ON THE USE OF REAL-TIME OBSERVATIONS TO CONFIRM ETA MODEL PREDICTED TRENDS IN THE TEMPERATURE STRUCTURE OF THE LOWER ATMOSPHERE TO FORECAST A SOUTHWESTERN KANSAS FREEZING DRIZZLE EVENT

Ryan C. McCammon
National Weather Service Office
Dodge City, Kansas

I. INTRODUCTION

A prolonged freezing drizzle event occurred over a relatively small portion of the Dodge City County Warning Area (CWA) (Figure 1) from 2331 UTC 30 January 1999 through 1137 UTC 31 January 1999. This resulted in ice accumulations of up to one half inch. The impact of this icing was an increase in traffic accidents and the snapping of large tree limbs, including one that fell onto a parked vehicle (Figure 2).

Figure 1 >Figure 1. Dodge City County Warning Area with data locations cited in paper.



Figure 2

Figure 2. Tree limb on top of parked car in Dodge City resulting from ice accumulation.



A recent study by Bernstein and Brown (1997) indicated a relative maximum in freezing precipitation events (Figure 3) over much of the Dodge City, Kansas , CWA. Given this, a need for further study to predict such phenomena seemed warranted. The purpose of this study was to compare and to contrast ETA sounding and partial thickness forecasts to the evolution of the real atmosphere so as to validate the ETA's sounding forecasts as a useful tool for predicting precipitation type for southwest Kansas.

 

Figure 3
Figure 3. Frequency of freezing precipitation (FZRA & FZDZ) in hours per year. Climatology taken from 30 years of data from 207 stations, (Bernstein and Brown 1997).

 



To assist in verifying ETA sounding forecasts for this event, a combination of surface and upper air data were employed. Wind data from a nearby wind profiler and upper air sounding data from an Atmospheric Radiation Measurement project (ARM) sounding site were also used to evidence indirect but important impacts on precipitation type. The results demonstrate the continuing need for such data sets.

II. SYNOPTIC SITUATION

At 0000 UTC 29 January, a shortwave trough existed at 500 mb over the Arizona/New Mexico border. By 0000 UTC 30 January, this wave developed a closed, cyclonic circulation (also evident at the 850 and 700 mb levels) over southwestern Texas. Over the next 36 hours, the closed low system moved northeast to northern Arkansas by 1200 UTC 31 January. Throughout the upper low's movement, low-level moist, warm air flowed northwest from the Texas/Louisiana Gulf Coast region. Specifically, during the afternoon of January 30, warm moist air moved over a relatively shallow, cold and dry layer near the earth's surface located from north-central Kansas to western Nebraska and eastern Iowa. The dry air mass is demonstrated by Figure 4. The shallow cold, dry air advected southwest throughout January 30 into early January 31. Light precipitation occurred periodically over most of western Kansas during the afternoon and evening hours of January 30. At 1920 UTC 30 January, light rain began at Dodge City and ended at 2130 UTC. Freezing rain began at 2230 UTC and changed to freezing drizzle at 2331 UTC. The freezing drizzle continued for 12 hours, until 1137 UTC 31 January. This prolonged duration produced the heavy accumulation of ice and resultant problems.

 

Figure 4 Figure 4. 1800 UTC 30 January regional surface chart. Station model is standard.

 



III. FORECAST PROBLEM: FORECASTING PRECIPITATION TYPE

The main forecast challenge was whether low-level warm air advection would be sufficient to change freezing precipitation to liquid. With model forecasts indicating the potential for prolonged precipitation from 0000 UTC 31 January to 1200 UTC, the precipitation type decision was critical. A precipitation-type forecasting technique that utilizes thicknesses of various isobaric layers (e.g., 1000-700 mb, 850-700 mb) can be found in a paper by Keeter and Cline (1991). Typical precipitation-type partial thickness thresholds have been identified for western Kansas. For instance, Phillips (1999) suggested typical rain/snow partial thickness thresholds of 1540 m for 1000-850 mb and 2860 m for 1000-700 mb. For this forecast situation, these thresholds were relevant because the ETA-derived predicted partial thicknesses for these isobaric layers suggested the precipitation would be liquid (Figure 5). Given that the surface temperatures were expected to be sub-freezing due to evaporative cooling, a logical conclusion would be to forecast a freezing precipitation event for Dodge City, Kansas during the period from late on January 30 to early on January 31.

