, plus winds and temperatures from the Oklahoma Mesonet.
- Where is the main initiation focus for storms?
- In what general direction do they propagate?
- Is this consistent with your expectations?
Norman, OK Radar
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6. At 4:00 am CDT on the 14th, the Norman WFO issued the following Warning Decision Update.
- What does it tell you about using radar-estimated precipitation for the rest of the day?
Norman WDU
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7. The rains continued into the afternoon and died down by 5pm. Examine the 48-hour rainfall totals, the latest Flash-Flood Guidance, and the forecast discussion from the Norman WFO.
- What portions of Oklahoma received the most rainfall?
- How have the guidance values changed in this region, compared to those from the day before?
- What are the main hazards facing southwestern and southern Oklahoma?
48-hour rainfall
One-hour FFG
Three-hour FFG
Six-hour FFG
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8. It's 7:00 pm on the 14th. Examine the Mesonet surface conditions and the 500 mb chart.
- From what direction are the 500-mb winds blowing in the region?
- What are the 500-mb wind speeds?
- How is the surface front aligned, according to Mesonet data?
- Where would you expect storms to initiate, and in what general direction would they travel?
- Is there sufficient moisture to sustain heavy precipitation?
500mb
Mesonet Map
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9. Examine the following 4-hour loop, valid from 7:00 pm to 11:00 pm on the 14th. BREF1 from Norman is shown, plus winds and temperatures from the Oklahoma Mesonet.
- Where is the main initiation focus for storms?
- In what general direction do they propagate?
- Is this consistent with your expectations?
- Where do flooding hazards appear most significant?
Norman, OK Radar
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10. Examine the storm-total precipitation at about 3:00 am on the 15th.
- Where has the most rainfall ocurred? How much has fallen?
- How is the axis of rainfall aligned with the front? With the 500-mb winds?
Storm-total Precipitation
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11. Here is the Mesonet "Rainfall vs. FFG map at 3:00 am on the 14th.
- Which counties show rain-gauge data exceeding FFG?
- Which counties show rain-gauge data approaching FFG?
- How many rain gauges go into the determination for each county?
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12. Examine the one-hour radar precipitation accumulations from KTLX ending at about 3:00 a.m. on the 15th.
- How much rainfall occurred in Stephens County between about 9:00 p.m. and 3:00 a.m.? How about Carter County?
- Do these accumulations exceed the 6-hour FFG product?
- How much rainfall occurred in Stephens County between midnight and 3:00 a.m.? How about Carter County?
- Do these accumulations exceed the 3-hour FFG product?
- Look at the one hour precipitation accumulations for northwest Carter county. Which of the one-hour accumulations exceed the one-hour FFG guidance?
One-hour rainfall
One-hour FFG
Three-hour FFG
Six-hour FFG
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