P4.6

P4.6 DOCUMENTATION OF THE OVERLAND REINTENSIFICATION OF TROPICAL STORM ERIN OVER OKLAHOMA, AUGUST 18, 2007

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Figure 1. Track of Erin’s center at 6 h intervals (black symbols) and forecast of 12 h positions after 12 UTC 18 August 2007 (Source: U.S. Navy).


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Figure 2. Oklahoma Mesonet meteograms from Watonga and Ft. Cobb, OK, showing sharp pressure fall and strong wind shift between 1000 and 1300 UTC (0300 to 0600 CDT) as eye of redeveloped Erin passed. Note wind gusts between 70 and 80 mph (31-36 m s-1) at both locations.

 

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Figure 3. Total rainfall (inches) observed by Oklahoma Mesonet, 0000 UTC 18 to 0000 UTC 19 August 2007.

 

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Figure 4. Severe weather reports for 8/18/07 to 8/19/07 from Storm Data, via SPC. Red paths denote tornadoes with F (EF) Scale ratings. Blue dots signify convective wind gusts ≥50 kt (25 m s-1) severe criteria, with values in kt. No hail reports occurred with this phase of Erin’s lifespan.

 

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Table 1. Severe weather reports in Oklahoma associated with Erin (August 19) (Source: NWS Forecast Office Norman)

 

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Figure 5. Enhanced infrared satellite images at (a) 0040, (b) 0240, (c) 0440, (d) 0640; (e) 0840; and (f) 1040 UTC during rapid intensification of Erin as the storm crossed into southwest Oklahoma. Expansion and cooling of cloud tops occurred through 0840 UTC, with warming from about 1040 UTC onward.

 

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Figure 6. (Base reflectivity (0.5° tilt) from KTLX for (a) 1102; (b) 1152; and (c) 1224 UTC representing snapshots of the evolution of the precipitation-free eye over central Oklahoma.

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Figure 7 – Base reflectivity (1/2 deg tilt) from KTLX (Oklahoma City radar) at 1024 UTC overlain on conventionally plotted METAR and Oklahoma mesonet surface data, illustrating closed wind circulation collocated with Erin’s redeveloped eye. Note also the warm core structure evident in the surface temperature observations (°F).

 

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Figure 8. Subjective analysis of isobars (black) and isotherms (dashed red) at 1007 UTC 19 August 2007. Outside storm-scale cold pools generated by convection around the center of the storm (and evidenced by the outflow boundaries), the surface temperature field indicates that the cyclone was warm core.

 

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Figure 9. Conventional plot of Oklahoma Mesonet observations at 0620 UTC 19 August 2007. Wind barbs represent gusts in mph, with temperature and dew point values in °F. Over the next 10 h wind fields were strongest near the ground, and weakened with height, as is expected for warm core lows.

 

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Figure 10. 72 h backward trajectory analysis (tick marks at 6 h intervals) of the 10 m AGL (surface) parcel, ending at the star (Oklahoma City) at 1000 UTC 19 August. The graph below, read from right to left, shows relative humidity of the parcel with time following the trajectory, with tick marks corresponding to those on the planar map. Courtesy NOAA.


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Figure 11. Plots obtained by NCEP reanalysis for 12 UTC 19 August 2007 of (a) 1000 mb heights (m) overlain with total precipitable water in the column from the surface to the top of the atmosphere (kg m-2); (b) 500 mb heights (dm) overlain with θ (K); and (c) 500 mb heights (dm) overlain with 1000 mb divergence (10-5s-1).


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Figure 12. 500 mb temperatures (oC) and 850 mb heights (dm) at 12 UTC 19 August 2007, obtained by NCEP reanalysis. Note the collocation of the warmest temperatures 500 mb temperatures with the lowest heights at 850 mb (and other lower levels, not shown). Baroclinic systems would show colder temperatures on the western portion of such cyclones.


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Figure 13. (a) Composite soundings and hodographs for right-front quadrant of landfalling tropical systems (from McCaul 1991) (bold—fast moving systems; light—slow moving systems); and (b) ADAS sounding and hodograph for KOUN (Norman, OK) at 1000 UTC 19 August 2007.

 

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Figure 14. Percentage of normal precipitation (top) and observed soil moisture (Fire Weather Index units, FWI) over Oklahoma for July 2007 (Source: Oklahoma Climatological Survey).


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Figure 15. 2006 and 2007 Oklahoma precipitation monthly totals relative to normal (Source: Oklahoma Climatological Survey).

John P. Monteverdi*