 

Figure 5 Figure 5. 30-hour ETA forecast of 1000-700 mb thickness valid 0600 UTC 31 January.

 



However, forecasters opted for rain as the dominant precipitation form, largely because of the low-level warm air that was in place during the afternoon of January 30. The Saturday afternoon area forecast discussion further illustrated this thinking, "...do not expect much in the way of falling temperatures through midnight and with layer of warm air just above the surface remaining in place through at least 0600 UTC, expect most of the precipitation will fall as rain." However, sounding forecasts from the 0000 UTC 30 January and 1200 UTC ETA valid at 0600 UTC 31 January (latter sounding presented in Figure 6) indicated saturated, sub-freezing conditions from the surface to 800 mb, surmounted by a dry layer. The 0000 UTC 31 January ETA sounding forecast valid at 1200 UTC (Figure 7) showed the strongest freezing drizzle signal of all sounding forecasts. Thus the ETA's forecasts were depicting that a classic freezing drizzle sounding (McNulth, Griffin, and Fuchs 1995) would occur from 0600 to 1200 UTC. To a lesser extent, the NGM and AVN forecast soundings from January 30 hinted at this same freezing drizzle scenario, although the low-level predictions from both models were warmer than the ETA's solution. All of the model runs, particularly the ETA, were consistent from run-to-run with the portrayal of the freezing drizzle sounding.

 

Figure 6 Figure 6. 30-hour ETA sounding forecast for Dodge City valid 0600 UTC 31 January.

 



 

Figure 7 Figure 7. 12-hour ETA sounding forecast for Dodge City valid 1200 UTC 31 January.

 



IV. DISCUSSION

Significant lower tropospheric warm air advection had occurred prior to the period 0000 UTC 31 January through 1200 UTC, the period of time that the ETA forecasted the freezing drizzle sounding. The main question, therefore, was whether the warm air advection would change any precipitation to all liquid. In order for the ETA forecast solution to be achieved, a "source" for low-level cold air was necessary. That source was less than 32°F dew point air that had the potential to cause evaporational cooling, provided precipitation could occur within this environment. A lower tropospheric dry dome of air over Nebraska was such a source. The North Platte, Nebraska soundings from 0000 UTC and 1200 UTC 30 January plus the Valley, Nebraska soundings from 1200 UTC 30 January and 0000 UTC 31 January (Figure 8) showed the low-level dome of dry air. This dry dome was evident throughout 30 January with the 1800 UTC surface chart showing dew points (°F) ranging from the lower 30s in western Kansas to the middle 20s in Nebraska and eastern Iowa (Figure 4).

 

Figure 8 Figure 8. 0000 UTC 31 January UTC Valley, Nebraska sounding.

 



In order to increase the magnitude of low-level dry air flow from Nebraska to southwestern Kansas, surface to 800 mb wind trajectories would have to be from the north to northeast during the afternoon of January 30 into the next morning. Haviland wind profiler data showed winds in the lowest 1000 m backing from east/southeast to north/northeast from 1245 UTC 30 January until 2100 UTC (Figure 9), apparently confirming such low-level dry air advection toward southwest Kansas. With freezing rain and drizzle already occurring by late afternoon on January 30, additional wet-bulb cooling in the surface to 800 mb layer would support a continuation of the freezing precipitation threat.

 

Figure 9 Figure 9. Haviland, Kansas wind profiler data. Ordinate is height in meters above mean sea level. Wind barbs in knots with colors corresponding to speed.

 



Dodge City soundings from 1200 UTC 30 January and 0000 UTC 31 January verified that low-level cooling had taken place during the day, further indicating that ETA forecasts for 0000 UTC 31 January to 1200 UTC were reasonable. Additionally, data from the Hillsboro, Kansas ARM site from 0300 UTC 31 January and 0600 UTC (Figure 10) also confirmed the low-level sub-zero °C temperature profile's persistence. The 1200 UTC 31 January Dodge City sounding (Figure 11) verified the ETA forecast of a classic freezing drizzle sounding. In fact, the 1200 UTC 31 January Dodge City sounding was even colder in the surface to 800 mb layer than the ETA had predicted.

 

Figure 10 Figure 10. 0600 UTC 31 January Hillsboro, Kansas sounding with wet-bulb curve in purple.

 



 

Figure 11 Figure 11. 1200 UTC 31 January Dodge City sounding.

 



V. SUMMARY AND CONCLUSION

ETA forecast soundings 18 to 30 hours prior to the onset of the 12-hour freezing drizzle event predicted a vertical thermodynamic profile supportive of freezing drizzle by 1200 UTC 31 January. Contrarily, tremendous low-level warm air advection occurred prior to and through the early part of the event, leading forecasters to believe that freezing drizzle would change to all liquid before the ETA forecast verification times of 0600 UTC 31 January and 1200 UTC. However, dry air existed in the surface to 800 mb layer to the north and northeast of the Dodge City CWA. Haviland profiler data and surface observations helped detect this dry air advecting southwest toward southwestern Kansas throughout January 30. Precipitation falling into the low-level dry air helped maintain the temperature below 32°F from the surface to 800 mb. Hence, evaporative cooling offset the warming due to advection such that there was the production of a sub-freezing layer at and just above the earth's surface to support freezing drizzle.

As a result, an environment favorable for freezing drizzle evolved in western Kansas. Since model forecast soundings showed the lowest sub-freezing layer to be below 800 mb, a partial thickness forecasting technique was of limited value in determining precipitation type for this particular case. Indeed, all thicknesses using 1000 mb as the lower level are compromised because that level is below the surface (Dodge City being near 925 mb). The better vertical resolution of ETA model forecast soundings was superior.

Overall, the ETA model forecast soundings for Dodge City anticipated the event well. Haviland wind profiler and Hillsboro ARM sounding data corroborated the ETA forecast sounding by showing the potential for evaporative cooling to counteract the low-level warm advection. These two data sources were extremely valuable in tracking the ETA model predictions versus the real atmosphere's evolution. In this particular case, the ETA model forecast soundings for Dodge City accurately predicted the evolution of the vertical temperature profile.

VI. ACKNOWLEDGMENTS

I would like to thank Edward Berry (SOO at Dodge City) and Steve Hunter for their input regarding the improvement of the manuscript and of the graphics. I also would like to thank Elizabeth Page (of COMET) for providing data for this study.

VII. REFERENCES

Bernstein, B., and B. Brown, 1997: A Climatology of Supercooled Large Drop Conditions Based Upon Surface Observations and Pilot Reports of Icing. Preprints, 7th Conf. on Aviation, Range and Aerospace Meteorology, Long Beach, CA, AMS (Boston), 82-87.

Keeter, K., and J. Cline, 1991: The Objective Use of Observed and Forecast Thickness Values to Predict Precipitation Type in North Carolina. Weather and Forecasting., 6, 456-469.

McNulty, R., J. Griffin, and M. Fuchs 1995: Winter Precipitation Type. NOAA, Technical Attachment NWS CR-88-4., DOC/NOAA/NWS Central Region Scientific Services Division, Kansas City, MO., 29 pp.

Phillips, G., 1999: Critical Thickness Study. Available as an unpublished manuscript located in the Dodge City Winter Weather reference manual.


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